## PHYS208 Fundamentals of Physics II

### Quiz 9 -- LR Circuit

To prevent damage from arcing in an electric motor, a discharge resistor is sometimes
placed in parallel with the armature. If the motor is suddenly unplugged while running,
this resistor limits the voltage that appears across the armature coils.

Consider a 12 V dc motor that has an armature
with a resistance of 7.5 ohm and an inductance of 450 mH.
Assume the back emf in the armature coils is 10 V
when the motor is running at normal speed.

a) Calculate the maximum resistance *R* that will limit the voltage across the armature
to 80 V when the motor is unplugged.

b) How long does it take for the motor to de-energized?
That is, after what time will the *energy* in the inductor be 99% dissipated?

I have forgotten the source of this problem, a long-forgotten textbook I believe.
My apologies to the author. I am responsible for part b however...

"http://www.physics.udel.edu/~watson/phys208/quiz9.html"

Last updated Nov. 21, 1997.