The difference between synchronous motor and asynchronous motor
The working principle of an asynchronous motor (induction motor) is to generate an induced current in the rotor through the rotating magnetic field of the stator, generating electromagnetic torque, and the magnetic field is not directly generated in the rotor. Therefore, the rotational speed of the rotor must be less than the synchronous speed (there is no such difference, that is, the slip rate, there is no rotor induced current), so it is called an asynchronous motor: and the synchronous motor rotor itself generates a fixed-direction magnetic field (using a permanent magnet or The DC current is generated. The stator rotating magnetic field "drags" the rotor magnetic field (rotor) to rotate, so the rotor speed must be equal to the synchronous speed, which is also called synchronous motor.
When used as an electric motor, most of them use asynchronous machines; the generators are all synchronous machines. The difference between synchronous motor and asynchronous motor:
When a three-phase alternating current passes through a winding of a certain structure, a rotating magnetic field is generated. Under the action of a rotating magnetic field, the rotor rotates with the rotating magnetic field. If the rotating speed of the rotor is exactly the same as the rotating magnetic field, it is a synchronous motor; if the rotational speed of the rotor is smaller than the magnetic field The speed, that is to say the two are not synchronized, is the asynchronous motor. The asynchronous motor has a simple structure and is widely used. The synchronous motor requires the rotor to have a fixed magnetic pole (permanent magnet or electromagnetic), such as an alternator and a synchronous AC motor. The stator speed is less than the rotational speed of the rotating magnetic field and is therefore called an asynchronous motor. It is basically the same as the induction motor.
s = (ns - n) / ns. s is the slip ratio, ns is the magnetic field speed, and n is the rotor speed.
Fundamental:
(1) When a three-phase asynchronous motor is connected to a three-phase AC power source, the three-phase stator winding flows through a three-phase magnetomotive force (stator rotational magnetomotive force) generated by a three-phase symmetrical current and generates a rotating magnetic field.
(2) The rotating magnetic field has a relative cutting motion with the rotor conductor, and according to the principle of electromagnetic induction, the rotor conductor generates an induced electromotive force and generates an induced current.
(3) According to the law of electromagnetic force, the current-carrying rotor conductor is subjected to electromagnetic force in the magnetic field to form electromagnetic torque, which drives the rotor to rotate. When the motor shaft has a mechanical load, it outputs mechanical energy outward.
Features:
Advantages: simple structure, convenient manufacturing, low price and convenient operation.
Disadvantages: Power factor lag, light load power factor is low, and speed regulation performance is slightly worse. Mainly used for electric motors, generally do not make generators!
An asynchronous motor is an AC motor whose ratio of the speed of the load to the frequency of the connected grid is not constant. Asynchronous motors include induction motors, doubly-fed induction motors, and AC commutator motors. Induction motors are the most widely used, and in general, induction motors are asynchronous motors without causing misunderstanding or confusion.
The stator winding of a common asynchronous motor is connected to the AC grid, and the rotor winding does not need to be connected to other power sources. Therefore, it has the advantages of simple structure, convenient manufacture, use and maintenance, reliable operation, low quality and low cost. Asynchronous motors have higher operating efficiency and better working characteristics, and they are close to constant speed operation from no-load to full-load range, which can meet the transmission requirements of most industrial and agricultural production machinery. Asynchronous motors are also easy to generate various protection patterns to suit different environmental conditions. When the asynchronous motor is running, the reactive power must be drawn from the grid to make the power factor of the grid deteriorate. Therefore, synchronous motors are often used for driving high-power, low-speed mechanical equipment such as ball mills and compressors. Since the speed of the asynchronous motor has a certain difference in rotational speed with the rotating magnetic field, the speed regulation performance is poor (except for the AC commutator motor). It is economical and convenient to use DC motors for transportation machinery, rolling mills, large machine tools, printing and dyeing and papermaking machinery that require a wide and smooth speed range. However, with the development of high-power electronic devices and AC speed control systems, the speed control performance and economy of asynchronous motors currently suitable for wide speed regulation are comparable to those of DC motors.
Synchronous motors, like induction motors, are a common AC motor. The characteristic is: in steady state operation, there is a constant relationship between the rotor speed and the grid frequency n=ns=60f/p, ns is called synchronous speed. If the frequency of the grid is constant, the speed of the synchronous motor at steady state is constant and independent of the magnitude of the load.
