Permanent magnet DC motors use permanent magnets to create the magnetic field required for the motor, eliminating the need for a separate power source for excitation. In the electric motor courseware section, when explaining the principles of electric motors, the magnetic field is described as being generated by permanent magnets; the eight-slot DC motor model uses a U-shaped permanent magnet to generate the magnetic field. In the past, due to the poor magnetic properties of permanent magnets—their weak magnetic force and susceptibility to demagnetization—they were only used in low-power motors, primarily in toys and educational instruments. The rapid development of permanent magnet motors in the last decade or so is attributed to the rapid advancements in permanent magnet technology. Permanent magnet DC motors have expanded from toys, instruments, and household appliances to large motors in vehicles and other applications.
The working principle and structure of a permanent magnet DC motor are similar to those of a regular DC motor, except that permanent magnet poles are used instead of current-excited poles. This section introduces the structure of a small permanent magnet DC motor. The basic structure of a permanent magnet DC generator is the same and will not be described again.
This is a four-pole DC motor. The image below shows the four permanent magnet poles of the motor, arranged on the same circumference.
Permanent magnet poles
Neodymium iron boron (NdFeB) is currently the best permanent magnet material, possessing extremely strong magnetic force and high coercivity. High-performance permanent magnet motors often use NdFeB for their magnetic poles. The magnetic poles are fixed inside the housing, which acts as the magnetic yoke, providing the magnetic circuit for the motor.
permanent magnet DC motor stator
In the diagram below, the magnetic circuit of the motor is represented by blue magnetic field lines.
Magnetic circuit of permanent magnet DC motor
Permanent magnet motors do not require excitation current and generate less heat, but the housing is mostly sealed to prevent iron powder contamination. Heat is generally dissipated through multiple heat sinks on the outer periphery of the housing. The end cover plate extends outside the housing and also serves as the motor feet. The whole structure constitutes the stator frame of the motor, and the motor is mounted and fixed by the motor feet.
permanent magnet motor stator housing
The rotor of a permanent magnet DC motor consists of a rotor core, rotor windings, commutator, and shaft, just like the rotor of a regular DC motor. For more information, please refer to the DC motor section of the motor courseware.
permanent magnet DC motor rotor
Current flows through the brushes and connects to the rotor windings. A four-pole motor typically has four sets of brushes, all mounted on a brush holder to form the brush assembly. The diagram below shows the relative positions of the rotor and the brush assembly.
permanent magnet DC motor rotor and brush
Insert the rotor and brush assembly into the stator, as shown in the figure below.
Permanent magnet DC motor assembly diagram
The image below is a cross-sectional view of a permanent magnet DC motor with the end cover closed.
Permanent magnet DC motor structure diagram
The image below shows the appearance of a permanent magnet DC motor.
