The working principle of asynchronous motor (induction motor) is to produce induced current in the rotor by the rotating magnetic field of the stator, which generates electromagnetic torque. The rotor does not directly generate a magnetic field. Therefore, the speed of the rotor must be less than the synchronous speed (without this difference, namely slip rate, there will be no induced current in the rotor), and therefore it is called asynchronous motor. For synchronous motors, the rotor itself generates a fixed-direction magnetic field (produced by permanent magnets or direct current), and the stator's rotating magnetic field "drags" the rotor's magnetic field (rotor) to roll, so the speed of the rotor must be the synchronous speed, and therefore it is called synchronous motor. Most motors are asynchronous when used as motors; generators are all synchronous.
When a three-phase AC passes through a certain structure of windings, it generates a rotating magnetic field. Under the action of the rotating magnetic field, the rotor rotates with the rotating magnetic field. If the speed of the rotor is completely consistent with the speed of the rotating magnetic field, it is a synchronous motor; if the speed of the rotor is less than the speed of the magnetic field, that is, they are not synchronized, it is an asynchronous motor. Asynchronous motors have a simple structure and are widely used. Synchronous motors require the rotor to have fixed poles (permanent or electromagnet), such as AC generators and synchronous AC motors.
The speed of the motor (stator speed) is less than the speed of the rotating magnetic field, hence it is called an asynchronous motor. It is basically the same as an induction motor. s=(ns-n)/ns. s is the slip rate, ns is the magnetic field speed, n is the rotor speed.
Basic principle:
(1) When a three-phase asynchronous motor is connected to a three-phase AC power supply, the three-phase stator winding passes through three-phase symmetrical current generated three-phase magnetic potential (stator rotating magnetic potential) and produces a rotating magnetic field.
(2) This rotating magnetic field has a relative cutting motion with the rotor conductor. According to the principle of electromagnetic induction, the rotor conductor generates induced electromotive force and induces current.
(3) According to the law of electromagnetic force, the current-carrying rotor conductor is subjected to electromagnetic force in the magnetic field, forming an electromagnetic torque, driving the rotor to rotate. When the shaft of the motor carries mechanical load, it outputs mechanical energy externally.
Characteristics:
Advantages: Simple structure, easy to manufacture, cheap price, convenient operation.
Disadvantages: Lagging power factor, low power factor under light load, slightly poor speed regulation performance.
Mainly used as a motor, generally not as a generator!
An asynchronous motor is an AC motor, and the ratio of its speed under load to the frequency of the connected power grid is not a constant relationship. Asynchronous motors include induction motors, double-fed asynchronous motors, and AC commutator motors. Induction motors are the most widely used, and under normal circumstances without causing misunderstanding or confusion, induction motors can generally be referred to as asynchronous motors.
The stator winding of a general asynchronous motor is connected to an AC power grid, and the rotor winding does not need to be connected to any other power source. Therefore, it has advantages such as a simple structure, easy manufacturing, use, and maintenance, reliable operation, small size, low cost, etc. Asynchronous motors have high operating efficiency and good working characteristics, and they run at nearly constant speed from no-load to full-load range, which can meet the transmission requirements of most industrial and agricultural production machinery. Asynchronous motors also facilitate the generation of various protective forms to adapt to the needs of different environmental conditions. When asynchronous motors are running, they must draw reactive excitation power from the power grid, making the power factor of the power grid worse. Therefore, for driving large-power, low-speed machinery such as ball mills and compressors, synchronous motors are often used. Since the speed of an asynchronous motor has a certain slip relationship with its rotating magnetic field speed, its speed regulation performance is relatively poor (except for AC commutator motors). For transportation machinery, rolling mills, large machine tools, dyeing and papermaking machinery, etc., which require a wide and smooth speed regulation range, using DC motors is more economical and convenient. However, with the development of high-power electronic components and AC speed regulation systems, currently, the speed regulation performance and economy of asynchronous motors suitable for wide speed regulation have become comparable to those of DC motors.
Synchronous motors and induction motors are commonly used AC motors. The characteristic is that during steady-state operation, the rotor speed and the power grid frequency have an unchanging relationship: n=ns=60f/p, where ns is called the synchronous speed. If the power grid frequency remains unchanged, then the synchronous motor's speed during steady-state operation is always a constant regardless of the load size.
Synchronous motors are divided into synchronous generators and synchronous motors. Modern power plants mainly use synchronous motors as AC machines.
Working principle
◆ Establishment of main magnetic field: Excitation winding passes DC excitation current to establish alternating polarity excitation magnetic field, i.e., establishing the main magnetic field.
◆ Current-carrying conductor: Three-phase symmetrical armature winding serves as the power winding, becoming the carrier of induced electromotive force or induced current.
◆ Cutting movement: The prime mover drives the rotor to rotate (inputting mechanical energy to the motor), and the alternating polarity excitation magnetic field rotates with the shaft and successively cuts across each phase of the stator winding (equivalent to the winding conductors cutting the excitation magnetic field in reverse).
◆ Generation of alternating electromotive force: Due to the relative cutting motion between the armature winding and the main magnetic field, an alternating electromotive force whose magnitude and direction change periodically will be induced in the armature winding. Through the lead-out wires, AC power can be provided.
◆ Alternating nature and symmetry: Due to the alternating polarity of the rotating magnetic field, the polarity of the induced electromotive force alternates; due to the symmetry of the armature winding, the three-phase symmetry of the induced electromotive force is ensured.
Operating modes
◆ The main operating modes of synchronous motors are three: as a generator, motor, and compensator.
As a generator, synchronous motors operate in their primary mode. As a motor, it is another significant mode of operation for synchronous motors. The power factor of synchronous motors can be adjusted, and in situations where speed adjustment is not required, the use of large synchronous motors can improve operational efficiency. In recent years, small synchronous motors have started to be used more in variable frequency speed control systems.
Synchronous motors can also be connected to the power grid as synchronous compensators. At this time, the motor does not carry any mechanical load, and by adjusting the excitation current in the rotor, it supplies the power grid with the required rational or capacitive reactive power to achieve the purpose of improving the power factor of the power grid or regulating the voltage of the power grid.
Synchronous generators and other types of rotating motors are composed of two major parts: the stationary stator and the rotatable rotor. Normally, they are divided into rotating-field type synchronous motors and rotating-armature type synchronous motors.
The most common type is the rotating-field synchronous generator, where the inner circle of the stator core evenly distributes stator slots, and these slots contain three-phase symmetrical windings arranged according to rules. This kind of synchronous motor's stator is also called the armature, and the stator core and winding are also called the armature core and armature winding.
The rotor core is equipped with paired magnetic poles of a certain shape, and the magnetic poles are wound with excitation windings. When DC current flows through them, a pole-alternating distribution magnetic field will be formed in the air gap of the motor, called the excitation magnetic field (also known as the main magnetic field, rotor magnetic field).
The prime mover drives the rotor to rotate (inputting mechanical energy to the motor), and the alternating polarity excitation magnetic field rotates with the shaft and successively cuts across each phase of the stator winding (equivalent to the winding conductors cutting the excitation magnetic field in reverse).