Why the rotor winding of three phase wound rotor induction motor is star connected?

The stator can either have a star or delta connection. Many motors have a winding scheme and coils with six connections that connect the windings in either star or delta. The rotor of a squirrel-cage induction motor has many rotor bars connected at both ends. That is essentially a delta connection because it is the only way the rotor bars can be connected to form a closed circuit. On the other hand, a wound rotor always has a star connection because that allows the external connection of resistors in series with the winding.

The advantages of Star Connection are as follows.

• The phase voltages in the star connection are 57.7 % of the line voltages, i.e., the rotor winding in the star connection is less exposed to voltage as compared to the delta connection, which in turn proves more economical if we consider insulation, breakdown strength, the requirement of conductor material, etc.
• Availability of neutral in the star connection: if the neutral is grounded, then it also provides a path for the Zero-Sequence currents during faults, whereas, in the delta connection, the zero-sequence currents flow within the delta circuit, hence increasing the load on the winding.
• Star connection reduces the number of slip rings required to connect the external resistance to the rotor of the induction motor. With a star connection, only 3 slip rings are required. Whereas in a delta-connected rotor, 6 slip rings are required (2 for each phase)
• Vline= √3 Vph or Vph=Vline/√3 in a star connected, i.e., Vph is Reduced to √3 times the line voltage in a star-connected system. Now, Voltage is related to insulation, and insulation is related to cost, i.e., the higher the voltage, the higher the insulation and the higher the cost.
• In the star connection, additional external resistance may be inserted in the rotor circuit to increase the starting torque and decrease the starting current. As the motor speeds, these external resistances are cut out of the rotor circuit.