Page 98 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

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.
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: This consists of two sets of conductor bars called outer and inner cages
as shown in Fig 4. 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 bars 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. As such 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.
Fig 5 shows the exploded view of 3 phase squirrel cage induction motor.

Slip and rotor speed: The speed at which the rotor rotates is called the rotor speed or speed of the motor. The
difference between the synchronous speed and the actual rotor speed is called the `slip speed’. Slip speed is the
number of revolutions per minute by which the rotor continues to fall behind the revolving magnetic field.
When the slip speed is expressed as a fraction of the synchronous speed, it is called a fractional slip.

\ fractional slip S = N s - N r
N s
N s N - r
Then percentage slip (% slip) = x 100
N s

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