CN106208555B - A kind of extended method of built-in brshless DC motor pole embrace - Google Patents

Abstract

The invention provides a method for expanding the pole arc coefficient of a built-in brushless DC motor. Firstly, the permanent demagnetization magnetic field strength is calculated according to the maximum demagnetization magnetic potential of each pole and the demagnetization magnetic field strength at the inflection point k at high temperature when the built-in brushless DC motor is in rated operation. The minimum edge thickness of the magnet; then, use AutoCAD to redesign the shape and size of the permanent magnet according to the minimum edge thickness, and measure the angle between the edges of the permanent magnet; finally, calculate the brushless according to the angle between the edges of the permanent magnet and the angle corresponding to the pole pitch Extended pole arc factor for DC motors. The method of the invention makes full use of the structure of the permanent magnet groove, expands the edge of the permanent magnet and increases the width of the permanent magnet, so that the pole arc coefficient is expanded, which not only avoids the generation of sharp demagnetization, but also avoids the permanent magnet in the process of expanding the pole arc coefficient. Magnets are wasted. After using the method of the invention to expand the pole arc coefficient, the air gap magnetic density is closer to the square wave, and the torque ripple, vibration and noise of the motor can be reduced through the square wave control.

Description

An Extended Method of Pole Arc Coefficient of Built-in Brushless DC Motor

technical field

The invention belongs to the technical field of motors, and in particular relates to a method for expanding the pole arc coefficient of a built-in brushless DC motor.

Background technique

Rare earth permanent magnet motors have the advantages of simple structure, convenient maintenance, stable operation, reliable performance, high power density and good speed regulation performance, and have been widely used in the fields of wind power generation, electric vehicles and CNC machine tools; at the same time, for permanent There are more and more studies on the design and optimization of the magneto body. The research content mainly includes the new structure design of the motor and the optimization of the motor parameters to achieve the goals of reducing torque ripple, improving power factor, and obtaining a reasonable air gap flux density. Among them, the pole arc The coefficient is a key parameter in motor design, which directly affects the air-gap magnetic density waveform. When the pole arc coefficient increases, the air-gap magnetic density distribution will gradually change from sine wave to square wave, and the better the square wave characteristics of the motor's back EMF, the better the rotation speed. The torque ripple will be reduced, the motor will run more smoothly, and the electromagnetic vibration and noise of the motor will also be improved. Therefore, in the design of the brushless DC motor, it is hoped that the pole arc coefficient is as large as possible. The built-in brushless DC motor is a special structure of the brushless DC motor. The magnetic steel is embedded in the rotor, which has higher reliability. In the design of the built-in brushless DC motor, it is necessary to comprehensively consider the motor volume and power density. The value of the pole arc coefficient will be limited by the size of the rotor of the motor. If a large pole arc coefficient is blindly pursued in the design process, the part of the magnetic isolation bridge of the rotor will become smaller, which will increase the flux leakage of the brushless DC motor.

In order to reduce magnetic flux leakage, the two ends of the permanent magnet slot of the built-in brushless DC motor will adopt a triangular structure, that is, the sharp corner of the permanent magnet slot. In general design, the permanent magnet will not be embedded in the sharp corner of the permanent magnet slot. When the pole arc coefficient of the designed high power density built-in brushless DC motor is relatively small, the pole arc coefficient can be expanded by using the slot corner of the permanent magnet. The method is to fill the slot corner with permanent magnets to increase the width of the permanent magnet , but there is a problem in the expansion process: the permanent magnet slot tip is thinner as it gets closer to the edge position. Since the permanent magnet filled into the slot tip is consistent with the shape of the slot tip, when the permanent magnet is embedded in the slot tip Or when the groove is completely filled, the thickness of the edge of the permanent magnet along the magnetization direction is very small. As the operating temperature of the motor increases, the demagnetization curve of the permanent magnet bends, and the armature magnetic field will cause irreversible demagnetization at the edge of the permanent magnet. This phenomenon is called sharp Corner demagnetization, sharp corner demagnetization will cause permanent magnets near the edge to fail, and blindly increasing the width of the permanent magnet, especially when filling the sharp corners of the slot, will cause permanent magnets to be wasted due to partial failure. Therefore, the expansion of the pole arc coefficient will be limited by the demagnetization of sharp corners. The built-in high power density brushless DC motor is small in size, and the arrangement space of the permanent magnets is limited. The pole arc coefficient cannot obtain the ideal value during the design process. It is necessary to find a way to avoid the sharp corners of the permanent magnets of the built-in brushless DC motor. Polar arc coefficient expansion method for demagnetization.

Contents of the invention

technical problem to be solved

In order to make up for the defect that a large pole arc coefficient cannot be obtained in the process of designing a brushless DC motor with large power and small volume, improve the performance of a square wave controlled brushless DC motor, and avoid waste caused by sharp-angle demagnetization of permanent magnets, the present invention proposes An extended method for pole arc coefficients of built-in brushless DC motors considering the effect of demagnetization at sharp corners.

