Automatic Star Delta Starter

What is Star Delta Starter?Most small size induction motors are started directly by direct  on line starter, but when very large motors are started with this way, they cause a disturbance of voltage on the supply lines due to large starting current surges.

To limit this starting current surge a large induction motors are started with reduced voltage and when they run up to near full speed ,they connected with the full supply voltage

For reduction of starting voltage two method are used one is star delta starter and another is auto transformer starting .

star Delta Starter

Working Principle of Star Delta Starter

In this method the motor starting voltage is reduced so that it limit the starting current surges.In this technique the motor winding initially connected with Star configuration and due to which its reduced the voltage across each winding.This also reduced the torque of motor by a factor of three.

After specific period of time which is set with the help of timer ,the winding are reconfigured as delta and the motor run normally. Star-Delta starters are the most commonly used in our daily life.

There is only two electromagnetic contactor which are closed during running of motor .These two contactor are named as Main contactor and Delta contactor. The third Contactor which is Star Contactor and that only become On when then motor is initially staring for a specific period of time which is set with the help of timer in automatic star Delta Starter.

Start delta Starter circuit

The current in Star connection is one third of the current in Delta connection.

Star Delta Starter Consist Following Units

The Following unit consist in star Delta Starter

1-Electromagnetic Contactor (Main,Star and Delta Contactors)

2-Timer or Time Relay 1 No

3-Three pole thermal over current Relay 1 No

4-four Pole Circuit Breaker MCCB 1No.

Start Delta Starter Power Circuit

The  main circuit breaker is used to supply initial power to the power circuit of SDS.The main Contactor is connected to the voltage source R ,Y,B to the primary terminal of the motor U1 ,V1 ,W1.

In initial operation, the main Contactor and the Star contactor are closed  and then after a period of time ,the star Contactor is opened and then Delta Contactor is Closed. The control of star and delta is carried out with the help of timer. The star and delta Contactor are electrically interlocked and it will be prefer if it is mechanically interlock

Power circuit SDS

The star Contactor which used to short the motor terminal U2,V2 ,W2 so that it obtain the star Connection of the motor.Due to star Connection the one third   of DOL current is provide to the motor ,thus reducing the high inrush current  inherent with large capacity motors at startup

The controlling and interchanging of star and delta connection is achieved with the help of control circuit

Star Delta starter Control Circuit

The ON button starts the circuit by energizing star contactor coil and timer coil circuit. when the star contactor energized , the star main and auxiliary contactor change its position from NO to NC.

When the Star auxiliary Contactor(which is contacted on main contactor coil circuit) become NO to NC .then its energized the main contactor coil and main contactor’s main and auxiliary contact cange its position from NO to NC.This whole operation will happen in a friction of time.

After pushing ON push button switch ,the auxiliary contact of the main contactor which is connected in parallel across the ON push button will become NO to NC which provide the latch to hold the main contactor coil activated after releasing the ON push Button switch.

Control Circuit of SDS

The Star Contactor remain activated until the timer contact become NC to NO .

Once the timer reached its specific time ,the timer’s auxiliary contact which is connected with star Coil circuit will change its position from NC to NO and at the same time the timer Auxilary Contact which is connected with Delta contactor activated coil will change its position from NO to NC so Delta contactor  Coil  become energized and Delta contactor become On operating the motor with Delta connection.

The normally close auxiliary contact of both the Star and Delta Contactor are interlock with each other so that it prevent to activate both the contactor coil activated simultaneously .

The Control circuit also provide two type of interrupting circuit to shutdown the motor .The OFF push button switch break or shutdown the motor when necessary by pushing .The thermal overload relay also protect and shutdown when the motor is over current  by NC contact becoming NO.

automatic SDS

In some cases it is necessary to change from star connected winding to delta connected winding during starting. Power and control circuit can be designed by  two ways- Open transition or Closed transition.  

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