AC or DC
This question comes up a lot. So going through the gearbox, AC is more expensive and normally comes as a whole system with a matched controller and motor. These systems often offer a complete solution that also includesa motor controller, a DC-DC converter and 12V controls to hook directly to the donor car systems. This all sounds great and it is, but it comes at a price. Another advantage of the AC motor is that many systems offer motors that freely rev up to 13,000 rpm and this means it is easy to run a single gear ratio and ultimately replace the gearbox, clutch flywheel etc. with just a simple reduction box and a differential that are not too difficult to get hold of. Another advantage of AC systems is that generally they normally have regenerative braking to put charge back in the batteries at different levels when you release the throttle and when the brake pedal is applied. This also reduces wear on the mechanical brake pads and provides the drag that you would normally expect from a petrol engine when you release the throttle. DC systems can also have regenerative braking, but it is not so common. I have a DC motor and controller with regen braking, but I will explain more about controllers in another posting. DC motors are generally cheaper, but often limited to around a nominal 5,000 rpm and maxing at around 7,000 rpm by over-revving. So this makes the gearbox essential. I know from experience that I can drive comfortably from rest to 40 miles per hour in 2nd gear only. I can take this up to over 50 mph but the acceleration is much slower after 40 and that is about 5000 rpm, so it is over-revving. Changing into 3rd gear gives me a sharp rise in electric current, but the acceleration comes back in a lump and feels good. This then takes me up to a comfortable 55mph and can pull away from zero, but it can be a little sluggish on the initial pull away, but still drivable. 4th gear will give a good top speed, but pull away from rest is really poor and forces full current draw. 5th gear can be used on it's own, but the pull away is unacceptable I find. With an AC system revving up to 13,000 rpm it would be quite comfortable to have a gear ratio and differential equivalent to about 2nd gear and this could go from rest to a good high top speed. This is the sort of performance you could expect from a Tesla Roadster. The Tesla uses an AC motor.
The first photo below is an ADC FB1-4001 from Advanced DC Motors and this is one of the most commonly used motors in electric car conversions. This is what I have in the Probatron (http://www.probatron.blogspot.com/). The second photo would have been my preference and is the Warp 9 motor from Netgain. It is a little bit more powerful than the ADC motor, but also a little more expensive. Being in the UK, I decided to arrange to have an ADC FB1-4001 shipped over from America and I arranged the shipping myself. I could not do this for the Warp 9 as they have a UK distribution network that stock them and wouldn't sell direct to me. ADC have a UK sales rep that basically takes my money and arranges the shipping for a small commission. I saved several hundred pounds this way. Both of these motors will run up to a nominal 5,000 rpm and can rev up to about 7,000 rpm, but this is not recommended. The ADC is rated at 19kW continuous and 176kW peak power. I believe the last time I looked the Warp 9 was rated at 22kW continuous and I would expect slightly more peak power than the ADC motor. Both motors are rated at 144 Volts, but can work comfortably at 156v. My system works at 120v, but I would like to have a higher voltage. The voltage is almost like the engine size. Higher voltages make it much easier to develop higher power as greater efficiency is possible. This is another advantage of AC motors, the efficiency is slightly better than DC motors. All electric motors can typically produce efficiencies of over 80% while petrol engines are hard pushed to make 25%. The problem then is down to battery weight and cost. Again this is the subject of another posting.