Due to their wide range of applications, motors come in many varieties, series, and specifications, and their structures also vary. However, regardless of the type, motors structurally consist of three main parts: the stationary part, the rotating part, and the transition parts. The stationary part is usually called the stator, the rotating part is called the rotor, and the transition parts include electrical transitions (such as slip rings, commutator, and brush mechanism) and mechanical transitions (such as end covers and bearings). The main components of the stationary and rotating parts are the core and windings, followed by structural components used for mechanical support or fastening. Today, Ms. will briefly describe the typical structural characteristics of asynchronous motors.
Structure of a closed motor
Taking a standard B3 motor as an example, its basic characteristics are: horizontal design, base with feet, two end-cap bearings, single shaft extension (double shaft extension is also possible upon request), and internal and external fans. The base is made of cast iron with cooling fins to increase the heat dissipation area and improve cooling conditions. The top of the base has eye bolts for lifting, and the bottom has two small round holes for draining condensate. Looking from the drive end, there is a square junction box on the right side of the base for introducing the power cable. The inner cavity of the base is cylindrical without longitudinal ribs, ensuring good contact with the stator core and facilitating heat dissipation.
The stator core is axially secured in dovetail grooves on its outer circumference using fasteners. This simple structure ensures that the outer circumference of the core is almost entirely in contact with the inner circumference of the frame, which is beneficial for heat dissipation. This structure can be achieved using an external press-fit process.
The aluminum cage rotor is fitted to the shaft with a heat-shrink sleeve (or the shaft can be knurled and then cold-pressed for assembly). The end ring and the blades of the internal fan are integrated, simplifying the manufacturing process and improving reliability. Several small pillars for placing balance weights during balancing are also cast on the end ring. The internal fan accelerates the circulation of hot air inside the machine, facilitating better heat exchange with the base and end covers. The external fan is a radial centrifugal fan made of aluminum or plastic, and its shroud is fixed to the base with screws.
The front and rear end caps are identical, and the bearings are single-row radial ball bearings.
Protective motor structure
The basic features of this motor are: a base with feet, two end-cap bearings, and a generally symmetrical radial ventilation system (usually self-ventilated, but duct ventilation can be made as needed). The core contains ventilation channels. The base is made of cast iron with longitudinal ribs on the inner circumference to form ventilation channels with the outer circumference of the core. For large-size motors, due to their greater weight, there are two eye bolts on the top of the base. Viewed from the drive end, the terminal box is located in the center of the right side of the base. The stator core of large-size motors uses an internal press-fit structure, meaning the laminations are directly stacked inside the base. After stacking, they are secured axially with pressure rings and arc-shaped keys.
The stator cores of 3 kV and 6 kV motors use open slots, while those below 500 V use semi-open or semi-closed slots. The ends of the stator coils are secured with spacer pads and binding ropes, or with polyester-sheathed fiberglass ropes. For higher speed motors, the ends of the stator coils are further reinforced with insulated end clamps to enhance their ability to withstand the electromagnetic forces generated by the inrush current during startup.
The rotor core is directly fitted onto the shaft and fixed circumferentially and axially using flat keys and ring keys. The yoke has axial ventilation channels to facilitate cooling. When the motor capacity is large, the rotor core is often mounted on a support, with sufficient axial ventilation channels between the core and the support ribs and loads. Each end of the rotor core has a pressure ring (fixed to the support by an arc key) to secure the core axially and support the coil ends; grooves are also cut into these rings to hold counterweights.
The rotor employs a double-layer wave winding, with semi-formed, insulated copper busbars (called half-coil) inserted into semi-closed slots, bent, and then connected. Except for the slot containing the slanted conductor (also called a skip-layer coil), each slot typically has two conductors. The coil ends are secured with non-woven fiberglass tape. The rotor winding leads are routed via cables through the central hole of the shaft from the non-drive end, then connected to the slip rings. This shortens the distance between bearings and reduces shaft deflection. Several fan blades, evenly spaced along the circumference, are mounted on the parallel connectors at the ends of the rotor's half-coil windings, functioning as a fan.
The slip rings and brush assembly are protected by a protective cover. During startup, the brushes contact the slip rings to allow for the connection of an external starting resistor. After startup, the handle is moved to the running position, the brushes and slip rings separate, and the three slip rings are short-circuited by the short-circuit ring. Some wound-rotor motors do not have a brush lifting device; in this case, the brushes are always in contact with the slip rings, resulting in increased mechanical losses and brush wear during operation, but the motor structure is relatively simple.
To prevent localized air circulation, baffles are installed inside the end covers. Utilizing the air pressure generated by the fan blades at both ends of the rotor windings and the radial ventilation channels in the rotor core, cooling air enters the motor through the air inlets below the end covers on both sides. Part of the air blows across the stator winding ends and enters the back of the core, while the other part passes through the radial ventilation channels in the rotor and stator cores and enters the back. Both then converge and escape through the air outlet at the bottom of the frame.
Due to the larger load on the transmission end, a single-row radial short cylindrical roller bearing is used there. A single-row radial ball bearing is used on the non-transmission end. Roller bearings can also automatically adapt to axial movement caused by shaft expansion and contraction due to temperature changes, as well as axial movement caused by permissible machining and assembly deviations of components. To facilitate assembly and disassembly, and to keep the bearings clean during assembly, disassembly, and maintenance, both bearings are fitted with bearing sleeves.
Box-type motor
This is a promising new structural type that has emerged in recent years. It is currently widely used in both AC and DC motors abroad, especially in medium and large asynchronous motors.
The main components of a box-type motor include the housing, base, stator, rotor, and bearings. Since the housing does not bear any load, its structure is lightweight; it can be a single piece or composed of upper and lower parts. Typically, for split housings, the lower half of the housing is identical for motors of the same frame size, but the upper half varies depending on the motor's protection type and cooling method. The base, needing to bear the weight of the entire motor and related forces, requires high strength. Depending on the situation, the motor bearings can be rolling or sliding bearings. The bearing housing is divided into upper and lower halves at the center of the shaft for easy disassembly.
The advantages of the box-type structure are mainly: (1) The components are highly interchangeable (e.g., the base, iron core, bearing housing, etc. are all interchangeable). (2) By using different outer covers, motors with different protection types and cooling methods can be easily derived, making standardization easy. (3) The stator iron core is externally press-fitted, so the stator iron core press-fitting, unloading, impregnation and base and outer cover processing can be carried out simultaneously, shortening the production cycle; and it is also convenient to use the overall vacuum and pressure impregnation process to improve the insulation quality. (4) Less machining, high air gap uniformity, convenient unloading and easy disassembly.
Since the 1970s, my country has also begun to adopt this structure in medium-sized asynchronous motors, but some problems still need further research and solutions. For example, there are currently two structures: one with adjustable air gap during assembly and one without. Which one is better needs further research and resolution. The former is simpler to machine, but the assembly workload is large; the latter's machining workload is not necessarily less than that of the ordinary structure, and it requires some special machine tools. Secondly, with the box-type structure, the amount of cast iron parts is reduced, but the amount of steel plate used increases. In addition, in terms of total manufacturing time, it significantly reduces high-voltage motors, but increases low-voltage motors, and so on.