Technical solutions

A method for expanding the pole arc coefficient of a built-in brushless DC motor is characterized in that the steps are as follows:

Step 1: Calculate the minimum edge thickness h ₁ of the permanent magnet according to h ₁ =F _adm /H _k , where, F _adm is the maximum demagnetization magnetic potential per pole when the built-in brushless DC motor is in rated operation, and H _k is the permanent magnet at temperature T The demagnetization magnetic field intensity at the inflection point k on the demagnetization curve of the magnet material, where T≥150°C; the calculation formula of F _adm is Among them, I _a is the rated operating current, W _Φ is the number of series turns of each phase winding, K _W is the winding coefficient, and p is the number of pole pairs;

Step 2: Make the edge thickness of the expanded permanent magnet equal to h ₁ . According to the principle of keeping the edges of the permanent magnet parallel before and after the expansion, use AutoCAD to draw the shape of the expanded permanent magnet, mark the size of the permanent magnet, and measure the edge of the expanded permanent magnet at the same time The angle α ₁ between the permanent magnet edges is the angle between the upper ends of the two edges of the permanent magnets of each pole and the intersection points A, A' of the slots and the rotor axis O;

Step 3: According to α _pmax = α ₁ /β, the maximum value of the pole arc coefficient α _pmax of the extended built-in brushless DC motor is obtained, and the expansion range of the pole arc coefficient is (α/β, α _pmax ]; where, β is the angle corresponding to the pole pitch, and α is the angle between the edges of the permanent magnet before expansion.

Beneficial effect

The method for expanding the pole arc coefficient of a built-in brushless DC motor proposed by the present invention fully utilizes the structure of the permanent magnet slot, expands the edge of the permanent magnet, increases the width of the permanent magnet, and expands the pole arc coefficient; utilizes the rated work of the motor The ratio of the maximum demagnetization magnetic potential of each pole to the demagnetization magnetic field intensity at the inflection point k determines the minimum edge thickness of the permanent magnet, which not only avoids the generation of sharp demagnetization, but also avoids the waste of permanent magnets during the expansion of the pole arc coefficient. After using the method of the invention to expand the pole arc coefficient, the air gap magnetic density is closer to the square wave, and the torque ripple, vibration and noise of the motor can be reduced through the square wave control.

Description of drawings

Fig. 1 is a calculation flowchart of a pole arc coefficient expansion method of a built-in brushless DC motor according to the present invention.

Figure 2 is the demagnetization curve of NdFeB 35SH at 150°C.

Figure 3 is a schematic diagram of the rotor before and after the expansion of the pole arc coefficient of the built-in brushless DC motor.

Detailed ways

The present invention will be further described below in conjunction with the accompanying drawings and embodiments, and the present invention includes but not limited to the following embodiments.

Since the pole arc coefficient of the built-in brushless DC motor is related to the spatial distribution of the permanent magnets on the rotor, the wider the permanent magnet width, the larger the pole arc coefficient of the motor. If the slot corners are filled with permanent magnets, the width of the permanent magnets can be expanded, and the pole arc coefficient will increase accordingly, making the air gap magnetic density of the brushless DC motor closer to the square wave. The square wave controls the torque ripple of the brushless DC motor and Noise will be reduced. However, when the working temperature of the motor is higher than a certain value, after the demagnetization magnetic field generated by the winding exceeds a certain value, the demagnetization curve of the permanent magnet will drop sharply, and the new recovery curve will no longer coincide with the demagnetization curve, resulting in irreversible demagnetization, and the permanent magnet after demagnetization The magnetic density provided to the outside decreases sharply, and repeated work even leads to the complete loss of magnetism of the permanent magnet. Therefore, when the permanent magnet expands to the sharp corner of the slot, it is necessary to check the minimum thickness to avoid demagnetization at the sharp corner and prevent waste of the permanent magnet.

Based on the above thought, as shown in Figure 1, the specific process of the permanent magnet motor pole arc coefficient expansion method considering the influence of sharp angle demagnetization of the present invention is as follows:

1) Determine the demagnetization magnetic field strength H _k at the inflection point k of the permanent magnet demagnetization curve

Sharp-angle demagnetization occurs under high temperature conditions. The internal temperature of the high-power-density permanent magnet motor that works for a long time is above 100°C. The operating temperature of the motor in this example is about 150°C. The minimum edge thickness of the permanent magnet calculated based on the magnetic field strength, as shown in Figure 2, the demagnetization magnetic field strength at the inflection point k is _Hk =487000A/m;

2) Determine the maximum demagnetization magnetic potential F _adm per pole when the brushless DC motor is rated to work

When the motor is in the rated working state for a long time, the temperature rises, and then the demagnetization magnetic field generated by the motor winding current will cause irreversible demagnetization of the permanent magnet. Therefore, to estimate the minimum thickness of the permanent magnet, it is necessary to first determine the maximum demagnetization of each pole when the motor is rated. Potential, the calculation formula is Among them, I _a is the rated operating current, W _Φ is the number of series turns of each phase winding, K _W is the winding coefficient, and p is the number of pole pairs. The schematic diagram of the built-in brushless DC motor rotor of this embodiment is shown in Figure 3, its rated current I _a = 50A, the number of series turns of each phase winding W _Φ = 42, the winding coefficient K _W = 0.96, and the number of pole pairs p = 3 , then the maximum demagnetization magnetic potential per pole can be calculated as F _adm =581.9A.

