Basic Structures and Working Principles of Electric Motors

Pictures showing the inside structure and components of an electric motor in sectional view

Figure 1: The inside structure and components of an electric motor.

An electric motor is a device that converts electrical energy into mechanical energy. The general principle of its work is: appropriate magnetic and conductive materials constitute magnetic circuits and circuits that conduct electromagnetic induction with each other. In this way, electromagnetic power can be generated to achieve energy conversion. From the perspective of composition, the motor is mainly composed of a stator and a rotor. The energized coils (i.e., the stator windings) generate a rotating magnetic field and act on the rotor to generate a magneto-electric rotational torque.

Basic Structures of Electric Motors

The electric motor consists of two parts: a coil that can be turned and a stationary magnet. In a motor, the part that can rotate is called the rotor, and the part that does not move is called the stator. When the motor is working, the rotor rotates rapidly in the stator. Take the structure of a three-phase asynchronous motor as an example:
1. Stator (the stationary part)
Stator Core: part of the motor magnetic circuit on which the stator windings are placed.
Stator winding: part of the motor circuit. When connected to three-phase alternating current, it can generate a rotating magnetic field.
Frame: fixing the stator core and the front and rear covers to support the rotor. It plays the role of protection and heat dissipation.

2. Rotor (the rotating part)
Rotor core: part of the magnetic circuit of the motor. The rotor windings are placed in the core slots.
Rotor winding: cutting the rotating magnetic field of the stator generates induced electromotive force and current, and generates electromagnetic torque to make the motor rotate.

3. Other components
End caps: supporting.
Bearing shaft: connecting the rotating part and the stationary part.
Bearing cover: protecting the bearing.
Fan: cooling the motor.

The Working Principles of Electric Motors

The working principle of a simple electric motor shown in a block diagram

Figure 2: The working principle of a simple electric motor.

The operation of a motor involves several related physical principles. Two relatively important principles are introduced. The first is the principle of electromagnetic induction discovered by Michael Faraday in 1831. If the conductor moves in a magnetic field, or if the strength of a stationary conducting loop is changed, a current will be set or induced in the conductor. The second is the inverse of this principle: the principle of electromagnetic reaction, first discovered by Andrea Ampere in 1820. If an electric current passes through a conductor in a magnetic field, the magnetic field exerts a mechanical force on it.

Taking a three-phase asynchronous motor as an example, when the three-phase asynchronous motor is connected to a three-phase AC power supply (each with a difference of 120 degrees in electrical angle), the three-phase stator winding flows through the three-phase magnetomotive force (stator rotating magnetomotive force) generated by the three-phase symmetrical current and generates a rotating magnetic field. The magnetic field rotates clockwise along the inner circular space of the stator and the rotor at a synchronous speed. Then, the rotating magnetic field and the rotor conductor have a relative cutting motion. According to the principle of electromagnetic induction, the rotor conductor (the rotor winding is a closed path) generates an induced electromotive force and generates an induced current. The direction of the induced electromotive force is determined by the right-hand rule.

According to the law of electromagnetic force, under the action of the induced electromotive force, an induced current basically consistent with the direction of the induced electromotive force will be generated in the rotor conductor. The current-carrying rotor conductor is subjected to an electromagnetic force in the magnetic field generated by the stator (the direction of the force is determined by the left-hand rule). The electromagnetic force generates an electromagnetic torque on the motor rotor shaft, which drives the motor rotor to rotate in the direction of the rotating magnetic field. When there is a mechanical load on the motor shaft, the mechanical energy is output.