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Differences between three-phase asynchronous motors and synchronous motors

Release time:

2024-04-18

Whether it is a synchronous motor or an asynchronous motor, they both consist of a stator and a rotor. As the name suggests, the stator is the fixed part of the three-phase asynchronous motor and the rotor is the rotating part of the motor.

　　The stator of a three-phase asynchronous motor is divided into a stator core and stator windings, and the rotor is also divided into a rotor core and rotor windings. For example, the figure below shows the stator core and stator winding (coil) of a motor.

　　The main function of the stator of a three-phase asynchronous motor: When the stator coils are connected to a three-phase alternating current, the stator coils generate a rotating magnetic field, which rotates at a speed related to the frequency of the power supply.

　　We can simply understand that when three-phase AC is applied to the stator coil, the stator coil will produce a rotating magnetic field, just like a magnet rotating in the stator core. If the power frequency is increased, the rotational speed of the magnet will also be increased; on the contrary, if the power frequency is decreased, the rotational speed of the magnet will be decreased. Therefore we can control the frequency of the power supply to adjust the rotational speed of the stator field of the motor.

　　Suppose at this point, if a small steel ball is placed in the inner ring of the stator core, three-phase alternating current is applied to the stator windings of a three-phase asynchronous motor. We can see that the small steel ball will adhere to the core and rotate with the magnetic field.

　　Synchronous and asynchronous motors have the same stator windings, the main difference being the rotor. Let's look at their basic principles.

　　Principle of synchronous motor

　　Assuming that permanent magnets are made into a three-phase asynchronous motor rotor, the rotor permanent magnets will rotate with the magnetic field of the stator windings because the rotor permanent magnets and the stator magnetic field are attracted to each other. The faster the magnetic field of the stator core rotates, the faster the rotor (motor) speed will be. Since the speed of the motor is the same as the speed of the magnetic field, it is called a synchronous motor.

　　Since the magnetic field strength of permanent magnets is relatively weak and the load is relatively light, many synchronous motor rotors use electromagnets instead of permanent magnets, i.e., the rotor is made into an electromagnet. The electromagnetic coil is called the excitation winding.

　　Principle of Asynchronous Motors

　　According to the structure of the rotor, asynchronous motors can be categorized into two types, i.e. squirrel-cage asynchronous motors and wire-wound rotor asynchronous motors. The commonly used squirrel-cage asynchronous motor is used here as an example.

　　The rotor of the squirrel-cage asynchronous motor is generally a cast aluminum rotor, and its windings are copper or aluminum strips with the ends of the windings welded together, just like a rat cage.

　　When the stator windings are connected to three-phase alternating current, the rotating magnetic field of the stator will cut the rotor windings, thus inducing current in the windings (aluminum strip). After passing through the windings (aluminum strip), the current will be accelerated by the electromagnetic force in the magnetic field.

　　As the three-phase asynchronous motor rotates faster and faster, the two remain relatively stationary when the rotor speed = stator magnetic field speed. At this point, there is no relative motion between the rotating magnetic field and the rotor windings (i.e., the rotor windings are not cut off), and the rotor will lose its electromagnetic force and decelerate. When the motor decelerates, the relative motion between the rotating magnetic field and the rotor winding reappears, and the rotor will be accelerated by the electromagnetic force.

　　Simply put, AC asynchronous motors obtain rotational force based on the relative motion between the rotating magnetic field and the rotor. Once the speed of the three-phase asynchronous motor reaches the speed of the magnetic field, it will lose rotational force and decelerate; the deceleration will be followed by relative motion, so it will accelerate again. Thus, the motor is always in a state of acceleration and deceleration. Since the speed of the motor does not coincide with the speed of the magnetic field, it is called an asynchronous motor.

　　Advantages and disadvantages of both and their applications

　　The speed of a synchronous motor is synchronized with the speed of the stator magnetic field. Irrespective of the load on the motor, the speed of the motor does not change as long as the motor does not go out of step. The motor speed can be precisely adjusted by simply adjusting the frequency of the power supply, which is suitable for precise speed regulation applications. Synchronous motors are not widely used as asynchronous motors and are mainly used for large generators due to their high precision, complexity of manufacture, high cost and maintenance difficulties.

　　The speed of an asynchronous motor is lower than the speed of the stator field, which is related to the load. The speed of the motor will vary with the load. Asynchronous motors are widely used because of their simplicity, low cost, ease of installation and durability. Asynchronous motors account for at least 80% of the motors in China.

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