Why Induction Motor Takes High Starting Current?

Working Principle of Induction Motor:

The induction motor draws high starting current as compared to the running condition. The starting current of the induction motor is about 6 times of the motor full load current. An induction motor of rating 11 KW,22 Amps,440 volts takes high starting current about 132 Amps. The current reduces as the motor accelerates towards its base speed or synchronous speed.  

When three phase supply is fed to the stator of the induction motor, the motor takes magnetizing current to set up the rotational magnetic flux in the air gap.The flux travels through the air gap and the short-circuited rotor conductor cuts the magnetic flux. The voltage is induced when the rotor conductor cuts the magnetic flux. The current starts flowing in the rotor conductor. Due to an interaction of the rotor current and the main flux, the torque is produced.

Why Induction Motor Takes High Starting Current?

What happens at the starting of the motor? The voltage induced in the rotor depends on the relative speed of the synchronous speed of the rotation magnetic field and the speed of the rotor. At start, the rotor is at standstill so its speed is equal to zero. At starting, the difference between the speed of the synchronous speed of the rotating magnetic field and the speed of the rotor is maximum. The difference between the synchronous speed and the rotor speed is called slip of the motor.The slip of the motor is;

s= (Ns- Nr)/Ns *100   ———(1)  

Where,  
s = Slip
Ns= Synchronous speed of the motor= 120 f/P
Nr = Rotor speed

As the speed of the rotor is zero at start,the rotor conductor will cut the maximum flux and the maximum voltage will be induced in the rotor.As the motor starts accelerating the speed of the rotor will align in the direction of synchronous speed of the motor and the slip will get reduced.The voltage induced in the rotor conductor can be expressed as;

Er = s* Es ————(2)  

Where, Er = Rotor Voltage
s  = Slip
Es = Stator Voltage

induction motor slip and rotor EMF

At start, the slip of the motor is equal to unity and the induced rotor voltage is equal to the stator voltage.The rotor induced voltage goes on decreasing as motor accelerates towards its base speed.

Er = Es    When Nr =0 & slip=1

It is clear that the induced rotor is maximum at the starting of the motor.

The rotor impedance is highly inductive at starting time.The rotor inductance is ;

Xr= 2πfrL

Xr = 2π(s*fs)L

The rotor reactance depends on the slip. At start, the reactance of the rotor is high because the slip of the motor is equal to unity.  

The impedance of the rotor circuit is;
Zr=Rr+jsωL
The rotor current is ;
Ir= sEs/(Rr+jsωL)
Ir= Es/(Rr/s+jωL)   

The value of Rr/s increase as the slip gets decrease.When the induction motor is started the rotor reactance is more than the rotor resistance and because of the large Xr/Rr ratio the motor takes large inductive current. Moreover, because of the large Xr/Rr ratio, the power factor of the motor is very poor. The equivalent circuit  of induction motor is as given below.

equivalent circuit of induction motor

When the induction motor is switched on, the value of Rr/s is less and the value of Xr is constant, the value of Rr/s increases as the slip of the motor decrease with increase in the motor speed.At start Xr>Rr/s and when the motor starts accelerating the Rr/s becomes more than the Xr and the motor current decreases.

From above discussion, it is clear that at start the rotor is highly inductive and the maximum voltage induces in the rotor hence the rotor draws very large current. The current starts to reduce as the motor speeds up because the ratio Rr/s increase with a decrease in the slip with motor acceleration.

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