Click here to read about the AmpFlow, a highly efficient and powerful custom motor designed for BattleBots.
BioHazard '96 to '99 used two permanent magnet DC motors that I ordered from a surplus dealer. BioHazard 2000-2003 uses the A28-400 AmpFlow.
I paid $50 each for the surplus motors and I wasn't expecting to get much for this price, since top-of-the-line motors in this size range can cost over $1000 each. After careful examination and testing of the motors, Team BioHazard decided they were keepers, but first they had to undergo a thorough tune-up.
These motors are rated at 1/4 horsepower at 12 volts. My plan was to run them at 24
volts. When you double the voltage, you also double the amperage that a motor can draw.
This means that the power that can be produced goes up by a factor of four. Running a 1/4
horsepower motor at one horsepower is a severe strain, so I put them through the following
eight-step process to bring these motors up to competition quality:
5. SQUARE DRIVE. The output shaft is 1/2" diameter with a flat on one side. The
idea is to mount a pulley or sprocket onto the shaft with a set screw tightened onto the
flat. This may work for a 1/4 horsepower motor, but it is totally unacceptable for a one
horsepower motor that undergoes frequent and violent changes of direction. I machined the
round shaft into a perfect 3/8" square shaft. I bought some sprockets with 3/8"
diameter holes and broached them. Now they have 3/8" square holes that fit perfectly
onto the shafts. I use a total of 24 sprockets for power transmission and speed reduction,
and none of them have set screws. I could have used a key instead of the square shafts
buts the sprockets are too small in diameter to allow room for a keyway. If you learn just
one thing from this page, let it be this: SET SCREWS SUCK.
8. BREAK IN. A motor should be broken in before running it at full power if it is new,
or has either new brushes, or a newly-trued commutator. The object is to get the brushes
properly seated on the commutator and make sure that they have the largest possible area
of contact. This can take several hours of low voltage operation, several hours that I
didn't want to spend, so I machined the brushes to exactly match the radius of the
commutator. I then broke each motor in by running it for about 15 minutes in each
Motor Facts and FAQs:
To get an estimate of your motor's horsepower, you need to know just one fact: the current draw when stalled.
Here is the formula:
For a heavyweight robot, (220 pounds), shoot for 1.5 horsepower per motor minimum, 2.5 horsepower for a fast robot.
If you double the voltage, you also double the current that the motor can draw. If your battery can put out that much current, and your electronics can handle it, you will get four times the horsepower by doubling the voltage. The RPM of the motor will also be doubled and the motor will get four times as hot, (heating is proportional to current squared).
You can limit the power that your motor draws by putting a resistor in series with the motor. If you double the terminal resistance of a motor, you can double your voltage, but still only draw the same number of Amps through the motor. This will double your horsepower. I consider this a last resort because your efficiency goes out the window.
If you do this, make sure your resistor can handle the power. Your best bet might be a simple coil of heavy gauge wire with high temperature insulation. 12 gauge wire has a resistance of 1.6 Ohms per 1000 feet, so if (for example), your motor has a terminal resistance of .055 Ohms, you would need a coil with 30 feet of 12 gauge wire, (about 1/2 pound). Interestingly enough, this technique has very little effect on the motor's top speed.
If you wire two motors in parallel they will each be able to deliver their full power, so if your battery can put out that much current, you will get twice the horsepower. If you wire them in series, you double the resistance that the current has to pass through, so you will only have 1/2 of the current, and 1/2 of the power. Putting two motors in series gives you only 1/2 of the power of one motor alone.
To change the timing of a motor you should rotate the brushes relative to the magnets.
On some motors this is easy to do, but if the brush holders and the magnets are mounted on
the same part of the motor housing, then you will have a much bigger job to alter the
Yes, you can rewind the motors to increase either torque or RPM (but not both). I have
rewound motors myself, but I don't recommend that you do it yourself unless you have a lot
To get more horsepower you should use fewer turns of a larger gauge wire. This will reduce the terminal resistance of the motor, and increase the RPM. The motor will develop more horsepower, but it will be at a less manageable RPM.
In either case, you run the risk of damaging the magnets. With more turns, it is possible to generate higher magnetic fields that can demagnetize the magnets. With fewer turns, the motor will draw more current, which means it will run hotter, and that can also demagnetize the magnets. The damaging magnetic fields will probably be reduced with fewer turns, but if the current gets high enough, they might be the same or even stronger. This is the same type of problem that can happen if you pump more voltage into a motor than that for which it was designed.
That is an easy question! Check out the AmpFlow.