A: The FF-180 motors are used in a wide range of products (electric toothbrushes, cordless shavers...) so you can count on finding them somewhere. Try a web search.

• Firgelli Robots has a bare FF-180 gearmotor in stock.

• Pololu robotics has a 35:1 FF-180 gearmotor, but no bare motors.

A: I'm gonna say no. The motor's output is well less than one watt, and we recommend a minimum 4 watts per pound of robot. Move up to at least the FF-050 motor for a beetle.

Q: Are any FF-050 motors still around?

A: Bare FF-050 motors are available direct from BaneBots and many other sources [August, 2016].

A: I'm not current on brushless ESCs. I think this is a perfect question for the RFL forum.

Q: Does the Skyartec 18A Brushless ESC work legtally with the Spektrum AR6110 DSM2 Microlite?

A: See above.

A: I'll assume that you know that either of those motor choices would be massive overkill -- that's why you refer to your design as a 'super wedge'. The gearing and wheel diameter selection will hinge on the arena size in which you expect to compete.

The HTI motor does not bolt right up to the current BaneBots P60 gearbox for the RS-550, and there is no 26:1 ratio currently offered for the BaneBots P60 or P80 gearboxes that mount the RS-775. You'd have to modify the gearbox to mount your HTI motor to the RS-550 version of the P60 gearbox.

The DeWalt motors are both more powerful and reliable than the HTIs, but they are also considerably more expensive. Given that you are already overpowered, you will need to make your own decision as to which path to take. Either way, the 'bot is going to be a real handful to drive. Consider adding a peizo gyro to the radio system if it's uncontrollable.

I tried using the HTI motors with a 20:1 reduction, 3" wheels, and 16.8v driving it on a ~30 lb robot (1 per side), but they smoked. I opened one up and discovered that the brush arms are alarmingly thin copper leaf springs. Do you think cobalts would perform better, or that I should try a motor from a name brand drill?

A: I'm really surprised that you cooked the HTI motors at that voltage and gearing. I would have thought you could push a brick wall at full thottle 'til the battery went flat and not stress that drivetrain. The motors were only drawing about 15 amps to break the wheels loose - very disappointing!

The critical factor in drive motor selection is not whether the motor is brushed or brushless, but rather the design purpose of the motor. Hobby brushless motors and the Astroflight Cobalts were built to spin model aircraft propellers. They are not suited to applications where they bog down and pull high amperage for extended periods. A quality drill motor, in contrast, is made to produce big torque and survive bogging down at high amperage.

There are very good reasons why so many successful robots use drill motors. Save the brushless and cobalt aircraft motors for weaponry. DeWalt rules the drivetrain kingdom!

A: Mark J. here: I'm puzzled - what makes you believe it's shorted to the case? What symptoms does it show? Does it run?

A continuity or resistance tester will tell you quickly if it's shorted - put one lead on the case and the other to either of the input leads. If there's current flow, it's shorted. Unless there is something very obviously awry with the brush holders, the problem is likely insulation melted off the armature windings. That isn't something that can be reasonably repaired -- spend $17.50 and buy a new one.

A: You're providing way too little information for me to give you an answer. Read #21.

A: RS-370s are commonly run at 12 volts, but overvolting always carries some risk. See #25 for guidance.

A: No - the gear reduction ratio is too large. Even if greatly overvolted the robot would be way too slow with any reasonably sized wheels. See #21 for help in selecting suitable motors.

A: Greater motor power does not translate directly into greater pushing force. Pushing force is limited by the weight on the driven wheels, the tire compound, and the arena surface. Excess power will simply spin your wheels.

How much speed you can use is limited by the size of your arena and your skill as a driver. More speed than you can control is more dangerous to your robot than to your opponent.

Torque can be turned into RPM and vice-versa by changes in gear ratio. The question you want answered is 'how many watts of output power should my motor have?'

See #21 and numerous posts in this archive for assistance in selecting a motor.

I know that you don't have enough information to diagnose the problem exactly, but could you tell me what are several possibilities?

A: Things to check:

• 'Thick, dirty, and caked' are not words I want to hear describing gearbox grease. Flush out the old grease and replace with 'slick, clean, and smooth' new lithium ball bearing grease.

• Don't assume that it's the gearbox that is jamming. Debris can enter the motor as well. Check and clean.

• Examine the bearings carefully. A small metal shaving can cause big trouble there. Clean with a solvent and check for any lack of smoothness when spinning. Re-lubricate before re-assembly.

A: It isn't just that motor -- BaneBots has dropped its entire line of small planetary gearmotors and replaced them with larger, heavier gearboxes aimed at BotsIQ and FIRST teams. Rumor is that there just weren't enough sales to justify ordering fresh stock of the small gearmotors.

A few of the old gearmotors are on their 'Closeout Specials' page, but when they're gone I don't think you'll see any more."

Q: Following up on my question regarding the banebots motors, what would be an alternative beetleweight 2-wheel drive motor that isn't too heavy?

A: The B16 is about as close a match as I can find. At 11.1 volts the performance is similar to the BaneBots RF-370 16:1 gearmotor at 7.4 volts, and it weighs about an ounce less."

Q: Should I keep the banebots already in my robot, or should I replace them with the B16s?

A: I don't see an advantage to switching unless you're out of spares and worried about it. You can still get replacement motors from BaneBots - it's the 24mm gearboxes that have gone away.

The B16 has a little less torque and a little less speed, so I'd stick with the BaneBots for as long as I could.

Q: I don't have any spare banebot motors. Should I still stick with the current ones?

A: Your call. If I had a 'bot design that exposed the wheels to impact damage I'd want spare gearboxes in my repair kit. You'll have to judge for yourself how likely it is that you'll need spares.

A: The Scorpion HX is available by special order direct from Robot Power. I suspect the increasing popularity of brushless weapon motors has caused a drop in demand for three-channel ESCs with brushed weapon motor support.

You can always go with a two channel ESC for the drive and add a stand-alone single channel ESC for the weapon motor - your choice of brushed or brushless.

A: All of those solenoids are big, heavy, and likely have more capacity than you need -- but I need to know full details about what you're doing with that 550 motor before I can recommend a control device for it. I may be able to come up with something smaller and lighter.

Q: i am planning to use the motor to drive my 3kg sumo robot. i would need something powerful enough to control the motor.

A: You REALLY don't want a sumo robot controled by solenoids. A solenoid is a simple on/off switch. It will provide either no power or full power - nothing in between and no reverse.

To have any reasonable control of the motor you will require an Electronic Speed Controller (ESC). An ESC will be lighter, will require no interface to work with your R/C system, and will provide full-range speed control in both forward and reverse. See #21 and multiple posts in this archive for help in selecting an appropriate ESC for your specific drivetrain needs.

P.S. - That motor is wild overkill for a 3kg sumo bot. It will overpower the tire grip, spin the wheels, and add no additional push.

Q: can you recomend one?

A: In order to recommend an ESC I need much more information about your robot. I know it's weight (6 kg) and the type of motor you plan to use (Duratrax 550). In addition I need to know:

• the number of motors to be used [2, 4, more?];
• the battery voltage [12 volts?];
• the gear reduction ratio from the motor to the wheel; and
• the wheel diameter.

Example: in a 6 kg robot with two Duratrax 550 motors running at 12 volts with 3" wheels and a 16:1 gear reduction -- each motor will consume 14.3 amps in a full throttle stalled push, so a 20 amp ESC would do very nicely. Changes to the wheels, gearing, or voltage could change the amperage requirement by a large margin!

The calculation above confirms my opinion that the motors are massive overkill for a 6kg robot: the maximum traction the robot can provide requires less than 10% of the maximum torque of the motor to overcome. That much power will be useless in a small sumo arena. Pick some smaller motors!

A: Previously answered - with a diagram. Search this archive for "Can you run 4 motors off a 2 channel speed controller?".

A: That's a mighty thick dustpan, Anthony. You sure of that measurement? The test of a 'best' robot is winning matches, so we'll wait and see about your hammerbot.

The Team Tentacle Torque & Amp-Hour Calculator says your current drivetrain is really slow. Even running at 12 volts, top speed with 1" tires is well less than 0.5 MPH. It's hard to find something that would work well in a beetle with only 1" wheels. The BaneBots 10:1, 25mm FF-180 Gearmotor [no longer available] is a possible choice since your wheels are well protected. Drop the voltage to 6.0 volts and you'll get zero to five MPH in less than 5 feet with 1" wheels. Amp drain is less than 2.5 amps at full push.

Q: hello,
You said [in the archive] that the [Park] EFL-370-1360 [Outrunner brushless motor] is more likely to melt then the EFL-370-1080, i was wondering why is this and how does it melt or burn out?
thanks alot i love the site!

A: It's typical for a brushless motor of a specific physical size to be offered in several different versions based on the diameter and length of the wire used to wind the stationary magnet coils. Larger diameter wire of a shorter length gives a motor that spins faster, produces more power, and consumes more amperage. Amperage produces heat.

In robot weapon applications, brushless motors are stressed close to their maximum ability to get rid of that internal heat before it builds up and literally melts some motor component. The Park EFL-370-1080 is wound with a greater length of smaller diameter wire than the EFL-370-1360. It will produce less internal heat and has a better chance of surviving in a robot weapon application.

A: The Ampflows run so quietly that you might forget to turn it off. Combat robot builders wouldn't care if they screamed like banshees but if you have a suitable power supply and want a quiet, reliable, and compact motor you are well advised to buy an Ampflow.

A: See #23 for an explanation of the difference and a short discussion of correct use of each type in combat robots. Many additional posts on this topic are in this archive.

A: See #25. You haven't told me what use you plan for this motor, but such a mild overvolting (12 volt nominal to 14.4 volt) will likely be fine.

Q: I will be using it in a heavy pushing and lifting.

A: Read #25. If you properly gear the motor to keep it well above stall, you should be fine. If you let it stall it can fry even at its rated voltage.

A: Directly drive? How are you going to do that? Something has to interpret the output signals from the radio receiver and use that information to control the power flow from the battery to the motors. That has to be either a speed controller or a crude relay system with an interface. You can plug a servo directly into the receiver only because it has a small Electronic Speed Controller (ESC) built in -- you can't just plug a motor directly into the receiver.

For a start I'd suggest you browse this archive to get some background and useful links for ESC selection and usage.

A: As discussed previously in this archive, it is our opinion that brushless motors are in general   >> NOT SUITABLE <<   for use in combat robot propulsion drivetrains. Briefly:

• Hobby brushless motors are designed for model aircraft. They pack a great deal of power into a small and light package. They are able to do this only by operating high up in their RPM range in their peak efficiency zone.

• If pressed in the mid to low RPM range where combat robot propulsion motors commonly operate, brushless motors will rapidly overheat and 'melt down' due to the small cooling surface area provided by their diminutive size and mass.

• Brushless motors from quality manufacturers come with complete and accurate data. If you buy a cheap knockoff you generally get poor data, if any. It is the amperage ratings for hobby-grade Electronic Speed Controllers for brushless (and brushed) motors that get grossly exaggerated.

• Performance for all types of electric motors will change when the motors get hot. This is not a condition unique to brushless motors.

Q: My second question about brushless motors is in regard to using them to drive a weapon. This year we would like to drive our weapon weighing about 5 lbs at 15,000-20,000 rpm. We realize it will take quite a bit of power to get a weapon up to this speed, as brushless motors are designed to turn lightweight airplane propellers, not heavy, destructive weapons. Therefore, we have been considering using the combined power of two brushless motors to drive our weapon. However, we are worried that the motors might spin at slightly different speeds, and therefore will constantly be fighting each other and draining power and performance. Is this true? If we could in fact run two brushless motors on our weapon, could we even possibly run both of them on the same shaft? Thanks so much!!

A: Motors for rotary weapons spend most of their time sustaining spinning mass at high speed, which is entirely within the design scope of brushless motors. However, there are serious drawbacks to spinning a weapon at 15K+ RPM. Check the archive for discussions of problems with spinning a rotary weapon at very high speed (lack of 'bite', gyroscopic effect, etc.)

You will be interested in the Team Run Amok Spinner spreadsheet [requires Excel] which models rotary weapon performance based on the mass and shape of the spinning element and the power input from the weapon motor[s]. The spreadsheet graphs RPM and energy level of the weapon over time and can calculate the approximate battery power required.

I have seen many robot weapons powered by multiple motors. A full explanation of why this isn't a problem would take too long, but consider:

• In a tug-of-war, would someone offering to help pull on your end of the rope drain your performance if they weren't as strong as you?

• Do two people on a tandem bicycle have trouble because their pedal-power isn't exactly matched?

A: I'm guessing that you have a BaneBots 36mm gearbox fitted with the RS-385 motor [others were offered] and that you are happy with the performance of that motor in your drivetrain design.

It isn't out of the question to run the RS-385 at 13.2 volts [I've seem them run at 12 volts] but it's really pushing it, and the gear ratio would be very wrong. You could also 'turn down' the drive motor voltage with the 'Travel Adjust' function on your transmitter, but since you asked for a replacement I'll find one for you.

The simple replacement would be a 'long can' version of the same motor, like the RS-395. Speed and acceleration at 13.2 volts are a little better than the RS-385 at 7.2 volts. Maximum amperage draw in your drivetrain is about half that of the RS-385. Weight is about 0.6 ounce greater per motor. The motor shaft has to be shortened a bit to fit within the gearbox.

The battery capacity requirement for the RS-395 is less than that for the RS-385. You haven't told me which of the HK-30 series motors you plan to run or details of the weapon, but you should still be fine running two drive motors plus an HK-30 weapon motor from your 2300 mAh pack.

Q: Thank you so much! This will help me enormously in my design, and the Scorpion motor I would be using would have ben the HK30/1000kv to a 1:1 ratio or someting similar. Thanks again!
Daniel

A: You're welcome, Daniel. The RS-395 makes a nice drive motor for hobbyweights but is generally overlooked.

You don't mention what type of weapon you plan to spin with the HK-30, but you probably know I don't like direct drive spinners -- a weapon spinning 13,000 RPM isn't going to get any 'bite'. See the archive for a full discussion.

A: Mark J. here: in most cases, the answer is 'no'. Although the input signal from a common receiver would be the same going to two speed controlers, each ESC has its own internal 'clock' that regulates the power output pulses. The two ESCs would sometimes cooperate, and sometimes be completely out of synch. This applies to both brushed and brushless motor controllers.

However, there is a special case where two ESCs can be used to drive a single motor. Some large robot motors, (Ampflow, Bosch GPA) have two pairs of brushes set perpendicular to each other in the commutator plane. If you electrically isolate these brush pairs, each pair can be driven by its own ESC and the amp draw will be split between the controllers. I have seen this done successfully, but it is not a recommended approach -- buy a properly rated ESC and avoid risky design.

A: Mark J. here: electronic speed controllers are very carefully designed to make best use of their electronic components. Modifying is both risky and ill advised. Read Chuck McManis' article on Understanding MOSFET Current Ratings for a good background in power transistor application.

The Infineon BTS7960B power chips used by the Scorpion XXL are not well designed for external heat sinking, but you might gain a very little capacity by adding a dab of heat sink grease to each of the power chips on the bottom of the board and mounting the ESC with the chips pressed down to an aluminum panel. My advice: mount the XXL in a well protected area with good ventilation and let it operate as intended. If you need more current capacity, buy a higher rated ESC.

Q: What do you think about attaching heatsinks to the MOSFETs, sealing the assembly from the air (to protect against condensation), and putting the board in a very cold environment between matches, like a refrigerator or ice bath?

A: like I said, the TO-263 case on the BTS7960B does not lend itself to an add-on heatsink. You would need to remove each chip from the board and re-mount them to gain access to the metal heat plate on the back - a tricky process with a surface-mount chip. You could also replace the BTS7960B chips with BTS7960P chips and gain a TO-220 case with extended metal heat sink tab, but that's a lot of work (and again tricky).

Chilling the whole ESC would be like putting a light bulb in the refrigerator -- a couple seconds after it was switched on, it would be up to the same temperature an unchilled controller would be. You have to gain greater heat dissipation if you're going to gain current capacity. My advice given above stands: if you need more capacity, buy a different ESC.

A: Mark J. here: in theory a fanless Victor 883 will handle 30 amps continuous. Team Test Bot uses fanless Victors to control DeWalt drive motors in their current 30-pound robots, but I figure their motors are only drawing about 15 amps. I assume you're not really pulling 30 amps 'continuously' for three minutes.

Victors have no current limiting circuitry and no heat sink; they rely on airflow to dissipate heat from the power FETs. That makes them comparable to a hand grenade, and removing the fan is like pulling the pin. With no heat sink to provide 'thermal mass', a quick power peak can cause an equally quick temperature rise and an ESC failure with absolutely no warning.

I can't recommend a specific maximum power rating at which a Victor 883 is 'safe' without the fan. If I did want to run a Victor 'sans fan' I'd add at least a small aluminum heat sink to each FET to provide a little thermal mass and heat dissipation boost.

A: Immersion in clean fresh water shouldn't cause any direct damage to small gearmotors, but if you let them sit around damp they may quickly rust internally. I'd suggest a tear-down ASAP to dry everything out. Put a small drop of oil on each of the bearings as you reassemble, check that everything turns freely, and you should be ready for a low speed test.

Thanks so much for your time.

A: You haven't told me what type of robot this is and what your performance expectations are. I can't recommend a drivetrain without knowing what will be expected from it. Fifteen pounds plus another thirty pounds of magnetic downforce -- must be some kind of pushybot, no?

