Bench test

I tested the motor + drive system last night, and overall it was a success. I ran the motor up to 7500 rpm with no problems. The cooling fan sounds like a jet taking off at that speed, though. It's nice to have a lot of cooling air, though. Unloaded, the motor consumes about 750 watts at top speed, which seems reasonable. I can run the motor down to about 30 rpm, where it uses about 50 watts. It would have been nice to get an idea of torque output, but I don't have an easy way of loading the motor.

The motor controller has two modes: V/f and sensorless vector. At its simplest, V/f mode makes the voltage to the motor linear with frequency, so for example, to run the motor at 30 rpm, the drive would supply 10 Volts at 1 Hz, and at 7500 rpm it would supply 230 Volts at 250 Hz. This is fine for constant load applications, but if the load increases too much, the motor can stall. In sensorless vector mode, the drive uses a mathematical model of the motor to calculate the torque applied at any time. It can then change the voltage and frequency to match a requested torque or speed. This is much better for an electric car, because motorists are used to an accelerator pedal that requests more torque from the motor. Because the motor's rated capacity is much smaller than the drive's (5 HP vs 30 HP) the drive was unable to model the motor, so sensorless vector mode is not working. I will see if I can input parameters specific to the motor and get it working. Otherwise, I will probably need a bigger motor eventually. In the meantime, I can set the acceleration and deceleration times for the drive to be long enough so that everything works. This just means that the drive will take the car's inertia into account a little better, and won't ask the motor to turn at 7000 rpm when the car's speed is telling the motor it must turn at 500 rpm.

Now to get rid of that pesky gas engine so I can start putting things in the car...

The motor drive is here!

I ordered a motor drive last week from the internet. In the past, these were the kind of things you would have the purchasing representative from you company call the regional sales rep about, a price would be quoted, and a lot of paperwork generated. Now, there are a couple of places to get industrial equipment on line. I bought this one from factorymation.com, as they had brands that I had heard of before. Whatever that's worth.



The drive weighs about 30 kg.

The business end: This is where the cables from the batteries and to the motor will run. The fans dissipate waste heat. The unit is about 90% efficient, but that's still nearly 2kW of waste heat at full power.

DC or AC?

Most electric cars built by hobbyists (see http://www.austinev.org/evalbum/ 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 http://acpropulsion.com/ and http://metricmind.com/for 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:
http://www.austinev.org/evalbum/1149

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.

Why?

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.