Friday, June 22, 2007

DC or AC?

Most electric cars built by hobbyists (see for a nice collection of home-built and other electric vehicles) run on DC - direct current. It's pretty simple - get a bunch (4-15) of batteries, hook them together, and run a motor with it. Speed/torque control is done by limiting the current through the motor by an electronic chopper (Pulse Width Modulation). AC systems, on the other hand, need higher voltage (300 Volts DC for a 240 Volt motor, or around 600 Volts DC for a 480 Volt motor) This means a string of 26 batteries to run a 240 Volt motor. Also, instead of a single switch to control current, as in the DC case, AC motor drives need to have 6 switching transistors and a bunch of digital signal processing logic to produce the three sine waves needed to run a 3 phase motor. AC Propulsion and Siemens and a few others make automotive-type AC drives. See and examples. These go for about $20k, so they're quite out of most hobbyists' reach. So why bother with an AC conversion?
  • Pure snobbery
  • Simpler, more efficient motors
  • Regenerative braking
  • Higher voltage means lower current, which means lighter wires and other components
An interesting industrial-component AC system from Australia is here:

Here's the new motor. It's a 5 horsepower (3.75 kW) 3 phase, 240 volt induction motor (framesize 112M -- ebay $150).
My approach with this conversion is to use off-the-shelf industrial components as much as possible. Industrial AC motor drives are coming down in price significantly these days, and AC induction motors have always been cheap compared to their DC counterparts. This motor was manufactured in 1980 and apparently sat in a warehouse for the last 27 years, as it was brand new in the original crate.

The old motor

Here's the old gas motor, which is for sale, by the way. Abingdon's finest.

Begin at the beginning

Here's the car. Not exactly Kermit-green, but kinda looks like a frog. I'm a big fan of cars with names. As far as I know, this car doesn't have a name yet. So. Little green MG, I dub thee Kermit The Car.


I've started this journal as a way to keep track of my progress converting a 1970 MGB GT to run on electricity. Why a pushing-40 British car? Why rip out a perfectly good gas engine and replace it with something that belongs on an air compressor? I don't know. I probably won't know when I'm all done.