I can tell you that any of the motors you mention are overkill for a 15 + 30 pound robot. The DeWalt setup in 'high' gear with 3" wheels will break the tires free at only about 1/6th of the available torque, and acceleration would be blinding because you're only dealing with 15 pounds of true mass. Power is NOT going to be a problem.

I can also tell you that the DeWalt Powerdrive is a very reliable drivetrain that can take enormous abuse. They have powered a great many successful robots. I have no experience with the Magnum gearmotor line and cannot comment on their quality.

The HTI motor is commonly run at 24 volts in combat robots. The Mini EV and RS-750 class motors are commonly run at 18 volts but should be OK at your suggested voltage in such a light robot.

With the information you have provided I really can't offer any additional guidance.

A: It isn't clear from your question whether you are looking for upgraded replacement gears for the Harbor Freight gearboxes or for tougher gearmotors to completely replace the HF units.

Getting a better set of gears for the planetary gearbox isn't really an option. Several problems:

• There are several versions of HF 18v cordless drill;
• Finding individual gears with the correct pitch, tooth count, width, and bore is unlikely; and
• Buying individual gears in small quantity is expensive.

A: You're not doing anything wrong with your search, you're just searching for three incompatible criteria in an ESC: good, light, and 65 amps. I can get you any two of those but not all three. You might consider a small power polenoid.

A: We've never used AstroFlight motors, although they have found application in a number of successful robots. The 'cobalt series' are all quite small and light for their output and have proven to be high-quality. It would help if you told me what use you had planned for the motors.

A: Previously discussed. Short answer: hobby brushless motors are built for model aircraft and airplane guys don't buy motors based on torque. Our Run Amok Spinner spreadsheet includes a calculator to estimate stall torque from specs that brushless motors do list.

A: We have no experience with the Magnum 550. It looks like the old style Banebots gearbox, but I can't comment further.

A: Not very much info to work with... I need some clarification.

When you say channel 2 does not work, do you mean that in mixing mode (DIP switch 1 'up') both motors run forward and backward in response to channel 1 input, but that you have no steering control from channel 2 input? What happens if you swap the ESC plugs in the receiver -- plug Ch 1 into Ch 2 and vice versa?

Q: In mixing mode, the motors had no response to the Turn(ch.2) whether the the Ch.2 was plugged into the aileron channel or elevator channel, the motors would not respond to the steering

A: That rules out a problem with the radio. One more quick test: set DIP switches 1 and 3 'up' (on) and the rest down (off). Leave the 'flip' wire unplugged. If you still get no channel 2 response I suspect that your ESC is faulty -- time to send it back.

A: I suggest that you:

1. Read the Sabertooth 5 RC manual.

2. Read the recent Sabertooth questions in this archive.

3. Charge your battery.

4. Try again.

5. Write back and tell me what the status lights on the Sabertooth are doing.

A: That flashing blue light likely means the ESC is in 'lithium mode' and is attempting to count off the number of lithium cells it detects in your battery pack. Continued flashing means that it isn't happy with what it finds. Are you using a lithium battery? Is it fully charged? Check the manual and correct any ESC DIP switch setting errors. Switch #3 should be down (off) for lithium batteries and up (on) for other battery types. Charge your battery before continuing.

A steady dim blue light indicates that the ESC has power and functioning correctly. Turn on the transmitter and the blue light will brighten, indicating the presence of a radio signal. You should be set to go.

A: Yes, but the key word is 'sometimes'. Victors rely on airflow rather than heat sinking to stay healthy. Without the airflow they will handle a great deal less power. Victors don't have current or thermal limiting circuits and will simply melt if they can't get enough cooling. I don't have a factor to de-rate a Victor without the fan -- if you try it (I wouldn't) you're on your own.

If you're hurting for weight/space, you might consider the SyRen 25 amp controller. It is less than half the weight of the Victor 884, more compact, and has no fan. Rated at 25 amp continuous, 45 amp peak - it has the current/thermal limiting circuitry that is missing in the Victor and is less likely to melt if overloaded.

A: See #21.

Thanks, Carlo Bertocchini

A: Glad to help, Carlo. Thank goodness for 'search and replace'. Also changed all the links to point to your AmpFlow website.

A: I suppose you've tried to run the motor with unsatisfactory results? You can try putting it back in the vise 90 degrees off from the way you squashed it before and see if you can bend it back sorta round. That's not exactly a high-precision repair, but you don't have anything to lose by trying. If all else fails it will make a very pretty refrigerator magnet.

A: Stalling a motor for two minutes(!!) is a very bad thing. It is entirely likely that the motor was heat damaged during that time. The wire insulation on the armature may be cooked, the commutator may be warped or pitted, and the brushes may be burned. Normally I would be suspicious that the new gearbox had some type of clearance problem when installed and was binding -- but if the motor is slow and overheats even when not attached to the gearbox then it's toast. Replace it.

A: NO!! BaneBots will sell you the correct 2.3mm pinion gear for a Speed 400 motor. Use it!

Q: Speaking of which, how do you take the motor out of the banebot gearmotors?

A: Remove the screws holding the gearbox together and pull the gearbox free from the motor mounting plate. You'll see the two screws that hold the motor in place.

Q: Pinion guy again. I was actually hoping to put a speed 400 into the 24mm 16:1 gearbox. My hope is to use 2 16:1 banebot motors to power a hammer. The arm is 6 inches and the weight is 2.5 ounces. Could they survive being directly connected to the hammer arm?

A: The Speed 400 does not have the same mounting screw spacing as the BaneBots 24mm gearbox -- you'll have to modify the mounting plate a little. BaneBots does sell a 2.3mm pinion for the 24mm gearbox, and this modification has been done before.

I'm curious -- how did you settle on two of the Speed 400 motors conected to a pair of the 24mm 16:1 gearboxes? Why not the 9:1 or the 20:1? Why two? I haven't done the calculations for the weapon, but your choice seems kinda arbitrary.

I can't tell you how long the motors would survive as hammer actuators. Anytime you use a component for something it isn't built to handle, you're on your own as far as reliability goes. Just guessing, I don't think the gears are going to last long -- the sudden stop is going to be hard on them.

A: Mark J here: the short answer is that greater power handling requires an increase in the capacity and number of power transistors and power capacitors. Increasing the number and size of those components also increases the need for cooling systems (fans, heat sinks) to remove heat build-up. For a full answer see 4QD-TEC: Electronics Circuits Reference Archive - PWM speed control.

A: One of the benefits of paying for a quality brand motor is that it comes with specifications that can be relied upon to be accurate. A Chinese knock-off may or may not come with specs -- if it does you shouldn't count on their accuracy (IMHO).

A: You need to remember that brushless outrunner motors are designed for model airplanes where they are commonly mounted behind a propeller that pumps a whole lot of air around them. Their amperage rating already assumes continuous airflow cooling -- they should be de-rated for still air applications (like robots). The killing heat is deep in the dense and compact windings of the motor that don't have enough surface area to dissipate heat any faster. If there was a way to get more power per ounce out of these motors, the airplane guys would already be doing it.

A: That's right, Kv is no-load RPM per volt. If the motor in question will operate at 20 volts, it would spin (with no load) at 40,000 RPM (2000 times 20) at that voltage.

Q: Is there a limit to the speed of brushless motors? What if I have a 5000 kv brushless inrunner at 20 volts, would it spin at 100,000 rpm? Is that possible?

A: Mark J here: any specific brushless motor system has an RPM limit. Brushless motors come with an operating voltage range, and pushing a high-performance motor beyond that range is done at your own risk. The RPM limit may be the point where:

• the maximum switching speed of the motor controller is reached; or

• rotational forces cause bearing or structural failure.

A: Second question first - you have to have SOME WAY to control motor direction and speed. I have seen (and built) some slow moving robots with motors controlled by power relays that provided very simple forward/off/reverse control of each motor. While this is adequate for slow robots the savings in expense or weight is small because large power relays are both expensive and heavy, and you will also require an interface between the relays and the radio system. Electronic Speed Controllers (ESCs) are used because they are the best option for weight, control, and reliability.

Back to the first question - different manufacturers rate maximum current capacity in different ways, and trying to compare numbers can be very frustrating! Ratings are often quoted for a 'cold' (room temperature) ESC and with an associated time limit, such as '220 amps for 30 seconds'. There may also be a higher 'peak' amperage rating that may be sustained only for an instant. As the temperature of the ESC rises, the amperage capacity will drop. Mounting the ESC to a large heat sink may extend the time capacity at a given amperage. A well designed ESC will have 'current limiting' circuitry that will reduce the current as the temperature rises to prevent thermal failure.

The highest believable and substantiated amperage rating that I know of for an 'off the shelf' ESC is 320 amps (cold) 240 amps (hot) for the 4QD-300.

Note that an ESC may not have to handle the maximum (stall) amperage rating of a specific motor. If the motor is geared such that it provides enough torque to spin the wheels before the motor stalls, the maximum amperage actually used will be reduced. The Team Tentacle Torque & Amp-Hour Calculator can estimate maximum amperage consumption for specific motors in specific applications.

Q: Aaron, in response to your expensive and heavy relays: [link to R/C 10 amp on/off switch deleted] [endorsement of specific website deleted]

A: Mark J here: the R.C switch for which you sent a link is not suitable for forward/off/reverse control of a motor. That type of control requires a pair of double pole double throw (DPDT) relays wired into a 'H-bridge configuration' -- the switch in question is a single pole single throw (SPST) switch with a fixed trigger point which cannot be used in an H-bridge.

Aaron's comment about heavy and expensive relays refers to high-amperage relays required to handle hundreds of amps, but suitable smaller mechanical H-bridges are also relatively heavy and offer little savings compared to ESCs of similar amperage ratings.

A: Mark J here: my previous statement applies to the use of hobby brushless motors in combat robot drivelines -- they work well in properly designed spinning weapon applications.

The first problem with using hobby brushless motors for robot drivetrains is finding a suitable reversing brushless speed controller. A combat robot that can't back up is entirely useless.

The second problem is the nature of the loads placed on drivetrain motors in robotic combat. Hobby brushless 'airplane' motors pack a great deal of power into small packages. As long as these motors are used for power output in the higher RPM range at which they are designed to operate their amperage consumption stays within reasonable bounds and everything is fine. In R/C cars and similar applications, these motors only rarely and briefly operate in the lower RPM range.

In combat robot drivetrain applications motors are often 'bogged down' in pushing matches and in rapidly alternating forward / reverse maneuvers -- with a resulting sharp increase in amperage consumption. More amperage produces more heat. These motors are so small compared to their amperage consumption under these conditions that they simply cannot dissipate enough heat and they 'melt'.

Q: Hey, the guy with the armor and brushless motor questions again. I recently picked up two speed controls (for brushless motors) that have reverse, forward and braking. $37 and they do 100 Amps continuous, 580 amp bursts (which I think should cover the "high" amperage for pushing). I'm using two motors with a combined 2000 Watt power. I'm pretty sure they will get extra-hot, however I'll have fans and also the match is not very long. If worse comes to worse I'll buy extras.

As for the operating in the high RPMs, I'm pretty sure I'll have enough power to spin the wheels the entire time so it'll stay up in the high RPMs :).

The motors I'm using are 320kV, so I'll only need a 3.4:1 gear reduction to run 15 ft/s at 14.8V (4s lipo). The problem I'm running into is that it is hard to find pulleys for large pitch timing belts where I can get a 3.4:1 gear reduction and still have the taller gear stay under my 1.75" robot height. Any suggestions for my 3.4:1 gear ratio? I could do a double gear reduction, however I'd like to minimize the number of belts. Currently the only pitch that works is MXL (2.03mm) which I'm pretty sure will not be able to handle the torque created by my brushless monsters (at 1/4" belt width).

My entire drive system has to fit within (inner dimensions) a 1.5" extruded square aluminum tube. Any suggestions would be nice.

A: I have several comments:

• If you believe those numbers on the $37 speed controller, I have a bridge in New York City I'd like to sell to you. ESCs from the R/C industry are very well known for wild power rating inflation. The ratings are typically calculated from the instantaneous capacity of the individual components at room temperature. Push them past a fraction of the quoted amperage in the real world and you get thermal runaway and a very large puff of smoke. Those ratings are IMHO a joke. Fortunately, your design will not push the controllers anywhere close to capacity.

• Be aware that brushless ESCs often have a 'soft start' feature that ramps up power rather than switching it on abruptly. This can seriously impact motor performance in a combat robot setting. There may also be a delay in switching between forward and reverse operation that can be very frustrating.

• Two thousand watts is a ludicrous amount of power to put into a featherweight robot. Some quick 'educated guess' calculations on the Team Tentacle Torque & Amp-Hour Calculator indicate that you may be able to use less than 10% of the available motor torque without spinning the wheels. Loading the motors this lightly will certainly keep them from bogging, but it's massive power overkill. You could use much less powerful brushed motors and achieve equal performance.

• A box full of spare motors will not do you any good when the motors in the 'bot fail during a match and put you out of the tournament. You don't get to call 'time out' while you go off to replace a molten motor. If you were loading the motors to take full advantage of their power, fans wouldn't be much help. When bogged down, these motors could be less than 50% efficient in their use of electrical power -- the rest goes to heat. Think about how hot a 500-watt light bulb the same size as your motor would get and how quickly that would happen. There is neither enough surface area nor a sufficient internal heat dissipation path to get that heat away from the core of the motor quickly enough to prevent meltdown.

• Information on calculating maximum torque loading for MXL timing belts can be found at the RoyMech website. You don't need to design the drivetrain on the maximum torque the motors CAN produce, but only on the torque the motors will actually need to produce before the wheels spin. I don't think the MXL belts running on small pulleys will suffice, but you'd best run the calculations to check. I suspect that you'll need larger belts and a two-stage reduction to get to 3.4:1.

Q: [Received before above answer was published] Okay, about the brushless motors, again (sorry). I'll measure the resistance when I get them but I'm estimating around 30mOhm (from similar motors, these didn't have a rating!). This results in ~ 500 amps at a stall, within the capabilities of my controller but I doubt within the capability of the motor. As the motor heats up it will not behave ohmicly, correct? It's resistance will increase so this number will actually be much much lower.

I'm sure I could drive within these constraints (IE attempt to not stall it for more than brief moments) however this is not practical. I think I would be better off going with a higher-resistance motor (which is usually proportional to motor size) so that stall currents are not so large, and even then the motor should be able to handle it.

In fact my earlier statement about the wheels spinning could also be true... I may have enough torque to where I can, in fact, not stall the motor (for more than brief moments) and this set up will actually work fine.

I know you prefer questions that require brief answers, but I'm finding it hard to find information on brushless motors being stalled (because no one uses them in these applications). Thanks again..

A: No apology needed. I know how much thought goes into designing a robot. You're just getting a little ahead of yourself (and me).

You've learned one of the reasons why I recommend using brand name brushless motors: they come with full specifications. The Chinese knock-offs are inexpensive, but you have no idea what you're getting. Mounting one of these in your sportflier airplane to give it a try is one thing, but designing a combat robot around a pair of them is another matter.

Heat will raise the terminal resistance of the motor, but not by a great deal. The motor impedance raises linearly as the motor gains RPM due to backward-flowing electromotive force (EMF) that resists the current flow. That's why it's so important to keep the RPM up in the high range and to avoid stalling the motor.

As noted in the preceding comments: given the power of these motors, the weight of the robot, and the gear reduction chosen, there's no way that you're going to be able to stall the motors in this robot short of welding the axles to the chassis. You can expect to use only around 10% of the stall amperage (and torque -- they are linearly related) of the motors before the wheels spin freely.

You've stumbled across one solution to using brushless motors in combat robots: overkill. Selecting smaller, lower-output motors would appear to make sense, but their lower power also means that it's easier to bog them down and drop them into the amp consumption danger zone. No, if you really want to use hobby brushless motors in a combat robot I think you want to stay well up in the 'overkill' zone.

There is a very good reason why no one uses brushless motors in applications where they stall -- they melt! And quickly!! I think that's all the information you need to know about stalling them.

Q: Of course I do not believe 580 Amp ratings, but like you mentioned I'm banking on overkill (in every aspect). You mentioned using lower-powered brushed motors, however after a lot of research I found that most builders use overpriced victor speed controllers @ $200 each.

I think I'd be better off sticking with overkill at $140 (2 motors 2 speed controllers) than going with under-kill at $400+. I'm also fairly confident that at 3.2lbs (motors, ESCs batteries and wires) this will be the lightest 2000 watt drive system with 5Ah of 16V that I can find (probably lighter than brushed systems at lower power too).

As for the soft start I'm sure I can mess with the programming to fix that, if it happens to be a problem. What are your thoughts on the weight/cost/effect when comparing to a lower powered brushed system?

A: Victor speed controllers are not overpriced. They are designed and built specifically for combat robotics. Builders use Victor controllers because they will deliver every last amp promised in their specifications and do not fail in the middle of a match. You will discover the true value of this after you have played around with those hobby ESCs for a while.

The motors and controllers you have proposed do NOT constitute a 2000 watt combat robot drive system. The components are undocumented and unproven, the specifications are fictional, and if output should ever approach 2000 watts multiple components will fail from thermal overload. The drive MAY work in your proposed featherweight only because the traction available to a 30-pound robot will limit the effective output to around 350 watts.

My opinion continues to be that hobby brushless motors intended for model aircraft use are unsuitable for combat robot drivetrains for the reasons previously given. Perhaps you will prove me wrong.

