CIM Motor Failure

[Richard Wallace]

Originally Posted by Brandon Holley

..., we went through the debugging process, power the drive side with just 1 CIM and then alternating to make sure they both were "driving". We then switched the Victors that were powering them to ensure they were both "driving".

The motors always appeared to be functional, as in the robot would drive. However, during eliminations, or heavy testing, the robot would begin to pull slightly towards the "cold motor" side. We replaced the motors, and the issue has generally gone away.

This is an excellent example of debugging. Your problem was apparently solved by replacing the motor, so you now know that the PWM channel, the Victor, and the motor position on your gearbox are no longer suspect.

Did you measure the free current of the "bad" motor, after it was removed from the system? Often a bad motor will draw more current than normal when running free. If you don't have a convenient way to measure it, see me at IRI -- I have a test set you can use.

As Al pointed out above, it is difficult to cool CIMs. They don't have a flow-through path for cooling air, so the heat developed in their rotor assemblies stays trapped inside. The rotor heats up much faster than the case, so a hot case indicates a VERY hot rotor. In more quantitative terms, Al's "too hot for the finger" test indicates case temperature above 60 Celsius, and the pictures he posted indicate rotor insulation temperatures above 180 Celsius.

I've been a motor guy for a long time. CIM's are some of my favorite motors because they can really take the heat. But even a CIM can be overcooked. Thanks for the cautionary story, Al.