Question

From magnetization characteristic at If = 7.1 A, Ea = 225 V at 1000 rpm. What will be the terminal voltage at speed 950 rpm?

(a) 225 V

(b) 235 V

(c) 214 V

(d) 220 V

The question was asked in class test.

This key question is from Graphical Representation of External Characteristics topic in division DC Generator of DC Machines

Answer 1

The correct option is (c) 214 V

The best I can explain: Speed is directly proportional to the back emf of a machine. So, Ea = 225 V at 1000 rpm

Ea (950 rpm) = 225 x 950 / 1000, this will give speed of given DC machine at 950 rpm. Upon calculations we get, Ea = 213.7 V= 214 V approx.