Question

If a DC shunt motor is working at full load and if shunt field circuit suddenly opens _____________________

(a) Will make armature to take heavy current, possibly burning it

(b) Will result in excessive speed, possibly destroying armature due to excessive centrifugal stresses

(c) Nothing will happen to motor

(d) Motor will come to stop

I got this question in a job interview.

The origin of the question is Operating Characteristics of DC Shunt Motor topic in section Characteristics of DC Motors of DC Machines

Answer 1

Right choice is (a) Will make armature to take heavy current, possibly burning it

To elaborate: At no load or lower loads, there is possibility of excess speed in such cases but here, when full load is given, armature circuit will draw very high current in order to maintain back emf. So, if fuses or circuit breakers are not used then, armature circuit may burn.