INDUSTRIAL AUTOMATION | Draw star/delta starter circuit diagram for 3-phase induction motor semi-automatic type. Explain its working.

 Semiautomatic Star-Delta Starter for 3- Induction Motor: 

The power and control circuit of Semiautomatic Star-Delta starter for 3-

induction motor is shown in the figure (a) and (b) respectively. In power circuit,

three contactors are required to connect motor to supply and to connect its

windings first in Star and then in delta. 

Referring to the control circuit, the motor is operated in following sequence:

i) When ON push-button is pressed, the Star-contactor coil S is energized through  normally-closed OFF push-button, pressed ON push-button and NC contact D. Therefore, Star contactor S is opearted and motor windings are connected in star. 

ii) After energizing Star-contactor S, the NO contact S1 get closed and the Main-contactor M get energized. In power circuit, the closing of M contactor connects the motor to supply and it is started as star-connected motor. The M contactor is held ON through the closed NO contact M1.

iii) The motor continues to run in star-connection as long as ON push-button is kept pressed. 

iv) When it is ensured that the motor has picked up 75% of its rated speed, the pressed ON push-button is released. The opening of ON push-button causes interruption of current in coil S and Star-contactor is de-energized. Therefore, the contact S is opened, however contactor M is held ON through closed NO

contact M1 

v) After de-energizing Star-contactor S, the motor winding is disconnected from star-connection and due to closing of NC contact S2, the Delta-contactor  D is energized. 

vi) On energizing contactor D, the motor is reconnected in delta-connection. Thus motor then continues to run as delta-connected motor. 

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