Methods Starting of Three Phase Induction Motor

 Methods Starting of Three Phase Induction Motor

The induction motor is fundamentally a transformer in which the stator is the primary and the rotor is short-circuited secondary. 

At starting, the voltage induced in the induction motor rotor is maximum (s=1). Since the rotor impedance is low, the rotor current is excessively large. 

This large rotor current is reflected in the stator because of transformer action. 

This results in high starting current (4 to 10 times the full-load current) in the stator at low power factor and consequently the value of starting torque is low. 

Because of the short duration, this value of large current does not harm the motor if the motor accelerates normally.

However, this large starting current will produce large line-voltage drop. 

This will adversely affect the operation of other electrical equipment connected to the same lines. 

Therefore, it is desirable and necessary to reduce the magnitude of stator current at starting and several methods are available for this purpose.

Methods of Starting Three Phase Induction Motors

The method to be employed in starting a given induction motor depends upon the size of the motor and the type of the motor. 

The common methods used to start induction motors are:

1 Direct-on-line starting 

2 Stator resistance starting
3 Autotransformer starting 

4 Star-delta starting
5 Rotor resistance starting

Methods 1 to 2 are applicable to both squirrel-cage and slip ring motors.

However, method 5 is applicable only to slip ring motors. 

In practice, any one of the first four methods is used for starting squirrel cage motors, depending upon, the size of the motor. 

But slip ring motors are invariably started by rotor resistance starting.

Except direct-on-line starting, all other methods of starting squirrel-cage motors empiry reduced voltage across motor terminals at starting.