Constructional Features of Induction Machines : Rotor Construction

Stator construction of Induction motor is already discussed in my article Stator of Induction Motor and in my current article I'll discuss construction of Rotor for three phase induction motor.

Rotor

The building construction of rotor are somewhere similar with that of stator. It is also made up of thin laminated silicon steel sheets of low losses and high permeability. There are two types of rotor commonly used for Induction motor. These are :

1. Squirrel Cage Rotor
2. Slip Ring Rotor

Squirrel Cage Rotor

This is also termed as Cage rotor. It consists of a cylinder shape rotor made up of laminated thin sheets of silicon steel. In such rotor slots to place winding are cut nearly parallel to the shaft but with an angle of inclination. Each slot contains an insulated conductor either of Aluminum (small rating machines) or Copper (Large rating machines) is inserted which is connected to other conductors with the help of end rings (made up of conducting material same as that of conductor but heavy in size) to short circuit the rotor bars and to produce rotor current.

Slip Ring Rotor

Slip ring rotor is a bit different from Squirrel cage rotor. In slip ring type rotor, rotor conductors are not short circuited at the end with the help of end rings but are taken out with the help of slip rings. These slip rings are mounted on the shaft and external resistances may be connected to these slips rings as shown below in the diagram and hence rotor resistance may be varies as per our requirements.

Comparison of Cage and Slip Ring Rotors

We have seen both type of rotors of Induction motors. These are Squirrel cage rotor and Slip ring rotor. Now its time to compare both these rotors, which one is better lets see.

Advantages of Squirrel Cage Rotor

• It has simple construction so easy to construct/manufacture.
• It is cheap as compare to slip ring rotor.
• It require less maintenance as no fraction is there.
• Frictionless rotor results in higher efficiency.
• It has no contacts so low chances of sparking

Advantages of Slip Ring Rotor

• High starting torque may be obtained by adding resistance.
• High starting resistance means low starting current.
• Speed control with change of rotor resistance is possible.