Star Delta Starter Working Principle and Circuit Diagram:
This star delta starter method of starting of cage induction motors is based upon the principle that with 3 windings connected in star, the voltage across each winding is 1/√3 i.e. 57.7 % of the line-to-line voltage whereas the same winding connected in delta will have full line-to-line voltage across each.
The star delta starter connects the three stator windings in star across the rated supply voltage at the starting instant. After the motor attains speed the same windings, through a change-over switch, are reconnected in delta across the same supply voltage.
The basic diagram of connection is shown in Fig. 1.71(a). An actual starter incorporates under voltage and overvoltage coils, as shown in Fig. 1.71 (b). The star delta starter is also provided with a mechanical interlocking device to prevent the handle from being put in the ‘Run’ position first.
Since at starting instant, the stator windings are connected in star, voltage across each phase winding is reduced to 1/√3 of line voltage and, therefore, starting current per phase becomes equal to Isc/√3.
Starting line current by connecting the stator windings in star at the starting instant = Starting motor current per phase = Isc/√3.
Starting line current by direct switching with stator windings connected in delta = √3 Isc.
Hence by star delta starter line current is reduced to one-third of line current with direct switching.
Hence with star-delta switching, the starting torque is also reduced to one-third of starting torque obtained with direct switching.
This method of starting of cage motors is simple, cheap, effective and efficient since no power is lost in auxiliary components. This method is also suitable for high inertia and long acceleration loads.
This method needs a motor to be delta-connected for normal operation and all the six terminals of the 3-phase stator windings are to be brought out. The reduction in voltage is fixed and starting torque is also low. So this method is limited to application where high starting torque is not the essential requirement e.g. machine tools, pumps, motor-generator sets etc. This method is unsuitable for line voltage exceeding 3,000 V, because of excess number of stator turns required for delta connection.
Such star delta starters are employed for starting 3-phase squirrel cage induction motors of rating between 4 and 20 kW.
Precaution while handling Star delta starter:
The initial current flowing when the motor is started in star is 57.7% of the short-circuit current in delta together with a transient in each phase. The transient currents decay rapidly but the steady state is not reached until the motor has attained 70 per cent of its synchronous speed. The change-over from star to delta connection should not be made until the motor attains about 90 per cent of synchronous speed, otherwise there will be a current surge considerably greater than full-load current which may even be greater than the standstill current with star-connection.