Squirrel cage Induction Motors

Squirrel cage Induction Motors

✓ Stator Construction

The induction motor stator resembles the stator of a revolving field, three phase alternator. 

The stator or the stationary part consists of three phase winding held in place.

The slots of a laminated steel core which is enclosed and supported by a cast iron or a steel frame as shown in Figure a.

The phase windings are placed 120 electrical degrees apart and may be connected in either star or delta.

Externally, for which six leads are brought out to a terminal box mounted on the frame of the motor. 

When the stator is energized from a three phase voltage it will produce a rotating magnetic field in the stator core.

Diagram

Rotor Construction

The rotor of the squirrel cage motor shown in Figure b contains no windings. 

Instead it is a cylindrical core constructed of steel laminations with conductor hars mounted parallel to the shaft and embedded near the surface of the rotor core.

These conductor bars are short circuited by an end rings at both end of the rotor core. 

In large machines, these conductor bars and the end rings are made up of copper with the burs brazed or welded to the end rings shown in Figure b.

In small machines the conductor bars and end rings are sometimes made of aluminium with the bars and rings cast in as part of the rotor core. 

Actually the entire construction (bars and end-rings) resembles a squirrel cage, from which the name is derived.

The rotor or rotating part is not connected electrically to the power supply but has voltage induced in it by transformer action from the stator. 

For this reason, the stator is sometimes called the primary and the rotor is referred to as the secondary of the motor.

 

Since the motor operates on the principle of induction and as the construction of the motor with the bars and end rings resembles a squirrel cage, the squirrel cage induction motor is used.

The rotor bars are not insalated from the rotor core because they are made of metals having less resistance than the core. 

The induced current will flow mainly in them. 

Also the rotor bars are usually not quite parallel to the rotor shaft but are mounted in a slightly skewed position. 

This feature tends to produce a more uniform rotor field and torque. 

Also it helps to reduce some of the internal magnetic noise when the motor is running.