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How is an electric motor's power rating defined?

  1. May 29, 2014 #1
    So, I'm not an expert on electric motors. I have yet to take a formal class on electric motors. But I am however in a position where I'm required to size a couple electric motors for a project.

    I've been using software that is supposed to help size motors depending on the application and I recently came across something that I can't explain.

    Specifially, I found the amount of power that was required to move a mechanical load. This ended up being around 10 Watts. However, this load had a high torque but moved rather slowly which is what lead to the low calculated power. But when I tried finding a suitable motor, the program was offering motor solutions of 3 to 4 kWs.

    This made no sense to me. Why would a 10 Watt load require such a high powered motor? I then realized that because the torque was so high, this meant there would be a large current associated with it. I interpretted this as if only high powered motors could handle this high of current, so that's why I was bumped into that range.

    But I'm still not happy with this answer. Could anyone explain this to me?
  2. jcsd
  3. May 29, 2014 #2
    There are 3 big things I'm aware of. Voltage limit, current limit, and the amount of magnetic material. The max voltage is typically determined by the insulation on the wires. When voltage gets too high, it breaks down the insulation and jumps from wire to wire, burning up your motor in the process. The current limit is primarily affected by how much heat the windings can take. More current means a lot more heat that needs to be dissipated. So essentially these two, voltage protection and heat dissipation are the big ones. Also note that this means electric motors can often be run past their current rating for short periods of time, and you can water cool a motor to increase the current it can handle.

    The third big limitation is how much raw magnetic material is in the motor. Motors work by converting electrical energy into magnetic flux, then from flux into mechanical motion. But the magnetic material can only absorb a certain amount of flux before it is saturated. Excess flux is wasted and won't have any contribution to mechanic power. Flux is directly tied to torque, so the greater your need for torque the bigger the motor is going to be.
  4. May 29, 2014 #3


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  5. May 30, 2014 #4

    jim hardy

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    Classic case of blind trust in a computer program : "Garbage in Gospel out".
    Thank goodness you have common sense - some folks would've ordered the multi horsepower motor.

    hp = 2 pi X torque(ft-lbs) X rpm / 33,000

    If that motor sizing program is giving preposterous results figure out which of the terms it's not getting right. You haven't said a thing about it. Does it expect you to account for external reduction by gears or belt ?

    old jim
  6. May 30, 2014 #5
    So in the case I listed, do you think the torque required would require a certain amount of current only available in larger powered motors? Thus the reason a smaller motor wouldn't work? Simulation results showed that the smaller motors could handle the amount of current without a gearbox.

    Other than that, thank you for your explanation.
  7. May 30, 2014 #6
    for your situation, I think you are more limited by the flux saturation than the current. Either way, you need gearing.
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