Rotor of 3 phase induction motor

It is a similar to rotor of a DC machine with semi-closed slots depending on the type of rotor and winding induction motor are classified into two types:-

1. Squirrel cage                                                                                                                                              
2. Slip ring /wound rotor

Squirrel cage rotor:- Squirrel cage rotor use a laminated cylindrical core having parallel slots on its outer Periphery . Copper or aluminium bar is placed in each slot. All bar these are placed in the slots thars are joined by metal rings called end Rings are permanently short circuited winding which is indestructible. The rotor is not connected electrically to the electrical supply but has current induced in it by Transformer action from the stator winding.

Squirrel case Rotor

These induction motor which employs squirrel cage rotor are called squirrel cage induction motor. we are most of the times used squirrel cage rotor in 3-phase induction motors because it has a remarkably simply and robust construction enable it to operate in the most adverse circumstances. That's why it suffers from the disadvantage of a low starting torque it is because of rotor bars are permanently short-circuited and it is not possible to add any external resistance with the rotor circuit to get a high starting torque.

Slip ring /wound rotor:- Wound rotor consists of a laminated cylindrical Core and carries a three phase winding similar to the one on the stator. The rotor winding is uniformly distributed in the slots in the entire rotor and is usually star-connected. The open ends of the rotor winding are brought out and joined to three insulated slip ring mounted on the rotor shaft with one brushed resting on each slip ring. The three brushes are connected to 3-phase star-connected Rheostat. At starting the external resistance is added in the rotor circuit to give a large starting torque. These resistance are gradually (step by step) reduced to zero as the motor run up to speed. the external resistance are used during starting period only. when the motor attend normal Speed, the Three Brushes are short-circuited so the wound rotor run like a squirrel cage Rotor.

Slip ring Rotor

 Operating principle:- When a three phase supply is given to stator it produce rotating magnetic field at (Ns)synchronous speed. It cuts the rotor with relative speed  (Ns - 0) and produce voltage, current and torque. The rotor rotates in the same direction to the magnetic field according to lenz's law. actually it went to to catch synchronous speed but could not catch it and sleep back with the speed known as slip speed and run near to the synchronous speed the operating principle is mutual induction quit Similar to Transformer it is singlely excited machine.