Saturday, November 1, 2008

Still learning

I have learned quite a bit about motors since setting out to rewind mine. First, I was wrong about my old motor being a 2 pole machine. It turns out that most 4 pole motors use something called consequent, or phantom poles, where there are 6 coil groups, 2 for each phase, so identical windings to a 2 pole motor. The difference is, the coil groups opposite each other (in the same phase) are wired to produce the same magnetic polarity, say North, in the air gap. This produces two consequent South poles at 90 degrees to the two North poles, making four total poles for each phase.

As far as the rewinding, I have put that off. I have bought a nicer motor-- aluminum frame, inverter duty, and will probably rewind eventually. For now, though, I will convert it from star to delta topology, which is as simple as bringing the central star point out in three leads. This has the effect of converting a 208 volt motor to 120 volts, meaning I can run at 3033 rpm instead of 1750 at full (240) volts. This means I can run the motor at 10 Hp continuous instead of the 5.5 rated. Since peak torque is roughly 2.5 times rated torque for a motor of this type, I should be able to get 25 Hp peak. How long I can keep that up will depend on my cooling scheme, which I haven't really settled.

Monday, September 22, 2008

Winding update

It turns out that I made an error in assuming that there must be an even number of slots per pole. There is a method of winding called lap winding (as opposed to concentric winding) that allows almost any number of slots per pole by winding coils with constant pitch. I will post more as soon as I learn how to do that in this case.

Sunday, September 21, 2008

Nameplate mystery

I decided to tackle removing the motor and beginning the process of rewinding by having a look at the old windings. It turned out that the nameplate was wrong--- It's not a 4-pole motor at all. It is a 2-pole motor, meaning the full-load speed was around 3500 rpm, not 1750 at the nameplate advertised. There are 36 slots in the stator, meaning that 6 slots are used for each pole, times 2 poles, times 3 phases. I would like to increase the number of poles to increase the torque. Rewinding for 4 poles is impossible, as 36 slots divided by 3 phases gives 12. 12 divided by 4 poles gives 3 slots per pole. The number of slots per pole have to be even, though. The number of poles also has to be even, and there are only two even numbers that multiply together to get 12 -- 6 and 2, so this motor has to have to have 6 poles with 2 slots per pole or 2 poles with 6 slots per pole. Two slots per pole isn't enough because it results in a poor approximation of a sinusoidally varying magnetic field around the stator, so I guess I'm stuck with 2 poles. This means that at 180 Hz, the motor is spinning about 10000 rpm, which isn't too good for the transmission, I'm sure.

Friday, September 19, 2008

Heat death

Since the last post, I drove the car back and forth to work (2.5 mi each way) a few times, and enjoyed getting the control settings to be a little more driveable. The car accelerates well, but has only a 35mph top speed. The problem seems to be that the torque falls off fairly rapidly as the motor is driven above 60Hz because of field weakening. The way induction motors work, they need a voltage proportional to the frequency to maintain constant torque. As my drive system peaks out at 240 volts or so, the torque falls off, because I am not able to force enough current though the windings. The solution to this is to rewind the motor to run at a lower voltage, say 80 volts. Then, we don't run out of voltage in a 240 volt drive system until we hit 180 Hz. The result is that the motor will have usable torque over a much wider speed range.

Luckily, I was able to speed up the process of rewinding the motor by overheating it to the point the windings shorted. I thought it would be a good idea to drive down to Stanford and back (10 miles) on a hot day, and as I was about to pull into my place, the inverter shut down because it had detected a fault. I knew perfectly well what had happened. I had been meaning to install some sort of temperature instrumentation, as well as a blower for cooling air, but had been putting it off. I figured I would test my luck with my $150 ebay motor...

I've never rewound a motor before, so I am practicing with a smaller motor from my blower, which I also got on ebay, and which I will be installing in the car when I have the motor off. I bought the blower with a 575 volt motor, which I will rewind as a 220 volt motor and run off a smaller inverter. Here's the stator of the small motor:


It's a 2-pole, 1/2 hp motor, with 4 slots per pole, star wound. The windings are 25 AWG, which I am replacing with 20 AWG to do the voltage conversion. There's about 2.5 lbs of copper in this, which works out to about 1000 feet of 20 gauge.

