Starting Methods of 3 Phase Induction Motor

In this article we discuss about following aspects about starting of induction motor.

• Why we need different starting method to start induction motor?

• Why starting current of induction motor is high?

• Starting method of induction motor?

 

Why does an induction motor need a starter?

 

At starting if we apply rated voltage to stator very high current flows through the rotor circuit as rotor is short circuited it will produces high current in stator which is called inrush of current which may damage the motor. So that’s why we need the starter to reduce the starting current. There are two different ways to reduce the starting current these are

• By reducing E₂ induced emf in rotor (it is applicable in squirrel cage as well as slip ring induction motor.)

• By increasing rotor resistance R₂ rotor resistance (it is not applicable in squirrel cage induction motor)

Why does induction motor draw heavy Current at starting? 

 

In previous article we studied about the working principle of induction motor in which we studied that the induction motor is working on the principle of Faraday’s Law of Mutual Induction which is quite similar to transformer. Generally we can say that induction motor is nothing but a rotating transformer with secondary short circuited. In transformer no load current is quite small where as in induction motor is large i.e. 30% to 50% of rated current due to air gap between stator and rotor. At no load slip of the induction motor is zero. When induction motor is loaded induced emf in rotor sE₂ will increase because slip will increase which will increase the rotor current I₂

Starting methods of induction of motor

 

• Direct On Line Starting

• Stator Resistance Starting

• Auto Transformer Starting

• Star Delta Starting

• Rotor Resistance Starting

DOL Starter

As the name implies, Direct Online Starter is used to start the induction motor at rated voltage. It does not control the starting current and torque consequently. Therefore it is used to start the motor which get quickly accelerated. As motor get quickly accelerated then slip get decreases quickly as slip decreases, induced EMF in rotor is also decreases therefore current get also decreases. DOL starter is basically an assembly of contactor and a protection device (relay, circuit breaker etc.). Contactor is an electromagnetic device which behaves like a switch. Contactor continuously make or break the contact of each phase of motor from supply. Protection device is used to protect the motor from over voltage due to over loading.

DOL Starter is used for the induction motor rating less than 5KW.

Stator Resistance Starter

The basic principle of this method is reduced voltage starting. In this method additional external resistance is connected in series with stator winding. This will cause additional voltage drop across the stator winding due to this voltage induced in the rotor circuit is reduced simultaneously rotor current get reduced due to which starting current get reduced.

This method is used to start squirrel cage as well as slip ring induction motor.

This method is not so popularly used in high rating machine because in this type of starting losses associated with stator increases. 

Autotransformer Starter

The basic principle of this starter is reduced voltage starting. In this method an autotransformer with % tapping is used by which voltage across the stator is reduced correspondingly starting current is reduced.

This method is generally preferred for high rating machine.

This method is economically expensive of because autotransformer.

 

Star Delta Starter

The basic principle of this starter is conversion of delta connection into star, by converting delta into star, voltage across each phase is reduced by √3 times. This method is only used for delta connected motor. In this method, at starting a delta connected motor is intentionally connected in star by use of triple pole double through switch. By which voltage across each phase is reduced by √3 times correspondingly starting current is reduced by √3 times. After some period of starting, machine is reconnected into delta by using triple pole double through switch.

This method is most popularly used in industrial application as it simple cheap and efficient.

This starter is reduced voltage only by √3-times means 57.3%. But autotransformer starter reduced voltage to multiple level. For autotransformer starter with 57.3% tapping, it is equal to star delta starter.

 

Note :- These three methods which we discussed above are reduced voltage starting and can be used for slip ring induction motor as well as squirrel cage induction motor.

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