Star Delta starter comes under the category of starters using reduced voltage method for starting the motor. Just like all other starters, the basic aim to use this starter is to decrease initial high starting current generated by the motor at the starting procedure. It is used for the purpose of limiting the voltage drop and interference in the main voltage line.
One way of reducing the voltage at the starting time of motor is to configure its windings in star delta connection. When the motor is started, the windings of motor is configured in star mode. This star mode decreases the voltage by 1/√3 of the normal applied voltage. As soon as the motor picks up the speed, the feeding supply of the motor is reverted back to normal supply.
In Star delta starter, each of the 2 points of the phases of the winding is connected to the terminals of starter. As we have already discussed, the motor windings are configured in star connection in starting phase and converted to delta connection in the normal running phase. The voltage between every phase at the time of starting of motor is 57.7 % (1/√3) of the applied voltage . This in turn results in decrement of the starting current. It also reduces the torque applied to the motor in starting to the 1/3 of the value which we may get in case of direct online starter.
This type of starter (star delta starter) is normally used to start big motor having high drive loads. The star delta starter is the most popularly used starter after direct on line starter.
If we see the below mentioned figure, we can observe that the main parts constituting Star delta starter are :
- Contactors ( 03 in no)
- Timer ( 01 in no), and
- Bimetallic OLR (Over load relay) (01 in no)
The contactors which are used in Star delta starter are small in size as compared to the the only contactor of D.O.L. starter. It is because in case of Star delta starter, the contactors have to handle only the winding current which is 57.7 % (1/√3) of the normal load current.