Induction Motor Speed


Thread Starter


There is a nameplate of 37 KW Induction motor

380 V, 50 Hz, 80.1 A, 2850 RPM, P.F 0.87

400 V, 50 Hz, 77.9 A, 2870 RPM, P.F 0.84

415 V, 50 Hz, 76.8 A, 2880 RPM, P.F 0.82

Why with the Increase of Voltage

RPM increases?

Power Factor Decreases?

In text (Electrical Machines) I have seen no relation which directly describe relation between speed and Voltage applied ?
Aghvel... It has to do with the characteristic torque-speed curve of the motor, especially the region between breakdown-torque and the rated output-torque.

A) RPM Increase
As voltage increases "slip" decreases. Remember "slip"” is the difference between rated-synchronous speed (supply frequency) and rated-load speed. Also recall that this region of the curve is almost a straight line inversely proportional to output-torque. That is, the slope of the torque-speed curve between breakdown-torque and rated-torque changes. The result is an increase in speed for rated-load torque.

B) PF Decrease
The higher voltage increases the magnetic core’s excitation current. Thus the reactive component of input-current goes up resulting in a higher input kVA. Power Factor is the ratio of input kW divided by input kVA. But input kW equals efficiency times output kW. Thus, PF = input kW / input kVA. And, because input kVA is now larger PF is reduced!

Regards, Phil Corso