Important that the pause between star contactor switch off and Delta contactor switch is on correct. This is because Star contactor must be reliably quenched before Delta contactor is activated. It is also important that the switch over pause is not too long.
For 415v Star Connection voltage is effectively reduced to 58% or 240v. The equivalent of 33% that is obtained with Direct Online (DOL) starting.
If Star connection has sufficient torque to run up to 75% or %80 of full load speed, then the motor can be connected in Delta mode.
When connected to Delta configuration the phase voltage increases by a ration of V3 or 173%. The phase currents increase by the same ratio. The line current increases three times its value in star connection.
During transition period of switchover the motor must be free running with little deceleration. While this is happening "Coasting" it may generate a voltage of its own, and on connection to the supply this voltage can randomly add to or subtract from the applied line voltage. This is known as transient current. Only lasting a few milliseconds it causes voltage surges and spikes. Known as a changeover transient.
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