How Are Squirrel Cage Induction Motors Constructed?

We have seen the classification of A.C. motors and discussed in detail the working principle, concept behind rotor rotation and many other aspects of induction motor. Now let us discuss in detail, the most common form of induction motor, the “Squirrel cage” induction motor. The reason for the name “Squirrel cage” is because of the type of rotor used in these motors. Almost 95% of the induction motors used is of squirrel cage type. And in the previous article on Types of Electrical Motors, we have classified Induction Motors based on the type of rotor used. They are

1. Squirrel Cage Induction Motors &

2. Slip-Ring or Wound-rotor Induction Motors

Any Induction Motor has a Stator and a Rotor. The construction of Stator for any induction motor is almost the same. But the rotor construction differs with respect to the type which is specified above.

Stator:

The stator is the outer most component in the motor which can be seen. It may be constructed for single phase, three phase or even poly phase motors. But basically only the windings on the stator vary, not the basic layout of the stator. It is almost same for any given synchronous motor or a generator. It is made up of number of stampings, which are slotted to receive the windings. Lets see the construction of a three phase stator. The three phase windings are placed on the slots of laminated core and these windings are electrically spaced 120 degrees apart. These windings are connected as either star or delta depending upon the requirement. The leads are taken out usually three in number, brought out to the terminal box mounted on the motor frame. The insulation between the windings are generally varnish or oxide coated.

The Rotor: Squirrel Cage Rotor:

This kind of rotor consists of a cylindrical laminated core with parallel slots for carrying the rotor conductors, which are not wires, as we think, but thick, heavy bars of copper or aluminium (aluminium) or its alloys. The conductor bars are inserted from one end of the rotor and as one bar in each slot. There are end rings which are welded or electrically braced or even bolted at both ends of the rotor, thus maintaining electrical continuity. These end rings are short-circuited, after which they give a beautiful look similar to a squirrel thus the name.

One important point to be noted is that the end rings and the rotor conducting bars are permanently short-circuited, thus it is not possible to add any external resistance in series with the rotor circuit for starting purpose. The rotor conducting bars are usually not parallel to the shaft, but are purposely given slight skew. In small motors, the rotor is fabricated in a different way. The entire rotor core is placed in a mould and the rotor bars & end-rings are cast into one piece. The metal commonly used is aluminium alloy. Some very small rotors which operate on the basis of eddy current, have their rotor as solid steel without any conductors.

1. It helps in reduction of magnetic hum, thus keeping the motor quiet,

2. It also helps to avoid “Cogging”, i.e. locking tendency of the rotor. The tendency of rotor teeth remaining under the stator teeth due to the direct magnetic attraction between the two,

3. Increase in effective ratio of transformation between stator & rotor,

4. Increased rotor resistance due to comparatively lengthier rotor conductor bars, &

5. Increased slip for a given torque.