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.

[motor circuit]

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...

Last updated Nov. 21, 1997.