This week, we were hard at work in the 2008 Electric Cadillac Escalade EXT conversion and we are at a very rewarding phase of the build where things move along quite quickly.
Tim Catellier of Chandler Arizona slipped in and mounted a small explosive device under the hood and this kind of wrecked our week.
Tim, an EVCCON 2011 attendee, and so of course a rank up in standing over "ordinary" viewers, inquired about a pleasant little puzzle he was having with his BMW Z3 conversion. He had been at EVCCON 2011 with his erstwhile assistant, his FATHER, and we were privileged to examine first hand and up close the particulars of this Zilla/Netgain build using CALB cells - excellent in all respects. He in turn claims the drag racing and autocross was the most fun he'd had in a car EVER with his pants on and no music.
It seems he had developed a "frame leak." I'm actually extraordinarily pleased with this. We had done a show that prominently featured our efforts to chase down a frame leak on the 2009 Mini Cooper Clubman. We described what they were, how you could detect them, why it was important for your personal safety NOT to have one, and how to chase them down. We have since had a regular flow of viewers who HAVE found frame leaks subsequently.
While most were minor leaks in the 30 ma range, some have been serious. So I'm kind of pleased this particular episode caused a lot of people to check their systems for leaks, and more pleased that they have largely been successful in addressing them. I'm not precisely a safety Nazi. Life's a bitch, and then you die, and ho hum. But this one has caused us some nasty shocks in the past and so it deserved some attention. Apparently the problem was not as rare as I thought. It wasn't just us. And so, as Marthaa Stewart says, "That's a good thing."
The check is simple. Connect a multimeter between any battery terminal and frame ground. You should see a varying voltage of 4 to 6 volts. It will probably jump around. This is because the pack "floats" separately from frame ground. If you read ANY steady voltage above a volt or so, you have a frame leak. If you read DIFFERENT steady voltages based on which battery terminal you measure to ground, you certainly have a frame leak.
If you connect a 12v light bulb between the terminal and the frame and it fails to light, it is probably a very minor leak of 30 or 40 ma usually through some of your instrumentation. If you get any light, you have a serious situation that can be life threatening in the right environmental conditions - like sweaty skin.
In any event, Mr. Catellier had one. ANd after carefully and logically troubleshooting the problem, isolated it to the MOTOR. DC series motors of course have brushes and a commutator. Carbon conducts. And a buildup of carbon dust within the motor is not unusual. Sufficient carbon dust can short the field windings to the case. Actually this is so common that a leak of less than 30 ma is not even considered a problem by the motor manufacturers. Tim's was more in the 800 ohm range which into a pack his size, is pretty stout.
He tried blowing the motor out with compressed air and failed to eliminate the problem.
George Hamstra, of Netgain Motors had previously rebuilt this same motor and wanted a clean slate on this problem. After examining photos of the brushes, he asked Tim to send in the motor and he shipped him a brand new motor from Warfield, complete with the latest Helwig H60 RedTop brushes.
It was a non-trivial amount of work to swap out the motor. But Tim again enlisted the aid of his erstwhile assistant, DAD, and they were able to do it in some four and a half hours. The frame leak was gone. And they reassembled the car.
He was back on the road and drove it to work and back the next day. But there was an interesting difference. Tim had kept careful records over the past 26 months and tracked energy usage, electricity costs, gasoline prices, and right down to the dollars dimes and ducats saved in not purchasing gasoline, all in a handy spreadsheet, just as a proper Computer Systems Administrator is wont to do.
The car averaged 380 Wh per mile for over two years. In fact, Tim and I had actually had a conversation earlier about this as we have a pretty strong rule of thumb that a car will use about 1 Wh of energy to move 1 Mile over time and average, for each 10 lbs of car. Tim's car weighed in at EVCCON 2011 at 3285 lbs. And 380 Wh/mile somewhat exceeds our rule of thumb. I was unable to account for this or provide a persuasive theory. Maybe my rule of thumb not so good medicine in all cases.
On his drive to work and back, Tim noted an energy usage of 280 Wh/mile. This is kind of dramatically better.
He describes this somewhat better than I do on his blog at http://evz3.blogspot.com/
I received an e-mail message regarding this. In discussing the issue, he refined his results with total driving over the past 680 miles - 332 Wh/mile average. At 3285 lbs, I like this number for by now obvious reasons.
I initially suggested that the only thing I could imagine would cause such a DRAMATIC disparity in two otherwise identical 11 inch motors was the advance timing. The Netgain motors come marked with alignment marks for CCW, N, and CW timing. This corresponds to counter clockwise as viewed from the drive end of the motor, neutral, and clockwise respectively.
Most explanations of electric motor theory are technically correct as far as they go, but of necessity incomplete. First, some of the factors are difficult to explain. And second, after 150 years some of them we really don't know exactly. For some reason, "square" motors - that is motors with a certain ratio between diameter and length, run better than others. A certain amount of mass of iron in the case is simply required for efficient operation. We CAN make motors lighter. That's not necessarily a good thing. We know that the field windings set up a magnetic field that causes armature rotation. But were you aware that there is an interaction where the armature rotation then affects the magnetic field - in fact rotating it in the direction of the armature?
