Most domestic motor manufacturers prefer cast iron frames, but with the continuous extension of national environmental protection policies, box-type frames will extend from large-scale products to smaller ones, especially for small-batch products with special requirements. Box-type frames can compensate for the time and cost of casting molds for cast iron frames. Compared with corrugated steel plate frames, box-type frames are more widely used in large motors in my country. Today, Ms. Can will specifically discuss motors with box-type frame structures.
Introduction to Box-Type Motors
Starting in the late 1950s, Westinghouse Electric Corporation (WH) in the United States first developed the F/A series of box-type motors. Subsequently, the box-type structure was widely used in medium and large AC and DC motors abroad. In my country, box-type motors have been developed since the 1970s. Therefore, all newly designed basic series of medium-sized high-voltage induction motors in recent years have adopted the box-type structure.
There are many specific types of box-type motor structures, with variations in the shape, processing, and assembly methods of their components. The base of a box-type motor is typically welded from steel plates. Other major components include the outer casing, base, stator, rotor, and bearings. The outer casing, not bearing load, serves only as protection and forms part of the cooling system; it can be made of steel plates approximately 2mm thick. The outer casing can be a single piece or composed of upper and lower parts. The base must bear the weight of the entire motor and various forces or torques, requiring high strength and rigidity; it is usually welded from thicker steel plates (casting is also possible). The stator is an independent component completely separate from the base and outer casing, integrally integrated into the base. The shaft support can use hydraulic bearings or sliding bearings, both mounted in independent bearing housings. For ease of disassembly, the bearing housing is divided into upper and lower halves at the shaft centerline.
Main advantages of box structure
First, the components are highly interchangeable. Not only are motors within the same series interchangeable, but synchronous motors and induction motors are also interchangeable, as in the Westinghouse F/A series mentioned above. In the early 1980s, my country introduced Siemens technology to produce six types of large rolling mill DC motors, and apart from the bearings, the stator, rotor, brush holder, end cover, and base frame were all interchangeable.
Secondly, for motors with split enclosures, various protection types and different cooling systems can be easily derived simply by using different enclosures. In this way, by merely changing the local structure of the motor enclosure, the needs of different users can be met, thereby shortening the production cycle and reducing production costs.
Third, it enables the external pressing process for stator cores in small and medium-sized motors. This allows for the simultaneous processing of stator core pressing, unwinding, impregnation, and base and outer casing on two separate production lines, shortening the production cycle. Furthermore, the externally pressed stator cores facilitate the use of integral, vacuum, pressure impregnation (VP1) processes after unwinding, improving insulation quality and reducing winding temperature rise. In addition, using appropriate expansion molds during stator core stacking can completely eliminate the need for machining the inner diameter of the core (usually done on a vertical lathe), reducing the no-load additional losses of the stator teeth.
Fourth, it requires less machining, is easy to remove from the production line, and is easy to disassemble. The box-type steel plate base is much lighter than the traditional cast iron base and has an aesthetically pleasing design. Furthermore, the large footprint, poor working conditions, and severe environmental pollution of foundries, coupled with high remediation costs, have further spurred the development of box-type steel plate motors.
Generally, the vibration absorption performance of a box-type steel plate base is worse than that of a cast iron base. Therefore, attention should be paid to reducing motor vibration and noise during design and manufacturing.
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