Robot Motors- Ampflow Motors Information

Ampflow Motors have been specifically designed and battle-hardened for use in the BattleBots robot combat competition. They have also been used in applications like motorized scooters, electric bicycles, and small electric motorcycles with excellent results. Take a look at the specifications of the Ampflows. We think that you will agree that these are the ultimate motors for combat robots.

We offer three different motors for combat robots:

A40-300 A28-400 A28-150
3.8 Horsepower 4.5 Horsepower 3 Horsepower
4" Diameter 3" Diameter 3" Diameter
6.9" Long 6.7" Long 4" Long
3840 oz-in Torque 3720 oz-in Torque 1970 oz-in Torque
84% Efficiency 83% Efficiency 82% Efficiency
24 Volts 24 Volts 24 Volts
4000 RPM 4900 RPM 6000 RPM
11.9 Pounds 6.9 Pounds 3.8 Pounds
Built-in Capacitors Built-in Capacitors Built-in Capacitors
Ferrite Magnets Neodymium Magnets Neodymium Magnets

Click here to go back to the main Ampflow motor page or to purchase


The quest for the perfect motor is one that all BattleBots competitors have gone through. There are several choices available but as many competitors have found, only a few of them will work well in battle.

What makes a good motor?
There are several things you should consider when choosing a motor. The most obvious are power, weight, and efficiency, but there are several other factors that are just as important.

  • Does the motor have a usable RPM range?

  • Will it put controller-destroying voltage spikes on your electrical system?

  • Is the thing solidly built, or will it fall apart in the middle of a battle?

  • Can it withstand the heat build-up of high-Amperage operation?

  • Will the motor use the limited power you have on-board efficiently, or will it drain your batteries before the match is over?

  • Does it produce radio noise and interfere with your radio control?

  • Is the motor and shaft designed for easy mounting?

Let's look at each of these questions in detail and see how the Ampflows handle them.

Horsepower and Torque
Raw horsepower is very important in combat. If you watch the battles carefully, you will see that the robot with the most pushing power is often the one that wins. The horsepower and torque of these motors are higher than other reversible, permanent magnet motors in this size range. Some gearmotors can't even achieve 3840 oz-in, and that is after multiplying the torque with a gearbox. Remember our torque figure is taken directly from the motor shaft before any gear reduction. (Please note that this is the theoretical peak torque when stalled. Operating any high-performance motor while stalled will damage it). Other popular motors put out 1 to 2 horsepower each. With two Ampflows you will have up to 9 horsepower at your command!

RPM Range
Usually, the higher the voltage and RPM of a motor, the higher its efficiency will be. Some motors have high efficiency, but at a no-load speed of 20,000 RPM or more. Gearing such high RPM down to a usable wheel speed takes several stages of gear reduction. This is heavy and wastes power and the efficiency advantage usually disappears. The tricky part is making a moderate speed motor that is also highly efficient. The relatively low RPM of our motors makes the speed reduction much easier. Most people use one or two stages of roller chain and sprockets to achieve the speed reduction. Gearboxes for the A28s are available here.

Solid Construction - Easy Mounting
The Ampflow motors are made from two sturdy aluminum castings and a seamless body. Some motors use a rolled can with the magnets glued in place. Motors with rolled construction have been known to rip open under severe full-throttle direction changes. That can't happen with our seamless design. The output shafts are 1.75 inches long and 5/8" diameter for the A40 and 1/2" diameter for the A28s. The long shaft length and a 3/16" keyway (1/8" for the A28s), make it easy to mount pulleys, sprockets, and gears in just the position you need them. A second 1/2" diameter shaft extends 3/4" from the rear of the motors. This is very handy for mounting encoders, fans, tachometers, brakes, or any other devices you might need. If you don't need the rear shaft, you can cut it off flush with the face of the motor.

Some motors use an internal fan to keep them from overheating. The Ampflows are efficient enough to operate without a fan. This has three advantages: (1) The motor housing is completely sealed so nothing can enter the motor and damage it; (2) The sealed motor tends to contain interference-causing radio frequency noise that would otherwise escape through the ventilation holes; (3) The power that would normally be used to run the fan can go directly to driving your robot.

The armature rides in two large high-quality ball bearings and it is dynamically balanced using epoxy rather than by the normal method of drilling the laminations. Drilling can cause eddy currents in the laminations, which increase motor heating.

