Motor testing - what do you use

When people have “bad” motors they tie a loose overhand knot into the cable and give it to me. I get a stack and start the repair process.

Up until a few moments ago I would plug them into a signal splitter, put an axle and an 84 gear on it and try it out. In most cases I can hear the gears grind and know it’s a rebuild. Other times the motor is “just fine” and I untie the knot and put it back into the parts inventory.

I said up until a few moments ago when I burned out the last of my signal splitters. (4 in the last 10 days) It turns out that motors (and servos) that have shorts in them will burn out the circuits in the splitter. I had been checking them with an ohm meter for solid shorts in them but I appear to have missed one.

At $20 each they are the same price as motors, might as well toss the motor rather than testing it.

I’m looking for a good, robust way to test motors. Any ideas?


we use a gear on an axle and turn it ourselves, but that doesnt show anything but gears

we use an axle and the wrench and just slowly turn it

To test the gears, my team puts an axle in the motor and turns it with whatever is nearest (e.g. a gear, a wheel, or pliers). If the gears are broken than it will sound and fell weird. Sometimes we can tell if the motor has an internal problem other than gears (such as an electrical problem) by doing this.

We also have a microcontroller that is programmed to run all of the motors at full power. To test the motors, we plug eight questionable motors to the microcontroller and apply a little torque to each motor. We can test a lot of motors with this method, because the microcontroller doesn’t have to be turned off between motors. Although I don’t know for certain, but I believe that the microcontroller breaker will trip if the motor has a short.

Funny how VeX calls the signal splitter a “no brainer” solution for a smaller remote controlled vehicle… when you have to make sure you won’t fry the electronics if there is a short.

with our club, almost everyone knows how to replace the gears, to test them we turn the motor by hand and listen to see if the motor is turning, we will also in cases where there is a doubt, just take it apart and see whats wrong, the 5 minutes to take it apart and see, is not a waste.

Cortex will also burn out if the motor wires are shorted.
Vex motors themselves have some overcurrent protection, but shorts wont be caught if they are in the cable before getting to the protection.

Brainstorming other ideas to test motors:

  • use a home brew power expander with a fuse, and a Series/parallel dual DVM harness with two $3 DVM from Harbor Freight. Add a $1 110V power switch, so you can turn off the power quickly if there is a short. The DVMs current path typically have a 10Amp fuse.
  • use a Y cable with a good motor and a motor to be tested, so the good motors shows that there is power available, and how much speed is expected. When the “good” motors dies, it shows you just burnt out the controller path, so thats a good time to toss the bad motor.