Function And Necessity Of A.C. Starters

Function Of Starter

Why we use Starter in Induction Motors?

The device which is used to start the 3-phase induction motor is called a starter. The function of the starter is to limit the starting high current to a safe value.

Necessity Of Starter 

Rotor current at standstill, 

The magnitude of E₂ depends upon the flux linking with the rotor conductors and its relative speed. The strength of the rotor flux depends upon the applied voltage. 

At the instant of applying rated voltage to the stator winding, rotor is stationery and as such the slip is unity. So if full rated voltage is given to the stator winding, then the magnitude of the emf induced in the rotor conductors will be high, because the relative speed between the rotor conductors and stator revolving flux is very high i.e. equal to the synchronous speed of the stator flux. Further the rotor conductors are short circuited and thus have low impedance.

Hence, the current drawn by the stator winding or motor is very large, approximately 5 to 7 times the full load current.

The effects of high starting current are as follows:

The copper losses occuring in starter and rotor windings due to high starting current are extremely large, which would produce a lot of heat inside the machine and may damage the insulation of the windings. Moreover, there will be a dip in the supply voltage.

The starting current drawn by the motor can be reduced to a permissible value by 

1. Applying reduced voltage to the stator winding.
2. Inserting the resistance in the stator circuit. 
3. Inserting the resistance in the rotor circuit.

In the case of slip ring induction motors it is possible to add external resistance in rotor phases as wound rotor has 3-phase star connected winding to limit the starting high current.

However in the case of squirrel cage motors, the rotor has copper bars, which are short circuited at both ends by end rings. Hence it is not possible to add any external resistance in series with the rotor circuit for starting purpose. 

So in the case of squirrel cage induction motors, the initial high current is controlled by applying reduced voltage to the stator winding during the starting period and the full normal voltage is applied when motor has picked up speed to about 70 to 80% of its normal speed.

Methods Of Starting For 3 Phase Induction Motors

Starting current of three phase induction motors can be reduced to a safe value by following methods:

For squirrel cage induction motors:

(1) DOL (Direct Online Starter)

(2) Primary Resistance Starter

• Manual Primary Resistance Starter
• Automatic Primary Resistance Starter

(3) Auto Transformer Starter

• Manual Auto Transformer Starter
• Automatic Auto Transformer Starter

(4) Star - Delta Starter

• Manual Star - Delta Starter
• Automatic Star - Delta Starter

For slip ring induction motors:

(1) Rotor Resistance Starter

• Manual Rotor Resistance Starter
• Automatic Rotor Resistance Starter