These motors work with alternating current (AC).
To control their speed, unlike DC motors where the armature coil current is varied, in AC motors, it is controlled by varying the frequency. For torque control, the voltage and frequency at the stator terminals are regulated.
Types of AC Motors
For the classification of these motors, the current they use to operate is considered: single-phase and three-phase.
And considers the coupling of the magnetic fields of the rotor (armature) and stator (inductor). If they turn in unison (at the same time), they are synchronous, and if they turn with a current offset, it means they are asynchronous and are called induction motors.
These are the motors that run on single-phase alternating current.
It should be clarified that some of these motors have extremely low starting torque and sometimes need an extra pair of coils just for starting as they cannot start rotating on their own.
Types of Single-phase Motor
Single-phase Synchronous Motor
This is arguably the same as the DC motor with series excitation but with single-phase alternating current.
The motor’s operation is based on the coupling of magnetic fields that rotate simultaneously, which is synchronous.
For this purpose, the rotor has coils connected to a collector connected in series with the inductor coils. These motors use brushes or carbons to apply the current to the rotor.
Therefore, the same motor can operate with both alternating and direct current; therefore, it is called “the universal motor.”
In these motors, the stator creates a rotating magnetic field that is out of phase. For this, there are two pairs of perpendicular coils. One of them is connected to the alternating current generating an oscillating magnetic field. In the other coil, there is a capacitor which is responsible for offsetting the current reaching the coil by 90° with respect to the current of the inner coil (rotor or armature), so the magnetic field produced by the second coil will also be offset with respect to the previous one.
The composition of both fields is a sum of vectors, and it’s resultant rotates in the motor’s space.
The rotating magnetic field induces a current to the rotor conductors. This is because there is always a variation of magnetic flow, and that is why the rotor rotates at a slower speed than the speed of synchronism at which the magnetic field rotates. Then a Lorentz force is produced on the rotor conductors.
These motors work with a three-phase alternating current. They are more robust and therefore are the most used in industrial machinery.
Types of Three-phase Motors
Three-phase Synchronous Motor
This motor works with a three-phase alternating current. It is generally said that it is the big brother of the single-phase motors.
These motors create a rotating magnetic field in the same way as single-phase synchronous motors.
The operation of these motors is totally analogous.
Similarly, as the single-phase asynchronous motors have a rotating magnetic field, a current is induced in the rotor by the rotating magnet at a higher speed than the rotor; the Lorentz force also plays a role.
The rotating magnetic field is achieved by connecting each of the coils to a line of the three-phase current.
The rotor can have different connections, depending on the motor. They commonly have a squirrel cage connection, and other so-called short circuit motors have a wound rotor with the advantage that its rotation’s speed and torque can be regulated with short circuit potentiometers.