[patrickrd]

This is for a project not related to FIRST (and possibly related to the team listed in my signature), but I was hoping that there would be someone out there in the FIRST community who might be able to help out with a problem my team is having.

We are using very expensive motors that we purchase from overseas, and they are rated at 6 volts. However, in past years, we have driven the same motors (with different gearheads) at up to 13 volts. We have never (not once) burnt out a motor in previous years at these voltages. In previous years, we used PWM at 4000 hZ and ~13 V peak to control the speed of the motors. This year, we are also using PWM, but at 400 hZ and ~15V peak.

We are burning out motors (we've burnt out about 10 so far) and have tried everything we can think of to keep them from burning out. They burn out (we have verified the brushes get fried or broken off) seemingly at random -- not necessarily due to excessive use -- and the motors often are not even that warm when we touch them after they malfunction. We have changed our control algorithms to reduce the maximum duty cycle to less than 40%, and even that does not stop them from burning out.

I think it is the PWM frequency that is too low, and as a result the 15V spikes are being applied too long and the very tips of the brushes are heating up VERY fast during this time, causing failure at the very tips. But I can not find any books or web sites that treat PWM any differently than an average voltage.

So my question to you engineers is this: Have you seen anything like this, and do you think raising the PWM frequency back to 4000 hZ would solve this? Our electrical engineers are hesitant to switch to 4000 hZ because they say performance drops as the PWM frequency increases... But I think a small performance drop is a small price to pay if the motors stop failing. The lead time is 6-8 weeks on new motors, and with the competition 2 weeks away, we can not afford to burn any more out!

Thanks very much!
- Patrick