This is a starting method that reduces the starting current and starting torque. Star delta starter design normally consists of three contactors, an overload relay and a timer for setting the time in the star-position (starting position).
For the star delta starter, a motor must be in delta connected during a normal run and the main purpose is to be able to use star delta starter.
Star delta starter received the starting current is about 30 % of the starting current during direct on line start and the starting torque is reduced to about 25 % of the torque available at a D.O.L start.
Star delta starter only works when the application is light loaded during the start. If the motor is too heavily loaded, there will not be enough torque to accelerate the motor up to speed before switching over to the delta position.
Description of Star Delta Starter Operation
For star delta starter,the basic function is to enable the motor to start and the motor windings are configured in a star formation to the supply voltage.
The voltage applied for star delta starter to the individual motor winding is therefore reduced by a factor of 1√3 = 0.58 this connection amounts to approximately 30% of the delta values. The starting current is reduced to one third of the direct starting current.
Due to the reduced starting torque, the star-delta-connection is suitable for drives with a high inertia mass but a resistance torque which is low or only increases with increased speed. It is preferably used for applications where the drive is only put under a load after run-up.
After motor run-up, in most cases an automatic timing relay controls the switch-over from star to delta. The run-up using star connection should last until the motor has reached the approximate operational speed.
so that after switching to delta, as little post acceleration as possible is required. Post-acceleration in delta connection will instigate high currents as seen with direct on-line starting.
The duration of start in star connection depends on the motor load. During delta connection, the full mains voltage is applied to the motor windings.
To enable a switch-over from star to delta, the six ends of the motor winding are connected onto terminals. The contactors of a star-delta starter switch over the windings accordingly.
How star delta starter works?
Starting in star, the main contactor connects the mains to winding endings U1, V1, W1.The star contactor shorts winding endings U2, V2, W2.
After successful run-up, the star contactor switches itself off and the delta contactor connects terminals U1/V2, V1/W2,W1/U2.
When changing from star to delta, we should pay attention to ensure it running in the correct phase sequence.Incorrect phase sequence can lead to very high current peaks during the cold switch-over pause, due to the easy torque reduction following re-start.
High peaks current can damage the motor windings and stress the control gear unnecessarily. The rotation of the motor has to be considered as well.If wrong rotation,we should change the rotation by switch the phase.I discuss detail about How to change star delta starter motor rotation in my separate post.
For star delta starter circuit diagram,wiring technique and motor base termination,Please read my post for star delta motor connection.Also for simple star delta control circuit wiring and types of star delta starter,please refer to my post for Electro mechanical star delta.
For more detail,please refer to ELECTRIC MOTOR HANDBOOK :-