Question

Net field current in a compound motor is given by ______________

(a) If (net)= If + (Nse/Nf) Ia

(b) If (net)= If – (Nse/Nf) Ia

(c) If (net)= If + (Nf/Nse) Ia

(d) If (net)= If – (Nf/Nse) Ia

I have been asked this question during a job interview.

This intriguing question originated from Operating Characteristics of DC Compound Motor in division Characteristics of DC Motors of DC Machines

Answer 1

Correct option is (a) If (net)= If + (Nse/Nf) Ia

To elaborate: Net flux in a compound motor is determined by addition of shunt field flux and series field flux. Thus, for calculating effective flux, net field current is find out. If is the constant value given for shunt field, while If net takes the presence of series field also.