A: I've answered this question previously as part of related questions, but I'll take this opportunity to give a complete answer. The answer is yes, multiple brushed motors may be controlled by one channel of an Electronic Speed Controller. There are a few conditions:

• All motors providing power to one side of the robot should be wired in parallel to one ESC channel.

• The combined amperage draw of all motors connected to one channel should not exceed the capacity of that channel. The Team Tentacle Torque & Amp-Hour Calculator can assist you in determining the expected amperage draw for your motors as used in your robot.

• All motors connected to one channel of the ESC will respond together as a group - you will not have independent control over each individual motor.

• A channel on a brushless ESC cannot control more than one brushless motor.

A: Mark J. here: what you're asking for is called multi-quadrant control -- see this article on brushless motor control for an explanation. Multi-quadrant control is a fairly common feature on high-capacity ESCs for brushed motors, but I don't know of any hobby brushless motor controllers that include a generator mode.

You can find references to industrial four quadrant brushless controllers by searching for: brushless quadrant motor controller.

A: IFI does not quote peak current for the Victor 884. I'd hesitate to push it past 100 amps, and that only for a blink. The IFI controllers are fan cooled, have no current limiting circuit, and are very easy to fry if overloaded.

Q: Do the ifi victors 884 have a BEC?

A: Nope - no battery eliminator circuit.

A: Mark J. here: I'll give it a shot. First, a couple of clarifications:

• A kilogram is not a measure of weight or force - it is a measure of mass. One kilogram of mass exerts a downward force (weight) of (about) 9.81 newtons at the surface of the earth. Careless use of the kilogram as a unit of weight in physics can lead to confusion.

• Torque should not be measured in kg-cm as it makes no sense to express torque as a product of mass - it is correctly expressed in N-cm.

This available force is not directly related to the mass of objects the motor can 'pull' along a surface, it relates to the weight of objects the motor could lift. A rope wrapped around a 10 cm radius wheel on a 100 N-cm torque motor could exactly offset a mass exerting a 10 newton gravitational force on the other end of the rope -- the mass would neither rise nor descend. A smaller mass could be lifted by the motor, and a larger mass would overcome the force of the motor and descend.

A: Mark J. here: the RS-550 motor is overkill for a 3kg 'bot. Paired with the BaneBots P60 26:1 gearbox and 4" wheels, a pair of the motors would give a top speed near 9 MPH (plenty!) and still be able to break the wheels free when pushing using only 5% of its peak available torque. Huge overkill - better suited to a featherweight pusher.

The BaneBots RS-385 gearmotor would be a better match. It's lighter, costs less, will run great on a 2-cell LiPoly pack, and a pair will still deliver that 9 MPH top speed with 4" wheels and will break the wheels free when pushing just at peak horsepower output. I'd go with that!

Q: then what about RS-540 motor?

A: Let's not play 'then what about this motor?' You have my recommendation.

Given the very limited information you've provided, the BaneBots RS-385 motor is a great fit. Select the gear ratio to suit your wheel diameter.

If you'd like to evaluate other motors, try the Team Tentacle Torque & Amp-Hour Calculator -- it's what I use.

If you won't be happy without overkill, go for it.

Q: What motor other than the mabuchi motor will you recommend for a 3kg sumo robot? My requirement is a robot with unbeatable pushing power, and speed. the robot runs on 2 in wheels.

A: As previously noted, we do not compete in sumo and are not privy to the secrets that experienced sumo teams have for generating 'push'. However, we do know some things that won't work.

Excessive motor power is not the answer to unbeatable push. A wheel-driven vehicle can only produce a certain amount of 'push' before the wheels break free and start spinning. Power beyond that point simply spins the wheels faster without providing additional 'push'. What do you have against Mabuchi motors?

For best pushing power you will want the entire weight of the robot to be supported on powered wheels - that usually means 4-wheel drive. Are you designing a 4-wheel drive robot?

There is no point in gearing your motors for speed that the robot cannot attain within the confines of a small arena. I suspect you're really asking for quickness, but a very quick robot can be trouble in a small arena. I cannot recommend gearing without knowing the arena size. How large is your sumo arena (dohyo)?

Q: The ring is 154cm in diameter, i am running my robot on six 2in wheels and it is r/c controlled.

A: OK, a pair of BaneBots 36mm 20:1 RS-385 gearmotors [no longer available] driving 2" wheels will propel a 3 kg 'bot to a top speed of 4.7 MPH in 4.3 feet, crossing the ring in less than 1 second. They will be able to break loose all six wheels when pushing while only using about 1/4th of their available torque. You'll have your hands full trying to control that quickness and speed in such a small ring -- you may end up dialing back the throttle response to make it more controllable.

What type of drivetrain will you be using to power six wheels? You're not planning on six motors, are you?

Q: No, i will use 2 motors instead. drivetrain will depend on the motor. I need 2 with a combined 1/4hp motor. it is best if it comes with gearbox.

A: Why so much power? There is simply no way to use that much power in an arena 5 feet across -- it would be like trying to drive a rally car in your living room. You'd be more of a danger to yourself than to your opponent. You aren't the first builder to decide that massive power is the answer to the sumo challenge, but look around at the motors used by successful sumo competitors and you'll discover that much more moderate power is what wins.

I've given you my recommendation - twice. If you don't like it, feel free to use any motor set-up you please.

Q: Okay, I can forget about speed. But is there any motor with gearbox out there that fits my requirement of at least 100 rpm and produce alot of pushing power?

A: Why yes, there is! I don't know why I didn't think of this before: the BaneBots 36mm 20:1 RS-385 gearmotor spins more than 1000 RPM at 9 volts and has four times more torque than you can possibly use. MAYBE YOU SHOULD TRY THAT!

Thank you ! That makes my trip to the singapore robotic game 2011 much more easier.

A: Given no information about the design and performance needs of your robot, I can't even start to recommend a motor. See #21.

Take a look at the Team Tentacle Torque & Amp-Hour Calculator -- it will allow you to evaluate the performance of a wide variety of robot propulsion motors. The BaneBots 36mm 16:1 RS-540 motor might be a good place to start.

A: Mark J. here: the 'working' current of a motor depends on the load placed on the motor and the efficiency of the motor in that load range. For drive motors, the Team Tentacle Torque & Amp-Hour Calculator can provide an estimate of working current based on the specific motors, voltage, gearing, wheel diameter, and robot weight. For weapon motors, the Team Run Amok Spinner Excel spreadsheet can provide a similar estimate based on the specific motor, voltage, gearing, moment of inertia, and weapon usage.

A: The spindle motor from a hard drive is a brushless permanent magnet DC motor. Back before purpose-built brushless motors were available for model aircraft, builders were modifying brushless hard drive and CD ROM drive motors for hobby purposes. The computer drive motors were designed for long life and stable output, not the type of high output applications hobbyists required, but they managed enough success to prompt the development of the wide range of hobby brushless motors now available.

You will need a suitable brushless motor controller to operate your salvaged spindle motor, and you will not have much guidance on the acceptable voltage level, or the output RPM and torque. I can safely say that the motor would be suitable only for insect-class applications and would not perform as well as a similarly sized brushless hobby motor. I wouldn't bother with it.

A: Right. I want to know that for some words put together you can make readable question.

A: Neither option is particularly good. A re-bent tooth has been weakened and is likely to fail, while a missing tooth is just asking for trouble. I can't recommend going into combat with a weakened gearbox -- the safe thing to do is replace it.

Q: can I get a new gear or gear case or should I just buy the whole thing?

A: You could ask around on one of the forums to see who has a Copal gearbox left over from a blown motor, or you can buy a new 'whole thing' and have a spare motor ready in your parts drawer.

A: There are two versions of the Park 370 outrunner. Using the brushless stall torque calculator from the Team Run Amok Spinner Excel spreadsheet with the following data from the Hobby Zone website for the 1360 Kv model EFLM1205:

• 11.1 volts
• 1360 rpm/volt
• 100 m-ohms resistance

A: You can match the three wires from the ESC to the three wires from the motor any way you like. If the motor turns the wrong direction for your application, swap any two of the wires and the direction will reverse.

A: Brushed motors should be 'broken in' to allow the countour of the brushes to wear down to match the curve of the commutator. This will provide maximum contact area between the brushes and commutator which will minimize heat build-up, arcing, and potential damage when run at full voltage under load. The basic procedure is the same for all brushed motors:

• Place a small drop of light lubricating oil on the exposed motor and gearbox bushings. You can apply it with a toothpick. Make sure the motor shafts are turning freely.

• Hook the motor(s) up to a variable power supply (your electronic speed controller will do nicely) and run them unloaded at a steady, moderate speed for five minutes.

• Turn the speed up a little and run for another five minutes.

A: Let's start by getting the numbers right. The Pololu 20D44L weighs 1.7 ounces, just about 3/4 the weight of the B16 at 2.3 ounces -- not half.

There are a lot of factors that go into gearmotor design: design voltage, expected longevity, efficiency, required strength, etc. Let's compare the two at their best voltage. At 12 volts the B16 produces 12% more stall torque, 35% more RPM, and more than twice the total output power of the Pololu at 6 volts. I haven't used the Pololu motor, so cannot comment on its durability.

A: I have no idea. Automotive starter motors are generally inappropriate for use in combat robots. They are designed for high loads for very short time periods. Starters are usually 'series wound' motors that have a different torque curve than permanent magnet DC motors, which makes weapon design computation more difficult. Builders have tried to use starter motors for weapon power, but I don't recall any that were particularly successful. I recommend against their use.

A: Mark J. here: your approach is well thought out and will work -- if your ESC is actually limiting current the way you think it is. Many 'current limiting' ESCs don't directly limit current, they just dial back the PWM in response to the heat level being generated by the FETs. The 'programmable' current limit may just be setting the response to a thermistor on the heat sink. When cool, these ESCs will provide more than the limit setting; when good and hot, less. Check with the manufacturer to see how the current limit works with your ESC.

A: Mark J. here: Team Whyachi would have been my recommendation, and Odyssey would have been my backup suggestion to NiCads. You're on the right track -- best luck.

I'm the one trying to do an upgrade for power soccer, and I haven't been able to find a suitable gearbox for the Magmotor motors anywhere. Team Whyachi emailed me back saying that 16:1 was the maximum they could do.

A: Mark J. here: first, I think you mean 'higher' torque constant. Lower armature resistance from less and larger wire will result in greater current flow at a given voltage and greater stall torque. The torque constant equation is:

Sorry to hear that Team Whyachi cannot provide a suitable custom gearbox. I have no other contacts.

A: You've got to control the drive motors somehow -- you certainly can't just plug them into the receiver! The FingerTech tinyESC weighs only 3 grams and should work well in a fairyweight robot.

A: Only if the motor is spending time at stall, which a brushless motor shouldn't do! Hobby brushless motors are designed for aircraft use where they never stall and are rarely bogged down at low RPM. They generally come with specifications for maximum amperage and how long they can survive at that current level. For longevity you should limit the current to that maximum recommended level, either thru the ESC or thru the design and use of the weapon.

A: I would recommend not going cheap on critical robot elements. You know how well the Axi works, you know it is an exact replacement, and you don't know anything about the off-brand motors except that they are cheap. Save yourself frustration and disappointment -- buy a new Axi.

Q: Hi Aaron, I have done some more research about direct replacements for the Axi 2217/9D and found a KD A22-20L Outrunner. I know you said that going cheap is bad, but I've already killed 3 Axi's, and they are just not worth it for 90 bucks apiece.

This one doesn't show the internal resistance, so how can I figure out the stall torque? Would it be better than the Axi? BTW, the kv and length are better than the Axi for my purposes Thanks.

A: Mark J. here: if a part is failing, going to a cheaper part is NOT the solution. Why do you believe that a $14 chinese knock-off is going to be better than an Axi? It isn't. The Axi can handle 34 amps for a full minute. The knock-off dies after 15 seconds at 25 amps. If you've blown three Axis you're going to eat these like popcorn. The knock-off is also heavier, spins slower, and is less current efficient. Without an internal resistance number you can't calculate stall torque, but I'd bet that the Axi is better.

First, take a look at your design to see what is contributing to the weapon motor failures. Consider going to a less radical wire count -- the Axi 2217/16 or /12 would be more reliable, but would provide somewhat less power. Power comes at the price of reduced longevity.

A: Mark J. here: do you remember where the Spinner Spreadsheet 'said something' about this? Right there is a calculator for estimating stall torque of brushless motors. Fill in the light blue cells with voltage, rpm/volt, and internal resistance values and the spreadsheet will spit out the stall torque in bright red font to the right under the friendly label 'Stall Torque'.

If you prefer to do it longhand, the formula is:

A: According to The Builders Database antweight 'Gilbert' uses Maxon gearmotors.

A: Mark J. here: the claimed stall current for the B16 is 2.6 amps at 14.8 volts. I don't have a B16 in the shop to test and verify, and I'm concerned about the validity of that number because the same motor is rated at a considerably higher current consumption when attached to other gearboxes (B62, B104, B231). A specific motor should have the same stall current irrespective of the gearbox to which it is attached.

The Ant 100 ESC is rated 2 amps continuous, 5 amps peak. If the stall current figures for the B16 are correct, you should be fine with the Ant 100 -- but I don't trust those numbers. I would strongly recommend independently determining the stall current of your drive motors. Instructions for determining the stall current using the 'D-cell' method are in this archive - search for 'D-cell'.

The Tentacle Combat calculator suggests that I could have equivalent acceleration with lower amperage requirements if I were to switch to a pair of Magmotor A28-400 motors. The batteries presently last for over four hours of use, so we obviously don't draw 160 amps all the time. Is there any good way to verify that the Magmotor motors will provide better battery range than the NPC-T74s? It will require a lot of expense and fabrication, so I would like to do as much homework up front as possible.

A: Mark J. here: the Magmotor motors are much more efficient than the NPCs. The NPC-T74 requires about 70 amps at 24 volts to produce 1.4 horsepower. The Magmotor A28-400 will produce the same horsepower at the same voltage while consuming only 51 amps. Based on continuous output, switching to the Mags would extend battery life by about 37%. Your actual battery life may be different as I suspect you're not out there placing a constant load on the motors, but you will benefit from the greater efficiency.

HOWEVER...

The purchase of the Magmotor motors and fabrication of gearboxes represents a lot of hours and dollars. You might consider going to an alternate battery technology. Optima deep cycle batteries are well designed but all lead-acid batteries perform poorly under very high loads, delivering only a fraction of their rated amp-hour capacity. The amp-hour rating is based on a small constant drain that will exhaust the battery after 10 hours. High pulse loads can cut this rating WAY down. Other battery types offer much better performance under these conditions. A NiCad pack, for example, can deliver more than twice the run time of an identically rated lead-acid battery under very high loads. This sounds a lot easier than replacing your motors.

Q: Mark, thanks very much for the reply regarding NPC-T74 versus Magmotor motor efficiency. I had initially been looking at forms of lithium batteries, but the required capacity is well beyond the legal limit for air travel. I was in touch with Powerstream, and I was told that all of the large format NiMH battery manufacturers are out of business. Do you have any suggestions in locating or building a NiCad based pack in the 40+ AH range?

A: Get in touch with the good people at Robotic Power Solutions. Explain what you're doing and I'm sure they can build something for you.

A: It depends on what you're trying to do with that heavyweight. The 6 horsepower that a pair of A28-150 Magmotor motors can produce is certainly plenty to maneuver a weapon-bearing heavyweight quickly and efficiently around an arena. If you're building a thundering ram-brick to hurl itself as a blunt-force projectile and smash your opponent, you'll need more power.

You can calculate performance for a wide range of drivetrains, weightclasses, and designs at the Team Tentacle Torque & Amp-Hour Calculator .

A: Stall torque is 309 g/cm @ 7.2 volts. Stall amperage is 28 amps.

A: The four mounting holes are tapped for 8-32 machine screws.

A: Mark J. here: stall current is only a factor if you're going to stall the motors. Run your drivetrain design thru the Team Tentacle Torque & Amp-Hour Calculator and see how many amps the motors will pull to spin the wheels.

An example 2-wheel drive hobbyweight running two Johnsons ('small') with 4" wheels and a 12:1 gear reduction at 12 volts will pull less than 17 amps per side pushing at full throttle against an immoveable object. That would be well within the 20 amp continuous rating of the Scorpion XXL.

'Thermal Protection' is a good feature to have as a back-up, but you don't want to rely on it on a regular basis. If you overload the ESC badly it may 'smoke' before the thermal protection even notices that something is wrong.

Q: Thanks for the help! I am using the motors with the team whyachi t-boxes and on the team whyachi page it says the ratio for the t-boxes is: 11.52:1 what exactly does that mean? can I round it to 12:1 on the team tentacle calculator? Thanks

A: An 11.52:1 gear ratio means that the motor must rotate 11.52 times to get the output shaft to rotate once. The output shaft will have 11.52 times the torque of the motor and the output shaft will spin 11.52 times slower than the motor.

The Team Tentacle Torque Calculator will accept decimal gear ratios, so go ahead and enter 11.52. If you did need to round, I'd suggest rounding downward for a more conservative estimate of amperage use.

A: The Briggs & Stratton Etek is no longer made. Stocks ran dry a few years ago.

Q: does Brigs and stration manufacture the Etek-R Motor?

A: No, the Etek-R is made by 'Mars Electric'.

A: That particular motor has been unavailable for years. It was an 'inrunner' style brushless motor that ran at very high RPM. For a weapon drive, something like an Axi 4120/14 would produce comparable power at a lower voltage and at more manageable RPM.

