The three-phase asynchronous motor consists of two basic parts: a fixed stator and a rotating rotor. The rotor is installed in the inner cavity of the stator and is supported on two end covers with the help of bearings. In order to ensure that the rotor can rotate freely within the stator, there must be a gap between the stator and the rotor, called the air gap. The air gap of the motor is a very important parameter. Its size and symmetry have a great impact on the performance of the motor. Figure 2 shows the components of a three-phase cage asynchronous motor.

Stator

The stator consists of three-phase stator windings, stator core and frame.The stator three-phase winding is the circuit part of the asynchronous motor. It plays an important role in the operation of the asynchronous motor and is a key component for converting electrical energy into mechanical energy. The structure of the stator’s three-phase winding is symmetrical. There are generally six outlet terminals U1, U2, V1, V2, W1, and W2, which are placed in the junction box outside the machine base and connected in a star (Y) or triangle (Y) shape as needed. △), as shown in Figure 3.The stator core is part of the magnetic circuit of the asynchronous motor. Since the main magnetic field rotates relative to the stator at a synchronous speed, in order to reduce the loss caused in the core, the core is made of 0.5mm thick high-magnetic silicon steel sheets. Both sides of the silicon steel sheets are coated with insulation. Paint to reduce eddy current losses in the core.The base is also called the casing. Its main function is to support the stator core and also bear the reaction force generated when the entire motor is running under load. The heat generated due to internal losses during operation is also dissipated through the base. The bases of medium and small motors are generally made of cast iron. Large motors are inconvenient to cast due to their large body, so steel plates are often welded and formed.

The rotor of an asynchronous motor consists of a rotor core, a rotor winding and a rotating shaft.

The rotor core is also part of the motor’s magnetic circuit and is also made of silicon steel sheets. Different from the stator core punching, the rotor core punching has slots on the outer circle of the punching. After stacking, many slots of the same shape are evenly formed on the outer cylindrical surface of the stacked rotor core to place the rotor windings.
The rotor winding is another part of the asynchronous motor circuit. Its function is to cut the stator magnetic field, generate induced electromotive force and current, and receive force under the action of the magnetic field to make the rotor rotate. Its structure can be divided into two types: cage winding and wire-wound winding. The main features of these two rotors are: the cage rotor has a simple structure, is easy to manufacture, is economical and durable; the wound rotor has a complex structure and is expensive, but external resistance can be introduced into the rotor circuit to improve starting and speed regulation performance.
The cage rotor winding consists of guide bars placed in the rotor slots and end rings at both ends. In order to save steel and improve productivity, the guide bars and end rings of small-power asynchronous motors are generally cast from molten aluminum liquid at one time; for high-power motors, since the quality of cast aluminum is difficult to guarantee, copper bars are often inserted into the rotor core slots. , and then weld end rings on both ends. The cage rotor winding closes by itself and does not need to be powered by an external power supply. Its shape is like a cage, so it is called a cage rotor.

Air gap
The air gap of asynchronous motors is very small, generally 0.2 to 2mm for small and medium-sized motors. The larger the air gap, the larger the magnetic resistance. To generate a magnetic field of the same size, a larger excitation current is required. Due to the existence of the air gap, the magnetic circuit reluctance of the asynchronous motor is much larger than that of the transformer, so the excitation current of the asynchronous motor is also much larger than that of the transformer. The excitation current of the transformer is about 3% of the rated current, and that of the asynchronous motor is about 30% of the rated current. The excitation current is a reactive current, so the larger the excitation current is.