LIMITATION OF CURRENT OF THREE-PHASE MOTOR

Whenever possible a squirrel cage three-phase motor should be started direct-on-line ( D.O.L. ) by means of contactors. It must be taken into account that for a certain motor the torque and current values are fixed, irrespective the load, for a constant voltage. In cases where the motor starting current is excessively high, harmful consequences may occur:

a ) High voltage drop in the power supply system. Due to that, equipment connected to the system may be affected;

b ) The protection system ( cables, contactors ) must be over designed resulting in higher cost;

c ) Utilities regulations limiting the line voltage drop.

If D.O.L starting is not possible due to these problems, indirect connection system can be used so as to reduce starting current.

• Star-delta switch
• Compensating switch
• Series-parallel switch
• Electronic start ( Soft-Starter )