Question

For a constant emf, if field current is reduced then the speed of the DC motor will_____

(a) Remains same

(b) Increases

(c) Decreases

(d) Can’t say

The question was asked in an interview for internship.

My question comes from EMF and Torque Production topic in chapter Circuit Model, Emf and Torque of DC Machines

Answer 1

Correct option is (b) Increases

To explain I would say: When the field current is reduced, the field produced by the field winding also reduces. Thus, the term Φ from the emf equation also decreases. For all other parameters kept constant speed of the DC machine is inversely proportional to field. Hence, speed of DC motor will increase.