Does motor temp affect performance?

[Joe Johnson]

Originally Posted by NC GEARS

We just completed the Michigan State Championship last weekend and during all of the qualification matches we were running top notch. Drive fast, turn efficiently, cross bump with ease etc. Once we got into the elimination rounds, our robot performance started to drop drastically. We kept trying to figure out what was wrong but we really could not find a cause. In the actual Finals rounds we started to feel our CIMS (4 of them 2per side for drive train) and they were extremely hot, literally almost too hot to touch. We were curious as to if the temperature of the motors could have something to do with performance of our robot? If anyone has any info on how CIMs will work/break down, how temperature affects them, etc that would be great! Thanks.

Hot motors have less output primarily because the have higher resistance in the windings of the motors which means lower stall currents, which means lower stall torque, which means that for a given load you need to operate closer to the stall point, which means you are less efficient at turning electron energy into shaft energy which means you generate more heat which increases the resistance of your windings... ...rinse and repeat.

If you are SURE that nothing bad has happened to your drive train and that you are just overheating your motors, then I fully recommend buying a 24 can case of "Freez it" or similar product and icing the motors before and after every match*.

I recommend keeping ahead of the heat because by the time you feel that the outside can of a CIM is warm the inside is HOT HOT HOT. Freeze 'em early, Freeze 'em often. That'd be my motto. If it weren't illegal, I'd put a small hole in the can of the motor and drizzle the liquid from the can right into the motor body itself that way the evaporating liquid would directly cool the bits of the motor that need cooling the most. As it is, you'll have to cool the case and depend on thermal conductivity to do the rest.

I loved you in Ypsi. Good luck in Atlanta -- buy a 24 case really. You'll thank me (again, assuming you have 100% certainty that your drive system is as it should be and it is just repeated, frequent matches that are causing the problem)

Joe J.

*I know some who argue that the thermal shock is too much for a motor but in the years I have been doing FIRST, I have never seen a motor fail this way and I have used this method on many many robots.