Basic Structure Of A Three-Phase Asynchronous AC Motor

A three-phase asynchronous motor consists of two basic parts: a fixed stator and a rotating rotor. The rotor is housed within the stator cavity and supported by bearings on two end covers. To ensure the rotor can rotate freely within the stator, a gap, called the air gap, must exist between the stator and rotor. The air gap is a very important parameter of the motor; its size and symmetry significantly affect the motor's performance.

Stator: The stator consists of the three-phase stator windings, the stator core, and the frame.

The three-phase stator windings are the electrical circuit of the asynchronous motor and play a crucial role in its operation, being the key component in converting electrical energy into mechanical energy. The stator three-phase windings have a symmetrical structure, typically with six terminals U1, U2, V1, V2, W1, and W2, housed in a junction box outside the motor frame. They are connected in a star (Y) or delta (△) configuration as needed. The stator core is part of the asynchronous motor's magnetic circuit. Because the main magnetic field rotates relative to the stator at synchronous speed, to reduce losses in the core, it is made of 0.5mm thick high-permeability silicon steel sheets. Both sides of the silicon steel sheets are coated with insulating varnish to reduce eddy current losses.

The motor frame, also known as the casing, primarily supports the stator core and bears the reaction force generated by the entire motor under load. Heat generated by internal losses during operation is also dissipated through the frame. Medium and small motor frames are generally made of cast iron. Large motors, due to their larger size and the inconvenience of casting, are often welded from steel plates.

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

The rotor core is also part of the motor's magnetic circuit and is also made of stacked silicon steel sheets. Unlike stator core laminations, rotor core laminations have slots cut into their outer circumference. The stacked rotor core has numerous uniformly shaped slots on its outer cylindrical surface to house the rotor windings.

The rotor windings are another part of the asynchronous motor circuit. Their function is to cut the stator magnetic field, generating induced electromotive force and current, and under the influence of the magnetic field, causing the rotor to rotate. Their structure can be divided into two types: squirrel-cage windings and wound-rotor windings. The main characteristics of these two types of rotors are: squirrel-cage rotors are simple in structure, easy to manufacture, economical, and durable; wound-rotor rotors have a complex structure and are expensive, but external resistance can be introduced into the rotor circuit to improve starting and speed regulation performance.

The squirrel-cage rotor winding consists of conductor bars placed in the rotor slots and end rings at both ends. To save steel and improve productivity, the conductor bars and end rings of small-power asynchronous motors are generally cast from molten aluminum in one piece; for high-power motors, because the quality of cast aluminum is difficult to guarantee, copper bars are often inserted into the rotor core slots, and end rings are then welded to both ends. The squirrel-cage rotor windings close automatically, requiring no external power supply. Its shape resembles a cage, hence the name.

Air Gap: The air gap in an asynchronous motor is very small, typically 0.2–2 mm for small and medium-sized motors. A larger air gap results in greater magnetic reluctance, requiring a larger excitation current to generate the same magnetic field. Due to the air gap, the magnetic reluctance of an asynchronous motor is much greater than that of a transformer, thus the excitation current of an asynchronous motor is also much larger. The excitation current of a transformer is approximately 3% of its rated current, while that of an asynchronous motor is approximately 30% of its rated current. Since the excitation current is reactive, a larger excitation current is more desirable.