3) Calculate the minimum edge thickness h _min of the permanent magnet

According to the magnetic potential is equal to the distance multiplied by the magnetic field strength, the minimum edge thickness of the permanent magnet is equal to the maximum demagnetization magnetic potential F _adm divided by the magnetic field strength H _k , the calculation formula is h _min =F _adm /H _k , the minimum thickness of the present embodiment is _hmin = 1.19 mm.

4) Redesign the permanent magnet

The shape and size of the permanent magnet slot are determined by the prefetch pole arc coefficient α _p and the width c of the magnetic isolation bridge. The slot edge position forms the slot tip, and a part of this position is not filled with a permanent magnet. Among them, α _p = α/β, α is the angle between the edges of the permanent magnet, that is, the angle between the upper ends of the two edges of the permanent magnet of each pole and the intersection points A and A' of the groove and the rotor axis O, respectively, β is the angle corresponding to the pole pitch. In this embodiment, the prefetched pole arc coefficient α _p is 0.83, the angle between the edges of the permanent magnets is α=50°, and the angle β=60° corresponding to the pole pitch.

In order to avoid the occurrence of sharp demagnetization, the minimum edge thickness h _min of the permanent magnet is used as the boundary condition for the widening of the permanent magnet, that is, the edge thickness of the permanent magnet after expansion is h ₁ =h _min , relying on the groove structure of the permanent magnet, using AutoCAD to directly Draw the edge of the permanent magnet in the sharp corner of the slot. According to the principle that the edge of the permanent magnet remains parallel before and after expansion, the edge of the permanent magnet is still perpendicular to the edge AB of the slot, that is, h ₁ //h. As shown in Figure 3, the new permanent magnet obtained after expansion The magnet is the sum of the original permanent magnet and the extended part of the permanent magnet. During this process, the shape of the permanent magnet has changed. Using AutoCAD, the size of each part of the permanent magnet can be directly measured and marked.

After the width of the permanent magnet is expanded, the angle between the edges of the permanent magnet becomes larger, which can be measured by AutoCAD: new intersection points C ₁ and C ₂ are generated between the upper edge of the new permanent magnet and the slot, and are assisted from the rotor axis O to these two points line, measuring the angle between two line segments, that is, the angle α ₁ between the edges of the expanded permanent magnet = 55.8°.

5) Calculate the pole arc coefficient α _pmax of the extended built-in brushless DC motor

According to α _pmax =α ₁ /β expanded polar arc coefficient α _pmax , the expanded polar arc coefficient in this embodiment is α _pmax =0.93, which is much larger than the prefetched polar arc coefficient. It should be noted that α _pmax is the maximum pole arc coefficient that can avoid sharp-angle demagnetization, and any value between (α/β, α _pmax ] during optimization can avoid sharp-angle demagnetization.

Claims (1)

1. A method for expanding the pole arc coefficient of a built-in brushless DC motor, characterized in that the steps are as follows: Step 1: Calculate the minimum edge thickness h ₁ of the permanent magnet according to h ₁ =F _adm /H _k , where, F _adm is the maximum demagnetization magnetic potential per pole when the built-in brushless DC motor is in rated operation, and H _k is the permanent magnet at temperature T The demagnetization magnetic field intensity at the inflection point k on the demagnetization curve of the magnet material, where T≥150°C; the calculation formula of F _adm is Among them, I _a is the rated operating current, W _Φ is the number of series turns of each phase winding, K _W is the winding coefficient, and p is the number of pole pairs; Step 2: Make the edge thickness of the expanded permanent magnet equal to h ₁ . According to the principle of keeping the edges of the permanent magnet parallel before and after the expansion, use AutoCAD to draw the shape of the expanded permanent magnet, mark the size of the permanent magnet, and measure the edge of the expanded permanent magnet at the same time The angle α ₁ between the permanent magnet edges is the angle between the upper ends of the two edges of the permanent magnets of each pole and the intersection points A, A' of the slots and the rotor axis O; Step 3: According to α _pmax = α ₁ /β, the maximum value of the pole arc coefficient α _pmax of the extended built-in brushless DC motor is obtained, and the expansion range of the pole arc coefficient is (α/β, α _pmax ]; where, β is the angle corresponding to the pole pitch, and α is the angle between the edges of the permanent magnet before expansion.