A: To a certain extent, yes. Linear actuators are powered by electric motors: make the motor faster or more powerful and the actuator becomes faster or more powerful. Overvolting is a common way to accomplish this. Double the voltage and you will double both the speed and the lifting power of the actuator. You will also substantially shorten the life of the motor.

You can speed up an electric linear actuator but you can't make it break the laws of physics. Pneumatics are much better for producing large bursts of power.

A: See question #23 in the . You might also find useful information in the Wikipedia article on brushless motors.

A: The supply of D-Packs dried up a few years ago. They were really inexpensive (about $50) for a 2+ HP motor, but they were also inefficient amp-sucking death pigs. You get what you pay for. Save up and buy a pair of Magmotor motors instead.

A: Only if you want to go really, really slow. With 2.5" wheels at 6 volts the top speed is about 0.5 MPH. It would take more than 9 seconds to travel across an 8 foot arena!

Allow me to introduce you to the Team Tentacle Torque & Amp-Hour Calculator. You can select a motor from their long list and set the other details of your design (weight, wheel diameter, number of motors, voltage) and it will calculate performance figures. Use it!

Q: I looked at the Team Tentacle Calculator but I could not make up my mind for my ant weight wedge. I want a motor $35 or under and under 1.3 oz. Fast but still controlable. Please recommend one. Thanks

A: Hard to recommend a motor without knowing anything about your design and little about your expectations. I will say that there have been a great many successful antweights built using the Copal 50:1 gearmotor.

A: I don't have any information on 'Unspecified Threat', but they certainly aren't using servos for their hammers! Far, far too little power.

If you're not finding gearmotors for your hobbyweight, you just ain't lookin'. Browse this archive for discussions on appropriate gearmotors. Cordless drill motors are popular.

A: Well, you've got the whole "overpowered" thing pegged.

Twin A28-150s is a huge waste of money ($598) and weight (7.8 pounds) in a hobbyweight. You'll only be able to use 2% of the torque capability of the motors before the wheels spin, the top speed will be severely limited by the small size of a typical sub-light arena, and full-power acceleration is going to throw the 'bot off in an unpredictable direction. Not a great idea.

A: Simple: run it in both directions and listen to it. It will run faster and have a higher pitch to the sound it gives off when running in the better-timed direction. Break in the motor before you try to figure out any timing advantage -- it may change after the brushes are fully seated. Sanyo 50:1 gearmotors are supposedly 'neutral' timed, so there isn't going to be much difference.

A: A standard two-channel ESC will control two motors to perform the same function as your control system. The advantage is that a hobby ESC will allow smooth variable speed control of the motors in both forward and reverse to give greater control than the combined receiver / motor controller in your RTR boat.

You would need to determine the voltage and amperage draw of your motors to select a suitable ESC to handle the power requirements. You'd also very likely need to replace the entire radio system of your boat with a hobby-grade system in order to be compatible with the signal requirements of the ESC. By the time you've done all of that, you'd likely have been better off to have chucked the Ready-to-Run boat and bought something more capable.

Q: Can you help me find the torque curves for the A28-150 Magmotor motor? Thanks.

A: Mark J. here: all Permanent Magnet Direct Current (PMDC) motors have an inverse linear torque-RPM 'curve': they produce maximum torque at stall and zero torque at free-running RPM. If you know the stall torque and free-running RPM of a motor you can calculate the available torque at any speed.

Running at 24 volts the A28-150 Magmotor motor produces 1970 oz-in torque at stall (zero RPM) and zero torque at 6000 RPM (free running RPM). The torque formula for this motor is:

PMDC motors have a direct linear relationship between torque and amperage. For each amp the A28-150 draws it produces 5.1 oz-in torque.

A: Mark J. here: an Electronic Speed Controller (ESC) controls motor speed by switching battery power on and off thousands of times per second. The longer the ratio of switched 'on' time to switched 'off', the faster the motor spins and the more power it produces. At 'full power' the ESC passes (almost) the full voltage and current from the battery on to the motors.

By using the Adjustable Throttle Volume (ATV) feature on a computerized R/C transmitter you can adjust the radio so that the ESC will never receive more than a 'partial power' signal and will never deliver more than partial current/voltage to the motors, but there is no automatic voltage limiting.

The above explanation of how an ESC works is VERY abbreviated. If you're interested in details on ESC function see the 4QD-TEC: Reference Archive.

A: The idea behind breaking in a brushed motor is to seat the brushes against the commutator to prevent destructive arcing when the motor is run at full voltage (or overvolted) under load. This is usually done with a variable voltage supply, but there are other options:

• Put your 'bot together, prop the wheels up off the ground, and run your throttle trim up until the motors are running at a moderate, steady speed. Run them for about five minutes, then turn the speed up a little for an additional five minutes.

• Wire two (or three or four) Sanyos in series with the 7.4 volt battery and let them run unloaded for about ten minutes.

• Hook one motor up to a single alkaline cell (AA, C, or D) and run it unloaded for about ten minutes.

Make sure the motors and gearboxes turn over smoothly before starting and monitor the motors during break-in for overheating or other signs of trouble.

A: I've got a couple of questions:

Why that motor? The 36 volt DeWalt with gearbox weighs a little over 3 pounds and produces about 1 HP at 36 volts. The old-style 18 volt DeWalt with gearbox weighs a little over 1.5 pounds, produces 1.5 HP at 24 volts, and is a well proven combat winner.

Next, what do you plan on doing with a 12 pound robot with 2 or 3 horsepower? It will be very good at spinning its wheels and crashing into the far wall of the arena before you can react -- is that sorta what you have in mind?

A quick check at the Team Tentacle Torque & Amp-Hour Calculator shows that a hobbyweight with a 36 volt DeWalt and 4" wheels will spin it's wheels at about 5% of the torque potential of the motor. Give this a rethink.

Q: About the ramming at a high uncontrollable speed, that's kinda the point. I want to slam the other with as much KE as possible. And I also plan to run the drive with a chain a sprocket system, at a 25:1 gear ratio with five inch wheels.

A: Check your math: a DeWalt 36 volt motor geared 25:1 with 5" wheels will give a top speed of about 11 MPH - hardly high uncontrollable speeed. You've picked the wrong motor, the wrong gearing, and the wrong attack strategy. I have nothing against ram bricks, but the whole idea behind them is 'controllable' speed and accuracy.

Q: Well, what DO you recommend for a hobbyweight rammer?

A: I can't recommend specific motors without knowing more about the rest of your design and details on the arena in which you will be competing. I can tell you that trying to put down a lot of power with a two-wheel chassis will make keeping it pointing in a straight line very difficult. For a typical sublight arena I'd go for a four-wheel drive chassis with 500 to 600 watts total motor output.

A: Hi, Daniel. Team LNW's hobbyweight 'Defiantly Daft' (6 wins, 7 losses) is an able competitor, but it is not known for massive power. Why copy their motors?

My dad contacted Team LNW's Greg Schwartz and asked about the motors:

"We run Ryobi 9.6v drill motors in DD on 18.5v lipos. Had the g/b's modified by Gaussewave (Rob Purdy) so the output shaft is a 3" x 1/2" keyed shaft."

Greg noted that he plans to upgrade with motors and gearboxes from these 18 volt drills which feature hardened steel gears. Thanks, Greg!

Q: Thanks for the help!, I once tried to modify two 12 Volt Black and Decker motors but in the end it did not seem to work well. Maybe it was the drill motors themselves, because they were not cheap chinese drills? Maybe a pair of Harbor Freights would work better, or maybe BaneBots motors? [Daniel]

A: Plenty of hobbyweights are powered by BaneBots motors. They are simple to mount and have a large range of power and gear ratio options. Some builders really like them and some really don't. In a design like 'Defiantly Daft' with bearing support on the end of each drive shaft I think they'd be fine.

A: Shaft length is not included in a standard motor length measurement and is not included in the motor length given by the Robot Marketplace for the B16 motor.

The Robot Marketplace provides a great service to the robot community, but they are inconsistent in the way they report dimensions and performance figures. Mixing metric and English units or leaving key measurements out of drawings or tables is common.

A: Not even close. The Sabertooth 5 ESC can only provide 5 amps of continuous current -- strictly antweight/beetleweight class. The Sabertooth 25 ESC can provide 25 amps of continuous power, which is enough for most hobbyweights or a moderately powered featherweight. Read thru this archive for help in selecting a suitable ESC.

A: At 24 volts the Magmotor E-150 pulls about 64 amps at stall.

You can calculate the stall amperage of a PMDC motor from the Terminal Resistance and Voltage:

Most of the common robotics motors have their vital specs listed at the Team Tentacle Torque & Amp-Hour Calculator

A: I prefer speed controllers that have power limiting features that keep the ESC from frying if overloaded. Neither Vantec or Victor ESCs have any power limiting.

A: The Sanyo motors are rated for 5 volts. I know that many builders run these motors on 3-cell LiPolys (11.1 volts), but when you're frying as many of them as quickly as you are it means that your design loads them too heavily for that much voltage. Check the gearmotors after mounting to make sure there is no binding or interference with free rotation.

I'm also betting that you didn't break in these motors by running them in at a lower voltage before turning up the juice. Any brushed motor that you're going to overvolt should first be run-in under no load at about half its rated voltage for at least ten minutes to properly seat the brushes and bushings.

A: Mark J. here: you're in luck -- the Bosch GBA 750 from 'Run Amok' is sitting on my workbench and I've wanted an excuse to open it up to see how badly it was cooked at Robotica. I put a micrometer on a couple of the wires leading to the commutator: 0.036". According to a handy wire gauge chart that's AWG 19.

A: No, the Team Delta DeWalt mount fits only the old style 18V gearbox.

A: Mark J. here: No need for separate controllers for the turret motors -- this is likely just a set-up incompatability between the controller and the transmitter.

Check the Sabertooth 2x10 option switches first:

• Set option switch 1 'down' (no mixing).
• Set option switch 2 'down' (no exponential response).
• Set option switch 3 'down' if using Lithium battery, 'up' if not.
• Set option switch 4 'down' (no flip control).
• Set option switch 5 'down' (no auto-calibrate)
• Set option switch 6 'up' (timeout enabled)

Still no joy? Write back and let me know. There are more things we can try.

A: Mark J. here: generally, no. Speed controllers for hobby brushless motors monitor current flow to 'sense' the rotor position and determine when power must be switched to control rotation. Unless multiple motors connected to one ESC are mechanically locked in perfect synchronization the contoller would receive confusing information on rotor position and motor operation would be disrupted.

Electronic controllers for brushed DC motors do no position monitoring -- the commutator takes care of that -- so you can run as many motors off of one brushed ESC as you like, provided that you do not exceed the rated amperage of the controller.

A: A pair of 12 volt drill motors are enough to power a simple featherweight. I'm not certain of the RPM available from your specific Black and Decker gearbox (there are several versions), but a typical drill gearmotor supplies about 500 RPM. With six inch wheels that will give a top speed close to 9 MPH and plenty of torque. That's a good start.

Date marker: April 2009

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Q: I am considering 3lb. drive motors. I have built ants and 12lbs, but not beetles. What might account for the difference between the Copal SH50 and the standard Copal 50?

A second question relating to the first: the B16 is a very popular 3lb motor. Faster than Copals, but less torque at more than twice the weight. You frequently advise to "look at what works for others", so no arguing with B16's success. Can you help me understand what makes the B16 superior to the Copal in the 3lb class?

You have great stuff here. Thanks.

A: Copal makes relatively small runs of motors to custom order. Most of what the robot community sees are production run leftovers, and the specifications are subject to change.

The Copal SH50 has the same motor and gears as the standard 6 volt Copal 50, but the construction of the bearing support/mounting plate is different. This adds a little weight and changes the mounting hole spacing. It is not a direct replacement and may not fit a chassis designed for the standard Copal. Hence, it is being sold at a discount.

As for comparing the Copal to the B16: the difference is strength. The Copal gearbox is adequate for the forces encountered in antweight competition, but the stresses of mass and weaponry multiplied by three is too much to ask out of their lightweight construction. The B16 gearbox is a planetary design in a fully enclosed metal housing. The bearing support is much better and the gears themselves are much more robust.

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Q: Hey Aaron, is there a motor with a RS-385 mount-hole pattern? I need the same speed and torque too, but at 1.5 oz instead of 2.5 oz. Thanks.

A: If there was an exact replacement for the RS-385 that was 40% lighter, why would anyone use the RS-385? No such motor -- look for your weight savings somewhere else.

The closest I can come is the brushless Axi 2204/54 Gold Line. It has the same mounting hole spacing as the RS-385 (16 mm), is about the same diameter, has a little less RPM, a bunch more torque, and weighs in at less than one ounce. The shaft is larger, and you'll need a brushless motor controller. Retail price is around $75, versus about $2 for the RS-385. It would work as a weapon motor, but I wouldn't use it for propulsion: if you bog it down it'll melt.

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Q: Hey Aaron, any idea on what the motors and gearboxes are in this picture (third down on the right) [expired link removed]? Where can I buy them? Thanks.

A: Terrible photo, but I'm pretty sure they're Mini-EV Warriors. You can buy them at Robot Marketplace [no longer available]. The spur-gear reduction drive units shown are custom.

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Q: We are working on a research project for the Space Elevator. We are using a A28-150 Magmotor motor. Any recommendations on what type of motor controller to use? I'm pretty new to all this and don't know where to start.

A: Mark J. here: climbing a ribbon is a little different than combat robotics, but I think I can point you in the right direction. A few basics: you're controlling speed on a single motor and I assume you will want to control it in both directions (forward/reverse), so you will be looking for a single-channel dual-direction permanent magnet DC controller. A key factor in motor controller selection is knowing how many amps of current the motor will be consuming, and for how long. For example, here are the ratings for a popular line of robot controllers:

	Max Surge	30 Seconds	Continuous
RoboteQ ax1500XC	250 amps	60 amps	40 amps
RoboteQ ax3500	500 amps	120 amps	80 amps
RoboteQ ax2550HESC	1000 amps	280 amps	160 amps

If you exceed the maximum surge rating of the controller it will likely destroy the device. Exceeding the 30 second or continuous ratings will (for the RoboteQ and most other controllers) cause the controller to cut back the amperage output and reduce motor power.

The A28-150 Magmotor motor, like all permanent magnet direct current motors, consumes amperage in direct linear proportion to the torque it is producing. At zero (free running) torque, the motor draws only 3.5 amps. At maximum efficiency the motor produces one horsepower and draws 37 amps. At peak output the motor produces three horsepower and draws about 185 amps. At maximum (stall) torque, the A28-150 motor can draw close to 375 amps.

Once you have examined the torque demands your design will place on the motor, you can match up the amperage demands to a suitable controller. It does no particular harm to go 'overkill' here -- better to have a controller with too much capacity than too little.

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Q: Hey Aaron, can you use stepper motors to drive an ant or beetle?

A: Not a great idea. Commonly available stepper motors are a type of brushless motor designed to rotate in precise increments to position something accurately -- like the read/write head on a disc drive. They require special driver circuits and are not designed for high power output.

When brushless motors were first being used in model airplanes, some hobbyists found ways to convert specific stepper motors for aircraft applications. Now that brushless motors designed for the purpose are widely available there is no need to go thru the trouble. I'd suggest sticking with more conventional motors.

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Q: Are there any drill motors that weigh no more than 8 oz? I need the ones with the motor attached to the gearbox.

A: I don't have a weight chart for drill motors with attached gearboxes, but I'm pretty sure they're all over 8 ounces. Robot Marketplace has a wide range of gearmotors with weights specified in their descriptions.

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Q: Can a Speed 400 motor fit a Whyachi T-box gearbox? Where can I buy gearmotors that are 1.25" square?

A: The Speed 400 motor has different mounting hole spacing than the Whayachi gearbox, and the gear reduction is not correct for the Speed 400. The BaneBots 28mm gearmotors are 1.1" wide and high by 2.8 inches long.

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Q: Hey Aaron, is there any difference between the Banebots 28mm RS-385 gearmotor and the Banebots 36mm RS-385 gearmotor? Does one give better performance?

A: The RS-385 motors are identical -- the difference is in the gearbox. The heavier 36mm gearbox has a larger shaft and gears. It can take higher loads and more punishment from heavier robots or designs that place more stress on the gearbox. This comes with a weight penalty: the 36mm geabox weighs more than twice as much as the 28mm gearbox.

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Q: Where can I buy the Lynxmotion PGHM-09 or PGHM-14 Gearmotors?

A: The planetary gear PGHM-09 and PGHM-14 motors are no longer in production and are no longer available. Lynxmotion does still supply the spur gear GHM-16 and GHM-12 gearmotors with similar performance ratings.

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Q: Can I over-volt the BaneBots 16:1 28mm RS-385 gearmotor to 12v?

A: I wouldn't. If you want to run the motors from a 12 volt source, you can do so safely by turning back the transmitter 'Throttle Volume' setting for the channels that control the ESC. See our Transmitter Programming Guide for more information.

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Q: Hey Aaron, what's the longest shaft you ever saw on a motor? I need a motor with at least 50 oz/in of torque and a 18" long shaft. Diameter of the shaft and speed don't matter. Thanks!!

A: Not sure what the longest shaft I ever saw was, but it was a whole lot less than 18 inches!

