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Homework Help: Electric Motor and Power

  1. Jan 26, 2010 #1
    1. The problem statement, all variables and given/known data
    An electric motor operates at a steady state condition with a rotational speed of 850RPM while producing a constant torque of 5ft-lbf. A watt meter reads 700watts supplied to the motor.
    What is the output power at the motor shaft?
    What is the power lost in the motor which ends up to the surroundings?
    What is the motor efficiency?

    2. Relevant equations
    (a) Power=torque*angular velocity
    (b) Efficiency=output power/input power

    3. The attempt at a solution
    I converted the torque to 6.78N*m.
    Then I plugged in my values or angular velocity (850RPM) and torque into equation (a).
    My result was: 5800W
    Is this not too large? The input power was only 700W, shouldn't my output power be less than this? Wouldn't this mean that power is not lost, but gained?
  2. jcsd
  3. Jan 26, 2010 #2
    You should convert the 850 rev/min to radians/s.
  4. Jan 26, 2010 #3
    Thanks, my answer makes more sense now!
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