Finite element analysis of permanent magnet brushl

2022-09-21
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Finite element analysis of permanent magnet brushless DC motor for electric vehicle

Abstract: a permanent magnet brushless DC motor for electric vehicle is designed, which adopts external rotor structure. Its purpose is to be embedded in the wheel hub of the vehicle to realize direct drive without gear transmission device, and can realize low-speed, high torque, high efficiency and variable speed operation. This paper focuses on the finite element analysis of the magnetic flux, magnetic induction intensity and torque of this kind of motor with ANSYS software

key words: electric vehicle; Permanent magnet brushless DC motor; Finite element analysis

1 introduction

electric vehicles are favored by countries all over the world because of their less public hazards, less oil consumption, simple structure, easy maintenance and long service life. Compared with other types of motors with the same power, permanent magnet brushless DC motor has small volume and light weight, and has quite obvious advantages in quality, efficiency, price, etc. permanent magnet brushless DC motor has no brush, slip ring and other parts, simpler structure, more reliable performance, good environmental adaptability, and is more suitable to be used as the driving motor of electric vehicles. Because square wave power supply is generally used, under the same peak voltage and current, the torque generated by the interaction between square wave current and square wave magnetic field is larger, so permanent magnet brushless motor can output larger electromagnetic torque

with the rapid development of electronic technology and control technology, if the motor is directly installed in the wheel hub of the car, and the speed regulation and direct drive (2 or 4 wheels) are realized through electrical control, the complex gear speed change actuator can be omitted in the car, and the car structure is greatly simplified and the weight is greatly reduced. Advanced countries in the world have taken it as the development direction to carry out research, and some countries have developed prototype

however, due to the limitation of the inner diameter of the hub, the volume of the permanent magnet motor is limited, and it needs to produce high torque, which brings certain difficulties to the design of the motor. Moreover, the design of permanent magnet brushless DC motor can not simply apply the method of permanent magnet synchronous motor, nor can it simply apply the method of DC motor. According to the characteristics, we should find a more accurate design method [1]. Here, the design method of combining magnetic circuit calculation and magnetic field analysis is used to study the three-phase permanent magnet brushless DC motor. The control structure of electric vehicle is shown in Figure 1 [2]

2 electromagnetic calculation of motor

according to the calculation program we have compiled, a 8kw, 12 pole external rotor brushless DC motor is designed and calculated

2.1 basic input data

rated output power: PN = 8 000W

rated speed: NN = 685r/min

armature outer diameter: Da = 0.31m

armature inner diameter: d0 = 0.08m

number of slots: q = 45

remanence density: br = 1.05t

coercivity: HC = 800kA/M

number of phases: M = 3

.

2.2 calculation result

2.2.1 performance calculation

stator current I1 = 24.05a, electromagnetic torque TN = 112.33n m. Input power P1 = 8656.74w, back EMF e = 349.70v, actual speed n = 677.20r/min, efficiency η= 92.41%。

2.2.2 winding data

line load: a=311.11a/cm

current density: j=6.86a/mm2

thermal load: aj=2134.21a2/cm*mm2

2.2.3 magnetic circuit calculation

no load point standard unit value: B00 = 0.92536, H00 = 0.074 64

load point standard unit value: BN = 0.084196, HN = 0.15804

magnetic flux per pole: Φ= 0.004 390 5wb

air gap magnetic density: B δ= 0.767 6T

stator tooth magnetic density: BT2 = 1.763 5T

stator yoke magnetic density: bj2 = 0.257 1t

rotor yoke magnetic density: BJ1 = 0.855 6T

2.2.4 no load characteristic curve

no load characteristic curve is shown in Figure 2

2.2.5 working characteristics of different load rates

working characteristics are shown in Figure 3

3 finite element analysis

the electromagnetic ability of ANSYS program can be used to analyze various problems of electromagnetic field. ANSYS program provides rich expressions of linear and nonlinear materials, including isotropic or orthotropic linear permeability, B-H curve of materials and demagnetization curve of permanent magnets. The post-processing function allows the user to display the magnetic line of force and calculate the magnetic flux density and magnetic field strength, force, torque and other parameters. In this example, the 2-D static magnetic field analysis is applied, and the vector magnetic potential method is used to simulate various saturated magnetic materials and permanent magnets. Static magnetic field analysis mainly consists of six steps:

(1) establish a model

(2) create a physical environment and give characteristics

(3) partition

(4) add boundary conditions and loads

(5) solve

(6) post-processing

3.1 create a solid model in ANSYS

and take the calculation results as the input data for establishing the model. Enter pre-processing (PREP7) to start modeling. Use geometric elements and Boolean operations to produce basic geometric shapes. The difficulty of this model is to copy the groove, which can be mapped after rotating the work plane in the cylindrical coordinate system. Figure 4 is the schematic diagram of 2-D model

3.2 create a physical environment, give material characteristics

3.2.1 define unit type

use both units shown in Table 1 [3]

3.2.2 defining material properties:

a total of four materials are used, and different parts of the model are given different material values

