Friday, February 24, 2012

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.

Friday, February 10, 2012

Battling a Frame Leak

This is a god-awful long, and most likely boring story.  Once again I'm hoping I can serve as an example to all you fine readers,  for what not to do.

About two weeks ago I pulled the Z3 into the garage and plugged it in.  When I flipped on the charger I heard, what sounded like, a popping noise coming from the front of the car.  It sounded like the noise you hear when you plug a speakers into the audio port on your computer, deep and quick.  But there were some kids playing out front and I wasn't sure exactly where the noise came from.  But the charger continued to charge and everything on the car checked out.

The following day I went to plug it in after another day's driving and I listened closely, this time with the garage door closed and the hood up.  Sure enough, I heard the same pop, only this time the charger turned itself off.  You may remember about a year and half ago, I was measuring one of the batteries during a charging cycle when one of the probes slipped and made contact with the chassis and the terminal at the same time.  There was a pop, the end of my probe was vaporized, the charger shut off and the breaker for the 240 volt AC outlet tripped.  That little mistake cost me $150 because I had to send the charger back to Manzanita to repair the AC rectifier which I blew up in the process.

After this recent pop I figured 1. I must have a high voltage leak to ground and 2. I'm going to have to send my charger along with another $150 to Manzanita.  I decided to check for the leak and got out my multimeter.  Sure enough, if I measured at the most positive terminal on the battery pack, and the chassis, I could measure the full 160 volts of the pack.  Damn!  I went to the fourth battery from the end of the pack and measured there and got the expected 13.2 volts.  I wondered how much current could get through this leak, so I clipped a regular automotive tail light bulb to the positive terminal of that battery and the chassis and it glowed nice and bright.  If there were no leak, or no path for current, that bulb would not light up.

OK, it was time to start disconnecting things to isolate where the leak was.  This should be a pretty straight forward task, and it seemed to be at first, but it wasn't long before it got weird.  I'll explain.  I started by unhooking all the peripherals, one by one, using the multimeter after each to see if I had voltage to ground.  I disconnected the DC to DC converter, the charger, the Masterflux A/C system, the ceramic heating element and finally the Link-Pro meter.  By this time only thing hooked to the high voltage system where the Zilla controller, the motor and the batteries them selves, but still the leak was present.  I unhooked the cables leading from the Zilla to the motor and the leak was gone.  Ah ha, found it!

My guess was that through poor design, I'd placed a cable close to something sharp and it had rubbed through and shorted to the chassis.  In deed, I found what looked like a suspicious wear mark on top of one of the rubber boots, under which is one of the terminals of the motor.  But as it happened, it had not worn through, though it would have eventually.  But then I noticed that I'd done something else that was remarkably stupid.  The main high voltage fuse of the car, which is held in a special holder which does not keep the fuse from sort of sliding one direction or another, had migrated one direction and looked like it was touching a part of the chassis.  Well, there you go!!!

I fixed that problem and insulated the connection so that it can creep all it wants and will never short against anything again.  I checked all the other cables and connections and found them good, so I started putting everything together.  I got everything back together, put the multimeter on the car and... I could still read the pack voltage.  Ah crap!  But this time, there was an important difference.  When I performed the light bulb test, there was not enough current flowing through the chassis to make the bulb light up.  At this point, I'm thinking there must have been two leaks.  I'm assuming I fixed the more serious one, but there's still a smaller one.

Now's where things get strange.  I started testing again, trying to be as methodical as possible, but I kept getting strange results, or results I didn't expect and couldn't explain.  For a while I was certain there was a leak through the Zilla, but that wasn't the case.  You see the problem is that I've run up to the end of my knowledge at this point.  When it comes to electrical stuff and electronics, if I can't SEE it, I'm probably not going to understand it.  I can SEE a wire touching the frame.  But once that wire enters a device, as far as I'm concerned what goes on in there is simply magic.

I'll save you some of the wretched details, but suffice it to say after disconnecting everything one at a time, again, I was left with only the batteries connected to the Zilla and I still had a leak.  But then I saw that I still had the current sensing leads from the Link-Pro connected to the shunt.  I thought well that can't be the problem, but I'll disconnect them to be sure.  Lo and behold, the leak went away.  What the hell!!  Maybe that line got pinched or abraded.  Nope, it looked fine.  The meter is supposed to be completely isolated from the chassis because I've used an isolating DC to DC converter specifically to power the meter.  I measured the 12 V output of that DC to DC converter and found that I could read 12 Volts if I grounded to the chassis.  Well that's not supposed to happen!  There's my problem!!  Or so I thought.

I started putting everything back together, one thing at a time, taking measurements with each connection made.  For kicks I connected the Link-Pro's shunt lines, expecting to see the leak and it wasn't there.  Ah, for crying out loud!  In fact, I measured the output of the Link-Pro's DC to DC converter power supply and it now no longer has any connection to ground.  Still scratching my head, wondering what the hell is going on, I connect the motor back up and the leak reappears.  Right about now, I'm prepared to burn the car to the ground and walk away.

Ignoring the Link-Pro issues for now, I focus on the motor.  I took the Zilla out of the loop and had simply a small pack of 12 cells and the motor ready for testing.  I hooked the negative line from the battery to the negative input of the motor, which happens to be labeled 'A1'.  Then I put the multimeter on the positive end of the battery and the car's chassis and I could read the battery's voltage.  That means there is a path for current from the battery, through the motor, into the chassis and back to the battery.  I measured the resistance and found that it was 1.19K ohms.  Now that's not much, and if I plug that into an Ohm's law calculator taking into account the full 160V pack voltage, that means that leak can pass only 0.135 amps, or 135 milliamps. 

I don't think that's normal.  I believe that the high voltage fields are supposed to be completely isolated from the case of the motor, but I could be wrong.  I know that leak hasn't existed in the past.  Truth is, I have no idea what might cause that, but I have emails in to some experts who hopefully will help me with that.

The good news through all this is that at one point I had the battery pack isolated from everything else and was able to connect just the charger to it.  I crossed my fingers and threw the switch.  It came on, stayed on and was charging the batteries perfectly.  No need for repairs there.

As for the Link-Pro and it's DC to DC converter, I have no idea why it would pass through the high voltage at one point and not another.  I'm wondering if that little converter has capacitors that were storing the power.  I really have no idea.  Again, as far as I'm concerned, that thing is magic.  But I did order a replacement DC to DC converter for a whopping $8.50 that I'll keep handy in case I need to swap out the other one.

If you made it though this and are still awake, congratulations!  If you have any words of wisdom you'd like to share or guidance to offer, I'm keen to hear it.  Just leave it in the comments.