PHYSICS 2 !!!

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At its normal operating speed, an electric fan motor draws only 10.9% of the current it draws when it just begins to turn the fan blade. The fan is plugged into a 120.0-V socket. What back emf does the motor generate at its normal operating speed?

  • PHYSICS 2 !!! -

    The current when it "just begins to turn the blade" is a starting surge current. There is no back emf (Vb) at that time. Let R be the resistance of the motor coils.

    The starting surge current is 120V/R.
    The current at normal operating speed is (120 - Vb)/R

    (120 - Vb)/R = 0.109*(120/R)

    Vb = 120 - 13.08 = 106.9 V

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