Synchronous Speed and Slip of a 3-Phase Induction Motor

In an induction motor, the speed at which the rotating magnetic field (RMF) rotates is known as synchronous speed (NS).

The value of the synchronous speed depends upon the number of stator poles (P) in the motor and the supply frequency (f). Therefore, for a given motor of P-poles, the synchronous speed is,

$$Synchronousspeed,$$

$$N_s=\frac{120f/}{P}RPM$$

Where, Nr is the actual rotor speed.

Generally, the slip speed is expressed as a fraction of the synchronous speed is called the per-unit slip. The per-unit slip is usually called the slip and denoted by ‘s’. Thus,

• When the rotor is stationary, i.e., Nr = 0, then the slip, s = 1 or 100 %.
• The slip at full-load varies from about 5 % for small motors to about 2 % for large motors.

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