Why the rotor winding of three phase wound rotor induction motor is star connected?
The stator can be either star or delta. Many motors have a winding scheme and coils with six connections that allow the windings to be connected in either star or delta. The rotor of a squirrel-cage induction motor has the number of 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 star connection are 57.7 % of the line voltages, i.e. the rotor winding in star connection is less exposed to voltage as compared to the delta connection which in turn proves more economic 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 and 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 star connection, only 3 slip rings are required. Whereas in delta connected rotor 6 slip rings are required (2 for each phase)
- Vline= √3 Vph or Vph=Vline/√3 in 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 higher the voltage, higher the insulation and higher the cost.
- In star connection, additional external resistance may be inserted in the rotor circuit at starting to increase the starting torque and decrease the starting current. As the motor gains speed, these external resistances are cut out of the rotor circuit.