It would help if I knew what use you had for this motor. Having to guess at the application: you're going to need a suitable motor, a miniature shaft coupling, a length of shaft material, and at least one pillow block to support the long shaft. Do a web search for 'miniature shaft coupling' and 'precision shaft', or seach an industrial supply site like McMaster Carr. Robot Marketplace has miniature pillow blocks.

You might also be able to find suitable parts at a well-stocked R/C boat hobby shop. They have speciality parts to connect motors to long propeller shafts that might work for your application.

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Q: Hi Aaron. I just bought a Jeti Spin 75 Opto ESC and am trying to understand how to program it with the throttle stick and without a 'Spin Box'. The instructions are horrible. Does this controller have an auto detect feature for the # of LiPo cells? Does setting it for one of the aircraft types set it up automatically for voltage cutoff etc. Any help would be greatly appreciated. Thanks, George.

A: The Jetis are made in the Czech Republic and the translation of the manual to English is not so good. Yes, Jeti controllers auto detect the number of LiPo cells. Here is the set-up drill:

1. With the transmitter and receiver off, connect the ESC to the motor and to the appropriate receiver channel.
2. Push the transmitter throttle full on and turn on the transmitter. Hold throttle full on.
3. Connect the ESC to the battery.
4. Power the receiver.
5. After 5 seconds you'll hear four tones. Keep the throttle full on.
6. A few seconds later you'll hear a single tone, a pause, two tones, a pause, and so on up thru six tones. These tones represent the six available modes of the ESC.
7. Immediately after the tones for the mode you want sound, throttle back to zero.

You will most likely want Mode 3 (zero degrees timing advance, brake on) or Mode 4 (24 degrees timing advance, brake on). Check your motor manual for the amount of advance required -- outrunners with eight or more poles require mode 4. All standard modes gradually shut back when they reach 68% of the starting voltage or 12 volts.

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Q: Hi Aaron. Glad to find your site. I have built a video jib crane. It works great. At the end of the crane where my camcorder is mounted I want to motorize it so I can remotely rotate the camera from left to right. I have tried using a small DC motor and gear, but its way to fast. I need to turn very slow but smooth. My question is what motor (low RPM hi-torque) should I use. I wouldn't mind if the motor was direct drive, but its probablly too much weight on that little shaft. Anyway, I need HELP please. Thanks Mike

A: Combat robots don't do anything very slow and smooth, Mike. Abrupt and violent is more like it.

Go have a look at the motor selection at American Science & Surplus. At the moment they have a small "Tough Gear Motor" that runs on 6 to 12 volts DC motor and spins 3 to 7 RPM. Should be just what you need.

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Q: Would 4 Sanyo 50:1 gearmotors have the same amount of power and torque as 2 Copal 50:1 gearmotors?

A: Mark J. here: Don't confuse the relationship between 'power' and 'torque'; power is the product of torque and speed. Sanyos have less torque than Copals, but spin more than twice as fast.

There is also some confusion over the true output power of the Copals. The published performance figures are unclear about the voltage at which the measurements were made and the RPM numbers don't add up correctly.

My best guess for specs at 12 volts:

• Four Sanyos will deliver a total 28 watts of power with a combined 89.6 oz-in of torque and will spin over 1000 RPM.

• Two Copals will deliver a total 18 watts of power with a combined 118 oz-in of torque and will spin 500 RPM.

So, at the same voltage four Sanyo 4.5 volt 50:1 motors will give about 50% greater power than two Copal 6 volt 50:1 motors. The Sanyos spin twice as fast with less torque, but speed and torque can be adjusted with wheel size selection.

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Q: Can you use a flip channel on a speed controller for your weapon instead?

A: No. The 'flip channel' available on some speed controllers is an input from a spare receiver channel that changes the on-board channel mixing to correct the directional response of a robot when the 'bot gets flipped upside down (inverted). There is no power output corresponding to the flip channel input, so no possibility of running a weapon from it.

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Q: Hey. We are trying to find decent brushed motors to drive a BattleBots IQ Mini series bot, but either can't find any good enough, or they are too heavy. We were going to use two Planetary Gearmotor from BaneBots, but they are discontinued currently and RobotMarketplace didn't seem to help. Could you suggest a place to look for a 14.4v motor or suggest one? Much appreciated. ^_^

A: I'm guessing that you planned to use the 42mm BaneBots gearmotors. That's not a bad choice, but kinda overkill for a BBIQ Mini. A pair of the Banebots 36mm 16:1 RS-540 gearmotors will provide good performance. With 3" wheels at 14.4 volts the motors will give a top speed near 11 MPH, reach top speed in 21 feet, and will break traction and spin the wheels at less than 11 amps. Weight is 11.6 ounces. Don't worry -- they'll run fine at 14.4 volts. Just make sure you run in the motors unloaded for several minutes at reduced voltage to properly seat the brushes before putting them under load at full voltage.

If you want more power you can replace the RS-540 with the RS-550 motor. It bolts right up to the 36mm gearbox and will give more speed and power, but I really think the RS-540 is plenty for a 15 pound robot.

If you're looking more 'heavy duty', the Piranha 36:1 Gearmotor [no longer available] could work for you. The gearbox is very well constructed and has a 1/2" output shaft. With 6" wheels the top speed and max amp draw are comparable to the BaneBots RS-540 gearmotor but acceleration is better, reaching top speed in 14 feet. Weight is 1.5 pounds, price is about $94, and they are in stock.

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Q: I have a motor similar to Robot Marketplace's Mini-EV motor. The only markings on it are the numbers "344742". It's about 2" dia. x 3.25" lg w/ a 5mm dia. x 5/8 lg shaft. Any ideas about voltage/amp draw/RPMs? Thanks, Mike

A: Tough one, Mike. The motor could be almost anything. I don't recognize that number, but many DeWalt motors have six-digit numbers stamped on the case, and the size is about right. Browse thru the pictures and see if anything matches up.

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Q: Is there a way to find out what ESC we have? I tried Robot Marketplace and there wasn't anything there that looked like it. I got it from a guy who made Toro.

A: 'The guy who made Toro' would be either Reason Bradley or Alexander Rose of Inertia Labs. They used and sold the IFI Robotics speed controllers. If those don't look right I'm gonna need a decent description. Alternately, you can contact Reason and Alexander thru the Inertia Labs website.

Q: The speed controller is small and is for the insect classes. It has two receiver plugs and a black screw down terminal on the side. The circuit board is green and has two mounting holes. There is a black plastic thing sticking up on the board and another cylinder shaped thing that is half silver, half black. The letters and numbers on the cylinder say: 47 VFC 3T1. It looks like the flip channel plug was removed. Merci.

A: Yea, that's the Barello Ant 100. The color/style of the screw connectors changed with availability. The Barello speed controllers are no longer in production and Inertia Labs now sells them only with their full antweight kit, but you may be able to find a few in stock somewhere.

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Q: Does a featherweight robot need a speed controller to work?

A: Combat robots of any weightclass need some method of controlling the direction and power of the motors. It's possible to use relays with R/C interfaces to get forward/off/reverse motor operation, but for reliability and precision driving control you need speed controllers.

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Date marker: October 2008

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Q: I wired up one side of the drive on my Beetle to my Scorpion XL, but when I pushed the stick into full throttle on my Transmitter, only one of the motors moved, and it was hardly a crawl. There aren't any shorts or whatnot, and when I connected the motors straight to the batteries they blasted off at top speed, so I know the motors aren't a problem. I'm running four Banebots 28mm to a 7.4v lipoly. What could be wrong?

A: My first guess is that you didn't bother to read the Scorpion XL manual. You just wired it up and turned it on, didn't you? If you can't find the time to read the entire manual, at least read the Scorpion XL quick start guide with particular attention to the radio calibration step.

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Q: Hi Aaron. Where can I buy a 28mm 256:1 BaneBots gearmotor? BaneBots no longer sells them.

A: RobotShop was the last source I had -- they're gone now.

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Q: Do you know how high in volts a ML-50 50:1 geared motor can be run?

A: How far a motor can be overvolted depends in part on how much load is placed on it. If you set up the wheel diameter so the motor can spin the wheel before it bogs down below about half the free RPM of the motor, you will get much better results than if you let it get close to stall and start to pull high amp loads. The Team Tentacle Torque & Amp-Hour Calculator can help you select wheel sizes that will keep the amp load reasonable.

That said, I would be comfortable running the ML-50 around 18 volts -- but don't stall it!

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Q: How do I turn off the soft start feature on my Jeti 90 Amp ESC?

A: Mark J. here: Jeti ESCs are made in the Czech Republic, and the manuals are a somewhat difficult translation. Assuming that you have the Jeti Spin 99 Opto, there are two options in changing the programming of the controller:

1. If you have access to a 'Hacker Master Spin Jeti Programmer Box', you can use it to set the 'acceleration' parameter directly. The instructions are contained in the JETI-Box manual.

2. Without the programmer box, you can only assign the controller to one of the six pre-set modes. You'd probably do best with mode 4 'Glider outrunner' which enables motor braking and minimal start-up ramping. Instructions for setting modes are on page 3 of the Jeti Controllers SPIN Manual. Mode 4 is enabled in the setup sequence by waiting until after the fourth set of five repeating tones before pulling throttle to zero.

Remember that reducing the startup ramping will place additional load on the ESC, motor, and battery. This may significantly reduce their operating life.

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Q: I was thinking of using two Magmotor E-150 motors at 36 volts to push around my middleweight spinner. They would give me a good amount of power at a reasonable price. How does that sound?

A: For drive power, two Magmotor E-150s will cost about $160 and come without gearboxes. You'll have to fabricate your own gear reduction and drivetrain. Overvolted to 36 volts (not recommended for drive power) two will deliver 2.25 horsepower - overkill for a big-weapon middleweight. Top speed with 6:1 gearing and 3" wheels is about 10 MPH in 2.2 seconds. The motors would weigh 7.8 pounds without the drivetrain.

Four BaneBots 42mm 27:1 RS550 gearmotors will cost about $200 and come ready-to-mount with gearboxes -- no additional expense or effort required to fabricate a drivetrain. At 14.4 volts (maximum recommended) four will deliver about 1.5 HP. Top speed with 3" wheels is about 8 MPH in 2.3 seconds. The motors would weigh 4.5 pounds including the drivetrain.

Going with the Magmotor motors is false economy. You'll spend a lot more money to get your drivetrain assembled than the difference in cost. You'd also add a lot of weight. I'd keep it simple and run the BaneBots gearmotors.

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Q: Would two 36 volt Dewalt drill motors be good drive motors for a middleweight with a heavy weapon?

A: Massive horsepower overkill. They would be suitable for a middleweight rammer or wedge, giving a top speed near 20 MPH. With a large weapon, that much speed would make the robot as dangerous to itself as to the opponent.

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Q: What speed controller do you recommend that I use for four Dewalt 24 volt drill motors?

A: Speed controller selection depends on more than the motor(s) you plan to control -- it also depends on how you plan to use them. If your application runs motors heavilly loaded at or near stall they will require a controller that can either source the enourmous amperage draw they will require or a controller that can limit amperage to protect itself and the motors under those circumstances.

The Team Tentacle Torque & Amp-Hour Calculator can help you figure out how many peak amps your motor set-up will require in a specific robot. Use that for a guide in selecting a controller.

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Q: What would be a good motor for a truly uniquely powerful 60 pound wedge? How many per side? I really wanna push the limits here.

A: Mark J. here: if you want to 'push the limits', you should first know what the limits are. Acceleration and pushing force are both limited by the weight pushing down on the wheels. When you add more power than needed to break traction and spin the wheels at peak horsepower, additional benefits are very small. You can gear up to trade that excess torque for greater speed, but controlling a lot of speed in a small arena can become a real handfull.

Old-school lightweight 'Dr. Inferno Junior' used four 18 volt DeWalt drill motors at 24 volts. That's 6 horsepower in a 60 pound 'bot. Run that set-up thru the Team Tentacle Torque & Amp-Hour Calculator with 4" wheels. You'll see that the motors are wildly under-utilized; the wheels break traction and spin at less than 1/6th the stall torque of the motors.

I'd say that there's no good reason to push lightweight power limit any further than it's already been pushed, but people seem to enjoy ignoring my advice. If you're determined to demonstrate real insanity, how about 4 of the 36 volt DeWalt hammerdrill motors? That would set a 'high water' mark that would stand for some time.

Q: How about three 16:1 RS775 BaneBots 42mm gearmotors per side with 4 inch wheels at 16.8 volts?

A: Six RS775s at 16.8 volts would give you just about the same power as four 18 volt DeWalts at 24 volts. The difference in gearing would make the BaneBots setup considerably slower (15 MPH vs. 22 MPH). It would certainly be powerful, but not uniquely so.

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Q: You've said the DeWalt 14.4v old-style drill motor can power a wheel. Can it power 2 wheels?

A: The number of wheels is not an issue -- the weight of the 'bot and the expected performance is what matters. A second or third wheel can be driven by chain or belt from the hub of the wheel directly attached to the gearmotor.

A pair of DeWalt 14.4v gearmotors can provide good performance in hobbyweight or featherweight combat robots no matter how many wheels you choose to power with them. You can check the performance of all popular combat robot motors with a variety of gear, voltage, and wheel combinations at the Team Tentacle Torque & Amp-Hour Calculator.

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Q: Is it OK for a motor's stall amperage to exceed the ESC's normal ratings if the motor won't be stalling much, if at all? Could I use a 883 ESC with two 550 motors in parallel?

A: Mark J. here: different ESCs handle overload in different manners. Some have current limiting circuitry that allows them to cut back loads exceeding their capacity. Others, like the Victor 883, can handle only very brief overloads before they fail.

If this is for a drive system, the Team Tentacle Torque & Amp-Hour Calculator will provide the expected amperage needed to spin the wheels based on a given drivetrain configuration. Keep that under the max amperage for your ESC and you should be OK unless you somehow lock up the drivetrain.

If this is for a weapon system, the calculation is more difficult. Two RS-550 motors in parallel would pull 170 amps at stall -- almost twice the 90 amp continuous rated capacity of the Victor 833 SC. You could feather the throttle at the start of the spin-up, but you really don't want to be doing that in combat.

My advice: don't scrimp on your speed controllers. It's less expensive to buy what you actually need than to keep replacing inadequate equipment.

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Q: What do the last two numbers in an Axi motor's model number represent? Examples: 5345/14, 5345/16, 5345/18.

A: The Axi model number is a simple code:

• The first two digits represent the diameter of the motor stator in millimeters.
• The third and fourth digits represent the length of the motor stator in millimeters.
• The number after the slash is the number of turns of wire wound around each pole of the stator.

In general, fewer turns of (larger diameter) wire result in a motor with greater torque, more RPM, and larger amperage draw at a given voltage. Motors with a larger number of turns of (smaller diameter) wire can handle greater voltage and may or may not have greater power output at a lower amperage draw than the fewer turn motors when run at higher voltages.

See comments on calculating the output of Axi motors in this section of the archive.

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Q: Is it safe to run a BaneBots 540 motor at 14.4v?

A: Depends on what you mean by 'safe'. Any overvolted motor will have a reduced lifespan, and that reduction increases very rapidly with voltage increase and with the amount of time spent pushing near stall. If you gear the RS-540 to keep it from bogging down you'll be alright at 14.4 volts. I'd keep a couple of replacement motors in my spares box, but you're gonna do that anyhow, right?

Alternately, you could upgrade to the RS-550 motor. It bolts up to the BaneBots gearbox and will run all day at 14.4 volts without trouble; I've seen them run at 18 volts in combat. You'll get increased torque and reduced stall amperage, but you'll loose some RPM and gain a few ounces of weight. I'd go with the RS-550 if you're building a speedy 'bot that could bog down when pushing, and I'd stick with the RS-540 geared down well for a pushy-bot.

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Q: How does the new Etek-R compare to the original -- better, worse, or the same?

A: The published performance specs are the same: speed, torque, max current, power. However, it is heavier by more than 7 pounds (28 versus 20.8 lbs.) so you could say it's not as good as the original.

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Q: Our BBIQ robot uses two drill motors and gearboxes ripped directly from Harbor Freight drills for propulsion. The motors have begun to behave strangely, making sporradic sparks like there is an electrical arc inside the motor. This happens to both motors simultaniously and only at full speed. What gives? Should we purchase better motors?

A: I'm gonna have to guess at how long you've run the motors, what voltage you run them at, and what sort of abuse you've put them thru.

I suspect that you didn't break in the motor brushes properly before subjecting the motors to heavy load. New brushed motors should be 'run in' at about 1/4 speed for at least 20 minutes before turning up the voltage and the load. This allows the graphite motor brushes to contour to the radius of the commutator and maximize the contact area. Failure to do this can cause the brushes and commutator to overheat, deform, crack, or rapidly wear.

Remove the motor brushes and inspect them for signs of overheating or wear. Make sure they move freely in and out of the brush holder assembly without sticking. Minor defects can be corrected with a small file. Also inspect the commutator. A drop of WD-40 on a Q-tip can be used to clean the commutator. It may appear blue if it has been badly overheated.

Reassemble the motors and run them in for several minutes to re-seat the brushes. If this doesn't cure the problem, you've cooked the motors and they should be replaced. Many successful robots have run Harbor Freight drill gearmotors -- just make sure to break them in properly and they should work well for you.

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Q: Is the 'Max Motor Current' of the Etek-R its stall amperage?

A: No. Maximum motor current is the allowable current draw for a specific time interval. The Etec-R can be loaded down to draw 320 amps for 2 minutes before something ugly happens, or loaded to 150 amps for as long as you like. I'd estimate stall current to be over 800 amps at 48 volts.

