In a star-delta starter, the motor first accelerates in star, at reduced voltage and current; when the timer runs out, it switches to delta after a short gap. At changeover the motor sees full voltage again and a second current peak appears. How small this peak stays depends on how far the motor has accelerated in star.

  • Early changeover: If the switch to delta happens while the motor is still slow, the current surge is almost as large as with direct-on-line starting and the benefit of star-delta is lost.
  • Late changeover: If the motor has completed its acceleration but is still in star, it tries to carry the load with low torque; current rises again and the motor heats up.

To set it correctly, watch the current with a clamp meter at the first start: the moment when the star current has passed its starting peak and settled is the right point for changeover. The time varies with the inertia of the load; it is longer on machines with large pulleys or flywheels. The short dead time at changeover prevents the star and delta contactors from closing together and causing a short circuit; it must be kept along with the interlocking in the control circuit.

Reminder: star-delta starting can be used only on motors that run in delta on a 400 V supply, i.e. motors marked 400/690 V.