By inspection of fig. 1710, it is seen that the induced pole toward which the magnet is moving is of the same polarity as the magnet; therefore it is repelled, while the induced pole from which the magnet is receding, being of opposite polarity, is attracted. A torque is thus produced tending to rotate the cylinder.
It must be evident that this torque is greatest when the cylinder is at rest, because the magnetic lines are cut by any element on the cylindrical surface at the maximum rate.
Moreover, as cylinder is set in motion and brought up to speed, the torque is gradually reduced, because the rate with which the magnetic lines are cut is gradually reduced.
Ques. What is the essential condition for the operation of an induction motor?
Ans. The armature, or part in which currents are induced, must rotate at a speed slower than that of the rotating magnetic field.
In the elementary induction motor, fig. 1,710, the cylinder is the armature, and the rotating magnets are the equivalent of a rotating magnetic field.
Ques. What is the difference of speed called?
Ans. The slip.
Ques. Why is slip necessary in the operation of an induction motor?
Ans. If the armature had no weight and there was no friction offered by the bearings and air, it would revolve in synchronism with the rotating magnetic field, that is, the slip would be zero; but since weight and friction are always present and constitute a small load, its speed of rotation will be a little less than that of the rotating magnetic field, so that induction will take place, in amount sufficient to produce a torque that will balance the load.