Star-Delta starter is an electrical motor starting device, generally uses in big size motor to overcome high motor currents during starting. Star and Delta means here 2 separate states of motor running, first Star connection and then Delta connection.
The starting current of any heavy electric motor can be more than 4 times the normal load current it draws when it has gained speed and has reached its normal running condition. To overcome this initial high current enchanting problem, such arrangement needs Star connection at starting time and if Star connection has sufficient torque to run up to 75% to %80 of full load speed, then the motor can be connected in Delta mode.
When the motor connected to Delta configuration the phase voltage increases by 173% and phase currents increase by the same ratio. The line current increases three times its value in star connection.
What Problem Starting Motor without Star-Delta?
We already know, if the motor starts simply when connected in Delta, the starting current would be huge and just to be able to start the motor, not to running condition.
To do this it would require –
- Large capacity circuit breakers to allow the start-up surge current to pass without immediately shutting the motor off.
- Over sized 3-phase power service cables require just for starting time, but normal running time not necessary.
- Large size of coils and contacts on the relays or contractors need to control the motor, but normal running time need smaller sizes of them.
Sometimes, utility company also not permits big motor starting without Star-Delta starter because of system instability due to switching transient.
What Important During Star-Delta Transition Period?
It is important that the break between Star contractor switch OFF and Delta contactor switch is ON, because Star contactor must be reliably quenched before Delta contactor is activated. During transition period of switch-over the motor must be free running with little deceleration. It is also important that the switch over pause is not too long, it may generate a voltage of its own and this may add to or subtract from the applied line voltage.
Star Delta Starter
The Star Delta Starter is a very common type of starter and is used extensively as compared to the other type of starting methods of the induction motor. A star delta is used for a cage motor designed to run normally on the delta connected stator winding. The connection of a three-phase induction motor with a star delta starter is shown in the figure below.
When the switch S is in the START position, the stator windings are connected in the star as shown below.
When the motor picks up the speed, about 80 percent of its rated speed, the switch S is immediately put into the RUN position. As a result, a stator winding which was in star connection is changed into DELTA connection now. The delta connection of the stator winding in shown in the figure below.
Firstly, the stator winding is connected in star and then in Delta so that the starting line current of the motor is reduced to one-third as compared to the starting current with the windings connected in delta. At the starting of an induction motor when the windings of the stator are star connected, each stator phase gets a voltage VL/√3 . Here VL is the line voltage.
Since the developed torque is proportional to the square of the voltage applied to an induction motor. Star delta starter reduces the starting torque to one-third that is obtained by direct delta starting.