Direct-switching or Line starting of Induction Motors
Direct-switching or Line starting of Induction Motors:
This method of starting in just what the name implies—the motor is started by connecting it directly to 3-phase supply.
The impedance of the motor at standstill is relatively low and when it is directly connected to the supply system, the starting current will be high (4 to 10 times the full-load current) and at a low power factor. Consequently, this method of starting is suitable for relatively small (up to 7.5 kW) machines.
Relation between starling and F.L. torques. We know that:
If Ist is the starting current, then starting torque (Tst) is
If If is the full-load current and sf is the full-load slip, then,
When the motor is started direct-on-line, the starting current is the short-circuit (blocked-rotor) current Isc.
Suppose Isc = 5 If and full-load slip sf =0.04. Then,
The starting current is as large as five times the full-load current but starting torque is just equal to the full-load torque. Therefore, starting current is very high and the starting torque is comparatively low. If this large starting current flows for a long time, it may overheat the motor and damage the insulation.