A Star-Delta (or Wye-Delta) starter is used in electrical engineering to start larger three phase motors, typically over 4 KW (5 HP).
The Star-Delta starter is chosen, rather than the cheaper D.O.L (Direct On Line) for larger motors, because when a motor is started a surge current is generated for a short initial period. This surge current is typically 5 – 7 times greater than the normal operating current of the motor, and can cause problems due to the fact that suddenly a large load is appearing on the electrical circuit of your ship or building.
If you started a large induction motor, rated at 50 Amps, then the surge current being between 5 – 7 times that figure, would mean a current appearing on your electrical system of between 250 – 350 Amps. This surge current could potentially cause a ‘blackout’ on board a ship, or at least put strain on the generator.
Even on land, large surge currents are undesirable, and many countries require Star (Wye) – Delta starters to be used to start motors rated above 5 HP or 4 KW.
The Star-Delta Starter basically consists of three contactors, a timer, and an OCR (Over Current Relay).
When the Star- Delta is first switched on, the electromagnetic coils in the ‘Line’ & ‘Star’ contactors are energised. The magnetic field produced by the coils (in conjunction with the metal cores fitted) closes the contacts of both contactors. This allows voltage to flow through the line contactor into one side of each of the three (three phase) windings.
The Star contactor is also initially closed, and this creates a ‘star’ winding configuration, by shorting the other side of each of the three windings together.
Having the motor windings connected in Star means that surge current is reduced when the motor initially starts up.
At motor startup, a timer is also started. To be continued………………..
(c) 2013 Craig Miles, all rights reserved.