Question

What will be the effect of opening of field of a DC shunt motor while motor is running?

(a) The speed of motor will be reduced

(b) The armature current will reduce

(c) The motor will attain dangerously high speed

(d) The motor will continue to constant speed

I got this question in an interview for job.

My doubt stems from Operating Characteristics of DC Shunt Motor -1 in division Characteristics of DC Motors of DC Machines

Answer 1

Correct answer is (c) The motor will attain dangerously high speed

To explain: In a DC shunt motor if supply for the field winding is cut down, the speed would dangerously increase in order to maintain the back emf of the motor. For a constant back emf, flux is inversely proportional to the speed of DC shunt motor. So, if flux drops to zero theoretically speed will tend to infinity.