Homework Help: PHYSICS 2 !!!

Posted by HELP !!!! on Monday, February 11, 2013 at 11:08pm.

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 !!! - drwls, Tuesday, February 12, 2013 at 12:51am
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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