Three Magnetic Circuit Classification Methods for Permanent Magnet Motors

Classified by the position of the permanent magnet

According to the different positions of permanent magnets, they can be divided into rotating pole type and rotating armature type. Figure (a) shows a rotating pole magnetic circuit structure, where the permanent magnet is on the rotor and the armature is stationary. This structure is used in both permanent magnet synchronous motors and brushless DC motors; Figure (b) shows the zone magnetic circuit structure of a rotating motor, where the permanent magnet is on the stator and the armature rotates. The permanent magnet DC motor adopts this structure.

Classified by the type of permanent magnet material used

According to the types of permanent magnet materials in the motor, it can be divided into single structure and hybrid structure. In a motor, using only one type of permanent magnet material is called a single structure, and the vast majority of motors use this type of structure. If two or more permanent magnet materials are used in the same motor, it is called a hybrid structure. Hybrid structures typically use two types of permanent magnets with different performance characteristics, leveraging strengths and avoiding weaknesses, fully leveraging the advantages of permanent magnet materials, improving motor performance, and reducing manufacturing costs. The following figure shows the hybrid magnetic pole structure in a permanent magnet DC motor. The low coercivity permanent magnet 1 (such as ferrite) is placed in the front of the magnetic pole, and the high coercivity permanent magnet 2 (such as neodymium iron boron) is placed in the rear of the magnetic pole.

Classification by permanent magnet placement method

According to the different placement methods of permanent magnets, they can be divided into surface mounted (surface mounted) and built-in (embedded), as shown in the following figure. The permanent magnet with surface mounted magnetic poles faces the air gap directly, which has the advantage of convenient processing and installation. However, the permanent magnet directly bears the demagnetization effect of the armature reaction; The water magnet with built-in magnetic poles is placed inside the iron core, and the processing and installation process is complex, resulting in high magnetic leakage. However, more permanent magnets can be placed to improve the air gap magnetic density, reduce the weight and volume of the motor.

Classified by the shape of permanent magnets

When designing permanent magnets, it is necessary to ensure that they generate sufficient magnetic flux and electromotive force in the magnetic circuit. If different permanent magnet materials are used, the shape of the permanent magnet will also vary. Aluminum nickel cobalt has a high residual magnetic density and low coercivity, usually made into slender shapes; Ferrite and rare earth permanent magnet materials have high coercivity. Due to their relative recovery permeability close to 1 and high magnetic resistance, when the length of the magnetization direction increases to a certain extent, the length of the magnetization direction continues to increase, and the magnetic flux provided by the permanent magnet to the outside world increases very little. Therefore, a flat structure is usually used. According to the different shapes of permanent magnets, they can be divided into tile shaped, arc-shaped, annular, claw pole shaped, star shaped, and rectangular magnetic poles.