Each of the four brushes has its own heavy-duty motor lead made from mil-spec 10-gauge flexible stranded wire, (12-gauge for the A28s). Each copper strand is coated with pure silver to protect against oxidation and to get the lowest possible resistance in crimped connections. These leads are flexible but not "floppy", so they tend to stay where you put them. The four leads have high-temperature Teflon insulation rated for 200ƒ C, (392ƒ F), so they can handle high current without melting.

Getting high efficiency from a low-Voltage motor is not easy. There are many factors that will have an effect on efficiency. We have tweaked each of these factors to achieve the amazing efficiency of the Ampflows.

  A40-300 A28-400 A28-150
Peak Efficiency (PE) 83.7% 83% 81.9%
RPM at PE 3700 RPM 4500 RPM 5500 RPM
Horsepower at PE 1.1 HP 1.4 HP 1.0 HP
Current at PE 41 Amps 51 Amps 37 Amps
Range of 80%+ Eff. .6 to 2.0 HP .7 to 2.3 HP .6 to 1.4 HP
Range of 75%+ Eff. .4 to 2.6 HP .5 to 3.1 HP .4 to 2 HP

As you can see from the above chart, the efficiency of the motors is very high over a broad range of power. Please note that all our performance numbers are from motors that have neutral timing. Most motor manufacturers quote efficiency numbers from motors that have advanced timing. Advanced timing is a bad idea when you need to run the motor in both directions. Our motors are shipped with neutral timing for good operation in both directions. If you use the motor in one direction only, you can advance the timing and get even more RPM and power. (But do not run the motor backwards if the timing is advanced). Timing adjustment is easy with the Ampflows.

The armatures are wound with very heavy gauge wire, and all the space in the armature is used. We left no power-robbing "empty air" in the slots; they are packed with copper! The A28s have skewed armature laminations to eliminate the heavy "cogging" that results from using the extremely powerful neodymium magnets. The A40 motor uses straight laminations, but the cogging is kept to a very low level by using a 42-bar commutator. The huge 1.75" diameter of the commutator combined with the four massive brushes and the 42 bars enabled us to get high power and high efficiency from this smooth-running motor, (the A28s have 21 bars). As far as we know, the Ampflows have the highest efficiency of any 24-volt reversible permanent magnet DC brush motors in their size ranges.

Another benefit of the four-brush, 42 and 21-bar designs is the absence of excessive electrical noise. While all motors produce some electrical noise, cheaper motors with fewer commutator bars are more likely to send noisy voltage spikes back to your controller. This can destroy your expensive electronics. With more bars, each time a brush comes into contact with a new bar, the timing of the winding connected to that bar is closer to being optimal. In low-quality motors with fewer bars, each winding will pass through a wider range of angular offset from the magnets. This causes the brush to spark more and generates more electrical noise and voltage spikes.

Built-in Capacitors
BattleBots veterans know how important it is to reduce the radio frequency noise. Without capacitors most motors will produce enough RF noise to shorten the range of your radio control. This RF noise can sometimes make it impossible to control your robot. The Ampflows come with four capacitors built right in to the motors. Each of the four brushes has a capacitor wired to its nearest neighbor on each side. There is no connection to the motor housing. Shunting your RF noise to the housing, (and ultimately to your robot's frame), has unpredictable results and we don't recommend it. Mounting the caps inside the motor works better than mounting them on the outside but it can be difficult to do. We do the work for you so your motor is ready to run as soon as you get it!

Save Weight
Efficiency is important for several reasons. A high-performance motor will convert most of the power it draws into torque and horsepower, while an ordinary low-efficiency motor will turn much of your battery power into heat. The weight limits are strict in BattleBots and it is important to make the best use of every pound of battery you have on board. Two 12 volt, 16 Amp-hour lead-acid batteries should have enough capacity to power your dual Ampflow-equipped robot to victory in a three-minute match. Some other motors require 36, 48, or even more Volts to get sufficient power. More Voltage means more batteries and more weight. 24 Volts is all you need with the Ampflows.

Motors that are used in the heavy and super-heavyweight classes range from about 8 to about 26 pounds. Motors for the light and middleweight classes range from about 2 to about 8 pounds. The A40 weighs in at 11.9 pounds. The A28-400 tips the scale at 6.9 pounds, and the A28-150 is just 3.8 pounds.

     A28-150 - Just 3.8 Pounds!
Ampflow A28-150Using Your Ampflow motor
The high current these motors are capable of drawing requires the use of high quality controllers. The terminal resistance of the A40 is .050 Ohms. The A28-400 has a terminal resistance of .042 Ohms, and the A28-150 has a terminal resistance of .064 Ohms. That gives theoretical maximum current draws of 375 Amps (A28-150), 480 Amps (A40), and 570 Amps (A28-400). But in practice, you will never see this level of current. In order to draw that much current you would have to use a battery that stays at 24 volts while supplying high current; you would have to have no resistance between the motor and the battery; the motor would have to be totally stalled; and it would have to be at room temperature (the resistance of all motors increases when hot).

