Question

Calculate the total heat dissipated in a rotor resistor of 21 Ω when .81 A current flows through it.

(a) 13.77 W

(b) 12.56

(c) 16.78 W

(d) 13.98 W

The question was posed to me in an international level competition.

My doubt stems from Induction Motors in portion Speed Control of Direct Current Motors & Induction Motors of Electric Drives

Answer 1

Right choice is (a) 13.77 W

To explain: The rotor resistor is a linear element. It only absorbs real power and dissipates it in the form of heat. The voltage and current are in the same phase in case of the resistor so the angle between V & I is 90°. P=I^2R=.81×.81×21=13.77 W.