Electrical Schematic – Motor Starting System – Direct On-line

The stator is directly connected to the mains supply:

When it is switched on, the motor behaves like a transformer with its secondary, formed by the very low resistance rotor cage, in short circuit. There is a high induced current in the rotor which results in a current peak in the mains supply: Current on starting = 5 to 8 rated Current. The average starting torque is: T on starting = 0.5 to 1.5 rated T.


This system could be used when:

  • the power of the motor is low compared to that of the mains, which limits interference from inrush current
  • the machine to drive does not need to speed up gradually or has a damping device to limit the shock of starting
  • the starting torque can be high without affecting machine operation or the load that is driven.