Talking about the design characteristics of permanent magnet brushless DC motor

The permanent magnet brushless DC motor is an electromechanical integrated device integrating the motor body, the controller and the rotor position sensor. The working characteristics of the motor are the result of the interaction of the various components. To meet certain technical requirements, it is necessary to From the angle, the specific working mode of the permanent magnet brushless DC motor is determined, which mainly includes the number of motor phases, the winding connection mode, the inverter topology, the winding energization mode, and the rotor position detection mode.

At present, three-phase permanent magnet brushless DC motors are the most widely used. When the inverter adopts the three-phase three-state working mode of the half-bridge structure, it is mostly used for small-power high-speed motors; when the inverter adopts the three-phase six-state working mode of the bridge structure, it can be applied to various driving systems. Since the winding electromotive force is non-sinusoidal and contains a large number of higher harmonics, the three-phase windings are mostly connected by a star. When the motor volume is not high and the environment is not bad, the position sensor is generally used to detect the rotor position, so that the motor has better starting characteristics, anti-overload and impact resistance, and better dynamic characteristics. The permanent magnet brushless DC motor running at constant speed can adopt the position sensorless control mode. Since the rotor position sensor is not needed, the volume of the motor is reduced and the cost is reduced, but the starting is difficult and the dynamic characteristics are poor.

Electromagnetic load selection

Since the armature of the permanent magnet brushless DC motor is the stator, the heat dissipation condition of the winding is better than that of the DC motor, so the electric load selection can be appropriately higher than that of the DC motor. The magnetic load Bδ depends on the matching relationship between the permanent magnet and its external magnetic circuit. At the same time, the geometry, performance and magnetization mode of the permanent magnet also have a great influence on Bδ. Under normal circumstances, when using sintered NdFeB permanent magnet, the air gap magnetic density can be 0.7~0.9T; when bonding NdFeB, the air gap magnetic density can be 0.35~0.45T.


Determination of the number of poles and the number of slots

When the outer diameter of the rotor, the effective length of the core and the magnetic gap of the air gap are determined, the magnetic flux around the air gap in the motor is determined. If more poles are selected, the magnetic flux per pole is reduced, and the cross-sectional area of the yoke of the motor can be reduced. At the same time, the winding ends are shortened, the amount of copper is reduced, and the winding inductance is reduced, which is beneficial to the winding current commutation. However, if the number of poles is selected too much, the leakage flux between the poles of the rotor magnetic poles will increase, and the utilization of the permanent magnets will be reduced. At the same speed, the more the number of poles, the higher the alternating frequency of the magnetic field in the motor core, resulting in an increase in the iron loss of the motor and a decrease in efficiency. As the alternating current frequency increases, the switching frequency of the inverter switch increases, and the switching loss increases. Therefore, the overall efficiency of the motor decreases after the number of poles increases. The determination of the number of poles should take into account the performance and economy of the motor. When designing the motor, you can select several pole numbers and calculate the motor characteristics. After comprehensively comparing the performance, determine the appropriate number of poles.


After the number of motor poles is determined, the number of stator slots can be selected with reference to the principle of motor design. There are generally two options to choose from, namely an integer slot structure and a fractional slot structure. The fractional groove structure can effectively reduce the cogging torque, and the number of pole slots can be variously matched. Among them, the fractional slot motor has good manufacturability and is suitable for high-power production of small-power motors, but its permanent magnet utilization rate is low. Integral slot structure motors are often used in motors with higher power, and the utilization of permanent magnet materials is higher, but appropriate measures are needed to weaken the tooth phase torque.

After determining the main parameters of the motor, the operating characteristics of the motor can be calculated according to the field-circuit coupled electromagnetic design method, and the calculation results are compared with the design index requirements. If the requirements are not met, the corresponding design parameters are adjusted and recalculated until the design requirements are met. .

Reprinted from the network