775pro brush failure

This past weekend 3739 competed at the Waterloo district event here in Ontario and had a great time. We had our ups and our downs and are ready to do it all over again in week 6.

One of the “downs” we experienced was during our last qualification match. Our drivers came back to the pit telling us that the elevator wasn’t working properly. We noticed that our SRX was sending a full 12V to the motor (775pro) but nothing was happening. After determining that it was our motor at fault we proceeded to replace it. One removed we realized that there was something lodged inside the motor causing it to stall. We assumed it was a loose rivet we had drilled out or something similar.

One brief autopsy later and we found this.

Turns out the not so foreign object lodged in the motor was one of the brushes from the motor itself. We’re not certain what caused this and I haven’t seen this before. Thought some others in the community might find it interesting and/or enjoy speculating as to the cause.

That’s a common (for us) 775Pro failure.
Before we figured out and fixed some software glitches with our elevator calibration, we were burning these out regularly.
I don’t know if you were stalling the motor or not (we were) but these things generally don’t survive a stall like a CIM can.
There are other more detailed threads here in CD but without rotation the fan can’t cool the thing and all that current flowing through the brush holders heats them up and they fail.

Makes me seriously consider the Neo brushless for next year…

I’d hazard a guess and say that most of our 775pros with 2015 and 2016 date codes have ended their service lives in this state.

Having a brush fall out is a very common failure mode for the 775 pro motor. The first time this happened to us, we thought we sucked a foreign object into the motor. I’m guessing the copper tab that holds the brush undergoes thermal expansion and the graphite brush loses its press fit. Watch for the floating brush sparking as it case-shorts.

We have found that you can safely stall the 775 pro for a time at about 3.5 V.

You can burn the brushes without ever stalling the motor. Running at less than about 60% free speed at full voltage will do it.

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