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Homework Help: Calculate the Motor Efficiency when?

  1. Mar 27, 2012 #1
    An electric motor drives an electric generator.The 2hp motor draws 15A from a 120V D.C source and the generator supplies 5A to a 48Ohm LOAD. It is given that 1hp= 745.7W

    Calculate the motor efficiency.
    Calculate the generator efficiency.
    Calculate overall efficiency.

    Ok soo I went forward and did this.
    Since 1hp = 745.7Watt I assumed that the motor would be going at 2x745.7W = 1491.4W .
    So if you take the P = I.V and you insert 15A.120V = 1800W
    So using n = Po/Pin for Efficiency 1491.4/1800 *100 = 82.85% efficiency.
    Not sure if this is correct but now?
    I can say P = I^2 / R and get the Watt but to what comparison? Which Po and which Pin ??

    Please help
  2. jcsd
  3. Mar 27, 2012 #2


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    Homework Helper
    Gold Member

    Ignoring any rounding differences/errors, yes, that looks correct to me. :approve:
    To calculate the generator's efficiency, just stick with the generator. What's the input power of the generator?

    (Hint: the problem statement says, "An electric motor drives an electric generator." I'm pretty sure you should assume no losses between the output of the motor and the input to the generator in this case. Assume the power is transferred between them via a frictionless rotating shaft. The only losses are within the motor and within the generator themselves.)

    (Continuing the hint: Keep in mind that this is not the case where a generator drives a motor. In that case, there could be I2R losses in the cables/wires connecting them. But in this problem, the motor and generator are connected mechanically [not electrically], and I suspect you should take the connection as loss-less.)
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