Question

Describe the effect of the following loads on power factor :

(i) Induction motors.

(ii) Transformer.

(iii) Partly loaded motor.

(iv) Cage type motors.

Answer.

It is understood that there is a supply with a power factor [usually of 0.8] and effect of various given equipments to be noted when hey are connected to such a supply.

Induction motor.

When stand still – no effect on power factor.

When running it will cause a drop in power factor.

Impedance in running condition = √(R)2 x (SX)2 – cos Φ =R/2

S = slip,

R = rotor resistence

X = rotor reactance/phase

As X of the coil increases Z increases and cos Φ decreases.

Also when slip increases Z increases and cos Φ decreases

Transformer.

This when connected to supply, doesn’t really effect the power factor. But the power factor shall vary, when it is loaded and is dependant on type of loading

Partly loaded motor.

It is like a transformer whose secondary is partly loaded

This partly loaded motor will marginally improve the power factor as the slip is less, and will be a little vbetter than the fully loaded induction motor.

Cage type.

The power factor remains constant for all loads. However a cage rotor when running as a synchronous machine will lead to a leading power factor.