Starting of Three Phase Induction Motor & Methods for Starting


Necessity for Starting an Three phase Induction Motor :


     In a 3-phase Induction motor, magnitude of rotor induced emf is depends upon slip.

Rotor current,


     At starting, the magnitude of rotor induced emf and slip will be maximum. This emf circulates very high current through the rotor. The induction motor acts as a transformer having short circuited secondary at start. Hence a rotor current is high, consequently stator draws a very high current of the order of 5 to 8 times full load current.


     Due to such high current at start there is possibility of damage of the motor winding and can generate large line voltage drop, which may effect the working of other equipment's connected to the same line. Therefore, starter is necessary to limit the current drawn by motor at start.


     Starters not only limit the starting current but also provide protection to the motor against over loading and low voltage conditions. 



Advantages of Using Starters :


     The functions of starter are

1. To start and stop the motor.

2. To limit the starting ( or in rush ) current, when necessary.

3. To permit automatic speed control, when required and

4. To protect the motor and other connected equipment's from sustained overload. low voltage etc. conditions.



Methods of Starting an Three Phase Induction Motor :


    From the expression of current, it can be seen that starting current can be limited to safe value 

(a) By applying reduced voltage to the stator consequently reduces E2 or

(b) By adding resistance in the rotor circuit at start 


     The various types of starters are 


(i). Direct on line ( D.O.L ) starter ( full voltage starting )


     For small capacity motors having rating less than 5 hp, the motors can withstand high starting currents due to robust construction. Motors are directly switched on supply lines hence the starters is called as Direct online starter.


(ii). Auto transformer starter


     The voltage applied to the stator can be reduced to a desired value by using a 3-phase star connected auto transformer. It contains a suitable change over switch.    


(iii). Star-delta starter ( Reduced voltage starting )


     It was TPDT ( Triple pole double through switch ) which connects the stator winding in Star at start and then in Delta while normal running. Hence this starter is suitable only for motors which are designed to run with a delta connected stator winding.


(iv). Rotor resistance starter ( Used for slip ring motors )


     In this method stator current can be limited by introducing a variable resistance in the rotor circuit.


     First three starters are used for both squirrel cage and slip ring induction motor. It is therefore recommended that large three-phase squirrel-cage induction motors be started with reduced voltage applied across the stator terminals at starting. Small motors below 5 hp ratings may however be started direct-on-line ( D.O.L ). 



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