Even the best 12-volt batteries in the 16 Amp-hour size range have an internal resistance of at least .007 Ohms each. Your controller will have at least .002 Ohms. The motors have between .042 and .064 Ohms. Add to that another .004 Ohms for your connectors, switches, and wiring, and you will have a circuit with between .062 and .084 Ohms. This gives theoretical maximum current draws of 285 Amps (A28-150), 340 Amps (A40), and 390 Amps (A28-400). The actual figures will be even lower if you have any other load on the battery (due to battery voltage droop).

This puts the current draw within the range that some commercial speed controllers can handle. We suggest that you use controllers from Vantec, IFI, Robot Solutions, or Robot Power . The motors have seen action in the BattleBots competition with the Vantec RSFR38E, RSFR47E, the 4QD-300, and the OSMC and all controllers worked fine. We have also seen the motor used with two IFI Victors or Thors per motor (one on each set of motor leads); this has also worked fine. The motors have also been run with the Vantec RDFR47E and RDFR38E which are "dual" controllers that can operate two motors independently. We don't recommend the dual controllers, but they have worked. In a recent survey of all the people who used the Ampflows at BattleBots, there was not a single report of a controller failure!

There are a few ways to reduce the wear and tear on your controller. One trick is to gear your robot so that it would be impossible to stall the motors. The more speed reduction you use, the easier it will be on your motors and controllers. Ten MPH might not sound very fast, but in the cramped BattleBox it is very difficult for the larger robots to take advantage of speeds much in excess of this (unless your primary mode of attack is ramming). More gear reduction will also give you faster acceleration and maneuverability.

Another trick is to use batteries that can't produce this high current. If you are worried about your controller -  just don't use those batteries. You should still get close to the full 4.5 horsepower using most other batteries because, (like all PMDC motors) the Ampflows produce their maximum power at 50% of their maximum RPM, and the current draw at that speed is no more than 240 Amps, (280A for the A28-400, 180A for the A28-150). Here are some tips for battery selection.

The A40 and A28-400 have tested fine with momentary current draws of over 500 amps but if you are using any high-performance motors in your weapon and your weapon becomes stalled, you must back off on the throttle to prevent damage to the motors.

By the way, one of the "rules of thumb" for determining if a motor is high in quality and efficiency is the difference between the no-load current, and the maximum current. A good motor might be able to draw 50 times its no-load current when stalled. Some very inefficient motors can do no better than 10 times or so. With a no-load currents of  of 3.5 Amps (4.5 Amps for the A28-400), the Ampflows can draw an amazing 110 to 137 times their no-load currents!

Here are some tips for break-in, repair, timing adjustments, and technical specifications.

The A28-400 Ampflow
A28 Magmotor

It has long been the tradition in robot combat to use double the motor manufacturer's recommended maximum voltage. At 24 Volts the A40 Ampflow is already very powerful. At 36 Volts it will develop 8.6 horsepower, but you run the very real risk of damaging your motor. If you decide to take the risk, we can offer the following suggestions: Limit the maximum current to no more than 350 Amps (lower for the A28s); Use the highest possible gear reduction; Use the motor in one direction only, do not try to reverse it; And time the brushes for optimal operation in that direction. The A28-400 puts out a respectable 1.1+ horsepower at 12 Volts and over 2.5 horsepower at 18 volts. If you are using this motor in a light or middleweight and you are bumping up against the weight limit, you might want to consider eliminating some battery weight and running the A28s at a lower voltage. More tips on overvolting.

Choosing your motor
Which motor is best for your application? The A28s have incredible power to weight ratios and for most weight-sensitive applications these would be the best choice. The A40 weighs more than the A28s, but in some cases this is advantageous. The higher motor mass will decrease the rate of motor heating for a given level of power. If you need high power for several minutes (as in a continuously spinning weapon), the larger A40 might be a better choice. The following chart gives some general guidelines for selecting the best motor.

  A40-300 A28-400 A28-150
Lightweight Drive N/A Good Best
Middleweight Drive Good Best Good
Heavyweight Drive Good Best N/A
Super H.W. Drive Best Best N/A
Massive Spinner Best Good N/A
Light Spinner Good Best Good
Heavy NCW* Good Best Good
Light NCW* N/A Best Best

*NCW = Non continuous weapon (hammer, lifter etc.)

