How do AC motors work?

The motors base their operation on obtaining a rotating magnetic field. Within this rotating field there may be an electromagnet, which rotates at the same speed as the field. In this case we’ll have a synchronous motor. A second possibility is that an induction windings are present inside the field, which gives rise to an electrical current and therefore the force of Lorentz. The rotation will be slower than the rotary field, which is why the motor is called asynchronous or induction motor.

Regarding the supply current, we will have single-phase and three-phase motors. Similar to direct current motors, alternating current motors consist of a fixed part called a stator or inductor, equipped with magnetic field generating coils, and a rotor or inductor, also known as armature. Take a look at this text if you are interested or want to buy shaded pole motor.


It is a motor identical to the direct current motor with series excitation. But in alternating current, motor operation is based on the coupling of magnetic fields that rotate in unison.
In order for this coupling to take place, the rotor has coils attached to a collector formed by gaskets, in series with the inductor coils. A couple of brushes apply the current to the rotor. Therefore, the same motor can operate with both direct and alternating current. But it can also function as a dynamo. This is why it is called a universal motor, and is widely used in small household appliances.


In this type of motors, the stator generates a rotating magnetic field. Two pairs of perpendicular coils are available for this purpose. One of them connects directly to the alternating current, generating an oscillating magnetic field. In the other coil is inserted a capacitor whose mission is to offset the current that reaches the coil 90° (electric) with respect to the current of the previous coil, so that the magnetic field generated by this second coil will also be out of phase with respect to the previous one. The composition of both fields is a sum of vectors and the resulting one rotates in space. The rotating magnetic field induces a current in the conductors of the rotor (reason why the rotor is also called induced) whenever there is a variation in magnetic flux. This always happens, as the rotor rotates at a slower speed than the synchronous speed at which the field rotates.


Similar to single-phase motors, three-phase motors achieve a rotating magnetic field. The three-phase synchronous motor has a rotor consisting of an electromagnet. It is not a very current motor due to the complication of feeding the inductor with alternating current and direct current induction, but its rotation speed is fixed and equal to that of synchronism.


The operation of these motors is completely analogous to that of single-phase induction motors:

  • A rotating magnetic field
  • Rotor current induction due to the rotating field at a higher speed than the rotor itself
  • Lorentz force and magnetic attraction force

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