Star Delta starter for 3 phase motors
Why to use star delta starter for motors?
Starting large 3 phase motors directly online, causes a disturbance of voltage on the supply lines due to large starting current which affect other loads
To start these motors a voltage reduction starters are used, and then the motor have full supply voltage reconnected.
Star Delta Principle of working:
At the motor starting the motor windings are re configured to be connected as star, then re configured to be delta connected
This reduces voltage at starting to be just winding voltage which is 58% of the line and motor rated delta voltage
N.B: Reducing the voltage also reduces the torque by 1/3 of motor torque
The star delta starter is made by three contactors, one timer and a thermal overload for motor protection
Contactors size are smaller than the direct on Line single contactor used in a circuit because the fact that they are controlling winding currents only
The currents through the motor winding are (58%) of the line current
Main and delta contactor of star delta starter circuit are AC3 with 58% of the motor current rating
The star contactor carries star current only, current in the star configuration is 1/3 of the full voltage (delta) current of the motor
Therefore, sizing of the star contactor is 1/3 of sizing for direct on line
