Question

Practical reason behind speed of DC shunt motor is proportional to back emf only is ____________

(a) Back emf is equal to armature drop

(b) Flux is proportional to field current

(c) Flux is proportional to armature current

(d) Flux is practically constant in DC shunt motors

I have been asked this question in a national level competition.

The origin of the question is Operating Characteristics of DC Shunt Motor -1 in division Characteristics of DC Motors of DC Machines

Answer 1

Correct choice is (d) Flux is practically constant in DC shunt motors

For explanation: The field winding in DC shunt motor is connected in parallel to the armature winding and the supply. If we assume that the supply voltage for motor is constant then flux also becomes constant. At the rated speed the back emf also becomes almost constant if the load is same.