Compared with induction motors, permanent magnet direct drive motors do not require reactive excitation current, can significantly improve the power factor, reduce stator current and stator losses, and have no rotor resistance losses during stable operation. Therefore, while reducing total losses, they can also reduce The fan and corresponding wind friction loss making its efficiency 10 to 15 percentage points higher than that of induction motors of the same specifications. In addition, the permanent magnet synchronous motor can maintain high efficiency and power factor within the rated load range of 25% to 120%, making the energy-saving effect more significant during light load operation.
The magnetic pole forms of permanent magnet motors can be divided into surface type and built-in type according to the installation form; according to the excitation direction of the permanent magnet, they can be divided into radial structure, tangential structure and hybrid structure. The structure of the rotor permanent magnet of a direct drive permanent magnet synchronous motor.
Since the direct drive permanent magnet synchronous motor has a larger number of magnetic poles and the design and process of the hybrid motor are complex, it is less used. From the perspective of optimization of the low-speed and high-torque drive system composed of the inverter SPWM power supply and the matching operation of the permanent magnet motor, to ensure that the drive system has a sufficient linear adjustment range, the rated output frequency of the SPWM inverter should be as high as possible (generally in 25Hz or above); To reduce the cost and loss of the frequency converter, the rated output current of the frequency converter should be as small as possible. Therefore, a multi-pole structure should be used in the motor design to reduce the rated synchronous speed; in the case of large torque, if the rated current of the motor is reduced, each pole must have a strong enough excitation magnetic field. The magnetic field strength provided by the permanent magnet is related to its excitation area. In the tangential structure, the magnetic flux under one magnetic pole is provided by two adjacent magnetic poles in parallel, so that a larger excitation area can be obtained. Therefore, the tangential structure is very suitable for multi-pole permanent magnet direct drive synchronous motors.
When low-speed high-torque motors use fractional slot windings, pole-slot matching is generally selected so that the number of slots per pole per phase Q<1. Compared with motors with integer slot winding structures, the advantages of fractional slot windings, especially fractional slot windings with the number of slots per phase and pole Q<1, are reflected in:
1) The cogging torque amplitude of the fractional slot winding is small, which is beneficial for reducing the motor torque pulsation, improving the speed regulation accuracy, reducing the motor vibration and noise; improving the distribution effect of the winding, and improving the sinusoidal nature of the motor's induced back electromotive force;
2) The number of slots under each pole of the motor is reduced and smaller slots are used instead of smaller slots. The effective utilization area of the stator slots is larger and the length of the coil end can be shortened;
3) Fractional slot winding motors can obtain a concentrated winding design with a motor pitch of 1. At this time, each coil of the motor is wound around only one tooth, which shortens the circumference of the coil and the extension length of the winding end, reduces the copper loss of the motor, saves production costs, and improves motor efficiency.