There are several reasons why the rotor winding of a three-phase wound rotor induction motor is typically star connected:
- To reduce the starting current: The star connection reduces the starting current compared to a delta connection, as it reduces the impedance of the rotor circuit. This can be helpful in reducing the mechanical stress on the motor during starting and also reducing the voltage drop in the supply system.
- To improve the power factor: The star connection also improves the power factor of the rotor circuit, as it reduces the phase angle between the rotor current and the rotor voltage. This can help to improve the overall efficiency of the motor.
- One reason is that the star connection reduces the voltage exposure of the rotor winding compared to a delta connection. As you mentioned, the phase voltages in a star connection are 57.7% of the line voltages, which means that the rotor winding is less exposed to voltage. This can be beneficial in terms of the insulation required for the rotor winding and the breakdown strength and conductor material needed. A lower voltage means that the insulation and other components can be smaller and less expensive, making the motor more economical.
- Another reason is that the star connection provides a neutral point, which can be grounded if desired. This allows for a path for zero-sequence currents to flow during faults, rather than having those currents flow within the delta circuit and potentially increasing the load on the winding.
- The star connection also reduces the number of slip rings required to connect the induction motor's external resistance to the rotor. With a star connection, only three slip rings are needed, while a delta connection requires six slip rings (two for each phase). This can simplify the design and reduce the cost of the motor.
Finally, the star connection allows for the insertion of additional external resistance in the rotor circuit at starting, which can increase the starting torque and decrease the starting current. As the motor gains speed, these external resistances can be cut out of the rotor circuit, allowing the motor to operate at its normal speed. This can be useful in applications where a high starting torque is required.