Electric Vehicles and Electric Motors

A friend of mine finally got delivery of a Tesla Roadster.  This prompted discussion of the drive train and the fact that Tesla has had to go from two speed transmissions which were failing to a transmissionless drive train.  The ultimate mechatronic challenge, the electric car, is also a challenger in terms of the precise  application of electric motor technology.

But it has to be said that the motor and drive solution for the electric car is not where the problem has to be solved.  Any motor can be made to run an electric car.  What is critical is how you apply it.  The starting conditions require high torque at low speed and the running conditions require low torque at high speed.  So, typically, what looks like a small 5 to 15 horsepower running requirement at full speed, becomes a 150 horsepower starting requirement depending on how quickly you would like to start.  If you want to keep up with a Corvette, it uses 450 HP to start.

And this produces a lot of confusion.  Why not use at 2 speed transmission to help the situation.  Fine, but the ones that are available can’t handle the dynamic response of the electric motor.

Can electronics help this situation?  Interestingly, yes.  There is a control algorithm generally called vector control which allows you to manage the rotor torque and stator torque separately.  By varying the phase angle between the two, like advancing and retarding the timing of a mechanical distributor cap on an internal combustion engine, you get different speed torque curves out of the motor.  COOL!  Is there any downside to this?

Yes.  You need more current to produce more torque.  That doesn’t change.  So you have to be able to supply the current, and you have to be able to manage the heat.  The heat is transitory since you only need the high current during starting, but it is best to have sophisticated software running to keep track of the RMS temperature of the motor.  Lower operating temperatures mean longer life and reduced risk of demagnetizing the motor.

So, yes, you can run an electric car with a garden variety AC motor, and with good electronics, you can make it run fairly efficiently.  With higher efficiency motors, the benefit is increased driving range from a given power source.  High efficiency motors are frequently smaller and lighter weigh, but a weight savings in the motor of 50 or even 100 pounds is not that big a factor in the driving range when the curb weight of the vehicle is 3000 pounds.

Basically, its F=ma.  If you can reduce the mass of the vehicle, you reduce the battery payload required to power the car.  Aluminum space frames, like on the Prowler, have been studied by the car industry and can reduce curb weight by 400 pounds and reduce cost by 10% at the same time.  We need to bring all the mechatronic leverage to the situation that we can, if we are going to make electric cars that make sense.  Before its too late for Detroit.

6 Comments on “Electric Vehicles and Electric Motors

  1. It seems that people have forgotten or have not yet learned what was known 100 years ago. Each of the four wheels should be driven by its own electric motor. A pancake motor inside each wheel with planetary gears would be good but we don’t want to add weight to the wheels so the motors should be mounted inboard; each one having a planetary gearhead driving an axel. The front axels will have universal joints for steering. Four motors will provide 4 wheel drive without energy wasting differentials and planetary gearing will handle heavy loads efficiently.

  2. It runs quietly and is very responsive. I think he likes it a lot, and of course, it makes an important statement that he wants to make.

  3. Thanks for your post. I agree about the planetary solution. It makes even more sense when we know that a decent planetary can be produced very cheaply with sintered powder metal. I am not sure if a 2 or 4 wheel solution was the most common in the early 1900′s. Either would work better than a differential gearbox. We have electronic differential algorithms in common use on powered mobility devices like wheelchairs, so there is very little new development required.

  4. Of course you saw this:

    PML Builds 640hp Electric MINI


    While all the world’s most famous motoring names look on, a British company, PML, has chosen the UK’s most prestigious motor show as the venue to strip away all the misconceptions surrounding electric/hybrid vehicles and to showcase a truly awesome car. Featuring four revolutionary electric wheels, the standard BMW Mini One you can see on Stand 270 is predicted to have a top speed approaching 150mph and to out-accelerate a Porsche 911 Carrera from 0-60mph. Brake horsepower is a stunning 160bhp per wheel – 640bhp in total.

    The car, dubbed the Mini QED, has been designed to run for four hours of combined urban/extra urban driving, powered only by a battery and bank of ultra capacitors. For longer journeys at higher speeds, a small conventional internal combustion engine (ICE) is used to re-charge the battery. In this hybrid mode, fuel economies of up to 80mpg can be achieved.


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