The following table gives the synchronous speed for various frequencies and different numbers of poles:

Table of Synchronous Speeds
Frequency R.P.M. of the rotating magnetic field, when number of poles is
2 6 10 16 20 24
25 1,500 500 300 188 150 125
60 3,600 1,200 720 450 360 300
80 4,800 1,600 960 600 480 400
100 6,000 2,000 1,200 750 600 500
120 7,200 2,400 1,440 900 720 600
125 7,500 2,500 1,500 938 750 625

Ques. How does the slip vary?

Ans. It varies from about 1 per cent. in a motor designed for very close regulation to 40 per cent. in one badly designed, or designed for some special purpose.

Ques. Why is the slip ordinarily so small?

Ans. Because of the very low resistance of the armature, very little pressure is required to produce currents therein, of sufficient strength to give the required torque. Hence, the necessary rate of cutting the magnetic lines to induce this pressure in the armature is reached with very little difference between the field speed and armature speed, that is, with very little slip.

Ques. How does the slip vary with the load?

Ans. The greater the load the greater the slip.

In other words, if the load increase, the motor will run slower, and the slip will increase. With the increased slip, the induced currents and the driving force will further increase. If the motor be well designed so that the field strength is constant and the lag of the armature currents is small, the driving force developed or torque will be proportional to the slip, that is the slip will increase automatically as the load is increased, so that the torque will be proportional to the load.