Here's the rotor, along with the end bells and the impeller from the blower. Such elegant machinery.

Monday, August 18, 2008

Kermit comes home

Kermit left his home of the past 15 months at TechShop, and made the 10 mile trip back to my house in Mountain View this evening. It was a little bit nervewracking setting out on the open road for the first time, but we made it without assistance from AAA. I set the maximum current to the motor at 30 amps, and limited the maximum RPM to about 3600, so I definitely didn't set any speed records. It was a trick accelerating from a stop through certain traffic lights before they turned yellow again, and I'm sure I irritated more than one other driver. Accelerating from 0-5 mph was very difficult, with torque not picking up until the car got going a bit. Once rolling, though, it felt peppy, at least compared with the stodgy original acceleration.

I need to fiddle with the voltage, current and frequency parameters a bit until I get acceptable performance. Mostly, I wanted to be assured of getting home without overheating my motor, which was no problem. I could hold my hand on the motor once I got home.

It feels really good to have come this far. Even though the car is by no means done, it is a usable means of transport at this point, and I will be driving it to work and dragging groceries home.

I'll keep updating this space with further news as Kermit rolls on.

We don't need no stinkin' capacitors

One solution for the inrush shenanigans described in the last post is to construct an inrush limiting circuit with a large resistor and a contactor- the capacitor is charged through the resistor until the operating voltage is reached, when the contactor is closed, bypassing the resistor. The other way is to yank out the capacitor. The purpose of the capacitor is to filter the rectified AC mains supply so that it is a smooth DC source for the inverter. A battery is already a nice smooth DC source, so the capacitor isn't really needed. I already ditched the 20-plus pound inductor that was the other part of the line filter, so I had no qualms about getting rid of the capacitor. Luckily, the capacitor was its own easily removable module:
The other plus is now the inverter can be put in a much smaller box, and will have considerably less need for interior ventilation, making weather sealing a possibility.

Of course I tested the capacitorless inverter/motor setup the way my pappy taught me -- by revvin' er up real good a few times.

Sunday, August 17, 2008

Kermit on a roll

Well, the big day finally arrived. Kermit moved under his own power for the first time! It was very exciting doing a few laps around TechShop's parking lot. Matt Heck took some video, which I'll post here soon.

There are a few issues before I can drive it home, however. The first is that when I connected the pack to the inverter for the first time, the inrush current to charge the capacitors caused a spark that was, well, cinematic. Of course I had all my safety gear on (gloves, face shield, etc.) and I was expecting a spark, so no injuries or soiled britches, but jeez... the blinding flash and deafening KAPOW were pretty exciting. Obviously, we can't have that kind of drama each time we fire up Kermit to go get some milk.

The second issue is that the 12V system isn't working yet, both because the DC-DC converter isn't converting, and because interfacing with the MGB wiring is a real head scratcher.

The third thing is related to the control issues mentioned in the last post. As I was driving around the parking lot, it was very difficult to accelerate and decelerate smoothly. The good news is, there's plenty of torque.

Sunday, August 10, 2008

Control issues

The way the motor controller is currently hooked up, I have the throttle position sensor supplying a voltage to the speed control input of the inverter. The problem with this is that in all other cars, pressing on the accelerator gives more torque, not more speed. Some inverters will allow a torque input, but with my motor/inverter combination, that isn't possible. What my inverter does have, however, is an analog output that represents the current being given to the motor at any instant. If I connect an opamp with the positive input to the throttle position sensor and the negative input to the current output from the inverter, and feed the output of the opamp in to the speed control input, the throttle position sensor should control the current supplied to the motor. Current is related to torque, so this hack should make the car a lot more drivable.

I'll wait until it gets around the block under its own power to try this, though.

