Electric motors can be classified into DC motors and AC motors based on the type of input current:
DC Motors – Motors that rotate using direct current are called DC motors. Based on the different ways the magnetic field circuit and armature circuit are connected, they can be further divided into series-wound motors, shunt-wound motors, and compound-wound motors;
AC Motors – In AC motors, the powerful induced current (eddy current) is generated in the rotating magnetic field. The continuous cutting of magnetic field lines by the copper bars on the rotor, according to Lenz's law, creates an induced current that opposes the relative motion between the magnetic field and the rotor. Therefore, the rotor rotates with the magnetic field. However, the rotor's rotational speed is not as high as the speed of the magnetic field change; otherwise, the magnetic field lines would not be cut by the copper bars.
Motors that rotate using alternating current are called AC motors. There are many types, mainly including:
Rectifier Motors – These motors are created by using a series-wound DC generator as an AC motor. Because the alternating current reverses direction simultaneously in both the magnetic field and armature circuits, the direction of the torque remains constant, and the motor rotates continuously. Because this type of motor can use both AC and DC current, it is also called a "universal motor." Vacuum cleaners, sewing machines, and other household appliances often use this type of motor.
Induction Motors – These are devices that rotate the rotor by placing it in a rotating magnetic field, due to the effect of eddy currents. The rotating magnetic field is not created mechanically, but by passing alternating current through several pairs of electromagnets, causing their magnetic pole properties to change cyclically, which can be considered a rotating magnetic field. Three-phase induction motors (with three pairs of magnetic poles) are commonly used. The motion of a DC motor is exactly opposite to that of a DC generator. In a generator, the induced current is formed by the induced electromotive force, so they are in the same direction. In a motor, the current is supplied by an external power source, and the direction of the induced electromotive force is opposite to the direction of the armature current I.
Synchronous Motors – These are motors where the armature rotates from one pole to the next in the same period as the direction of the input current. This type of motor cannot start on its own; it must be brought to the appropriate frequency using another motor or special auxiliary windings before the alternating current can be connected. If a change in load causes a change in speed, the speed will no longer match the AC power frequency, leading to instability, potential stalling, or even damage.
