What is Synchronous speed? Relationship between speed and Frequency

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 can be expressed by the following mathematical expression.

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 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  rotated. 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 

Frequency of the poles passing in a one cycle

Hence, for P poles the frequency is;