(1) air gap and groove: air, material characteristics: μ R (murx) = 1

(2) stator: dw465-50, material characteristics: B-H curve

(3) rotor: 10 steel, material characteristics: B-H curve

(4) magnetic pole: permanent magnet, material characteristics: μ R (murx) = 1.05, HC (coercivity vector): mgxxmgyy depends on the specific position of the magnetic pole respectively. See Table 2 for the X and y components of the coercivity vector of the 12 magnetic poles distributed in Figure 2

3.3 dividing lattice

select smart mesh in free lattice division, and the size of intelligent units can be freely controlled. The sectioning results are shown in Figure 5

3.4 add boundary and load

generally, it is considered that the magnetic line of force is closed along the outer surface of the motor, and this boundary belongs to the first type of homogeneous boundary. In many cases, the outer surface of the motor shaft is also taken as the first type of homogeneous boundary, with a value of a = 0

the permanent magnet motor uses the permanent magnet as the excitation source, which can be set through the coercivity in the material characteristics, without additional excitation

when analyzing the no-load condition, it is not necessary to load; When analyzing the load condition, add load current to the area in the tank

3.5 solve [3]

enter the solver solution, you can choose any of the following solvers: frontal, jCG, ICCG, PCG solvers, and for 2-D models, it is recommended to use wavefront solvers. When analyzing the nonlinear electromagnetic field, ANSYS calculates the convergence criteria, and each balance iteration has the corresponding convergence criteria. You can turn on drawing solution tracking while the solution is in progress

3.6 post processing

enter the general post processor POST1, and you can observe the results of the whole model or part of the model for a specific load combination at a certain time. POST1 has many functions, including simple image display to complex data list

3.6.1 the basic data obtained after solving is the z-axis component AZ of the magnetic vector, from which the derived data can be obtained [4]

(1) calculate the magnetic flux Φ

according to Stokes' theorem:

through the surface a, the magnetic flux is equal to the closed line integral of the magnetic vector along the boundary line of this surface. This is usually much easier to calculate than using magnetic density B

(2) calculate the induced electromotive force

(3) draw the magnetic line of force

B of the two-dimensional magnetic field has only two components, such as BX and by, while a has only one component AZ, abbreviated as a. In a two-dimensional field, the magnetic lines of force are equal a lines

3.6.2 mapping results to a certain path

one of the most useful and powerful features of POST1 is that it can map any result data to any path of the model. In this way, many mathematical operations and calculus operations can be performed along this path, so as to obtain meaningful calculation results. Useful side benefit Lubrizol invented thermoplastic polyurethane (TPU), which can observe the change of result items along the path in a graphical or tabular way

(1) define the air gap in the motor as a path

(2) map the radial component br of magnetic induction intensity B to the path

(3) display data along the path, as shown in Figure 7

3.6.3 calculate torque by Maxwell stress tensor method [4]

Maxwell stress tensor method is a torque calculation method derived from electromagnetic field theory, which will cause great losses to cars, motorcycles, and various vehicles and machinery. In the two-dimensional electromagnetic field, the tangential electromagnetic force density acting on the motor stator or rotor, the electromagnetic torque is generated by the tangential force. If it is integrated along the circumference with radius r, the expression of the electromagnetic torque is:

br, B θ Are the radial and tangential components of the air gap magnetic density at radius r, respectively

4 conclusion

electromagnetic field is the core of motor energy transmission. The distribution of electromagnetic field parameters of motor can be calculated accurately by using finite element method. The predecessor of this company is the sub business group "material technology" under the Bayer Group of the global top 500. It can be seen that when the small angle reaches the preset value, Sys software is a powerful auxiliary tool for motor design. According to the working principle and structural characteristics of permanent magnet brushless DC motor, we studied its design method. The design program of permanent magnet brushless DC motor and the numerical analysis program of electromagnetic field of permanent magnet brushless DC motor are compiled. The calculation results of the former program are used as the input data of the latter program. We take the advantage of the fast speed of the magnetic circuit calculation method. A large number of calculations (including optimization calculation) are carried out by the magnetic circuit method. The magnetic field analysis method is only used to calculate parameters such as electromagnetic torque, and the number of calculations is less, In this way, after several rotation calculations of two programs, we can get satisfactory results. We believe that the method of designing permanent magnet brushless DC motor by combining magnetic circuit calculation and magnetic field analysis is successful. Through the magnetic field analysis, we found that there are still many problems to be further studied, such as the torque fluctuation. Using six phase or five phase, the torque characteristics of the motor are more reasonable

References:

[1] Zhou Yuanfang, Xiao Renshan, Liang Jingzhang, et al Design and research of rare earth permanent magnet brushless DC motor [J] Guangxi electric power technology, 1996 (2): 1

[2]King-Jet Tseng,puter-AidedDesign and Analysis of Direct-Driven Wheel MotorDrive[J]. IEEE Transactions on Power Electronics,1997,12(3):518.

[3] ANSYS electromagnetic field analysis guide [M], 2000

[4] Hu Zhiguang Analysis and calculation of electromagnetic field of motor [M] China Machine Press, 1982 (end)

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