This starting system can only be used with a motor where both ends of its three stator windings are fitted to a terminal board.
The starting current peak (SC) is divided by 3:
– SC = 1.5 to 2.6 RC (RC rated Current).
A 380V / 660V motor star-connected at its rated voltage of 660V absorbs a current Ö3 times less than a delta connection at 380V. With the star connection at 380V, the current is divided by Ö3 again, so by a total of 3.
As the starting torque (ST) is proportional to the square of the supply voltage, it is also divided by 3:
ST = 0.2 to 0.5 RT (RT Rated Torque)
- The motor speed stabilises when the motor and resistive torques balance out, usually at 75-85% of the rated speed. The windings are then delta connected and the motor recovers its own characteristics.
- The change from
- star connection to delta connection is controlled by a timer. The delta contactor closes 30 to 50 milliseconds after the star contactor opens, which prevents short-circuiting between phases as the two contactors cannot close simultaneously.
- The current through the windings is broken when the star contactor opens and is restored when the delta contactor closes. There is a brief but strong transient current peak during the shift to delta, due to the counter electromotive force of the motor.
- Star-delta starting is suitable for machines with a low resistive torque or which start with no load (e.g. wood-cutting machines). Variants may be required to limit the transient phenomena above a certain power level. One of these is a 1-2 second delay in the shift from star to delta.
- Such a delay weakens the counter-electromotive force and hence the transient current peak.
- This can only be used if the machine has enough inertia to prevent too much speed reduction during the time delay.