Star Delta

[sh69]

Hello friends,

Can you please let me know the difference between open transition and close transition in star-delta starters?

Thanks for your responses in advance.

 

[Marke]

Hello sh69

There have been a number of disscusions recently on the merits and characteristics of star delta starters. On of the problems associated with the classic star delta starter is that in operation, the motor is first connected in star producing 1/3 full voltage torque at one third full voltage current. The motor accelerates up to full speed, or the speed where the shaft torque is eaual to the load torque, and then the starter, after timing out, changes to delta by first opening the star contactor, and then closing the delta contactor. When the star contactor is open, current is not longer able to flow through the windings because one end is open circuit. At this time, we have a rotating magnetic field from the rotor spinning within the stator. This causes the motor to act as a generator. When the delta contactor closes, we are closing on to the some of the instantaneous line voltage plus generated voltage. This causes transient torque and currents that are much worse than full voltage conditions.
The open transition stage can be eliminated by adding an auxiliary delta contactor across the delta contactor with large resistors in series. Functionly, the sequence is to operate in star, then close the aux delta contactor, the open the star contactor, then close the delta contactor. To be effective, the current flow through the windings via the aux delta contactor must be significant so the resistors must be similar to those you would use for a primary resistance starter. Small radio type resistors do not provide the required curent.
Best regards, Mark Empson

http://www.lmphotonics.com