This is termed the armature effect and actually we DO know quite a bit about this. In the Netgain Warp 11HV, they employ inter poles to counteract the effect of this, and this is how this particular motor can handle higher voltages without arcing.
In the ordinary Netgain Warp 11, the CCW position literally rotates the brush ring a few degrees in the direction of armature rotation. This moves what is termed "the neutral plane" that same number of degrees. And so the brushes are only truly at neutral when the motor reaches a couple thousand rpm and the armature effect comes into play. Since it is OUT of neutral at low rpms, you would think this would cause arcing on startup. But in fact, the actually applied voltage to the motor is very low at that point and so arcing isn't as much of a problem. Under power AND RPM, the voltage is higher, and so that is where we want the brushes to truly be in the neutral plane.
And so we refer to this as ADVANCE TIMING. My theory was that if Mr. Catellier had his first motor accidentally at N or worse CW timing position, this would cause severe arcing during acceleration - and probably a LOT of carbon dust. That could seriously undermine his Wh/mile and explain his frame leak.
Unfortunately, brilliant though it sounded, it was another case of my trying to type myself smart in a fashion that would make the DIYElectric crowd cheer with pride.
Tim had of course noted and recorded the position on both motors. CCW. Theory busted.
Leaving what? The brushes. I'm accustomed thinking of brushes as just brushes and that their main characteristic is their life span, based on the hardness of the material. This also becomes a factor in seating the brushes, as they can take longer or shorter amounts of wear to properly seat in. But once they are in operation, there is little to choose between them other than how many miles you get before you get to change them. The technology is 180 years old. What's to know?
But in the spirit of rational inquiry, what else is there? That was the notable change between the two motors.
I happened to have a Netgain Warp 9, and an ADC 8 motor on a bench I had cleared off. We reassembled the Warp 9 using the new Helwig H60 Redtop brushes and painted the motor. The ADC took a little more work, we bead blasted the end bells, (and the armature unfortunately), revarnished the armature, and replaced the brushes and the entire brush mounting ring. I intend to use it on a lawn mower.
But we looked around and found the old brushes from the Netgain Warp 9. I wired up a JLD404 meter and a digital voltmeter with a contactor, one of our A123 13.5volt modules, and a switch. And we ran the motor with the new brushes, that had about 10 hours of seating on them. Then we changed the brushes to the old brushes. As it turns out, these aren't Helwig brushes at all. They are marked ML1683 and ML1684 H8 with a stylized chevron with a capital N in it. I have not determined who the manufacturer is, but Tom Brunka advises that it is a low voltage brush used on fork lift motors typically at 72v and below.
So we ran the motor again with the old brushes. The difference was astounding. For the same 129Ah of energy, the new brushes ran for 3 hours 38 minutes and 54 seconds. The old brushes ran for three hours, one minute, and 25 seconds.
So we reran the new brushes again. Along the way, we began recording RPM and commutator temperature along with current and voltage. The results are shown below.
If you clear out all the chaff, the new brushes use 485 Wh per hour to turn the motor at 2652 rpm while the old brushes require 535 Wh per hour to turn the shaft 2053 rpm. This is such an astounding difference that there must be something HUGE wrong with our methodology.
Except it rather persuasively reflects, in both direction and amplitude, what Tim Catellier reports.
I really have no further explanation for this. Here is the data and we would be very interested in direct attempts to reproduce either refuting or supporting. In the meanwhile, if you are driving a Netgain Motor a year old or more, you might invest in a set of Helwig H60 RedTops. Baseline your Wh/mile, change the brushes, and measure again. We would be very interested in what you find.
The current Helwig part numbers are:
For the Warp 9 = 10-621117-674-3-01
For the Warp 11 = 10-621117-674-3-02
I think Netgain is offering these brushes now at $150 per set of eight brushes. We will try to do the same on the EVTV online store as soon as possible.
We have also chased down the XSTURBOS guy and ordered 10 of his Garret blowers. We'll put together a little kit with the blower, the shroud for the motor to mount it on, and a relay at $495. George Hamstra of Netgain has already spoken for one for his Bricklin build that we are hoping to see at the EVCCON 2012. But hopefully we'll have these up on the web site this week or next as well.
Why? The main defense you have to carbon dust buildup in the motor is the motor fan. The newer motors actually have an improved fan on the drive end and these fans do a pretty good job at moving air through the motor, both to cool it and to clear the inevitable carbon dust. The problem is, that works real well at 3000 rpm. It works not so well at 300 rpm. And at 20 rpm it doesn't do squat. We have routinely begun using these 434 cfm blowers on all our Netgain motors. Any more heat you can remove is a good thing. And at low rpms, you still have fairly impressive air flow to remove carbon dust as well as heat.
Many people have run Netgain motors for years quite handily without an external blower. So this is overkill EVTV style. Still we recommend it.
Additionally, there are many less expensive blowers that are out there that will do just fine as auxiliary blowers for the motor. These look cool. For what i'm spending on the Escalade build, looking cool is kind of required.
Your mileage may of course vary....