Question

For a given DC series generator with critical resistance equal to 100 Ω, armature resistance is equal to 50 Ω, and series field resistance is equal to 20 Ω, is connected across load of 50 Ω. What will be the load voltage?

(a) 20 kV

(b) 0 V

(c) 2 kV

(d) Data insufficient

I got this question during an interview.

Question is from Characteristics of Separately Excited Generators in chapter DC Generator of DC Machines

Answer 1

The correct choice is (b) 0 V

To explain: Here, DC series motor fails to excite as addition of armature resistance, load resistance and field resistance is greater than the critical resistance of the machine by 20 Ω. Thus, machine fails to self-excite, as a result we’ll get zero terminal voltage.