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Q: What motor should I use for a lifter like Team Plumb Crazy's Wipeout?

A: Electric lifters require a lot of power and a very sturdy gearbox. I believe 'Wipe Out' uses a Magmotor power it's lifter -- possibly the A28-150.

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Q: What motor does the Beetleweight Nuclear Kitten use for it's weapon?

A: It appears to be a good old fashioned Small Johnson motor. Proves that you don't have to run expensive brushless motors to be competitive.

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Q: How should I mount my Sanyo micro motors?

A: The motor comes with two screws that fit holes in the gearbox faceplate. Assuming that you have a flat plate chassis, you can make a wide metal L-bracket that bends back under the gearbox and fastens down securely. Drill holes in the L-bracket to match those in the faceplate and fasten with the provided screws.

The far end of the motor should also be secured to the chassis. If the high-stress gearbox end is well mounted, a zip-tie thru the chassis and around the motor end will do to secure the other end in an insect-class 'bot.

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Q: One of the gears in my Sanyo micro stripped. Should I buy a new one or attempt to repair it?

A: I don't have a source of replacement parts for the Sanyo Micro gearmotor. You could spend a lot of time and effort trying to find a suitable gear -- I'd replace it and save the stripped motor for parts.

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Q: Are any of the Solarbotics gearmotors well suited for ants and beetles?

A: No. Their gearmotors are all very small, low-output units. Also, most are geared way too low to provide enough speed for a combat robot. You could use their GM18 mini metal gearmotors in a 150 gram fairyweight, but the peak power output is only about 1 watt.

Now, I have seen Solarbotics gearmotors used in autonomous ants and ants with really huge weapons that just need to be pushed around slowly. There are 'special circumstances' were you can get away with very low drive power, but most ants and beetles will require much more power to be competitive.

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Q: What is petrol? How does a 'petrol motor' work?

A: Mark J. here: you've been browsing British websites, haven't you? Petrol is what they call gasoline in the U.K. -- short for 'petrolium spirits'. A 'petrol engine' is your standard internal combustion engine (ICE).

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Q: What would be the best motors to use in a hobbyweight thwackbot? It's mainly being built for Robot Battles and NERC events, so I'm not worried about the RFL judging guidelines.

A: Mark J. here: a thwackbot needs enormous spinning speed and lightning acceleration to be effective, and that translates into power. This is the one application where I'd go for total motor overkill and reach for the big gun: DeWalt. A pair of 14.4 volt DeWalt drill motors and gearboxes locked into high range and overjuiced to 18 volts would give you a screaming blur of a thwacker. Yes, you can use the 18 volt DeWalts pushed to 24 volts if you really want to, but there's overkill and then there's total madness.

You're correct that NERC and Robot Battles are not RFL affiliates, but they may very well use a similar 'Damage / Aggression' scoring that works against thwacks. I'd check on this before building.

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Q: What motor powered the housebots 'Dead Metal' and 'Mr. Psycho'? How much torque does it have?

A: Dead Metal, like all the early HouseBots, was powered by Bosch GPA 750 motors: 97 inch pounds of stall torque at 24 volts. The motors were geared way down; none of the early HouseBots were very fast.

I never saw 'Mr. Psycho' in person. I'm told that it was propelled by twin 5 HP Briggs and Stratton motors at 48 volts, but I don't know which specific motors those are.

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Q: I found a Magmotor model 21231 on EBay. Would it be good for a lightweight spinner? What about the other Magmotors currently for sale on EBay?

A: Mark J. here: the Magmotor company makes dozens of different motors for a variety of purposes with a wide range of output power. The 21-230 series you found on EBay is a low-output 2.7 pound industrial motor that comes in several different windings (not specified on EBay) for use from 12 to 120 volts. Peak output is about 0.4 HP with perhaps 350 oz-in torque. The motor is too heavy for its output to be of much use on a combat robot.

You have to know what you're buying on EBay. Descriptions are often incomplete and sellers may not know much about what they're selling. Stick with motors you know.

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Q: What is a "coreless" motor?

A: A normal DC electric motor has a rotating armature of copper wire wound around an iron or steel 'core'. The core supports the copper windings and helps to direct the magnetic field, but it's heavy and adds to the bulk of the motor. The actual force generated to turn the motor is exerted against the copper windings themselves so, although convenient in manufacturing, the core is not required.

By embeding the windings in epoxy resin the steel core can be eliminated, resulting in a motor that is lighter and faster to respond. The down side is extra expense in manufacturing and less sustained power handling capacity due to the loss of the heat sinking and radiating effect of the armature core.

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Q: Will the AME 218-2003 gearmotor work on a 4-bar lifter?

A: Way too little information. Any gearmotor will 'work' on a 4-bar lifter, but I can't even guess at the performance of the resulting lifter since you gave me no clue about the 4-bar geometry or the weightclass of the 'bot.

As a general comment the motor is very heavy for the power it puts out, and the worm drive gearbox is built for windshield wipers -- not heavy lifting. There are better choices for a lifter motor.

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Q: Is the NPC-41250 motor a good choice for a 60 pound wedge or pusher? If not, which motors of a similar or lesser price would be?

A: The NPC-41250 is quite durable and easy to mount, but it is also fairly expensive ($150+) and very heavy (7.5 pounds) compared to gearmotors with similar performance. Worse, the gear reduction is too great for use with reasonably sized wheels: with 6" wheels at 24 volts it tops out at a hair over 3 M.P.H. -- way too slow!

If you want to stick with an NPC setup, look at the 'Build Your Own Gearbox Kit' at the bottom of the NPC-41250 page linked above. You get 14 M.P.H. for the about the same cost and weight.

There are plenty of popular alternative gearmotors. BaneBots offers a wide range of geaboxes that mate to a wide range of motors. Other good choices would be the popular cordless drill gearmotors: 18v DeWalt, Harbor Freight, etc.

You can 'test drive' a wide selection of motor/gearing/wheel/weight combinations at the Team Tentacle Torque & Amp-Hour Calculator.

Q: What batteries should I use for a pair of NPC 'Build Your Own Gearbox Kits' in a lightwight?

A: Here are the steps to estimate needed battery capacity:

1. Go to the Team Tentacle Torque & Amp-Hour Calculator.
2. Put in your motor type (NPC 02446), Weight Class (60 lb), Motors per Side (1), Wheel Diameter (6" 4"), and Gear Ratio (5.5:1 4:1).
3. Click the 'Battery Chooser' button.
4. Read off the estimated Amp Hours of battery capacity needed for 3 and 5 minute matches.
5. Compare that to the 'real' output of a specific pack by selecting that pack in the pull-down menu and checking the 'Reserve Capacity'.

I'd go with the BattlePack PN-3000-24.

Q: Would I only need one BattlePack PN-3000-24 with a pair of NPC gearbox kits?

A: The 'Battery Chooser' estimates the power consumption of all the motors entered into the calculator. I made an error when I first did the battery pack calculations for a pair of NPC BYO Gearbox kits. I thought the gear reduction was 5.5:1, but it's actually 4:1. That changes things a little.

With the correct gear ratio, the 'Battery Chooser' shows that a single BattlePack 3000-24 would be fine for two NPC BYO Gearbox kits in a lightweight with 4" or 5" wheels. The performance difference between wheel sizes is small -- I'd go with the 4" wheels in an arena up to 24 feet across; 5" if you fight in a really big arena or just wanna brag about top speed.

With 6" wheels, the 'bot could completely drain a BattlePack 3000-24 at the end of a tough 3 minute match. You do not want to run out of juice at the end of a match! A single BattlePack 3600-24 would be a better choice for 6" wheels.

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Q: What kind of motor is the NPC-41250: brushed or brushless?

A: National Power Chair (NPC) is a company based in Minnesota that rebuilds motors and gearboxes for electric wheelchairs. Their gearmotors became very popular for combat robots back in the BattleBots days. All of their motors are brushed, as are other industrial permanent magnet DC motors built for economy, reliability, and prolonged high torque output capacity.

Brushless motors are found in electronic and computer applications where tightly controlled speed and compact dimensions are needed. The model airplane industry adopted brushless motors because of their very high power to weight ratio. High performance brushless motors are designed for high speed / low load operation and are not useable in combat robot drive systems where they would experience prolonged high loads at low speeds. They rapidly fail under such conditions. Robot combat use of brushless motors is restricted to weapons.

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Q: Can you point me to some lightweights other than Tremor that use the NPC 'Build Your Own Gearbox' kits?

A: No, sorry. The NPC kit is simple, durable, and easy to mount but is less powerful than some other gearmotor options of similar weight. In my opinion, it's still ample power for a lightweight.

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Date marker: December 2007

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Q: I recently received a 'Super Rooster' speed controller from a friend. Would it be able to control a 14.4V Harbor Freight drill motor?

A: Depends on how much load you put on the Harbor Freight motor. For use as a hobbyweight drive motor with reasonably sized wheels, it should be fine up to its maximum rated 12 volts -- 14.4 volts might be pushing it. Controlling a big spinner weapon is likely not advisable. Note that the 'Super Rooster' does have a bothersome delay before it will switch from forward into reverse.

Hobby R/C ESCs are rated for operation of R/C cars -- not combat robots that spend a lot of time pushing. If the motor is stalled, your Rooster isn't going to live long. Don't trust the amp figures given by hobby R/C ESC manufacturers; they are purely theoretical and for instantaneous loadings only.

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Q: Could I use two NPC Black Max motors in a middleweight or would that be overkill? I plan to use it in wedge bot's drive.

A. The Black Max has been used in some terrifying robots, but it is much heavier (15+ pounds) and less efficient than some more recently available motors. The A28-400 AmpFlow weighs less than half as much and produces more horsepower at a similar amp draw, but I'd go with the still smaller and lighter A28-150 for a middleweight wedge. That would be plenty.

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Q: Is the Piranha motor as good as the Robot Marketplace advertises?

A. The text description of the Piranha gearmotor at Robot Marketplace only says is that the gearbox is, "strong, lightweight and reliable". It's too new to have a combat record -- I don't know of anyone who is using it. Like any new product, this one is 'try at your own risk'.

What you're really buying here is the gearbox. For $93.99 it comes with an inexpensive 550 Johnson motor attached, but any 540 or 550 size motor would bolt right up. There are plenty of other 550-size gearmotors available, most of them quite a bit less expensive than the Piranha. It does have a big 1/2" shaft and beefy dual bearings in a tough-looking case. Mounting should be easy. The 36:1 gear ratio is a little to high for my taste -- with the Johnson motor @ 18 volts and 3" tires, it would only give 7 MPH.

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Q: What does 'ESC' stand for?

A: ESC stands for Electronic Speed Controller.

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Q: Where can I find some discontinued 'EV Warrior' motors?

A: Mark J. here: the EV Warrior was an electric bicycle built back in the 90's. The company went bankrupt and thousands of the Bosch-built motors hit the surplus market at very reasonable prices. The motors were compact, weighed around 3.25 pounds, and pumped out about 1.5 HP when overvolted to 24 volts. A lot of combat robots made good use of the motors.

Unfortunately, the last source of these surplus motors dried up in 2005. You might get lucky and find a few on EBay, but buy a couple of spares -- overvolted EV Warrior motors were not known for longevity.

Q: If the 'EV Warrior' motor was so popular, why where they discontinued?

A: They were popular because they were dirt cheap -- as little as $15. They were only dirt cheap because a warehouse full of them was dumped on the surplus market after the electric bicycle company went bankrupt. When Bosch was making them they cost ten times as much and were not a bargain for robot builders.

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Q: How come brushless motors need to use a brushless ESC?

A: Brushed motors use a set of physical sliding contacts (brushes and commutator) to control the direction of current flow through the armature windings and keep the motor spinning, so all the ESC has to do is control the speed by limiting current flow.

Brushless motors have no sliding contacts and rely on an 'intelligent' motor controller to perform the duties of both the commutator and the speed controller. A brushless motor will not even run without a suitable motor controller.

See also a previous post on this topic in this section.

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Q: Would two 500 rpm Harbor Freight motors at 18v with 2" wheels make for a nice pushy hobbyweight?

A: Too slow! The Team Tentacle Torque & Amp-Hour Calculator shows a top speed less than 3 MPH for that configuration. It'd be OK with 6" wheels. Better yet, use the 900 RPM drill motors with 6" wheels.

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Q: The 42mm 16:1 BaneBots with the RS-775 motor says it can take up to 20 volts. Do you think this is exaggeration, or could 4 RS-775 16:1 motors take 18 volts direct driving 3.5" wheels in a 30lber?

A: BaneBots is, if anything, a little conservative with their voltage ratings. I've seen the RS-550 motors run at 18 volts in combat, so the RS-775 should take the voltage nicely.

Plugging the numbers into the Team Tentacle Torque & Amp-Hour Calculator indicates that 4 of these gearmotors would give a wild ride to a featherweight 'bot. Top speed comes out to about 14 MPH, which would come in 1.3 seconds or 18 feet. The tires would spin at less than 11 amps per motor (127.5 amps is stall current). Might be overkill.

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Q: I'm ready to change the spur gear drive train on my robotic gripper to the worm-wheel drive you suggested in an earlier post. I've found some worms and gears at the Martin Sprocket site. Now I need to select a good DC gearmotor.

I was using the Hitec Digital Robot Servo with an additional 4:1 spur gear drive before -- remember that it got hot at stalling when trying to hold things? I have a choice of a 5:1, 10:1 or 20:1 worm reduction for the final stage of the new motor. With size and weight in mind, what gearmotor would you consider to be the best choice to replace my servo?

A: Take a look at the BaneBots RF-370 20:1 25mm Spur Gearmotor. Coupled to a 20:1 worm drive, it will give better torque and speed than your 4:1 geared Hitec servo at the same 7.4 volts.

If you're willing to run at a lower voltage the BaneBots FF-180 30:1 25mm Spur Gearmotor is lighter and at it's maximum rating of 4.5 volts with the 20:1 worm drive would provide torque and speed similar to the 4:1 geared Hitec servo at 7.4 volts.

Q: Can the BaneBots FF-180 Motor be overvolted to 7.4 volts? Specs says that its operating voltage is from 3 to 4.5 volts.

A: The Mabuchi FF-180SH-2860 sold by BaneBots is a metal-brushed motor. Metal-brushed motors don't generally put up overvolting as well as carbon-brushed motors, particularly in applications where it will be even briefly stalled. I'd advise against trying to run it at 7.4 volts in your application.

Q: If I choose the 4.5V Banebot gearmotor for my gripper, what would be your suggestion for dropping the 7.4 volts from my LiPo battery to 4.5 volts? I tried to use a solid-state voltage regulator in a previous project, but I wasn't happy with the results. Should I try to regulate the voltage or get a different battery for the gripper?

A: Mark J. here: I don't know how you're controlling your gripper motor, and that makes a difference. The problem with mildly overvolting DC motors is the extra amperage that comes with the increased voltage -- not the extra voltage itself. Amperage creates heat and increased heat will destroy the metal brushes in the BaneBots FF-180 motor.

If you're using an R/C system with an Electronic Speed Controller (ESC) to control the BaneBots gearmotor you can limit the amperage to the motor either by setting the ESC to deliver less than full power, or by setting the Adjustable Throttle Volume (ATV) on your R/C transmitter to prevent 'full throttle'.

If the motor will be controlled by simple switching, adding additional resistance to the motor circuit will restrict the maximum amperage and will keep the brushes intact. How much extra resistance is needed to hold the stall amps constant at the new voltage?

Additional Ohms Needed = ( New Voltage - Original Voltage ) ÷ Original Stall Amperage

The FF-180 motor may be run reliably at 7.4 volts if you add ( 7.4 volts - 4.5 volts ) ÷ 6.7 amps = 0.43 ohm additional resistance. You'll have the same torque that the motor would deliver at 4.5 volts and gain greater speed due to the increased voltage.

How do you add 0.43 ohm that will carry 6.7 amps? AWG 22 gauge wire will carry 7 amps of current and has a resistance of 16 ohms per 1000 feet. A loose coil of 27 feet of 22 gauge wire will not overheat and works out to 0.43 ohm.

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Q: I'm designing a hobbyweight spinner that will use an AXI2826/10 for weapon power. I used the formula you gave to calculate brushless motor output:

Brushless Motor Peak Output (approximate watts) = ( Voltage2 ÷ Terminal Resistance ) × 0.25

Plugging in values for the Axi I get 2194 watts. Thats comparable to a Mini-Magmotor motor and only 1/6th the weight! Can that be right?

A: Mark J. here: Axi and other brushless hobby motors do put out a huge amount of power for their weight. They were designed for model airplanes that can't afford extra mass. They were also designed to spin along at high RPM and high efficiency and to never be bogged down into the rev range where they produce peak power. Their small mass means that they will fail very quickly from thermal overload if they are required to operate in a high-torque range for more than a very small percentage of their run cycle. We can just barely get away with using them for spinner motors.

Magmotor motors are designed for combat robot propulsion. Their extra size and mass enables them to perform for extended periods at near-stall speeds and in the max power range of their torque curve. All that extra mass is wasted if the motor spends most of it's time at low power output keeping a spinner weapon up to speed. Conversely, an Axi would fail quickly if used as a middleweight propulsion motor.

Also remember that my formula for estimating brushless motor output is theoretical. All sorts of things will work to decrease the actual performance in use: additional circuit resistance caused by the motor controller, inability of the battery to provide full voltage at large amp drain levels, power loss from the gear/belt drive, etc.

