Power Factor of Squirrel Cage Induction Motor and Slip Ring Motor

In this article, we will find out what is the reason behind the poor power factor of induction motor at no load and better at high load in either squirrel cage induction motor or wound rotor induction motor . Let’s start.

Induction motors work on the same principle as the transformer except they have a rotating part. As we know that the induction motor operates by the interaction of the stator field and the rotor field and as a result a torque is developed.

Now when the torque is developed and the induction motor is started then at no load it has a very low power factor typically between 15 to 20 percent lagging. 

Why is it that the power factor of an induction motor is poor at no load and increases at high load? 

Actually what happens is that at the starting of the induction motor, the induction motor draws a higher magnetization component of current in order to overcome the reluctance offered by the air gap between stator and rotor. Thus, the total current increases. 

Both of these stator and rotor fields are due to the currents which either have resistive (in phase) components or reactive (out of phase) components. The torque which is developed is dependent on the interaction of these phase components. 

Generally, a motor which draws a high starting current, will produce a low starting torque.

Because, a low rotor resistance will result in the current being controlled by the inductive component of the circuit, resulting in a high out of phase current and thus as a result a low torque.

We can also say that at the starting of a motor, electrical current is inversely proportional to cos Φ or vice versa.

We can say there is a high current so the power factor is low.

Load Current ( Ic ) and Magnetizing current ( Im )

Total current in both windings of a motor is the vector sum of :

  • Load Current ( Ic ) and 
  • The Magnetizing Current ( Im )

The magnetizing current ( Im ) is constant and it generally does not change with load. It lags the voltage by 90 degrees.

The load current ( Ic ) is proportional to load on the induction motor and is in phase with the applied voltage.

In the no load diagram, the load current ( Ic ) is small when compared to the magnetizing current ( Im ) thus the power factor is poor at no load.

power factor of squirrel cage induction motor
Figure shows the power factor at no load condition

In the full-load diagram, the load current ( Ic ) has increased while the magnetizing current ( Im ) remains the same. As a result, the angle of lag of the load current decreases and the power factor increases.

power factor of squirrel cage induction motor at high load
Figure shows the Power Factor at high load condition

Due to the inductive impedance between the rotor and the stator winding, the power factor of the induction motor is always lagging.

Inductive reactance of the rotor varies according to the slip of the motor.

Power Factor due to Slip in Induction Motor

As,

Effective Rotor Reactance = Slip × Rotor Reactance

At low load, the induction motor has less slip. 

So as discussed earlier, the vector sum of the load current at stator ( Ic ) and magnetizing current ( Im ) has a large phase angle with the applied voltage and the motor power factor is low.

Thus, at a lower or no load both rotor and stator are inductive and the pf is low.

At high load, the slip of the motor gets decreased.  With less slip, frequency of the rotor current gets decreased.

 fr = Slip × fs 

As a result, rotor inductance will be decreased.

Xr = 2 × Pi × fr × L 

As the rotor impedance is less inductive, the power factor of the induction motor improves when the motor is at high load.

Therefore, the reliable operation of the slip ring induction motor can be achieved if the motor is operating at greater loads.

I hope that doubt is clear now about the poor power factor of induction motor at no load in squirrel cage, slip ring. Thanks!

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