Page 96 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

The resultant stator field will rotate further 60’0. So that from the resultant flux, position 1-7 it is obvious that for
each cycle of applied voltage the field of the two pole stator will also rotate one revolution around its core. It
will clear that the rotating magnetic field could be produced by a set of 3 phase stationary windings, placed at a
120’electrical apart and supplied with a 3 phase voltage.
Classification
Three phase induction motors are classified according to their rotor
construction
Squirrel cage induction motor
Slip ring induction motor
Squirrel cage motors have a rotor with short circuited bars whereas slip
ring motors have wound rotors having three windings either connected
in star or delta,

Parts
Stator of an induction motor
There is no difference between squirrel cage and slip ring motor stators. The induction motor stator resembles the
stator of revolving field three phase alternator. The stator part consists of three phase winding held in place in the
slots of a laminated steel core which is enclosed and supported by a cast iron or a steel frame. The three phase
windings are placed 120 degree 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
energised from a three phase voltage it will produce a rotating magnetic field in the stator core.
Rotor of a squirrel cage induction motor
The rotor of the squirrel cage induction motor contains no windings. Instead it is a cylindrical core constructed
of steel laminations with conductor bars mounted parallel the shaft and embedded near the surface of the rotor
core. These conductor bars are short circuited by an end ring. At either end of the rotor core. On large machines
these conductor bars and the end rings are made up of copper with the bars brazed or welded to the end rings. In
small machines the conductor bars and end rings are made of aluminium with the bars and rings cast in as part of
the rotor core. The rotor bars are not insulated from the rotor core because they are made of metals having less
resistance then the core. The induced current will flow mainly in them. Also the bars are usually not quite parallel
to the rotor shaft but are mounted in slightly slewed 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.
End shields

The function of the two end shields which are to support the rotor Shaft. They are fitted with bearings and attached
to the stator frame with the help of studs or bolts.
Double squirrel cage induction motor.

Rotor construction and its working
The outer cage consists of bars of high resistance metals like brass and is short circuited by the end rings. The
inner cage consists of low resistance metal like copper and is short circuited by the end rings. The outer cage has
high resistance and low reactance whereas the inner cage has low resistance but being situated deep in the rotor
core has a large ratio of reactance to resistance.at the time of starting the rotor frequency is the same as the stator
frequency. Hence the inner cage which has higher inductive reactance offers more resistance to the current flow.
A very little current flows through the inner cage at the time of starting.

The major part of the rotor current at the time of starting could flow through the outer ring which has high
resistance. This high resistance enables to produce a high starting torque. As the speed increases the rotor
frequency is reduced. At low frequency the total resistance offered for the current flow in the inner cage reduces
due to reduction of reactance and the major pare of part of the rotor current will be in the inner cage rather than
in the highly resistant outer cage.

83

CITS : Power - Electrician & Wireman - Lesson 70-75