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Closed Transition Wye Delta starter


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Hi Tom,


Welcome aboard!


You are probably aware that under a standard star/delta arrangement, the motor is temporarily disconnected from supply during the transition from star to delta.


When the motor is reconnected to supply (through energising the delta contactor) the generated voltage from the spinning motor is not necessarily in phase with the supply voltage.


The worst case scinareo is that the two voltages add together thus exposing the motor to a maximum of twice supply voltage for a very short period of time, eg 40 milliSeconds. This causes a current transient of twice locked rotor current (typically 12 - 16 times motor rated current) and a torque transient of 4 times locked rotor torque. It is therefore fair to say that a star/delta starter causes more severe electrical and mechancial stress than Direct on Line or Full Voltage Starting.


The idea behind closed transition star/delta is to ensure the motor remains connected to supply (via resistors) during the transition from star to delta. This will act to greatly reduce the current and torque transients mentioned above to more acceptable levels.


Hope this helps.






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