Question

A synchronous motor absorbing 60 kW is connected in parallel with an continuous load of 240 kW operating at 0.8 power factor lag.What is the kVAR supplied by the synchronous motor to obtain a total power factor of 0.9 lagging?

(a) 35, leading

(b) 35, lagging

(c) 145, leading

(d) None of the mentioned

I got this question during an interview.

This intriguing question originated from Machine Applications in section Electrical Machine Applications/Armature Reaction and Commutation of Electrical Machines

Answer 1

The correct answer is (a) 35, leading

The explanation is: Total power = 240 + 60 = 300 kW

             cos(phi) = 0.90. Total reactive power = P*tan(phi) = 300*tan(25.8) = 180 kVAR Load kVAR = 240*tan(36.87) = 180 kVAR

kVAR supplied by the motor = 145-180 = -35 or k3 kVAR leading.