Why does AC induction motor rotate the same way?

Suppose an aluminium disc is suspended where it can freely rotate.

A magnet is placed above (not touching) the aluminium disc and made to spun.

This obvious causes a changing magnetic field.

By Faraday's Law, this will induce current in the disc below that oppose the motion of the magnet by Lenz's Law.

However, why does the disc then still spin in the same direction as the movement of the magnet?

I understand that the disc will induce a secondary magnetic field that slows the rotation of the magnet, but how come the disc still follows the magnet?

Let's start from the ground up. The experiment below illustrates the basic principle of an AC induction motor and I assume is what you are referring to (if not, it still works).

When the magnetic rotates, there will be a induced current in the aluminium disc. By Lenz's Law, this current will act in such a way as to oppose the change in relative motion that caused it. The result of that the aluminium disc will appear to chase the spinning magnet.

The part you get stuck at is why the disc is following the magnet? Thinking in terms of relativity, by following the magnet you are decreasing the relative motion between the disc and the magnet. Thus by decreasing the relative motion you are essentially minimising this change (Lenz law) to stay stationary.

• I think I get it now! Is it because if the disc moved the other way, then the relative motion is instead increased? So to decrease the RELATIVE motion they must chase each other! Right? Commented Oct 26, 2015 at 6:38
• @JohnSmith Yes. Decreasing relative motion = analogous to becoming stationary i.e. minimising change so they can become still again by chasing each other and trying to 'catch up' per se. Commented Oct 26, 2015 at 7:10
• No problem, don't forget to mark it as answered Commented Oct 26, 2015 at 7:12

"By Faraday's Law, this will induce current in the disc below that oppose the motion of the magnet by Lenz's Law."

This is pretty much your answer. The disk drags the magnet, opposing the latter's motion, and thus the magnet, by Newton III, exerts an opposite torque on the disk, which tends to increase the disk's angular momentum to match that of the disk. Steady state is reached when there is no slip (the term used in the context of AC induction motors), when the rotor rotates at the same speed as the spinning magnetic field. At all other times, when there is relative rotation, a back EMF is generated to produce eddy currents, which are dragged by the spinning magnetic field so as to oppose the slip.

• But doesn't Lenz's Law always say that it should oppose the motion which induced the current? Shouldn't it spin the other way instead? Sorry for my ignorance. Commented Oct 25, 2015 at 12:03
• The currents resist the motion of the magnetic field through the disc, distorting the magnetic field which transmits a force to the magnet, resisting the magnet's motion. However, in the process an equal and opposite force is applied to the disc, pushing it in the same direction as the motion of the magnet. Commented Oct 25, 2015 at 18:24

Lenz effect and the resultant combined forces is what moves the aluminun plate. So the resultant forces will dictate the movement of the plate.

In 1835 Heinrich Lenz stated the law that now bears his name. An electric current induced by a changing magnetic field will flow such that it will create its own magnetic field that opposes the magnetic field that created it. These opposing fields occupying the same space at the same time result in a pair of forces. These forces are felt when you turn a generator and generate electricity. The more current you generate, the greater the force opposing you. This force can also be felt if you try to drag a conductive, non-magnetic plate between the poles of a horseshoe magnet. The plate sees a changing magnetic field which creates a current in the plate, which creates its own magnetic field opposing the one that created it.

The plate moves in the direction of the magnetic because although the moving magnetic field is inducing an electromagnetic field in the plate the net force of the field in opposition and the magnets EM field will be in the direction of the magnet. Therefor it will follow the magnet.