Design and Technology of Permanent Magnet Direct Drive Motor

1. Energy-saving principle of permanent magnet motor

Compared with the induction motor, the permanent magnet direct drive motor does not require reactive excitation current, which can significantly improve the power factor, reduce the stator current and stator loss, and have no rotor resistance loss during stable operation. Therefore, while reducing the total loss, it can reduce the fan and the corresponding wind friction loss, so that its efficiency is 10 to 15 percentage points higher than that of the induction motor of the same specification. In addition, the permanent magnet synchronous motor can maintain a high efficiency and power factor in the range of 25% to 120% of the rated load, making it more energy-saving in light load operation.

2. Selection of rotor structure of permanent magnet direct drive motor

The magnetic pole form of the permanent magnet motor can be divided into surface type and built-in type according to the installation form; according to the excitation direction of the permanent magnet, it can be divided into radial structure, tangential structure and mixed structure. Direct drive permanent magnet synchronous motor rotor permanent magnet structure form.

Due to the large number of poles of the direct drive permanent magnet synchronous motor, the design and process of the hybrid motor are complicated, so it is used less. From the optimization point of view of the low-speed and high-torque drive system composed of the matching operation of the inverter SPWM power supply and the permanent magnet motor, in order to ensure that the drive system has sufficient linear adjustment range, the rated output frequency of the SPWM inverter should be as high as possible (generally above 25Hz); In order to reduce the cost and loss of the inverter, the rated output current of the inverter 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 strength of the magnetic field provided by a permanent magnet is related to its excitation area. The magnetic flux under one pole of the tangential structure is provided by two adjacent poles in parallel, which can obtain a larger excitation area. Therefore, the tangential structure is very suitable for multi-pole permanent magnet direct drive synchronous motor.

3. Selection of winding structure of permanent magnet direct drive motor

When the low-speed high-torque motor adopts fractional slot winding, the pole slot is generally selected to match, so that the number of slots per pole per phase Q<1. Compared with the motor of the integer slot winding structure, the advantages of the fractional slot winding, especially the fractional slot winding with the number of slots per pole per phase Q<1, are reflected in:

1) The small amplitude of the cogging torque of the fractional slot winding is beneficial to reduce the torque ripple of the motor, improve the speed regulation accuracy, reduce the vibration and noise of the motor, improve the distribution effect of the winding, and improve the sine of the induced back emf of the motor;

2) The number of slots under each pole of the motor is reduced, and the smaller slots are replaced by smaller slots. The effective utilization area of the stator slots is larger, which can shorten the length of the coil ends;

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 only wound on one tooth, which shortens the circumference of the coil and the extension length of the winding end, reduces the copper consumption of the motor, saves production costs, and improves the efficiency of the motor.