“Asynchronous motors are induction motors with different rotor speed n and stator rotating magnetic field n1. The working principle of the asynchronous motor is simply: when nn1, the mechanical energy is converted into electric energy, which is represented as a generator. When n and n1 are reversed, the electric energy and mechanical energy are all converted into the internal iron core and winding losses of the motor, which is represented as a braking state.

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Asynchronous motors are induction motors with different rotor speed n and stator rotating magnetic field n1. The working principle of the asynchronous motor is simply: when nn1, the mechanical energy is converted into electric energy, which is represented as a generator. When n and n1 are reversed, the electric energy and mechanical energy are all converted into the internal iron core and winding losses of the motor, which is represented as a braking state.

1. Types of asynchronous motors

1. According to the number of stator power phases: single-phase asynchronous motor, two-phase asynchronous motor, three-phase asynchronous motor;

2. According to the rotor structure: squirrel cage asynchronous motor, wound asynchronous motor;

Second, the working principle of asynchronous motor

1. Structure of asynchronous motor

The asynchronous motor is composed of two basic units: a fixed stator and a rotating rotor. The stator part mainly includes the stator core, stator winding, and base. The rotor part mainly includes the shaft, rotor core, and rotor winding (closed conductor). There are evenly distributed slots on the inner circle of the stator core and the outer circle of the rotor core. , Its role is to embed windings, as shown in Figure 1.

Figure 1 Structure diagram of asynchronous motor

There is a small air gap between the rotor and the stator (usually around 0.2~1.5mm for medium and small asynchronous motors) to ensure that the rotor can rotate freely in the stator.

2. Generation of rotating magnetic field of asynchronous motor

Taking a three-phase asynchronous motor as an example, Figure 1 connects the two radial ends of the stator core to form a phase. For example, the start end A and the terminal X are connected as a phase, the start end B and the terminal Y are connected as a phase, and the start end C is connected. It is connected with terminal Z as a phase, which constitutes a three-phase motor, and three-phase symmetrical alternating current is passed to the stator three-phase winding, that is, the amplitude is equal, the frequency is equal, and the phase is different, as shown in Figure 2.

Figure 2 Three-phase symmetrical alternating current

A three-phase symmetrical current flows through the three-phase stator windings to generate a rotating magnetic field in space, as shown in Figure 3.

Figure 3 The formation of the rotating magnetic field of an asynchronous motor

When the alternating current phase θ=00, IA=0, IB is a negative value, and IC is a positive value. At this time, there is no current at the terminal A and X, the winding BY current flows from Y into B, and the winding CZ current flows from C. Z flows out, and the direction of the magnetic line of induction in Figure (a) is obtained by the ampere right-hand screw rule (as shown by the dotted arrow).

When the AC phase θ=1200, IB=0, IA is positive, and IC is negative. At this time, there is no current at the wire ends B and Y, the winding AX current flows from A to X, and the winding CZ current flows from Z to C Outflow, the direction of the magnetic line of induction in Figure (b) is obtained by the right-hand spiral rule of Ampere (as shown by the dotted arrow).

When the alternating current phase θ=2400, IC=0, IA is negative, and IB is positive. At this time, there is no current at the terminal C and Z, and the winding AX current flows from X into A and the winding BY current flows from B into Y and flows out. , According to the right-handed spiral rule of Ampere, the direction of the magnetic line of induction in figure (c) is obtained (as shown by the dashed arrow).

When the alternating current phase is the same as the state in Figure (a), repeat the previous process, it can be seen that the three-phase alternating current in the stator winding changes for one cycle, and the composite magnetic field of the space also changes with the phase sequence of the alternating current for one week. Change, the space composite magnetic field keeps rotating, so a rotating magnetic field is formed. The direction of the rotating magnetic field is determined by the phase sequence of the alternating current connected to the stator, as long as the phase sequence of the alternating current is changed (for example, the connected phase A is changed to the phase B). The phase sequence of the rotating magnetic field changes accordingly.

3. The working principle of asynchronous motor

Because the rotating magnetic field continuously cuts the closed conductor in the rotor, induced electromotive force and induced current are generated, and then the electromagnetic torque is generated by the interaction of the induced current in the rotor and the rotating magnetic field, so that the rotor rotates in the same direction with the direction of the rotating magnetic field. For example, in a cage asynchronous motor, since the rotating magnetic field cuts the rotor conductor clockwise, it is equivalent to the conductor rotating counterclockwise. Using the right-hand rule, let the magnetic line of induction penetrate the palm of the hand perpendicularly, with the thumb pointing to the direction of the conductor, and the four-finger direction is the induced current (As shown in Figure 4, the direction of the rotor winding current of the cage asynchronous motor), and then apply the left-hand rule, the magnetic line of induction passes through the palm of the hand, and the four fingers point to the direction of current movement. The thumb direction is the direction in which the rotor receives the electromagnetic force. (As shown in the figure, the electromagnetic torque torque is in the T direction), the electromagnetic torque is formed under the action of electromagnetic force, and the rotor is driven to rotate in the direction of the rotating magnetic field.

Figure 4 Sectional view of cage asynchronous motor

In an asynchronous motor, in order to keep the rotating magnetic field always cutting the rotor conductor to generate an induced current, the rotor speed is lower than the speed of the rotating magnetic field.