When a star delta starter is employed, there can be very high current and torque transients occur during the start.
The problem is two fold. 1) During start, the motor impedance is low, independant of load, until the motor almost reaches full speed. If you change over from star to delta at less than about 85% speed, you will draw almost locked rotor current, resulting in a high start current until the motor reaches full speed. 2) While the motor is connected in star (wye) you have a rotating magnetic field in the stator which is rotating at line frequency. This induces a current in the rotor and developes a torque field at line frequency. When you open the star contactor, you have a magnetised rotor spinning at less than line frequency. The effect of this spinning magnetised rotor inside a stator is that of a generator. The frequency is related to the speed of rotation and is not synchronised to the supply. When you close in to delta, you are closing in on a non synchronised generator so there is a current transient due to the vector addition of the generated voltage and the line voltage. This current establishes a new field in both the stator and the rotor and once the field is established, the current is limited by the effective impedance of the stator at that speed. The current transient can be greater than 2 times the full voltage start current, and the torque transient can be greater than 4 times the full voltage start torque.
The result is more damage to the supply and the driven load than a Direct On Line (Across the line) or full voltage start.
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