Saturday, 3 September 2016

Why does Induction Motor has Poor Power Factor at No Load


Induction Motor when started at no load has very poor power factor in the range of 0.1 to 0.3. For understanding the effect of loading on the power factor of Induction Motor, we will consider three components of current flowing in an Induction Motor namely Magnetizing Current, Stator Loss Component of Current and Load Component of Current one by one.

Magnetizing Current Im:

As Induction Motor is an electro-mechanical device, therefore it will draw current to set up magnetic flux in the air-gap which is called Magnetizing Current. This Magnetizing Current lags the Stator Voltage V1 by 90°.

Stator Loss Component of Current:

Stator Loss Component of Current provides the Stator Iron as well as Copper Loss just like in a Transformer.

Load Component of Current:

The purpose of Load Component of Current is to balance the Rotor mmf which is demagnetizing in nature. Thus due to Load Component of Current a constant flux is set up in the air-gap of Induction Motor.

At no load, the power taken by the Induction Motor will be just enough to meet the no load losses like Friction and Windage Loss. Thus Induction Motor takes very less current from the supply main. Refer the phasor diagram below.



As shown in the phasor, Stator current at no load is I0 while I2 is load component of current. Mind that Load component of current is taken just to meet the rotor inertia and no load losses. It is quite clear that at no load component of current is very low as compared to the magnetizing current. This is because Induction Motor needs to set up rotating magnetic flux in the air gap and as because the reluctance of air is more, the magnetizing current taken from the supply main will be more. If we add magnetizing current and load component of current vectorically then I = I0+I2 will be much lagging and the angle Ɵ2 made by the resultant current I will be in the range of 75° to 85°, and therefore the Power factor will be much low in the range of 0.1-0.3.

Effect of Loading on the Power Factor of Induction Motor:

As the Induction Motor is loaded, the load component of current increases from its no load value. The increment of load component of current from its no load value is responsible for supplying the load torque. Thus the load component of current increases from its no load value of I2 to I2’. When I2’ is added with I0 vectorically the resultant current I = I2’+I0 makes an angle of Ɵ1 which is less than Ɵ2. Thus as we increase the load Ɵ1 decreases which in turn means an improvement in power factor.

The power factor of Induction Motor becomes 0.85-0.88 when it is loaded 80-90% but beyond this loading power factor decreases slightly because of prominent effect of Stator and Rotor leakage reactance drop.


Thank you!

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