AC Motors: UNIZOR.COM - Physics4Teens - Electromagnetism - Alternating Current Induction
Channel: Zor Shekhtman on UNIZOR Education
Date: 2020-08-23

UNIZOR.COM - Physics through a Looking Glass of Mathematician


Alternating Current Motors

Alternating current (AC) motors are used where the sources of electricity produce alternating current - in our homes, at industrial facilities etc.
For example, AC motors work in refrigerators, water pumps, air conditioners, large fans and many other devices.

The idea of AC motors belongs to Nicola Tesla, who invented it at the end of 19th century.
And it was a brilliant idea!


Firstly, as in the case of DC motors, let's replace the permanent magnet in a stator with two coils around an iron cores - electromagnets that create magnetic field. Notice, however, that the current in these electromagnets is alternating, which means that the magnetic field between these electromagnets is alternating as well in magnitude and direction.

Secondly, we will place a permanent magnet on an axis, so it freely rotates between the poles of the electromagnets

What will happen if we switch on the alternating current in the electromagnets?
Well, nothing noticeable. The problem is, the polarity of electromagnets will start switching with frequency of the AC - 50 or 60 oscillations per second in usual commercial wiring, and a permanent magnet in-between will be forced in two different directions with the same frequency and it will not have a chance to start rotating, it will only vibrate with the same frequency.

But let's manually start rotating the magnet in any direction strong enough to force it to rotate with significant angular speed.
At different positions the variable external magnetic field of electromagnetic stator interferes with rotating permanent magnet rotor, sometimes slowing it, sometimes speeding its rotation.
In a short while the rotation of the permanent magnet rotor will synchronize with variable external field of electromagnetic stator and rotation will be maintained with the same angular speed as the frequency of AC in the coils of electromagnets.

Our first version of an AC motor is functional, but it has two obvious disadvantages.
One disadvantage is the usage of permanent magnet, they are very expensive. We could not avoid it in a DC motor, but in an AC case we might think that variable magnetic field might help to use the induction effect to avoid it.
Another disadvantage is that switching the AC on does not really start the rotation, we manually started it, and only then, if we gave a strong push, it started to rotate and maintained this rotation.


Let's use a wire loop instead of a permanent magnet rotor. But, to act as a permanent magnet, it needs an electric current running through the wire, and we don't want any extra sources of electricity to connect to it, it's complicated, needs a commutator, like in the original DC motors.
Instead, we will count on the induction effect created by an alternating current in a stator, which creates an alternating magnetic field, which, in turn, induces the electric current in the wire loop of a rotor.

The induced electric current a wire loop of our rotor appears when a wire crosses the magnetic field lines. Magnetic field produced by a stator is directed along a center line XY and is changing in magnitude from a maximum in one direction (from X to Y) to zero, to a maximum in another direction (from Y to X), again to zero etc.

The problem is, if the rotor is standing still, its wire does not cross magnetic field lines. Therefore, no electric current will be produced in it, and there will be no rotation. Rotor needs an initial push sufficient to synchronize its rotation with the frequency of alternating magnetic field to continue the rotation. Our second disadvantage is still unresolved.


Let's imagine that, besides two main electromagnets with poles X and Y, we have another pair of auxiliary electromagnets in a stator, that are positioned perpendicularly to line XY. Let their poles be A and B and (very important!) the current in them, also sinusoidal, is shifted in time relatively to a current in main electromagnets by 90°.
So, when the magnetic field in one direction is maximum along line XY, it's zero along line AB; when it's zero at XY, it's maximum along AB. Then direction of current is changing and the magnetic flux is at maximum in an opposite direction along line XY and zero along AB; then it's zero along XY and maximum in opposite direction along AB.
So, the magnetic field is rotating, as the alternating current performs sinusoidal oscillations.
This rotation of a magnetic field causes the rotation of a rotor.