This article is focused on the discussion the most common form of induction motor, the “Squirrel cage” induction motor. It takes the name “Squirrel cage” from the fact that the type of rotor in these motors is called “Squirrel cage” rotor. Almost 94% of the induction motors used is the type of squirrel cage . And the induction motor can be classified squirrel cage induction motors and slip-ring or wound-rotor induction motors based on the type of rotor used.
Any induction motor contains a stator and a rotor. Any induction motor has the stator of almost the same structure. But the rotor construction differs with respect to the type which is specified above.
The stator is the outer parts in the motor which can be seen, which may form single phase, three phase or even poly phase induction motor. But basically only the windings on the stator vary, not the basic layout of the stator. It is almost the same for any given synchronous motor or a generator. It is made up of number of stampings, which are slotted to receive the windings.
This kind of rotor is composed of a cylindrical laminated core with parallel slots for carrying the rotor conductors, which are not wires, but thick, heavy bars of copper or aluminium or its aluminium alloys. The conductor bars are inserted from one end of the rotor and as one bar in each slot. There are end rings welded or electrically braced or even bolted at both ends of the rotor can maintain the electrical continuity. These end rings are short-circuited, after which they obtains a beautiful look similar to a squirrel to get the name.
There is one important point needed to be noted is that the end rings and the rotor conducting bars are permanently short-circuited, so it is impossible 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 some small induction motors, the rotor is constructed in a different way. The entire rotor core is put in a mould and the rotor bars and end-rings are coupled into one piece. The metal commonly used is aluminium alloy. Some very small rotors which depends on the basis of eddy current, own their rotor as solid steel without any conductors.