Star-delta starting reduces starting current, but it reduces starting torque by the same ratio, to about one third. That is why it does not work on every machine. The main cases where it is unsuitable:

  • If the nameplate says 230/400 V: this motor already runs in star on a 400 V supply; the delta step would overvolt the winding.
  • If the load needs high starting torque: the catalogue gives Ts/Tn = 2.2 for the 11 kW 160M-4; in star this falls to roughly 0.7 times rated torque. A compressor starting under load, a full conveyor or a crusher stopped with material cannot accelerate with this torque; the motor hangs in star and draws DOL-level current at changeover.
  • If mechanical shock is a problem: the sudden torque jump at changeover strains belts, couplings and gearboxes.
  • If starts are very frequent: every changeover means an extra current surge and extra heat.

In these cases, starting with the load relieved (such as an unloaded start arrangement on a compressor), direct-on-line starting, a soft starter with an adjustable ramp or an inverter is considered. The data needed for heavy-load starting is in our article starting under load information; compressor applications are on the compressor motors page.