What is Synchronous speed? Relationship between speed and Frequency

Synchronous Speed

When three phases which are 120 degree-electrical apart are fed to the stator winding of the induction motor, a rotating magnetic field is produced which rotates in the space and time. The speed of the rotating magnetic field is called the synchronous speed of the motor. The synchronous speed is denoted as Ns. The synchronous speed can be expressed by the following mathematical expression.

Synchronous speed Formula

 
synchronous speed formula
The synchronous speed of the motor depends on;
  1. Frequency of the supply source 
  2. Number of poles of the motor
The synchronous speed of the motor remains constant if the number of poles and the frequency are constant. The synchronous speed of the motor can be changed by changing the number of poles or by changing the frequency. The machine which runs at synchronous speed is called synchronous machine.
 
The synchronous speed of the motor having different number of poles are as given below.

 

 
No. of Poles
 
 
Frequency    (50 Hz)                             
 
Frequency(60 Hz)          
2
3000
3600
4
1500
1800
6
1000
1200
8
750
900
10
600
720
12
500
600
16
375
450
20
300
360


Relationship between Speed and Frequency

The frequency of the emf generated in the stator depends on the frequency at which poles are  rotating. In a one complete cycle of the voltage generated, a pair of field poles passes over the coil.

Let the speed of the rotating magnetic field is N revolution per minute(RPM).


n = N/60 revolution per second

Revolution/rotation per second = f

In a one complete cycle one pole pairs passes over the coil. Therefore, the frequency of one pole pair passing is N/60. In a one complete cycle P/2 poles passes over a coil.

Number of poles in a complete cycle 

relationship between pair of field poles and number of poles in induction motor

Frequency of the poles passing in a one cycle

conversion of motor speed from rpm to rps

Hence, for P poles the frequency is;

relationship between speed and frequency of induction motor

relation between synchronous speed and frequency

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