At the moment of starting, an induction motor behaves like a transformer with a short-circuited secondary. While the rotor is stationary, the stator field cuts the rotor bars at full supply frequency; a large current flows in the rotor and the stator draws a high current from the supply. As the motor accelerates, the speed difference between rotor and rotating field (slip) shrinks, rotor current decreases and supply current falls to the rated value.

A catalogue example: the 4-pole 11 kW 160M-4 model has a rated current of 20.4 A at 400 V and a starting current ratio Is/In of 7.7. With direct-on-line starting, the starting current is roughly 20.4 × 7.7 ≈ 157 A. This current lasts for the duration of the start; with a heavy load and slow acceleration, it lasts longer.

Practical consequences:

  • Line voltage dips during starting; other equipment on the same transformer may be affected.
  • Fuses and circuit breakers must have a characteristic that withstands the starting current for a short time.
  • Frequent starts increase the heat accumulated in the winding.

Starting current is not a defect but a physical property of the induction motor; on larger motors star-delta, a soft starter or an inverter is used to limit it. For values by model, see our motor current table.