Types of three phase induction motor rotor

There are two types of induction motor rotors:

1. Squirrel-cage rotor or simply cage rotor.
2. Phase wound or wound rotors. The motors that use this type of rotor are known as Slip-ring rotors.

Squirrel cage rotor:

Squirrel cage motor works on the principle of Electromagnetism. It consists of Rotor, Stator and other parts like bearings, cylindrical laminated core, shaft, etc.

The function of bearings in cage rotor motor is to reduce friction between the rotating and stationary parts of the machine. The rotor of the motor consists of a cylindrical laminated core with parallel slots for carrying the rotor conductors. The rotor conductors are not wires, but it consists of heavy bars of copper, aluminum, or an alloy. The shaft is used in the motor to transfer mechanical power from or to the machine. The stator is the outer stationary part of the motor.

Figure: Cage Rotor

The advantages of skewing of cage rotor conductors are:

1. It helps in reducing noise during the operation and production of uniform torque.
2. During locking, the rotor and stator teeth attract each other due to the magnetic field and this locking tendency is reduced in cage motor.

Wound rotor or slip ring rotor:

The wound rotor consists of a slotted armature. Insulated conductors are put in the slots and connected to form a three-phase double layer distributed winding similar to the stator winding. The windings of the rotor are connected in star.

Rotor windings are distributed uniformly and usually connected in the star with here leads brought out of the machine by via slip rings placed on the shaft. The slip rings are tapped using copper carbon brushes. Wound rotor construction is generally used for large size machine, where the starting torque requirements are stringent. External resistance can be added in the rotor circuit through slip ring for reducing the starting current and simultaneously the starting torque.

Figure: Slip ring induction motor

Difference between the cage and wound rotors:

The advantages of the cage rotor are as follows:

• Cage rotors have robust construction and are cheaper than wound rotors.
• These rotors do not have brushes because of which the risk of sparking is reduced.
• It requires very less maintenance.
• They have high efficiency and higher power factor.

The advantages of wound rotors are as follows:

• Wound rotors have high starting torque and low starting current in comparison with cage rotors.
• In the case of wound rotors, we can connect additional rotors in the rotor circuit to control the speed.