Magnetic circuit classification
Permanent-magnet circuits can be classified by the working state of the magnets into static permanent-magnet circuits and dynamic permanent-magnet circuits. A static permanent-magnet circuit has a fixed air gap and a steady operating state; it is typically found in instruments and loudspeakers. A dynamic permanent-magnet circuit has varying magnetic reluctance or an external magnetic field, so the magnet operating point changes accordingly. The magnetic circuit of a permanent-magnet motor is a typical example of a dynamic permanent-magnet circuit.
Permanent-magnet motors share the same armature structure as electrically excited motors; the primary difference is that the poles in permanent-magnet motors are permanent magnets. There are many magnetic-circuit forms for permanent-magnet motors and several ways to classify them.
1. Classification by magnet location
Depending on the magnet location, motors are classified as rotating-pole type and rotating-armature type. Figure (a) shows the rotating-pole configuration, where the permanent magnets are on the rotor and the armature is stationary; this structure is used in permanent-magnet synchronous motors and brushless DC motors. Figure (b) shows the rotating-armature configuration, where the permanent magnets are on the stator and the armature rotates; this structure is used in some permanent-magnet DC motors.
2. Classification by types of permanent-magnet materials used
By the number of magnet material types used, motors are classified as single-material structure and hybrid structure. A single-material structure uses one type of permanent magnet and is the most common. A hybrid structure uses two or more types of permanent magnets, combining different material properties to take advantage of their strengths and reduce cost. The illustration below shows a hybrid pole structure in a permanent-magnet DC motor: a low-coercivity magnet 1 (for example, ferrite) is placed at the front of the pole and a high-coercivity magnet 2 (for example, NdFeB) is placed at the rear of the pole.
3. Classification by magnet placement
By how the magnets are mounted, motors are classified as surface-mounted and interior-mounted. Surface-mounted magnets face the air gap directly, offering easier machining and assembly, but they are more exposed to demagnetization from armature reaction. Interior-mounted magnets are embedded within the core; they require more complex manufacturing and exhibit greater flux leakage, but allow more magnet material to be placed to increase air-gap flux density and reduce motor size and weight.
4. Classification by magnet shape
Magnet shape must ensure sufficient flux and magnetomotive force in the magnetic circuit. Different magnet materials favor different shapes. Alnico has high remanence and low coercivity and is often made long and slender. Ferrite and rare-earth magnets have high coercivity and a relative permeability close to 1, resulting in high reluctance; increasing magnet length beyond a certain point yields little additional external flux, so these materials are typically made flat. Common magnet shapes include segmental (tile), arc, ring, claw-pole, star, and rectangular poles.
(1) Segmental (tile)
Segmental poles are widely used in permanent-magnet motors. There are concentric-segment and equal-radius-segment types. Sintered NdFeB blanks are rectangular, so wire-cut machining is common; concentric-segment poles have lower material utilization, typically 40% to 50%. Equal-radius segment structures improve material utilization and reduce wire-cut costs, increasing magnet utilization to about 80%.
(2) Arc
Arc-shaped poles are magnetized along the arc. Each pole's flux is supplied in parallel by two magnet pieces; the long magnetization path suits Alnico magnets.
(3) Ring
Ring poles are a continuous circular ring, offering simple structure and easy machining and assembly; they are commonly used in permanent-magnet DC motors. Their main drawbacks are low material utilization and the presence of a magnetic field on the geometric neutral line, which is unfavorable for commutation in DC machines.
(4) Claw-pole
A claw-pole magnet assembly consists of a magnet ring and two flanges with claws. The magnet ring is magnetized axially; the flanges, typically made of low-carbon steel or stamped steel, have uniformly distributed claws with the number of claws equal to the pole-pair count. The magnet ring and flanges fit on the rotor shaft, with claws on the two flanges staggered by half a claw pitch to form alternating N and S poles. To prevent flux closure through the shaft, a nonmagnetic shaft or a magnetic shaft with a nonmagnetic sleeve is used. For long axial pole lengths, double- or multi-claw designs are used to ensure mechanical strength.
(5) Star-shaped
Star-shaped pole structures are relatively complex and are often produced by direct casting. Nonmagnetic material, such as aluminum alloy, is cast between poles to increase strength and provide damping. This type is easy to manufacture and assemble but is difficult to magnetize uniformly.
(6) Rectangular
Rectangular poles are made from one or several rectangular magnet blocks. They are simple to manufacture, easy to process, and have high material utilization. Interior permanent-magnet synchronous motors commonly use rectangular magnets. In magnet design, avoid a length-to-thickness ratio greater than 20 for rectangular magnets; ferrite magnet thickness should be greater than 2 mm.
