I will explain the working principle and differences between DC and AC motors in the simplest and most understandable way.
The image above shows the simplest physical representation of a DC motor.
C working principle
DC Motor
In a DC motor, current from the DC power supply flows from the positive pole to the left side of the brush. The brush and commutator rub against each other and current flows through the left commutator, which has two left and right segments, into the coil and out the right side of the coil. The current then flows back to the negative pole of the power supply, creating a closed circuit.
The coil is located in the magnetic field of the main magnetic poles (N and S) and is subject to an electromagnetic force. Because the two sides of the coil have different current directions (inward left and outward right), they are subject to electromagnetic forces of opposite magnitudes. These two forces create the electromagnetic torque, which causes the coil to begin rotating.
The coil is embedded in the rotor slot, causing the motor to start rotating. The reversible parts rotate with the rotating shaft while the brush remains stationary. After one rotation, the right coil moves to the left and the left coil moves to the right. However, the commutator ensures that the current in the left-hand coil flows in the same direction as the current in the right-hand coil, resulting in a constant direction of electromagnetic force received at the same position. This ensures cyclic rotation of the engine.
However, the coil's magnetic field changes when the coil rotates to different positions, causing the electromagnetic force in the coil to change as well. This makes the coil rotation unstable and slows down suddenly. To ensure uniform and stable coil force, multiple coils can be installed.
Then we get the following:
Even with this engine model:
Furthermore, the two outer magnetic poles of the motor are actually created by the excitation coils that generate electromagnets. In smaller motors, permanent magnets are used, while larger motors use electromagnets.
The model is simply a representation, but the actual engine rotor is structured this way.
AC Motor
AC motors are classified into two categories: synchronous motors and asynchronous motors. Synchronous motors are mainly used as generators while asynchronous motors are mainly used as electric motors. The focus of this discussion is on asynchronous motors.
Asynchronous motors are favored for their simple structure, low cost, ease of maintenance and reliable operation, which has resulted in their widespread use. Despite their simple structure, the operating principle of AC motors is actually more complex than that of DC motors, making a clear understanding of the technology more challenging.
A three-phase symmetrical alternating current is applied to the stator of the AC motor as shown in the figure above. The stator remains stationary and just a change in current can generate a synthetic rotating magnetic field. This magnetic field acts like a rotating magnet around the stator, making work easier.
A closed coil is located inside the stator, and an electromotive force and current are induced in the closed coil, resulting in the creation of an electromagnetic force. This causes the closed coil to rotate.
It can also be understood that there is a rotating magnet in the stator, and the closed coil inside the rotor works as an electromagnet due to induction charging. As the external electromagnet rotates, it causes the internal electromagnet to rotate, thus rotating the rotor of the AC motor.
The rotational speed of the stator magnetic field is called synchronous speed. The rotor, which is driven by the stator's magnetic field, rotates at a slower speed, known as asynchronous speed. This is where the term “asynchronous motor” comes from.
The rotor of an AC motor is a simple closed coil or conductor, often referred to as a “mouse cage” asynchronous motor due to its cage-like structure. The electromotive force and current inside the rotor are induced by the magnetic field of the stator, which is why the asynchronous motor is also known as an induction motor.
Therefore, three-phase asynchronous motor has several names, including AC motor, asynchronous motor and induction motor, each referring to it from a different perspective. If you have any further questions, feel free to ask in the comments and I will do my best to provide detailed answers.
