Question

For large motors what is the ratio of compensating winding is required for increasing the speed of the motor?

(a) 2:1

(b) 4:1

(c) 6:1

(d) 8:1

This question was posed to me in a national level competition.

My enquiry is from Speed Control Using Field Control of Series Motor topic in section DC Motors of DC Machines

Answer 1

Right choice is (a) 2:1

Easiest explanation: For motors requiring a wide range of speed control, the field ampere-turns are much smaller than the armature ampere-turns at high speeds causing extreme distortion of the flux density in the air-gap. This leads to poor commutation. Compensating winding can be used to increase the speed range which can be 2 to 1 for large motors.