Electrical Schematic – Motor Starting System- Star-Delta starting

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 principle is to start the motor by connecting the star windings at mains voltage, which divides the motor’s rated star voltage by Ö3 (in the example above, the mains voltage at 380V = 660V / Ö3).

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.