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.