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Generator size vs speed debate

  1. May 22, 2012 #1
    Over at the Airborne Wind Energy forums we are having a debate about the physics of a generator.

    The question was posed that if you could generate 1kw of power in the form of rotational torque at 2 different speeds, one begin half as fast as the other how much more would the generator weigh to convert that slower RPM.

    One poster is maintaining that it would weigh 4 times as much.
    Another is guessing that it would weigh twice as much.
    I guessed that it would weigh about 1.66 times as much.

    I believe we are presuming that both the RPMs fall in a range where a gearbox is not needed.

    I find myself unable to explain this well enough to convince even myself.

    We thought that we should be able to "look it up" but I have been unable to do so. A link would be great.

  2. jcsd
  3. May 22, 2012 #2
    I don't know much about electrical engines. I work with mechanical, but I assume many of the same principles apply. I've seen logical arguments that it would be a factor of eight. But in practice it is less than that. Sometimes not much less.
  4. May 22, 2012 #3
    Yes, like you, I have experience in various applications but not generators specifically. In small DC electric motors the ultimate power is limited by cooling. By changing the winding you can get different RPMs out of the same motor. Even as much as double. You can also adjust the windings until you have the same power output at a different RPM. But there is more to it than that. Efficiency would be better at one RPM vs the other. So to get the efficiency back to where it started the motor would have to get larger at the slower RPM. I am like 90% positive that it would not get 4 or more times larger.
  5. May 22, 2012 #4


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    In practice I don't see why you would need to change the weight by much. Just use a 2:1 drive ratio (either gears or a belt) to match the speed to whichever generator is lightest.

    To get a rough idea of how big the system would be, a car alternator generates about 1kW, and it only needs a cheap toothed belt to handle the power to drive it.
  6. May 22, 2012 #5


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    It depends on the generator. In an automobile alternator we can just increase the field current at lower RPM. What you have proposed is not really enough information to give a concrete answer.
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