A permanent magnet brushless DC motor is an electromechanical integrated device that combines the motor body, controller , and rotor position sensor. The operating characteristics of the motor are the result of the combined action of all its components. To meet certain technical requirements, the specific operating mode of the permanent magnet brushless DC motor needs to be determined from a system perspective. This mainly includes the number of motor phases, winding connection method, inverter topology, winding energization method, and rotor position detection method.
Currently, three-phase permanent magnet brushless DC motors are the most widely used. When the inverter uses a half-bridge structure with a three-phase, three-state operating mode, it is mostly used for small-power, high-speed motors; when the inverter uses a bridge structure with a three-phase, six-state operating mode, it can be applied to various drive systems. Because the winding electromotive force is non-sinusoidal and contains a large number of high-order harmonics, the three-phase windings are mostly connected in a star configuration. When the requirements for motor size are not high and the environment is not harsh, a position sensor is generally used to detect the rotor position, giving the motor good starting characteristics, overload resistance, shock resistance, and good dynamic characteristics. Permanent magnet brushless DC motors operating at constant speed can use a sensorless control method. Since no rotor position sensor is needed, the motor size is reduced, and the cost is lowered, but starting is difficult and dynamic characteristics are poor.
The selection of electromagnetic load is important because the armature of a permanent magnet brushless DC motor is the stator, and the heat dissipation conditions of its windings are better than those of a DC motor. Therefore, its electrical load can be appropriately higher than that of a DC motor. The magnetic load Bδ depends on the matching relationship between the permanent magnet and its external magnetic circuit. The geometry, performance, and magnetization method of the permanent magnet also have a significant impact on Bδ. Generally, when using sintered NdFeB permanent magnets, the air gap magnetic flux density can be 0.7~0.9T; when using bonded NdFeB, the air gap magnetic flux density can be 0.35~0.45T.
The determination of the number of poles and slots in a motor is based on the determination of the rotor outer diameter, effective core length, and air gap magnetic flux density. Once these parameters are determined, the magnetic flux around the air gap is also determined. Choosing a higher number of poles reduces the magnetic flux per pole and the cross-sectional area of the motor yoke; simultaneously, the winding ends are shortened, reducing the amount of copper used and the winding inductance, which is beneficial for winding current commutation. However, too many poles will increase the inter-pole leakage flux of the rotor's hydromagnetic poles, reducing the utilization rate of the permanent magnets. At the same speed, more poles result in a higher alternating frequency of the magnetic field within the motor core, leading to increased iron losses and decreased efficiency. As the alternating frequency of the current increases, the switching frequency of the inverter switches also increases, increasing switching losses. Therefore, increasing the number of poles reduces the overall efficiency of the motor. The determination of the number of poles should comprehensively consider both the motor's performance and economic efficiency. During motor design, several pole numbers can be selected, motor characteristics calculated, and the appropriate number of poles determined after comprehensive performance comparison.
Once the number of motor poles is determined, the number of stator slots can be selected by referring to motor design principles. Generally, two options are available: integer slot structure and fractional slot structure. The fractional slot structure effectively reduces cogging torque, and its pole-slot combinations are diverse. Fractional slot motors have good manufacturability and are suitable for mass-produced small-power motors, but their permanent magnet utilization rate is lower. Integer slot structure motors are mostly used in higher-power motors, and their permanent magnet material utilization rate is higher, but appropriate measures need to be taken to reduce tooth phase torque.
After determining the main parameters of the motor, the operating characteristics of the motor can be calculated using the field-circuit coupled electromagnetic design method. The calculation results are then compared with the design requirements. If the requirements are not met, the corresponding design parameters are adjusted and the calculation is repeated until the design requirements are met.
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