on video How Electric Motors Work - Three Phase AC Induction Motors




Electricity is now taken for granted, and with it the electric motors that run both the TGV and your washing machine.

Several electric motor technologies exist, such as the direct current (DC) motor with brush, the brushless DC motor but with a Hall effect magnetic sensor, or the synchronous alternating current motor. The latter uses the alternations of the direction of the current to attract then repel the magnets of the rotor and make it turn and to control the speed of rotation of the motor.

The one I'm going to talk about here, the AC asynchronous motor or induction motor, is more mysterious. Imagined and developed by Nikola Tesla, it is extremely simple in terms of components. Indeed, it has no magnets, no brushes and zero electronics on board. And yet… it spins!

How can this happen?

We will also see the trick that allows this engine to operate in generator mode.

General description

An electric motor consists of two main parts: the stator and the rotor.

The stator corresponds to the fixed part of the motor: it is so called because it is static, it does not move.

The rotor, on the other hand, is the entire rotating part from which the engine torque is taken.

The two parts of an electric motor: the rotor, bottom left, and the stator, top right (source)

In a brushed DC motor, the magnets are on the stator. The supply current is transmitted to the rotor coils via the brushes which come into contact with the rotor shaft. The rotation of the latter causes the various coils of the rotor to be either connected to the positive brushes, or to the negative, or to nothing. This switching allows the magnetic field produced by the coils to always be opposite the fixed field of the stator, and therefore to rotate.

In a brushless motor, the various coils are on the stator. The coils can be supplied with direct current, and in this case it is a Hall effect sensor which controls the switching between the coils; or alternating current, in which case the alternations of the current produce a rotating magnetic field in the stator, which drives the rotor and operates the motor.

We will see some of these elements in detail below, but here is a very general presentation. Just remember for the moment the existence of the stator and the rotor, and that there are different technologies of electric motors.

Principle of induction

Electromagnetic induction is the appearance of an electric current in a conductor placed in a varying magnetic field.

This is the principle of the electric generator: the latter, when it is activated by an external force (wheel of a bicycle in the case of a dynamo for example, a wind for a wind turbine). This force sets in motion a magnet placed in a fixed coil.

The magnet produces a magnetic field, but the rotation of the magnet will vary the direction of this magnetic field: the direction that varies is enough to have a varying magnetic field.

It is important that the magnetic field is variable, because only the variation of a magnetic field makes it possible to induce a current (this is the law of Maxwell-Faraday).

The reason for this is logical: when you place a conductive coil in a moving or rotating magnetic field, the field will act on the electrons and force them to circulate, forming an electric current. If the magnet does not move, the electrons are not drawn anywhere and there is no current.

In the case of an inductive charger, the charger emits a varying magnetic field into your phone. A coil in your phone then produces a current which is used to charge the battery.

In induction cookers, currents are induced directly in the pan: these currents heat the metal and the pan.

In an induction motor, the magnetic field will also produce a current, but not only!

Operation of synchronous AC motor

The purpose of the article is to explain the asynchronous (or induction) motor, but to go step by step, let's first talk about the permanent magnet synchronous motor: it is a little simpler. Both motors operate on alternating current, usually three-phase.

The permanent magnet synchronous motor has its coils on the stator and the magnets on the rotor.

First, let's limit ourselves to the stator. The three-phase current in the stator coils will produce a magnetic field inside the stator. Given the operation of the three-phase, the magnetic field which reigns in the cage of the stator is rotary: 

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