Back emf's

A rotating electric motor also develops an induced emf; this is called a "back emf".

The current through a rotating electric motor is greatly reduced because of its "back emf".

The current through a non-rotating electric motor may be great enough to damage the motor.

While the motor is running, the net potential difference is

V_net = V_ext - E = 12 V - 9 V = 3 V

so the current is

I = V/R = 3/6 = 0.5 A

However, . . .

While the motor is stopped the net potential difference is

V_net = V_ext = 12 V - 0 = 12 V

so the current is now

I = V/R = 12/6 = 2.0 A !!!!!

RL Circuits			Eddy Currents
Return to Ch 21, Electromagnetic Induction

(c) Doug Davis, 2003; all rights reserved