# Motor Tips

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Click here
to read about the AmpFlow A28-150 and A28-400.
Powerful custom motors designed for BattleBots.
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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.

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Motor Facts and FAQs:
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PMDC motors have the following characteristics:

- They produce their maximum torque at zero RPM.
- They produce zero torque at their maximum RPM.
- They develop their maximum horsepower at 50% of their maximum RPM.
- At 50% of maximum RPM, they produce 50% of their maximum torque.
- At maximum horsepower, they are no more than 50% efficient.

How to estimate motor power?

- 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: Stall current*Voltage*.00033 = Maximum horsepower (approximately).
- For a heavyweight robot, (220 pounds), shoot for 1.5 horsepower per motor minimum, 2.5 horsepower for a fast robot.

What happens if I run my 12 Volt motors at 24 Volts?

- 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).

I want to use two motors to drive a single set of wheels. Should I wire them in series or in parallel?

- 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.

How can I change the timing on my motors?

- 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 timing.
- To get neutral timing, adjust the brushes to get the lowest no-load Amp draw. This will have a big effect on the RPM.

Can I rewind the armature to increase the torque or RPM of my motors?

- Yes, you can rewind 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 of patience.
- To get more torque and lower RPM and horsepower, put more turns of wire on the armature. Your motor will then draw fewer Amps, and therefore produce less total horsepower. A motor with more turns will develop more torque per Amp, but it will draw fewer Amps, so it is hard to predict exactly how much torque or RPM it will develop after the change.
- 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.

Which motors should I use in my BattleBot?

- That is an easy question! Check out AmpFlow.