Basic Structure Of Three-phase Asynchronous Motor

A three-phase asynchronous motor consists of two basic parts: a fixed stator and a rotating rotor. The rotor is installed in the stator cavity and supported on two end caps with bearings. In order to ensure that the rotor can rotate freely inside the stator, there must be a gap between the stator and the rotor, called an air gap. The air gap of an electric motor is a very important parameter, and its size and symmetry have a significant impact on the performance of the motor.
stator
The stator is composed of three-phase windings, stator iron core, and machine base.
The stator three-phase winding is the circuit part of an asynchronous motor, which 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 three-phase winding is symmetrical, generally having six output terminals U1 U2,V1,V2,W1,W2, Place it inside the junction box on the outside of the machine base and connect it into a star shape (Y) or a triangle shape (△) as needed.
The stator core is a part of the magnetic circuit of asynchronous motors. Due to the synchronous rotation of the main magnetic field relative to the stator, in order to reduce the losses caused in the core, the core is made of 0.5mm thick high permeability silicon steel sheets stacked together. The silicon steel sheets are coated with insulation paint on both sides to reduce the eddy current losses of the core.
The machine base, also known as the casing, is mainly used to support the stator iron core and bear the reaction force generated during the operation of the entire motor load. The heat generated due to internal losses during operation is also dissipated outward through the machine base. The base of small and medium-sized electric motors is generally made of cast iron. Large electric motors are often formed by welding steel plates due to their large body size and inconvenient pouring.
rotor
The rotor of an asynchronous motor consists of a rotor iron core, a rotor winding, and a shaft.
The rotor iron core is also a part of the magnetic circuit of the electric motor and is made of stacked silicon steel sheets. Unlike stator core laminations, rotor core laminations are slotted on the outer circumference of the laminations. After stacking, many uniformly shaped slots are formed on the outer cylindrical surface of the rotor core to place the rotor winding.
The rotor winding is another part of the asynchronous motor circuit, which functions to cut the stator magnetic field, generate induced potential and current, and rotate the rotor under the force of the magnetic field. Its structure can be divided into two types: cage winding and wound winding. The main characteristics of these two types of rotors are: cage rotor structure is simple, easy to manufacture, economical and durable; The structure of the wound rotor is complex and expensive, but the rotor circuit can introduce external resistors to improve the 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 low-power asynchronous motors are generally cast in one go from melted aluminum liquid; For high-power motors, due to the difficulty in ensuring the quality of cast aluminum, copper bars are commonly inserted into the rotor core slots and end rings are welded at both ends. The cage rotor winding closes by itself and does not need to be powered by an external power source. Its appearance resembles a cage, hence it is called a cage rotor.
air gap
The air gap of asynchronous motors is very small, usually 0.2-2mm for small and medium-sized motors. The larger the air gap, the greater the magnetic resistance. To generate a magnetic field of the same magnitude, a larger excitation current is required. Due to the presence of air gaps, the magnetic reluctance of asynchronous motors is much larger than that of transformers, and therefore the excitation current of asynchronous motors is much larger than that of transformers. The excitation current of a transformer is about 3% of the rated current, and the excitation current of an asynchronous motor is about 30% of the rated current. The excitation current is reactive current, therefore the larger the excitation current.