Page 103 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

Slip and rotor speed: The speed at which the field rotate is called synchronous speed. It depends upon the
frequency of the supply and the number of poles for which the stator is wound

Hence
120F
NS =
P
Ns = synchronous speed in rpm

P = no of poles
F = frequency of the supply
Actual speed: The speed at which the rotor rotate is called rotor speed or speed of the motor.
SLIP
Absolute slip (slip speed): - The difference between the synchronous speed and actual rotor speed is called the
slip speed
S = Ns - Nr
Fractional slip – when the slip speed is expressed as a fraction of the synchronous speed it is called fractional slip
(Ns-Nr)
Fractional slip S = Ns
Percentage slip (%) -the slip is expressed in percentage then it is called percentage slip
(Ns-Nr)x100
S = Ns

Rotor frequency
The frequency of the rotor voltage is called rotor frequency. It is equal the product of the slip and the stator
(supply) frequency
Rotor frequency Fr = fractional slip x stator frequency
(Ns-Nr) X S
Fr =
Ns
Fr = S x F
At the time of starting rotor is at rest and slip will be equal to one. The rotor frequency will be the same as the
stator frequency. When the motor is running at high speed slip will be low and the frequency of the rotor will also
be low.
Torque
The twisting moment developed in the rotor shaft by the interaction of the magnetic field of stator by rotor is called
torque. In the dc motor the torque is proportional to the flux per pole and the armature current. Similarly, in the
induction motor the torque is proportional to the pole, rotor current, and also the rotor power factor.

Stating torque - the torque available in the rotor during starting is called starting torque.
Torque = K x stator flux x rotor flux x rotor power factor.
Tα Ir ФcosФ2
At starting rotor is stationary so S= 1
Rotor reactance Xr =2πsfL
A large starting torque rotor resistance should large
Voltage of the machine is reduced at starting the torque will be considerably reduced
Starting torque is directly proportional to applied square of the voltage
A maximum torque for a given machine for given speed rotor resistance must be small
Starters
Necessity of starter: If normal voltage is applied to the stationary motor, then as in the case of a transformer
very large initial excessive current will be drawn by the motor from the mains. This initial excessive current is
will produce large line voltage drop which in turn will affect the operation of other electrical equipment and lights

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