Brushes and Carbon Dust

While at EVCCON 2011 I had the good fortune to catch a talk given by Tom Brunka from the Helwig Brush Company.  Tom went into great detail about how brushes for DC motors work, the different properties they can posses depending on the way they are manufactured, and the care of brushes and motors.  On the face of it, it seems this talk would be astoundingly boring, and in fact I think Jack Rickard scheduled it for the last talk of the conference precisely because he figured it would be lightly attended and would be in no danger of running long and interfering with the events planned for later that day.  As it turned out, it was a truly fascinating talk and it went much longer than expected due to the fact that attendees started asking questions as soon as Tom finished and simply wouldn't let up.  Eventually Jack had to put a halt to the exchange so that we could all go outside and race our cars.  What a great time that was.

One of the things that Tom mentioned is that brushes will wear and break in at one rate, but once they are bedded in, they will wear at a completely different rate.  The wear of the break-in rate is much higher than standard wear rate.  But the interesting thing is, if you disturb the brushes, i.e. remove and replace them, lift them out of their holder, nudge them, breath on them, or even look at them askance, they will begin to wear at the break-in rate again.  This level of wear has a couple of effects.  First and perhaps most obvious, your brushes wear out faster.  As a direct result of that faster wear rate you encounter the second issue, which is an accumulation of the dust that wore off the brushes on the inside of the motor.  If you have the motor ventilated with forced air, some of that dust will be carried out of the motor's vents.  But not all of it.

Such is what has happened with my motor.  Faithful readers will remember that I have had the motor out of the car a couple of times due to a strange phenomenon where the balancing putty fell off the armature.  On both occasions George Hamstra at Netgain motors, shipped the motor back to Warfield electric and had them repair it.  In order to do so, they had to dis-assemble it.  Now I didn't look at the brushes when the motor came home from the first repair so I don't know if new brushes were installed, but I did look at them the second time and they were the same brushes that were in the motor when it left.  How do I know?  Well new brushes are not contoured to fit perfectly against the armature when they are new.  As I said earlier, they must be worn, or bedded in and this can take up to 5000 miles to do.  But since the brushes had necessarily been removed during the motor's repair, that meant they were going to go through another instance of excessive wear and re-bedding; meaning more dust.

What does this have to do with my current situation?  Well, all that carbon dust is conductive.  After all, it's through that very carbon that the current is pushed to the armature of the motor.  I have a fan on the motor, pushing 110 CFM of air through it and likely carrying out a good percentage of the carbon dust along with the heat it's meant to blow out.  But clearly not all the dust is going out.  Somewhere within the motor, some dust has piled up and allowed a small path for current to flow from the high voltage fields to the case.  That's not good but apparently not unheard of.  Normally, you simply blow the motor out with compressed air and the problem leaves the motor in a plume of black dust.

So, I charged up my compressor, took the shroud for the fan off the front of the motor and started blowing.  There was plenty of dust.  More than I expected.  I thought this was great, it meant I likely fixed the problem.  Unfortunately not.  The problem is that in order to get all the necessary components in the car for it to function, Or simply because I'm very bad at designing things, all the components are crammed in there so tight it's difficult for me to get a nozzle in there to blow out every nook and cranny.  But my blow gun's nozzle is right at the end of the trigger limiting where I can really point it.

I've exchanged some emails with George and he suspects that the brushes in the motor right now are a high performance brush called H49 which do great for racing, but tend to dust a lot.  He thinks that the H60 brushes, which are a harder, street grade brush would be better suited for my build with the added benefit that they will produce less dust.  He's decided to send me a set of these at no expense.  Well, no expense to me.  They normally run over $300 a set and George is picking up the bill.  To say that George is fed up with my motor is an understatement.  He's stated that he will not RMA the motor again.  Meaning, if it ever has another problem, he intends to replace it.  Believe me when I say that it's very comforting knowing that he will stand behind the motor so resolutely.  For now, I just don't see a reason to do that, and here's why.

The new brushes will arrive early next week.  I've ordered a blow gun for my compressor with a 12" flexible hose which will allow me to better squirt air in all the nooks and crannies inside the motor.  I'll remove the old brushes, and push 8 or 10 tanks of air through the motor, then replace the brushes.  If all goes well, I should see the short go away.  Depending on the temperature, and consequently the time of day, the resistance of that short fluctuates between 1.18K Ohms, and .85K Ohms.  That means a potential leak of current in the range of 135 to 188 milliamps.  If I can't resolve the short and I have to take the motor out of the car then I'll have to decide whether I want to try to disassemble it to clean it, or simply give George the go-ahead to send out a replacement.  I really don't want to do that though.