Why does an Induction Motor has Poor Power Factor at NO Load

The power factor of an induction motor is very low in the range of 0.1 to 0.3 when motor runs at no load.There are two components of the no load current- Magnetizing Current(Im) and Working Current(Iw) or no load loss component current.The motor takes magnetizing current for setting up the magnetic field in the air gap. The magnetizing current remains constant if the voltage and frequency is constant. The magnetizing current lags the stator voltage (V1) by 90°. The magnetizing current is watt-less current.

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The magnetizing current(Im) does not depends on the loading on the motor and, practically the magnetizing current has constant value in induction motor. The motor also draw the no load loss current(Iw) because of the eddy current and hysteresis loss The no load losses in an induction motor are fixed type of lossses, therefore the no load loss current component(Iw) remains constant. The no load loss current(Iw) is  does not depend on the loading on the motor. Thus the no load current(Io) is equal to the vector sum of the magnetizing current(Im) and working current(Iw). The phasor diagram of no load current is as shown below.

no load current of induction motor- Phasor diagram

The load current of an induction motor depends on the loading on the motor and varies according to the percentage loading on induction motor. The load current varies as the loading on the motor is varied.

When the motor is operated on no load, the motor draws very less load current. The load current is drawn to meet the torque requirement to drive its rotor. The magnetomotive force(MMF=NI) produced in the rotor is phase opposition in the MMF produced in the stator. The phasor diagram of the motor at no load is as given below.

phasor diagram of induction motor at no load

At no load, the rotor current is I2.The  phase angle between the Iand rotor induced voltage E2 is more hence the power factor of the rotor circuit is low. The equivalent magnitude of the rotor current is reflected in the stator side.The current reflected in the stator side is known as rotor referred current.The total stator current of the motor is equal to the vector sum of the rotor referred current plus the no load current.

no load current of induction motor formula

At no load, the Cosine of the phase angle between the stator current(I1) and the stator applied voltage(V1) is the power factor of an induction motor. The angle between the voltage V1 and current I1 is large when the induction motor runs at no load, therefore the power factor of the motor is about 0.1 to 0.3. 

The power factor of the motor gets improved with increased loading on the motor. With increased loading, the impedance of the rotor circuit gets decreased and the rotor draws less reactive current and thus the power factor of the motor improves. 

Let us take an example of how the rotor power factor improves with increased loading on induction motor. Lets the rotor resistance is 0.1 Ohm and rotor reactance is 0.672 Henery and the full load slip of the motor is 4%. The calculation of the rotor power factor at different loading on motor is calculated in below given table.

rotor power factor calculation

From above table it is very clear that the power factor of an induction motor is poor when motor is at no load or at light load. The reason of the low power factor is high rotor reactance at lower loading. The rotor reactance value gets decreased with increased loading on induction motor.

The power factor of the rotor circuit improves with increased loading and thus the motor power factor improves. The phasor diagram of an induction at load is as given below.

phasor diagram of induction motor at load and no load
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