The working principle of an asynchronous motor (induction motor) is that the rotating magnetic field of the stator induces a current in the rotor, generating electromagnetic torque. The rotor itself does not directly generate a magnetic field. Therefore, the rotor speed is always less than the synchronous speed (without this difference, i.e., slip, there would be no induced current in the rotor), hence the name asynchronous motor. In contrast, a synchronous motor's rotor generates a magnetic field in a fixed direction (using permanent magnets or direct current). The rotating magnetic field of the stator "drags" the rotor's magnetic field (the rotor) to rotate, so the rotor speed is always equal to the synchronous speed, hence the name synchronous motor.
When used as electric motors, most are asynchronous; generators are synchronous. The difference between synchronous and asynchronous motors:
When three-phase alternating current passes through windings of a certain structure, a rotating magnetic field is generated. Under the influence of this rotating magnetic field, the rotor rotates with it. If the rotor's speed is exactly the same as the rotating magnetic field's speed, it is a synchronous motor; if the rotor's speed is less than the magnetic field's speed, meaning 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 magnetic poles (permanent magnet or electromagnetic), such as AC generators and synchronous AC motors. When the motor's speed (stator speed) is less than the rotating magnetic field's speed, it is called an asynchronous motor. It is essentially the same as an induction motor.
s = (ns - n) / ns. s is the slip, ns is the magnetic field speed, and 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 windings generate a three-phase magnetomotive force (stator rotating magnetomotive force) through the three-phase symmetrical current and generate a rotating magnetic field.
(2) The rotating magnetic field has 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 current-carrying rotor conductor is subjected to electromagnetic force in the magnetic field, forming electromagnetic torque, which drives the rotor to rotate. When the motor shaft is loaded with mechanical load, it outputs mechanical energy.
Features:
Advantages: Simple structure, easy to manufacture, inexpensive, and easy to operate.
Disadvantages: Lagging power factor, low power factor under light load, and slightly poor speed regulation performance. Primarily used for electric motors, generally not for generators!
An asynchronous motor is an AC motor whose speed under load is not a constant ratio to the frequency of the connected power grid. Asynchronous motors include induction motors, doubly-fed asynchronous motors, and AC commutator motors. Induction motors are the most widely used, and in cases where this will not cause misunderstanding or confusion, induction motors are generally referred to as asynchronous motors.
The stator winding of a typical asynchronous motor is connected to the AC power grid, while the rotor winding does not require connection to any other power source. Therefore, it boasts advantages such as simple structure, ease of manufacturing, use, and maintenance, reliable operation, small size, and low cost. Asynchronous motors have high operating efficiency and good working characteristics, operating at near constant speed from no-load to full-load, meeting the transmission requirements of most industrial and agricultural machinery. Asynchronous motors are also easy to derive into various protection types to adapt to different environmental conditions. However, when an asynchronous motor is running, it must draw reactive excitation power from the power grid, which degrades the power factor of the grid. Therefore, synchronous motors are often used for driving high-power, low-speed machinery such as ball mills and compressors. Because the speed of an asynchronous motor has a certain slip relationship with the speed of its rotating magnetic field, its speed regulation performance is relatively poor (except for AC commutator motors). For transportation machinery, rolling mills, large machine tools, printing and dyeing machinery, and papermaking machinery requiring a wider and smoother speed regulation range, DC motors are more economical and convenient. However, with the development of high-power electronic devices and AC speed control systems, the speed control performance and economy of asynchronous motors suitable for wide speed ranges are now comparable to those of DC motors.
Synchronous motors, like induction motors, are commonly used AC motors. Their key characteristic is that during steady-state operation, the rotor speed and the mains frequency maintain a constant relationship: n = ns = 60f/p, where ns is called the synchronous speed. If the mains frequency remains constant, the synchronous motor's speed in steady state is always constant and independent of the load.
Synchronous motors are divided into synchronous generators and synchronous motors. Modern power plants mainly use synchronous motors for AC power generation.
Working principle
◆Establishment of the main magnetic field: A DC excitation current is passed through the excitation winding to establish an excitation magnetic field with alternating polarities, that is, to establish the main magnetic field.
◆Current-carrying conductor: The three-phase symmetrical armature winding acts as the power winding, becoming the carrier of induced electromotive force or induced current.
◆Cutting motion: The prime mover drives the rotor to rotate (inputting mechanical energy into the motor), and the excitation magnetic field with alternating polarities rotates with the shaft and sequentially cuts each phase of the stator winding (equivalent to the conductors of the winding cutting the excitation magnetic field in the opposite direction).
◆ Generation of alternating electromotive force: Due to the relative cutting motion between the armature winding and the main magnetic field, a three-phase symmetrical alternating electromotive force with periodically changing magnitude and direction will be induced in the armature winding. AC power can then be provided through the leads.
◆ Alternating polarity and symmetry: The alternating polarity of the rotating magnetic field causes the polarity of the induced electromotive force to alternate; the symmetry of the armature winding ensures the three-phase symmetry of the induced electromotive force.
Operating mode
◆Synchronous motors mainly operate in three modes: as generators, as motors, and as compensators. Operating as a generator is the most common mode, while operating as a motor is another important mode. The power factor of a synchronous motor can be adjusted. In applications where speed regulation is not required, using large synchronous motors can improve operating efficiency. In recent years, small synchronous motors have begun to be more widely used in variable frequency speed control systems. Synchronous motors can also be connected to the power grid as synchronous compensators. In this case, the motor does not carry any mechanical load and relies on adjusting the excitation current in the rotor to deliver the required inductive or capacitive reactive power to the power grid, thereby improving the power factor or regulating the grid voltage.
Like other types of rotating electrical machines, synchronous generators consist of two main parts: a fixed stator and a rotatable rotor. They are generally classified into field-type synchronous generators and pivot-type synchronous generators.
The most commonly used type is the rotary synchronous generator, whose stator core has stator slots evenly distributed on its inner circumference, and three-phase symmetrical windings arranged in a regular pattern are embedded in the slots. The stator of this type of synchronous motor is also called the armature, and the stator core and windings are also called the armature core and armature windings, respectively.
The rotor core is equipped with pairs of magnetic poles that are made into a certain shape. Excitation windings are wound around the magnetic poles. When a direct current is passed through them, a distributed magnetic field with alternating polarities will be formed in the air gap of the motor, which is 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 into the motor), and the excitation magnetic field with alternating polarities rotates with the shaft and sequentially cuts each phase of the stator winding (equivalent to the conductors of the winding cutting the excitation magnetic field in the opposite direction).
