Question

What will happen if shunt field suddenly opens, in a differential compound DC motor?

(a) Motor will first stop and then run in opposite direction as series motor

(b) Motor will not work and come to stop

(c) Motor will work as series motor and run at infinitely high speed

(d) Motor will work as series motor and run at slow speed

The question was asked in an internship interview.

The origin of the question is Operating Characteristics of DC Compound Motor topic in chapter Characteristics of DC Motors of DC Machines

Answer 1

Right option is (a) Motor will first stop and then run in opposite direction as series motor

The best I can explain: If shunt field is suddenly opened, flux due to shunt field will become 0. As, net flux is equal to shunt field flux – series field flux, the direction of flux will be reversed and thus motor will run in opposite directions.