The working principle of an asynchronous motor (induction motor) is to produce induced currents in the rotor through the rotating magnetic field of the stator, which generates electromagnetic torque. The rotor itself does not directly produce a magnetic field. Therefore, the speed of the rotor must be less than the synchronous speed (without this difference, i.e., the slip rate, there would be no induced current in the rotor), and hence it is called an asynchronous motor.
On the other hand, the rotor of a synchronous motor produces a fixed-direction magnetic field on its own (using permanent magnets or direct current). The rotating magnetic field of the stator "drags" the magnetic field of the rotor (the rotor) along, so the speed of the rotor must be equal to the synchronous speed, and hence it is called a synchronous motor. Most motors used as electric motors are asynchronous; generators are all synchronous.
The difference between synchronous and asynchronous motors: When three-phase alternating current 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 rotor speed is completely consistent with the speed of the rotating magnetic field, it is a synchronous motor; if the rotor speed is less than the magnetic field speed, 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 electromagnetic), such as AC generators and synchronous AC motors.
When the motor speed (stator speed) is less than the speed of the rotating magnetic field, 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 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, a three-phase symmetrical current flows through the three-phase stator windings, generating a three-phase magnetic potential (rotating magnetic potential of the stator) and producing 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 induced current.
(3) According to the law of electromagnetic force, the rotor conductor carrying current experiences electromagnetic force in the magnetic field, forming an electromagnetic torque, driving the rotor to rotate. When the motor shaft carries mechanical load, it outputs mechanical energy outward.
Features:
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 used as a generator!
An asynchronous motor is an AC motor, and the ratio of its speed under load to the frequency of the connected grid is not a constant relationship. Asynchronous motors include induction motors, doubly-fed asynchronous motors, and AC commutator motors. Induction motors are widely used, and in cases where misunderstanding or confusion will not arise, they are generally referred to as asynchronous motors.
In general, the stator winding of an asynchronous motor is connected to an AC grid, and the rotor winding does not need to be connected to any other power source. Therefore, it has the advantages of a simple structure, easy manufacturing, use, and maintenance, reliable operation, small size, and low cost. Asynchronous motors have high operating efficiency and good working characteristics, running at nearly constant speed from no-load to full load, meeting the transmission requirements of most industrial and agricultural production machinery. Asynchronous motors can also be made into various protective forms to adapt to different environmental conditions.
When an asynchronous motor runs, it must draw reactive excitation power from the grid, worsening the grid's power factor. 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 paper-making machinery requiring broad and smooth speed regulation ranges, using DC motors is more economical and convenient. However, with the development of high-power electronic devices and AC speed regulation systems, the speed regulation performance and economy of asynchronous motors suitable for wide speed regulation now rival those of DC motors.
Synchronous motors and induction motors are commonly used AC motors. Their feature is: during steady-state operation, the rotor speed and the grid frequency have a fixed relationship n=ns=60f/p, where ns is called the synchronous speed. If the grid frequency remains unchanged, then the synchronous motor's speed during steady-state operation is always constant and independent 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 the main magnetic field: Exciting the excitation winding with a DC excitation current establishes an alternating polarity excitation magnetic field, thus establishing the main magnetic field.
◆ Current-carrying conductor: The three-phase symmetrical armature winding serves as the power winding and becomes the carrier of the 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 together with the shaft and sequentially cuts across each phase of the stator winding (equivalent to the conductors of the winding 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, alternating current 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 methods
◆ There are mainly three ways to operate synchronous motors: as a generator, motor, and compensator.
Running as a generator is the primary mode of operation for synchronous motors. Running as a motor is another important mode of operation for synchronous motors. The power factor of synchronous motors can be adjusted, making them suitable for applications where speed adjustment is not required. In recent years, small synchronous motors have started to be used more frequently in variable-frequency speed control systems.
Synchronous motors can also be connected to the grid as synchronous compensators. At this time, the motor does not carry any mechanical load, and by adjusting the excitation current in the rotor, the motor sends out the required inductive or capacitive reactive power to the grid, thereby improving the power factor of the grid or regulating the voltage of the grid.
Like other types of rotating motors, synchronous generators consist of two main parts: a stationary stator and a rotatable rotor. Normally, they are divided into salient-pole synchronous motors and non-salient pole synchronous motors.
The most commonly used type is the salient-pole synchronous generator, where the inner circle of the stator core is evenly distributed with stator slots, and the slots contain three-phase symmetrical windings arranged according to rules. The stator of this kind of synchronous motor is also called the armature, and the stator core and windings are also called the armature core and armature windings.
The rotor core is equipped with paired magnetic poles of a certain shape, and the magnetic poles are wound with excitation windings. When excited with DC current, a polar-distributed magnetic field is formed in the air gap of the motor, called the excitation magnetic field (also known as the main magnetic field or 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 together with the shaft and sequentially cuts across each phase of the stator winding (equivalent to the conductors of the winding cutting the excitation magnetic field in reverse).