## 6 January 2018

### 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
Ns = 120f / p
Where
Ns = 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 (Ns ) 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 = [ ( Ns  – N ) / Ns ]
% S = [ ( Ns  – N ) / Ns ] × 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 = [ ( Ns  – 0 ) / Ns ]
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 = SNs
= ( Ns  – N )

### Rotor speed

• The slip S = [ ( Ns  – N ) / Ns ]
N = Ns  – SNs
N = ( 1 – S ) Ns
• Therefore the rotor speed is given by N = ( 1 – S ) Ns
• 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 = [ ( Ns  – N ) / Ns ] × 100 %
% S = [ ( 1500 – 1440 ) / 1500 ] × 100 %
= 4%
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