Kermit gets some batteries

I took a couple of days off of work to make a final push on getting the car rolling, and it paid off. I finished all the sheet metal work around the battery box, and gave it a shot of primer gray. I also finished up all the wiring. I used 3/4" flexible waterproof conduit run in the tunnel above the driveshaft and transmission from the battery box to the front of the car where the inverter lives. I got 70 cells (the remaining 28 are still being conditioned) and installed them:I was hoping to take it for a little spin with this partial pack, but 244 volts from these 70 cells was not enough to start up the DC-DC converters for the 12 volt system, and without 12 volts, the main contactor would not close, so no joyriding yet. I should get the remaining cells put in sometime this week.

Monday, July 7, 2008

Contact!

I bought what will (hopefully) be one of the last components for the car:

This is a sealed contactor, which will connect the battery pack to the motor controller when I turn the key.

Apologies to Pininfarina

I have finished the battery box and tacked it in its proper location in the car. Unfortunately, to avoid striking the rear differential, the box had to be moved back to the point where the corners protrude from the rear skirt of the car.


This spoils the lines of the car somewhat, but the protrutions will at least be hidden behind the bumper. I don't feel too bad about cutting up the car anymore, though. I wirebrushed off some paint to do the welds, and discovered quite a bit of bondo, so apparently Kermit has had more than one surgery.

Saturday, June 14, 2008

Throttle test

I did the first test of the throttle control of the motor and drive. Everything went well; the drive was flexible enough to account for the idiosyncracies of the ebay special throttle position sensor I am using. More good news, the motor drive will fit in the engine compartment, freeing up more space in the back, and simplifying the wiring. I am running off mains power here, but it was a good feeling sitting in my car and revving the electric motor.

Battery box

I have started a sheet metal box to hold the battery. This will be welded in place of the spare tire and gas tank. I need 100 cells, give or take, and the squarest pack I could make with about that number turned out to be 7 cells wide and 14 cells long, giving 98 cells. I started with a pristine sheet of 16 gauge steel:
Which got a little less pristine as I taught myself sheet metal fabrication:

The finger brake at TechShop wasn't big enough to bend all four sides of the box, so I had to do some old fashioned whacking it with a hammer and a dolly to get a nice fold on the last two sides. I won't show any closeups of the hammer marks.

Next, remove the gas tank. I'm going to try to find a new home for this, as it is in good shape.



Next, I had to grit my teeth and cut a bloody great hole in the bottom of my car. Not an easy thing to do with a classic car with decent sheet metal.



The hole will be bigger in the end, but I don't want to get too excited about cutting and end up getting rid of some metal that would help hold the box in later.

Test fit!


Astute readers will notice the complete lack of welds. Unfortunately, Saturday is welding class day at TechShop. so I wasn't able to get a welder all day. I need to weld some separators between the rows of cells as well.

Friday, April 11, 2008

Mmmm, toast!

I needed a way to discharge the battery pack during my BMS tests, so I headed down to my local K-mapart to pick up a space heater. It's spring here in Califoria, though so there weren't any. I figured a nice Toast-R-Oven uses lots of electricity too, and I can make toast with it later. It's pretty funny to go to the store to find something that uses the most electricity.

One Logger per Child

I got one of the xo laptops from the One Laptop per Child project while they were having their give one get one promotion. It's really a neat device, but I haven't had much time to play with it. It really makes a good datalogger, though. I can hook it up to the battery pack and collect data on charge/discharge cycles while using my grownup laptop for grownup things.


Thursday, March 27, 2008

The BMS finally works

It's been three months, but I finally have a working battery management system. Here I'm testing it with 24 cells in series and the Manzanita charger. I deliberately discharged one of the cells, and the system is bringing it back in balance as it charges the whole pack. It's been a lot of work, but this system should give the best life possible to my pack.

I'm sure there will be some fiddling with the control algorithms, but at least the hardware is done.

Tuesday, February 19, 2008

What a thousand boards looks like

The production run of BMS boards arrived today. Richard Hatfield of Lightning Motors went in with me on an order of 1000 to bring costs down.

Thursday, January 31, 2008

Beginnings of a battery pack

I've finally entered the home stretch with the BMS system, and have 3 final prototypes installed on cells for testing. Cat for scale.