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Electric motor questions

  1. Jul 2, 2011 #1
    What type of electric motor would be best for installing in a Mid-size car? I have a design for installing an electric motor in a car with a generator to power the motor and batteries with an inverter for starting the motor. The motor once started will run at a minimum 400 rpm, then increase in speed just like a normal engine when you press on the accelerator pedal.

    My questions are:

    1. What type of motor would be best, AC or DC and since it will be speed controlled would that matter?

    2. How many HP would the motor have to be to power a midsize rear wheel drive car.

    3. How much drag/loss of hp would a belt driven 230 volt generator create?

    Thats all the questions i have for now. Any help would be greatly appreciated.
    Last edited: Jul 2, 2011
  2. jcsd
  3. Jul 3, 2011 #2
    The motor should be DC i guess. Since, you're giving a direct current supply.
  4. Jul 3, 2011 #3
    Just to check my understanding of your concept. You are planning on having an engine-generator set provide power to an electric motor for the traction drive. I'm guessing you are not planning on using a multi-speed transmission between the electric motor and the drive wheels. This arrangement is basically that used by diesel-electric railroad locomotives.

    Classically series wound dc motors were used for such drive systems. Before power electronics, they were the easiest motors to control and provide the needed torque speed characteristics.

    Today, with the available power electronics, the motor of choice is a multi-phase ac induction motor (typically 3 phase). The motor(s) is driven and controlled by a variable frequency, variable voltage inverter.

    How much horsepower would you need. As a first estimate, look at the horsepower for comparable cars. That wouldn't be exact, but is a good first estimate, but probably greater than you could get away with. A more refined estimate would need to be based on operating scenarios that described such criteria as max. speed, max. acceleration, time to speed, frequency of accelerations, etc.

    Losses from a generator (i.e. inefficiency) would be on the order of 2% or less. However, when generating power, that power will appear as a load on the engine. Belt drives are pretty efficient, but why not make it a direct drive between the engine and generator.

    Typically the speed range of an electric motor is not wide enough for typical highway driving. There are some approaches that can help, but they add complexity. Consider the use of a multi-speed transmission to expand the motor's speed range.
  5. Jul 4, 2011 #4


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    Staff: Mentor

    All of it.

    Sorry, but what you describe is a perpetual motion machine and won't work. People don't just buy gas because they are suckers. You really need it.
  6. Jul 4, 2011 #5
    ok my design is to make an adapter to go from the drive shaft of the motor to the flywheel/torque converter for a normal automatic transmission. The generator would be belt driven off the adapter. there would also be an alternator to charge the 24 volt battery system, which would be used to start the motor via an inverter, and will also help support the motor when running at lower rpm's.

    Once you start to accellerate from a stop the generator will produce the power required to run the electric motor, In a sense it would be a perpetual motion machine, but i feel this is one that would be able to be made to work.

    Im not looking at making a vehicle to set land speed records or drag race records......i know the electric motors for that would weigh several tons.

    Many electric go carts have been designed that are beating gas engine powered carts.

    Anyone know what the hp to torque ratio is on electric motors?
  7. Jul 9, 2011 #6
    So the battery would power the motor, which would power a torque converter, which would run a transmission, which would power a generator (or alternator, same thing), which would in turn power the original battery....which would in turn drive the motor and on and on...

    A more efficient system would be to remove the torque converter because it robs some energy. An even more efficient system is to also remove the transmission, as it also robs some energy. Then you are left with a battery powering a motor, which powers a generator, which powers the battery.

    Unfortunately, the generator also robs some energy. A better system would be to remove that as well.

    Which leaves you with a battery powering a motor. Any of whatever complications you proposed adding to this is the equivalent of a Flintstones car...you know, where You use your feet to power the car which then in turn pushes You and somehow is supposed to get you places faster. I hope you see how the very idea is ridiculous, as you are better off simply walking.

  8. Jul 13, 2011 #7
    I would suggest that to survive on the highway in a mid-size car, you would need 50 plus HP. This should be enough to get you to a safe speed.
  9. Jul 14, 2011 #8
    A simple answer to your question is to look into brushless DC motors. You'll need a special motor controller to run it, but in principle they're much more efficient/powerful than standard DC motors and although they are technically AC internally, they can be powered from a conventional DC source (such as a battery) via the motor controller. They can also be operated as a generator if torque is applied externally to the motor shaft (you'll need a special motor controller to do so).

    The power/torque ratings of such motors are determined by the diameter of the rotor and the number of turns in the armature coils. It can get more complicated, but they usually state the motor's rpm range and amperage requirements which, combined with the operating voltage, can provide a loose estimate for converting into hp (i.e. 4kW electric is similar to 5.4hp mechanical not counting efficiency losses in the conversion).

    Hope this clears some things up
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