Question

The efficiency of the DC motor at maximum power will be ___________________

(a) 100%

(b) Around 90%

(c) Anywhere between 75% and 90%

(d) Less than 50%

This question was addressed to me by my school principal while I was bunking the class.

This question is from Efficiency of DC Machine- 1 in section Efficiency and Testing of DC Machines

Answer 1

The correct option is (d) Less than 50%

For explanation: For getting maximum power, derivative of power with respect to current is equal to 0. This is practically impossible to achieve as, current required is much more than its normal rated value. Large heat will be produced in a machine and efficiency of motor will be less than 50 %.