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Motor Tips                    

Click here to read about the Magmotor, 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 S28-400 Magmotors.

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:

1. BALANCE. These motors vibrated pretty badly at 12 volts, at 24 volts they reminded me of those "Magic Fingers" machines that you find in cheap motels. I took them apart and trued up the armatures on a lathe. That did the trick. No fancy balancing or drilling was required.

2. COMMUTATOR. While the armatures where on the lathe I trued the commutators. Be careful if you decide to do this to your motors because you could do more harm than good. The commutator should end up perfectly concentric with the motor shafts, and perfectly smooth and shiny.

3. LEADS. While I had the motors apart I examined the connection between the leads and the brush housings. I tugged on the wire, and it popped right off! No wonder these motors were surplus, it was a failure just waiting to happen. I securely soldered all the leads.

4. CAPACITORS. While I had my soldering iron out, I decided to install capacitors across the leads. This is very important. Capacitors quiet the radio frequency noise that the motors generate. This noise will interfere with your radio. I once built a robot for the FIRST competition that had a radio range of about 15 feet. When I added capacitors, the range increased to over 100 feet! Use .01 to 0.1uF ceramic disk capacitors. Solder one across the leads as close to the brushes as possible.

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.

6. FACEPLATE. The motors came with a heavy steel faceplate with three mounting holes drilled and tapped. The spec sheet said that the holes were on a one inch bolt circle, but when I tried to bolt the motors to my one inch bolt circle, they didn't match up. They weren't off by much, but it was enough to make the motor bind and overheat when tightened down. I made new faceplates from aluminum. The holes lined right up and I reduced the weight of each motor by almost a pound.

7. CONNECTORS. I installed a high performance connector on each motor for fail-safe operation. I use Astroflight connectors for wire up to about 8-10 gauge and Anderson Powerpoles for larger wire.

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 direction.

I went through a similar, (but not as elaborate), tune up for my linear actuator motors. One extra step that I took with them was to replace the bronze bushings with ball bearings. The surplus drive motors already had ball bearings.

Motor Facts and FAQs:

• 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 powerful are my motors?

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 don't think that my electronics can handle all that current. Can I get more power without burning up my motor and electronics?

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.

• 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.

Tools you should have for the job include a tachometer and the AstroFlight Whatt Meter. To get neutral timing, adjust the brushes to get the lowest no-load Amp draw. This will have a big effect on the RPM. You can use the tach to fine-tune two motors to run at the same RPM. I timed BioHazard's motors to slightly favor forward motion.

Here is a tip for using those tachometers that employ a light sensor to detect RPM: Turn off all the lights and use sunlight or a flashlight. The flickering of shop lights caused by the AC current will throw off the reading on the tach.

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

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 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 the Magmotor.

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