Electronic – Why is slip necessary in an Induction motor

The synchronous speed n_s of the stator field is directly related to the mains frequency and the number of (magnetic) poles in the stator and is expressed in revolutions per minute [rpm]:

$$n_s = \dfrac{120×f}{p} \text{[rpm]}$$

The rotor speed n_r is is the number of revolutions per minute of the rotor.

Slip is the ratio of the speed [difference between stator and rotor] to stator field speed. Slip is defined as:

$$s=\dfrac{n_s-n_r}{n_s} $$

So if the rotor is:

• at the same speed as the magnetic field then slip is 0;
• at a lower speed than the magnetic field then slip is positive (for normal motor mode this is in the order of 5%);
• at a higher speed than the magnetic field then the slip is negative. The motor is in generator mode.

Li-aung Yip explained in his comment that slip speed is probably defined by the difference in stator speed and rotor speed, in other words: "the slip percentage expressed in RPM":

$$s_{rpm}=n_s-n_r$$

The rated speed is usually about 90-95% of synchronous speed. You can guess the synchronous speed and number of poles by looking for a number of poles that gives a rated speed slightly less than synchronous speed.

In this case, I would guess you are dealing with a 60Hz 12-pole motor, which would have a synchronous speed of 600 rpm.

I've done a quick lookup table below for reference.

Poles    50hz    60hz
2        3000    3600
4        1500    1800
6        1000    1200
8        750     900
10       600     720
12       500     600
16       375     450
20       300     360
24       250     300
30       200     240
40       150     180