4: Motor

The motor is one of the key parts for your conversion and will have the most influence on what goes with it. First choice is gearbox or hub/wheel motors. There are many choices for hooking a motor to a gearbox, but for hub motors, they are nearly all DC permanent magnet motors or AC. I have no specific experience with hub motors, but many of the top speeders and sporty electric cars use in wheel motors. The Venturi Fetish uses the Michelin electric wheels. These are amazing for developing a complete new car as the wheel contains the motor, suspension and brakes, all you need to do as a vehicle designer is build a frame to fix it to. They are also very expensive. The Lightning car uses 4 AC motors for 4 wheel drive. These were a new design and also expensive, but huge performance comes at a cost. So the easiest and cheapest way to get a reasonable performance is what I want to focus on. For this blog, the design is continuing the theme of change as little as possible for the first build, then you can go back, redesign and upgrade later. At the same time I don't expect anything to be second rate, that has never been my objective. You need to be happy with what you have built. So in this way I will consider that you will keep the gearbox, clutch and flywheel from the donor car.

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.

The third photo below shows an AC motor that comes with one of the Azure Dynamics systems. This is rated at 55kW, but these tend not to have such a high peak power and this is due to a flatter efficiency curve. Originally the Ford Probe 2.5L V6 engine that I had was rated at 164 Horsepower, and the ADC motor equates to 135 HP peak, but it is not delivered like that. I have an 800 Amp motor controller running at 120v, so the most I can develop is 96kW, and this equates to about 128HP. That is what it feels like, but the delivery of the power is completely different to a petrol engine and much smoother as it has just one electric motor instead of six thumpers to turn the crank. My advice to keep a good level of power and keep the cost down is to buy a DC motor and my choices are shown here. I would like to delve into AC systems, but cannot afford this right now.