These motors are custom manufactured for us by AmpFlow. The motors are warranted to be free from manufacturing defects, and fully operational when you receive them. The object of the BattleBots competition is to destroy or immobilize your opponent. Obviously we can not offer a refund policy for motors that have been used in these conditions.

Customer Service
Please note that our custom motors are NOT the same as the standard motors from the Magmotor Company. These motors are not available directly from Magmotor. Modifications include: High-current brushes and shunts, brush holders and insulators, high-temperature springs, custom windings, capacitors, rear end bell, shaft, and oversize commutator. About the only things the custom and standard motors have in common are the armature laminations and the front mounting plate. We do all the customer support for our custom motors. Please send all questions, billing, shipping, and customer service inquiries directly to us. (All inquiries to sent to the Magmotor company will be forwarded back to us).

Price and Shipping
One of our hardest goals was to pack all of these features into a motor that could be priced reasonably. Using rare earth magnets is normally the only way to get high efficiency from a low-Voltage motor. While these magnets are very strong, they are also extremely expensive. The cost of rare earth magnets in a motor the size of the A40 would have been astronomical. We studied the alternatives and decided to go with a high-energy-product ferrite magnet known as "T9". The T9 magnet material along with the other design features listed above have enabled us to achieve the same high efficiency of rare-earth in a much less expensive motor. Our cost cutting has enabled us to price the A40 at a very reasonable price.

The A28-400 is smaller than the A40 so we can use neodymium rare earth magnets in that motor and still keep the price reasonable.

The A28-150 uses less neodymium than the A28-400 so we are able to price it even lower.

If you calculate the cost per horsepower, and the horsepower per pound of other available motors, you will see what great values these are.

"That's a beautiful motor! Wow. Good work. Great price. This should become standard equipment for all large robots."
          --Edwin Wise.   Mad Scientist and author of Applied Robotics

"As the BattleBot competitions continue to grow, participants are constantly trying to make their robots quicker, stronger and faster. With nearly four horsepower and 3840 oz-in torque, the 11.9-pound A40 Ampflow is ideally designed for our participants. Since power is often the deciding factor in BattleBot competitions, more and more participants are selecting Ampflows to power their robots with a lightweight, high-efficiency motor that provides plenty of power on demand."
Trey Roski.   CEO of BattleBots.

"As the first motor designed specifically for robotic combat, the Ampflow performs fabulously! It combines incredible power and some of the best features you could ask for when looking for a high-end motor for a BattleBot. It is the most powerful and finest constructed DC permanent magnet motor that I've ever used. I chose Ampflows for the weapon motor in Nightmare and I am definitely a very satisfied customer!"
          --Jim Smentowski.    BattleBots Champion (Nightmare, Backlash)

"The Ampflow packs a lot of power into a light-weight package. They give Minion the power it needs and I still have weight to spare!"
          --Christian Carlberg.    BattleBots Champion (Team Cool Robots)

"Get the best motors you can afford. Good motors are the foundation upon which all else rests. People can win with hokey electronics or even with inoperative weapons, but good motors make good robots."
          --William Gurstelle.    Author of Building Bots : Designing and Building Warrior Robots

"We were extremely pleased with the performance of the 4" Ampflow in our middleweight Heavy Metal Noise at BB 4.0. The motor supplied an insane amount of energy to our kinetic disk weapons. Time to spinup was minimal and after combat the motor was never more than mildly warm to the touch."
          --Jay Johnson.    BattleBots Competitor (Big Bang Robotics)

"I've been building robots as a hobby for about 30 years and this is definitely one of the best motors (power, weight, cost, quality), that I've ever encountered. Since I've never built a 'BattleBot' before, I decided to focus most of my attention on other aspects of the design and having this motor meant that I didn't have to spend many hours 'tweaking' an inferior motor just be competitive."
          --Billy Moon.    BattleBots Competitor

"I can attest to their super high quality of craftsmanship and design. We tested them with a few different controllers and monitored the results using a Tektronix digital scope and a digital ammeter. Results: This is the largest motor we have tested and amazingly it had the cleanest feedback signal of any motor yet. Generally the higher the quality of motor mechanics, windings, brush and commutator, the cleaner this signal is. This means these motors are less likely to blow a speed controller due to transient voltage spikes.
          These motors are VERY well built. We saw no appreciable heat up in the testing we did. We did a lot of stall and near stall testing. These are amazing motors and are ridiculously powerful."
Alexander Rose.    4/2001.   BattleBots Champion (Toro, T-minus, Matador)