Synchronous speed
 The speed of the stator magnetic field in the three phase induction motor is known as synchronous speed. The synchronous speed is given by
Where
N_{s }=
Synchronous speed in RPM
f = Supply
frequency and
p = Number of
poles
 The standard frequency in India is 50 Hz therefore the synchronous speed for different pole condition is
Number
of poles ( p )

Synchronous
speed (N_{s} ) in RPM

2

3000

4

1500

6

1000

8

750

Slip
 The difference between synchronous speed and actual speed of the induction motor as a percentage of synchronous speed is known as slip. Let the actual speed of the motor is N RPM.
% S = [ ( N_{s } – N ) / N_{s} ] × 100 %
Slip at starting
 The slip at the standstill condition or starting is unity because the rotor speed at the starting is zero in the three phase induction motor.
S = [ ( N_{s } – 0 ) / N_{s} ]
S = 1
Slip in running condition
 As the induction motor or rotor speed increases, the slip decreases. The value of slip under running condition is always less than unity.
Slip speed
 The difference between stator magnetic field speed or synchronous speed and actual speed of the motor is known as slip speed.
Slip speed = SN_{s}
= ( N_{s } – N )
Rotor speed
 The slip S = [ ( N_{s } – N ) / N_{s} ]
N = N_{s } – SN_{s}
N = ( 1 – S ) N_{s
}
 Therefore the rotor speed is given by N = ( 1 – S ) N_{s }
 The slip at starting is unity i.e. S = 1 therefore the rotor speed at starting is zero.
Significance of name plate speed
 Let us consider that the name plate speed is 1440 RPM. The name plate speed represents the rotor speed.
How to known synchronous speed from name plate speed?
 The nearest synchronous speed of the 1440 RPM is 1500 RPM therefore the synchronous speed of that induction motor is 1500 RPM.
How to know number of poles from name plate speed?
 As the name plate speed is 1440 RPM therefore the synchronous speed is 1500 RPM of that induction motor. There are four poles for 1500 RPM synchronous speed induction motor.
 If the name plate speed is 980 RPM, the synchronous speed of that motor is 1000 RPM. There are 6 poles for 1000 RPM synchronous speed or 980 RPM rotor speed.
How to know slip for a given rotor speed of 1440 RPM?
 As the name plate speed is 1440 RPM therefore the synchronous speed is 1500 RPM of that induction motor. Therefore the slip
% S = [ ( N_{s
} – N ) / N_{s} ] × 100 %
% S = [ ( 1500 –
1440 ) / 1500 ] × 100 %