Question

If the average coil emf of a DC motor is doubled and flux is halved (keeping other parameters constant) then its shaft speed will become ___________

(a) a. Twice of the original speed

(b) b. Square of the original speed

(c) c. Four times of the original speed

(d) d. Half of the original speed

The question was asked during an online interview.

My question is based upon EMF and Torque Production in section Circuit Model, Emf and Torque of DC Machines

Answer 1

The correct option is (c) c. Four times of the original speed

The best explanation: Induced emf in a DC machine is equal to,

From the emf equation we get speed of the shaft i.e. n α E/Z, when all other parameters are kept constant. So, when E is doubled n becomes twice the original, halving flux on reduced emf will quadrupled the speed of a DC machine.