Star-delta Starter of induction motor
The stator winding of the motor is designed for delta operation and is connected in star during the starting period. When the machine is up to speed, the connections are changed to delta. The circuit arrangement for star-delta starting is shown in Fig. The six leads of the stator windings are connected to the changeover switch as shown.
At the instant of starting, the changeover switch is thrown to “Start” position which connects the stator windings in star. Therefore, each stator phase gets V/√3 volts where V is the line voltage. This reduces the starting current. When the motor picks up speed, the changeover switch is thrown to “Run” position which connects the stator windings in delta. Now each stator phase gets full line voltage V. The disadvantages of this method are:
(a) With star-connection during starting, stator phase voltage is 1/√3 times the line voltage. Consequently, starting torque is (1/√3 )2 or 1/3 times the value it would have with D-connection. This is rather a large reduction in starting torque.
(b) The reduction in voltage is fixed.
This method of starting is used for medium-size machines (upto about 25 H.P.).
Relation between starting and F.L. torques:
In direct delta starting,
Starting current/phase, Isc = V/Zsc where V = line voltage
Starting line current = 3 Isc
In star starting, we have,
Note that in star-delta starting, the starting line current is reduced to one-third as compared to starting with the winding delta connected. Further, starting torque is reduced to one-third of that obtainable by direct delta starting. This method is cheap but limited to applications where high starting torque is not necessary e.g., machine tools, pumps etc.