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Q: Will your spinner spreadsheet work for brushless motors? They are very different than brushed in many ways, I know.

A: Mark J. here: they are not as different as you think. The shape of the torque, amperage, and power curves for brushed and brushless motors used in combat robotics are effectively identical. Both brushed and brushless are Permanent Magnet Direct Current (PMDC) motors -- the only substantial difference is that brushed motors use a set of physical sliding contacts to control the direction of current flow through the windings, while brushless motors use an electronic controller box for that purpose. The elimination of the sliding contacts reduces arcing and friction, making brushless motors more efficient.

The Team Run Amok Spinning Weapon Excel Spreadsheet works equally well for brushed and brushless motors.

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Q: I'm gonna use the Axi Gold 2826 to spin a vertical disk in a 12lber. I was wondering, which model should I choose; the /08, /10, or /12?

A: How many volts are you willing to run? Up to 12 volts, I'd run the 2826/8. For 4 or 5 cell LiPolys, switch to the /10.

Q: So in order to get the most power out of a Axi gold 2826, I should use maximum number of batteries allowed with the highest number 2826? For example, the most powerful Axi 2826 combination would be 2826/12 with 18 NiCd cells?

A: Mark J. here: NO -- stop looking for short cuts and do the math.

On paper the 2826/10 (0.042 ohm) on either 16 NiCads (19.2 volts) or 5 LiPolys (18.5 volts) puts out more power than the 2826/12 (0.062 ohm) on 18 NiCads (21.6 volts). The number after the slash on the Axi model number refers to the number of turns of wire around each armature pole. The sequence for the Axi 2820 series just happened to work out with a larger number of wire turns providing greater power at max recommended voltage.

Q: How are you finding the terminal resistance of the different axi's? I could do the math if I knew how to find that.

A: Most sites that sell brushless motors provide the 'internal resistance' (same as 'terminal resistance') for each motor in the specifications. I get my specs straight from the Axi website.

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Q: What would you estimate the power of the Axi 2820 to be in watts?

A: Mark J. here: from other equations I've given here, adjusted for the specs usually provided for them:

Brushless Motor Peak Output (approximate watts) = ( Voltage2 ÷ Terminal Resistance ) × 0.25

• Axi 2820/8 @ 9.6 volts = 883 watts output
• Axi 2820/10 @ 12 volts = 917 watts output
• Axi 2820/12 @ 16.8 volts = 1192 watts output

Note that this is peak output -- attempting to maintain this output level for any length of time would melt the motor. For comparison, the continuous rated output of the Axi 2820/12 is 285 watts.

Q: Say a hobbyweight has a spinning bar weapon with 3500 joules of energy. That means that the bar could be spun up by an Axi 2820 in about 3 seconds, since 1 joule is equal to 1 watt second. So once it spun up, how long could it spinning before it melted?

A: Your calculation of spin-up time is over-simplified. The output of a permanent magnet DC electric motor operating at constant voltage is dependent on the RPM of the motor, with peak output at 1/2 of the no-load RPM. The power consumption, torque, and total power produced by the motor all change with RPM as the weapon system spins up. A full analysis of weapon spin-up time is provided by our Team Run Amok Spinning Weapon Excel Spreadsheet.

Amperage creates heat. Spinning up the weapon requires high motor torque output and coresponding high amperage draw. Once the motor is up to a high-speed spin, the amperage draw is greatly reduced as very little power is required to maintain the weapon speed -- just enough to offset frictional drag. In this situation a properly geared Axi 2820 would spin along happily until the battery goes flat.

The problem of meltdown becomes a worry if the motor is used for robot propulsion and is used for extended periods in high torque/amperage pushing, acceleration, or rapid reversing.

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Q: What motor should I use: AC or DC?

A: Batteries produce Direct Current -- use a DC motor.

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Q: Would the Mamba Max [expired link deleted] be a good hobbyweight weapon motor?

A: Like most R/C car hobby motors, very little performance information is provided for the Mamba Max. From what is given there isn't any way to tell how much power it puts out. It does claim to spin 82K RPM at 14.4 volts, which could be difficult to gear down to useable weapon speed.

The Mamba Max is an 'inrunner' style brushless motor. Outrunner 'rotating can' style brushless motors are much more popular for weapon motors because they produce much greater torque at lower RPM. Stick with a proven motor.

Q: Which Feigao motors are best for hobbyweight spinning weapons?

A: I haven't seen the Chinese-made brushless Feigao motors used in robot combat, so I can't comment on their suitability. The limited information they give for their 540Y-15/7 outrunner motor looks about right for a hobbyweight weapon, but why not stick with one of the proven brushless motors?

Q: Alright, are there any hobbyweight weapon motors you can recommend?

A: It would really help if I knew something about your weapon other than it goes on a hobbyweight. Full Body spinners, drums, bars, and disks of varying sizes all have different power requirements. We've said all this before on the site:

• Look at what other 'bot builders are using in similar situations. You'll find a lot of builders using Axi brushless motors for spinner weapon power. The 2826 and 4120 series Axi motors are commonly used in hobbyweights and have proven to be powerful and reliable. Similar motors are made by Lehner, and Himax.
• The Team Run Amok Spinning Weapon Excel Spreadsheet will help you to calculate the kinetic energy and spin-up time for your proposed weapon and motor combination.
• Help in figuring out the stall torque of a brushless motor from commonly listed specifications can be found in our previous post on Brushless Motor Selection.
• 'Outrunner' style motors are preferred because they run at lower RPM with greater torque than 'inrunner' motors. Keeping a belt drive on an inrunner motor spinning at more than 60,000 RPM can be a problem.

Note that although brushless motors offer more power-per-pound, there have been many champion spinners built with brushed motors. There is a lot more to building an effective spinner weapon than picking the right motor -- just make sure the motor isn't the weakest link.

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Q: Where can I find information on the stall amperage and such for the Harbor Freight drill motors?

A: The Team Tentacle Torque & Amp-Hour Calculator has listings for 83 different combat robot motors, including the Harbor Freight 9.6, 12, 14.4, and 18 volt drill motor/gearbox combinations.

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Q: What Axi gold motor would be good for drive in a 12 pound wedge? I wanna try something different. I was thinking maybe the Axi 2814 would get the job done and them some.

A: Mark J. here: I estimate the peak power output of an Axi 2814/10 at a little over 830 watts. A pair of those in a hobbyweight would give more than 140 watts of drive power per pound of robot -- about eight times what you could effectively use in a small arena. The extra power will just be lost in wheelspin, and keeping it pointed in a straight line would be a real challenge.

You could go with a smaller Axi, but the motors have tight limits on their maximum sustained amp draw: they melt if pushed too hard. An Axi makes for a great weapon motor, but they aren't ideal as drive motors.

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Next Mystery Post »

Q: How can I compare brushed and brushless motors for use in spinner weapons? Brushless motors have specifications on the websites, but those specs are for completely different types of applications.

A: Mark J. here: the primary market for brushless hobby motors are model aircraft, and model aircraft builders don't care about the specs we use -- model aircraft don't stall their motors! You can find formulas and definitions for all the motor specs aircraft guys use at the bottom of the Aveox technical page.

The Team Run Amok Spinning Weapon Excel Spreadsheet needs only two key motor specs: stall torque and no-load RPM. Brushless motors usually have an RPM per volt specification that lets you calculate the no-load RPM at your voltage. The stall torque can be calculated if you can find the 'terminal resistance' for the motor:

Stall Torque (in-oz) = ( 1352 ÷ RPM per Volt ) × ( Voltage ÷ Terminal Resistance )

If you can't get the required info for the motor you're interested in, I have a couple of low-tech solutions:

• I've said this a dozen times before on this site: look at what other 'bot builders are using in similar situations. If someone else has already done the trail and error work, don't repeat it -- copy it.
• There is a 'rule of thumb' that model airplane guys use: a brushless motor puts out about twice the power of a brushed motor of the same weight. So, find a brushed motor that has the specs you need to drive your spinner and shop for a brushless motor that weighs half as much.

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Q: I just bought a shiny new brushless motor from BaneBots. How do I wire it to the brushless speed controller? All the wires are black, so which goes where? Thanks for your time.

A: Would you believe it doesn't make any difference? You've got three black wires with male connectors from the motor and three black wires with female connectors from the controller -- plug 'em in any way you like. If the motor spins backwards, switch any two of the connectors to reverse the spin direction.

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Q: I'm building a hobbyweight. I plan for it to be a fast and powerful wedge bot with two 36mm BaneBots motors for drive. I was previously planning on using the RS-540 125:1 with 3" wheels, but the Team Tentacle calculator says it would hardly even go 1 mph. What motor would you choose for this situation? Thanks for your time.

A: Your RS-540 motor is fine, but you picked the wrong gearbox. Try re-running the calculations with the 16:1 gearbox, and leave the 'Gear Ratio' box on the Team Tentacle Torque & Amp-Hour Calculator at 1:1. That will give you better than 9 MPH and lightning acceleration with 3" wheels -- perfect for a small arena.

Q: If I decided to build a pushy bot/rammer instead of a wedgebot, would the RS-540 16:1s still work well?

A: See the previous article on propulsion power requirements. The RS-540s will give you plenty of power for anything you might want to do with a hobbyweight.

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Q: When I wire two motors together on one ESC channel, is the amperage draw doubled?

A: If you wire two identical motors in parallel to a single channel of an Electronic Speed Controller, the potential total amp draw from the ESC will double compared to a single motor. Each motor will have 100% of normal power output.

If you wire two identical motors in series to a single channel of an Electronic Speed Controller, the potential total amp draw from the ESC will be cut in half compared to a single motor. Each motor will have only 25% of normal power output.

See a related post on multiple motors.

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Q: I just wired up my BaneBots brushless motor and speed controler. When I turn it on I get is a strange beeping noise and the shaft rotates about an eighth of a turn and jerks back into place. At one point, my dad dropped the motor into ranch dressing, but nothing actually got inside. Is it that that experience that screwed up the motor, or did I do something wrong?

A: Your dad treating the motor like a Buffalo Wing isn't the problem. Take a minute and read thru the instruction sheet for the ESC. If the battery voltage is too low, the BaneBots GWS ESC will not start-up and will make a continuous 'dong-beep' sound. Charge up your LiPoly (the ESC only works with LiPolys) and you'll be fine.

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Next Mystery Post »

Q: Which BaneBots motors can be used for 12 pound robots or above?

A: Mark J. here: the motors you will need depend on the style of 'bot you're planning to build. A 'bot with a massive weapon that just needs to push itself around the arena will need a lot less drive power than a ramming brick or wedge. It comes down to matching the power-to-weight ratio of your 'bot to the type of attack strategy you're planning.

Rule of thumb: 4 watts of drive power per pound of robot will adequately maneuver an active weapon robot around the arena. Rammers and fast wedges require about 16 watts per pound.

Some maniac builders pack as much as 40 watts per pound into their 'bots, but there really are limits to how much power can be used in a small arena. I've built three champion robots on the 4 watts per pound formula, so don't tell me it can't be done.

The BaneBots 36mm planetary gearmotors are popular choices in the 12 pound hobbyweight class. The output shaft is a beefy 3/8" diameter. Three different motors are available with the 36mm gearbox: RS-385, RS-540, and RS-545. BaneBots does not provide output power numbers for these motors, but they do provide stall amps and voltage. You can get a rough estimate of output power with the following formula:

Peak Output (approximate watts) = Voltage × Stall Amperage × 0.25

The formula gives the following outputs:

• RS-385: 30 watts
• RS-540: 125 watts
• RS-545: 110 watts

From the 'rule of thumb', a 12 pound wedge robot would require about 16 watts per pound × 12 pounds = 192 watts of power, so two of the RS-540 or RS-545 motors coupled to the 36mm gearboxes would give you plenty of push.

Which gear ratio will you need? See the Ask Aaron Optimum Gearing page.

For heavier robots, BaneBots offers a 42mm gearbox with a 1/2" output shaft attached to the RS-550 motor. The RS-550 puts out approximately 250 watts, so two of these would be adequate for a 30-pound featherweight. More power? It is possible to mount the RS-775 motor (370 watts) to the 42mm gearbox.

Want still more power? Use more gearmotors! Four RS-550s would be enough for a 60-pound lightweight wedge, and six RS-775s would power a 120-pound middleweight rammer.

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Next Mystery Post »

Q: Could I please have a more detailed explanation of your formula to estimate the output wattage of motors?

A: Mark J. here: at stall, a Permanent Magnet Direct Current (PMDC) motor produces maximum torque and will consume:

Power Consumption at Stall (Watts) = Voltage × Stall Amperage

The mechanical power output of a motor is a product of torque and RPM. At stall, a PMDC motor produces zero mechanical power: the torque is at maximum but RPM is zero. PMDC motor torque and amperage consumption decrease linearly to approach zero at no-load RPM ¹. At maximum no-load RPM the motor again produces zero mechanical power: the RPM is at maximum, but the torque is zero. The product of torque and RPM is power, and power reaches maximum at 50% of the no-load RPM where the motor will consume very close to 50% of the stall amperage:

Power Consumption at Peak Output (Watts) = Voltage × (50% of Stall Amperage)

The efficiency of a PMDC motor in converting electrical power to mechanical power varies with motor design, materials, and RPM. Approximately 50% efficiency at peak output is typical. This gives an estimate of power output as:

Peak Output (Watts) = Voltage × (50% of Stall Amperage) × (50% Efficiency)

The above equation simplifies to the equation I use to estimate peak motor output power:

Peak Output (Watts) = Voltage × Stall Amperage × 0.25

Brushless motors do not specify a stall amperage figure because it is the brushless motor controller that determines current flow at low motor speeds. However, brushless motor specs do commonly provide a Terminal Resistance that allows you to calculate a theoretical stall amperage:

Stall Current = Voltage ÷ Terminal Resistance

A little algebraic substitution combines the two equations above to provide a one-step shortcut that estimates peak output power from just voltage and terminal resistance:

Peak Output (Watts) = (Voltage² ÷ Terminal Resistance) × 0.25

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Q: What motor specs would you recommend for a middleweight 'bot using 2 drive motors? I'm planning on a wheel diameter of about 8 inches, tops.

A: How much drive power a combat robot needs depends on the design and attack strategy of the 'bot. A wedge or rammer will need a lot more drive power than a full-body spinner. Several top-ranked middleweight wedges run a pair of A28-150 "mini" Magmotor motors -- that's 6 horsepower worth of go-power! A middleweight 'bot with a large active weapon could get by with as little as 500 watts of drive power, but common practice calls for more like 1500 watts to keep the 'bot in the fight. One horsepower equals approximately 750 watts.

Wheel diameter isn't a factor in motor selection. For larger wheels, just increase the gear reduction to keep the torque high enough to avoid stalling the drivetrain. See the Ask Aaron Optimum Gearing page.

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Q: Why is the IBC Dual Motor+2AUX Speed controller so expensive? Would it be too small for a basic middleweight 'bot?

A: Mark J. here: Reliable robot motor speed controllers are expensive, but at around $260 the Robowars IBC controller isn't more expensive than comparable dual-channel controllers like the Vantec RDFR23, or Robot Solutions RS80D. Don't confuse the power ratings of robot controllers with the advertised ratings for R/C car controllers. The R/C car controller ratings are entirely hypothetical -- try to pull the full rated amperage thru an R/C car controller and you'll very quickly get an expensive puff of smoke. Remember: the most expensive speed controllers are the ones you overload and blow up!

The Robowars IBC controller is popular for hobbyweight and featherweight robots, but with a 50 amp-per-channel power rating I wouldn't try to build a middleweight around it.

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Q: What is the most popular weapon speed controller for the hobbyweight class?

A: Mark J. here: I don't think 'most popular' is a good way to select a weapon controller. The controller should be matched to the specific motor and weapon loading for your 'bot, not just to it's weight class. I've seen a very wide range of weapon motors and designs in hobbyweights. If you can tell me more about the weapon you plan to build, I can be of more help.

I'm planning to use two Mini EVs or HTIs to power a vertical disk weapon.

A full analysis of your weapon system power requirements and energy potential also requires the weight and diameter of your disk, and the speed reduction of your drive system. Assuming an 8" disk weighing 3 pounds and a 3:1 reduction, your twin-mini-EV powered weapon running at 12 volts would spin up to 3780 RPM in about 1.6 seconds with 530 Joules of energy, and will eventually reach over 1000 Joules. Killer for a hobbyweight!

If you have a no-slip drive system for the weapon (chain, gears) and switch power full-on, those twin Mini EVs will pull an average 117 amps during that 1.6 seconds, with a peak inrush of almost 170 amps. That's killer too, but not in a good way.

The easy solution is to use a hefty battery pack, a Victor 883 controller, and a belt drive. You'll be able to feed the power in quickly enough to keep a great spin-up time while avoiding that nasty sudden amp spike you'd get with a solenoid/contactor. A belt drive set-up with a little slippage will help keep you from melting down when the weapon stalls. The 833 is reversible to allow you to clear the weapon if it jams.

There are other options:

• Use a solenoid/contactor and a separate high-amp battery pack for the weapon. This will keep the big amp surge from glitching the other systems on your 'bot, but adds weight.

• Use a high slippage belt drive with a solenoid/contactor to avoid high amperage draw during spin-up. This will lengthen spin-up time and will be difficult to adjust properly.

• Use a lower capacity motor controller with a Team Delta Digital Servo Slower between your receiver and controller to slowly ramp-up the amperage delivered to the weapon motor during spin-up. This will lengthen spin-up time and you'll still need a high-slip belt drive to keep everything healthy when the weapon stalls.

I'd use the Victor for the best combination of reliability, flexibility, and performance.

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Q: Big weapon motors in larger weight classes draw a lot of current. I don't want to buy $100+ ESCs for them. I've seen that using a solenoid with a switch is a much cheaper way. Is using a solenoid common in the 12 pound class? Can you explain how do a use a solenoid and switch to control the weapon motor? Is it possible to run two weapon motors off a single solenoid? Thank you for your help!

A: Mark J. here: mechanical switching control of brushed weapon motors has a number of drawbacks. Solenoid operated switches are called 'contactors', and a reliable contactor isn't much less expensive than a weapon ESC. If you get a cheap one, the contacts can weld shut under the high current loading encountered when suddenly dumping full voltage to a loaded weapon motor. That high current draw is also hard on your battery pack and may cause a voltage drop that can glitch other 'bot systems. A smooth transfer of current available with an ESC avoids these problems.

A contactor requires an R/C interface to trigger, like the Team Delta solid-state R/C 'D' switch. The R/C switch plugs into your receiver and controls a small current load to activate the coil of the contactor. The contactor then switches the high current load to the weapon motor. You can control as many weapon motors as you like with a single contactor -- as long as the total current draw of the motors does not exceed the capacity of the contactor. This also applies to ESCs.

Oh, and did I mention that contactors are heavy? They are not commonly used in high power sub-light 'bots. I'll say it again, 'cheap' and 'robot combat' don't mix.

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Q: Do you know of any motors with similar performance to the AXI 2208?

A: It's not easy to compare performance figures for the brushless outrunner motors -- manufacturers often don't supply the type of numbers you really need for comparison. Also, there are different versions of the AXI 2208 (2208/34, 2208/26, 2208/20) that have very different performance figures.

Browse around the Aeromicro website. They have what specs are available for AXI, Park, Himax and other outrunner motors. The E-flite Park 370 Outrunner has seen good use in insect class 'bots and would be a good place to start.

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Next Mystery Post »

Q: I'm really on a tight budget for my new bot. I have heard that you could use a two-way toggle switch instead of a speed controller. How do I do this?

A: Not a toggle switch -- some very early robots used an R/C servo to close microswitch contacts to turn motors on and off. It was unreliable, and suitable only for very slow robots. Also, some early R/C dune buggies used a servo to rotate a mechanical speed controller based on power resistors - both heavy and unreliable. If you really need cheap, you'll be better off to hack a radio/controller module out of an R/C toy or pull the low-power speed controller out of a low-end servo.

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Q: Is there a website that shows different kinds of motors for use in combat robots?

A: There are lots of motors you can use in your robot. You can find pictures, descriptions, and specifications for dozens of popular motors at: Robot Marketplace.

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Q: Where can I find speed controllers to salvage?

A: Mark J. here: check #16 in the for general comments about cheap robot parts.

Speed controllers are the heart of your 'bot. They are key to the function and usability of the entire machine, and if they fail you're toast. To be useable in your 'bot, speed controllers must:

• Handle the current and voltage requirements of your motors;
• Transition seamlessly from forward to reverse motion;
• Interface with your R/C system; and
• Be small, light, and reliable.

You're just not going to meet those requirements from a salvage source.

I don't recommend scrimping on robot electronics -- you can bet your opponent hasn't.

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Q: Where can I find gearmotors to salvage?

A: Inexpensive cordless drills are a very popular source of compact and powerful gearmotors. If you really want to 'salvage' something, permanent magnet DC gearmotors are used in automobile electric seats, windows, and windshield wipers. Your local auto salvage yard can be a good source.

Note that the power and output speed for gearmotors varies widely. You may have some difficulty finding something that meets your needs, so be ready to improvise!

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Q: What motors would you recommend for a beetleweight thwackbot?

A: Thwackbots are not popular in the insect weight classes. They need time and a clear area to spin up that just doesn't exist in a small insect class arena. If you really wanna try a beetleweight thwackbot, the B16 High Speed Gearmotor might be worthwhile.

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Next Mystery Post »

Q: Would a Park 370 outrunner motor be able to spin up a 5-6oz drum weapon effectively?

A: Mark J. here: to calculate the spin-up time and energy for a specific rotary weapon system you need to know the stall torque and top speed of your electric motor, the shape and dimensions of the spinning weapon, the type of material you'll use to make the weapon, and the gear reduction between the motor and the weapon. With that information, you can use the Team Run Amok Spinning Weapon Spreadsheet to get the performance specs of the weapon.

It may be difficult or impossible to find accurate torque information for small hobby motors like the Park 730. If you run into this, my suggestion is to learn from the experience of other builders. Find a robot with a weapon set-up similar to the one you wish to build and see if it performs well. Based on what I've seen, a Park 370 outrunner should have no trouble spinning up a 6 ounce drum of reasonable proportions. I'd use a belt drive with a 2:1 or 3:1 reduction, depending on the diameter of the drum.

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Q: Where do you get Bosch motors? How heavy are they?

A: Bosch makes several motors used in combat robotics:

• The Mini EV Warrior is available at Robot Marketplace [no longer available]. I don't think Bosch actually makes this motor, but it kinda got lumped in with them. Weight is just over one pound with an output of about 1/3 HP at 12 volts.

• The Bosch GPB, also known as the `EV Warrior' is no longer available. It weighed 3.25 pounds and produced just about 1 HP at 24 volts. These motors were on the surplus market for as little as $15 each and became VERY popular.

• Run Amok used the large Bosch GBA 750, purchased from Team Delta. This motor was popular in England and even powered some of the Robot Wars Housebots. The GBA 750 weighs a little over 8 pounds and can produce more than 3 HP when overvolted to 36 volts, but is more often used at 24 volts.

Q: How much Horsepower does a Bosch 750 motor produce at 24 volts?

A: Mark J. here: There's a simple formula to calculate the horsepower of a DC permanent magnet motor at different voltages:

Horsepower at New Voltage = (New Voltage ÷ Old Voltage)² X Horsepower at Old Voltage

The Bosch 750 makes 3.375 HP at 36 volts, so horsepower at 24 volts = (24 ÷ 36)² X 3.375 = 1.5 horsepower.

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Q: What's a good website that sells motors besides RobotMarketplace and BaneBots?

A: Check the motors, mounts, and gearboxes at Team Delta.

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Q: There a lots of 550-size R/C motors with different features! I want to build a hobbyweight Full Body Spinner using two 550 motors. Can you recommend any that can fit my application?

A: Mark J. here: real specifications for R/C hobby motors are just not provided by the manufacturers. They offer all sorts of variation in magnets, armature winds, and can designs -- but they won't give real-world numbers on the performance differences. Without figures like amperage draw, torque curves, and real RPM it's a pure guessing game.

How did you decide that two 550 motors are the right number to power your FBS weapon without specs for the motors you want to use?? How are you going to select an ESC for the weapon if you don't know how much amperage the motors draw??

I'd stick with motors that provide real specifications, like the Mini EV [no longer available. If you go with a hobby 550, try to find a bot that's using something similar and use their experience. Otherwise, you're on your own!

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Q: What's the difference between a 'machine wound' and 'hand wound' motor?

A: Mark J. here: a 'machine wound' motor has the armature windings done quickly by a machine. The windings are not neatly and compactly arranged on the armature. A 'hand wound' motor is wound by a human who can take care with the wire and make the windings more regular and uniform. A hand-wound armature usually starts off better balanced and requires less drilling or grinding in the dynamic balancing process. The more uniform wire winds also result in a little more efficient use of available current. The overall difference is small and is of more interest to an R/C racer than a 'bot builder.

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Q: I want to build a hobbyweight 'bot. What drive ESCs and brushed weapon ESCs are available?

A: You can see a whole bunch of different drive and weapon Electronic Speed Controllers at Robot Marketplace - speed controllers page.

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Q: Does the Axi 2208-34 have more torque than the 2208-20?

A: Mark J. here: Axi brushless outrunner motors are built for the model aircraft market and the specifications they list are designed to make it easy for an airplane builder to pick the best Axi for their purpose. It's not easy to extract robot type specs from the information provided.

The simple answer to your question is no. At the same voltage, the Axi 2208/20 will produce both more torque and more RPM than the Axi 2208/34. It will also pull almost three times the amperage at stall and will require a larger battery and an ESC with greater power handling capability.

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Q: How many amps does a 19 turn hobby R/C motor draw?

A: Mark J. here: The manufacturers don't tell, and the R/C racers don't care. To find out, you'll have to buy one and do your own testing (or find someone who has). A procedure to measure the performance specs of a DC motor, including stall amperage, can be found here.

I dug thru my box of old R/C parts and found a 17 turn hobby R/C motor. A quick test using the 'D-cell method' shows 31 amps stall current at 7.5 volts. That seems a little low. A 19 turn motor would pull less stall current, but I can't say how much less.

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Next Mystery Post »

Q: What is the 'D-cell' method of determining motor stall current?

A: Mark J. here: there are several methods for determining the maximum current a permanent magnet DC motor will draw at stall. I think the simplest for most builders is the 'D-cell' method. You will need:

• a new, fresh alkaline D-cell; and
• an ammeter capable of reading at least 15 amps;

Lock the motor shaft to prevent turning, if required. Wire up the D-cell in series with the ammeter and the motor. Close the circuit, quickly read the current, and disconnect the circuit.

The reading provided by the ammeter is the stall current at 1.5 volts. Current is proportional to voltage, so the stall current in operation would be this reading times (V ÷ 1.5) where V is the operating voltage of your 'bot. This all assumes that your 'bot battery can actually deliver the calculated amps, and that the resistance of your speed controller and wiring is negligible.

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Q: I'm thinking about using two Speed 400 motors to friction drive the shell of a beetleweight Full Body Spinner. Do you think that might be too little power?

A: Mark J. here: an insect class FBS has a disadvantage -- it has to spin-up really fast because of the small arena it will fight in. That's why you don't see many beetle FBS designs. A pair of Speed 400s will give you about 150 watts of power -- too little to spin up a heavy shell before your opponent gets to you.

Q: If I still want to build a beetle class FBS, what kind of motors would be adequate?

A: Mark J. here: I've posted before about calculating power requirements for spinning weapons. 'Adequate' power depends on:

• the rotational inertia of the spinning weapon; and
• the amount of time available to spin the weapon up.

The rotational inertia of a FBS weapon is very high for the weight of the shell, and the time to first impact in an insect-sized arena is very short. An undercutter or vertical spinner can turn the weapon away from the opponent to gain spin-up time, but you can't do that with a FBS. You're also going to get slippage from your proposed friction drive at high power transfer levels, adding to the spin-up time.

The cardinal rule of FBS design is: "There is no such thing as too much power." For a beetleweight friction-drive FBS, there many not even be such a thing as enough power. I'd consider another design.

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Q: Aaron, I need a speed controller and despite what you say about not scrimping on them I just can't afford to fork out AUD$325 for a good ESC. Any suggestions on a cheaper option without resorting to microswitching servos? Thanks again, Nick.

A: I really don't think that there are worthwhile options to a good speed controller, but I have seen some other things tried:

You can use the Team Delta RCE225 dual ended switch. It plugs direct into your R/C receiver and will give forward/off/reverse control for a single motor (rated 24 amps) for $42.50. Works best on slow 'bots.

If you're really hurting for funds, I remember seeing some early 'bots that converted the variable speed trigger that came from the same cordless drill as their motor. They removed the trigger spring and used a servo to move the throttle. Another servo ran the converted forward/reverse switch. Clunky and hard to drive, but it did work.

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Q: Have you had any experience with Jeti or Phoenix ESCs? Is one more reliable than the other?

A: Sorry -- I've never used either a Jeti or Phoenix. Try asking that question at the Delphi Antweight Forum.

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Q: If a brushless motor says it draws 8 amps should I get an 8 amp ESC or is it OK to run it with an 18 or 30 amp ESC? Is it better to get a higher amp rated esc?

A: You need to be very careful when interpreting motor amperage specifications. Brushless motor specs may list a "maximum loading" which is a recommended amperage over an extended period of time that is much lower than the peak amperage draw! Electronic Speed Controllers are usually rated for the peak amperage they can provide over a very short time period.

It doesn't hurt to use an ESC with a higher amperage rating than you need, and it may keep you from cooking the ESC. When in doubt, use a higher rated ESC.

Date marker: February 2006

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Next Mystery Post »

Q: I recently purchased a brushless motor and I'm wondering what the three leads are for?

A: Tech question, Mark J. here: In a brushless motor, each of the three leads is directly attached to different spots in the stationary field windings of the motor -- just as a commutator would be attached to the rotating field windings in a brushed motor.

Since brushless motors have no commutator, they require some electronic 'intelligence' to correctly assign the current flow to the field windings of the motor as the armature rotates. This intelligence is built into the motor controller matched to the brushless motor you have selected. The motor controller also acts as speed controller on hobby brushless motors.

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Q: Do you need a speed controller for a weapon motor? If so why?

A: Mark J. here -- You can turn your weapon motor on and off with a simple mechanical relay or contactor, but that puts a really big momentary drain on the battery. The drain can drop the voltage so low that your radio receiver cuts out. A speed controller allows you to feed power smoothly to the weapon and avoid the voltage drop.

With some designs, it's also handy to be able to reverse the weapon direction if you're inverted or if the weapon gets fouled on something.

Also, the new 'brushless' motors require a controller to run at all. You can't just hook them up to a battery -- it won't work!

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Q: My Electronic Speed Controller (ESC) has a Futaba J-style plug. Can I plug it in to a JR brand receiver?

A: Technical question - Mark J. here: The wire colors are different, but the signal and polarity is the same on Futaba, JR, and Hitec radio systems -- they are fully interchangeable. Some Airtronics radio gear has the polarity reversed, so always check before mixing anything with Airtronics. More info on the different connector types can be found here.

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Next Mystery Post »

Q: Do you know of any speed controls able to control 4 motors and 1 weapon motor?

A: Technical question - Mark J. here: A twin motor controller can handle two (or more) motors in parallel on each channel (four total) if the total current draw is within the amperage rating of the controller. The specific controller recommendation would depend on the total draw for a pair of the motors you plan to use.

If you want four motors each with independent control, then you'll have to use two, two-channel controllers.

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Q: Are there any weapon motors you would recommend for a Fairyweight?

A: If you want to go hard-core there are some tiny new brushless outrunner motors like the MSYS-300T that could make you 'King of the Fairies'. Only 20 grams, plus 2 grams for the controller!

On a lower budget, I think I'd try a warmed-up 130 size motor like the 'Hyper Dash 2'. They're light (17 grams), cheap, and put out crazy RPM at low voltage.

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Q: Is there a website that shows different types of fairyweight motors?

A: I'm getting tired of plugging Robot Marketplace, but how about trying their geared motor section?

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Q: What are the differences between the Speed-300 and the Speed-280 motors?

A: They're the same size and they weigh about the same, but the Speed-300 spins faster, puts out about 3 times as much power, and consumes about three times as many amps. There's also a Speed T-280, and a Speed S-280 that each have their own specs.

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Q: The T-280 motor draws only 5 amp max current. Will I be able to use a drive channel of an antweight ESC to control the motor?

A: The T-280 draws 5 amps at max power output, but much more at stall. If your application makes sure it never stalls because of high gear reduction or slip clutches, and your ESC can really handle 5 amps for as long as you plan to abuse the motor, then maybe - but I wouldn't.

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Q: Aaron, can I use the motors found in R/C cars as motors for an antweight robot?

A: All sorts of motors are found in R/C cars. Many toys have the 130 or 230 class motors that are popular in antweights. Larger scale hobby R/C racers have motors too heavy for most antweight designs, but they might work well for beetleweight weapons or hobbyweight drives.

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Q: Are 130 class motors enough to power a fairyweight wedge robot?

A: The 130's are more than enough, but heavy. Take a look at the Sanyo NA5S motor/gearbox. Less than 0.3 ounce each!

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Last - Back to Top»

Q: What antweight brushless weapon motors are the best?

A: 'Best' depends on your design needs. The 'outrunner' style (Axi, Park) motors are widely used because of their high torque and reasonable RPM. I like the Axi 'cause it's easier to mount.

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Q: Dear Aaron, I am making an antweight thwackbot. What size motors should I use to make it effective?

A: You don't see many ant thwackbots! Thwackbots use their high turn-in-place speed to spin an attached hammer or axe up to dangerous velocity. In a small ant arena you'd better be able to spin up quickly. Remember to keep your track width as narrow as possible to maximize spin RPM. More power is better, but I think a pair of hot Speed-280s should be enough to get you some respect.

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Q: I'm thinking about building an antweight spinner. What kind of motor do you recommend for the weapon?

A: The 'Speed 280' motor is a good choice. It's inexpensive, light weight, and has enough power to be feared! The current trend in weapon motors is 'brushless' -- but these are expensive and require special controllers. Try a Speed 280 for your first spinner! They are used in R/C aircraft, so your local hobby shop may stock them. If not, check out the ant motor selection at The Robot Marketplace

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Q: Can a R/C car controller handle greatly increased electrical voltage?

A: Technical question - Mark J. here: NO! Most micro electronics have a very narrow range of operating voltage. Raising the voltage can fry them. Unless you're very sure you know what you're doing, keep the voltage to your electronics at the original specified levels.

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Copyright 2006, 2007, 2008, 2009, 2010 by Mark Joerger -- all rights reserved.