Efficiency Improvement of Permanent Magnet BLDC Motors for Electric Vehicles

A permanent magnet Brushless DC (BLDC) motor has been designed with different rotor configurations based on the arrangement of the permanent magnets. Rotor configurations strongly affect the torque and efficiency performance of permanent magnet electric motors. In this paper, different rotor configurations of the permanent magnet BLDC motor with parallel the Halbach array permanent magnet were compared and evaluated. Many applications of electric drives or air-crafts have recently preferred the surface-mounted permanent magnet design due to its ease of construction and maintenance. The finite element technique has been used for the analysis and comparison of different geometry parameters and rotor magnet configurations to improve efficiency and torque performance. A comprehensive design of a three-phase permanent magnet BLDC 35kW motor is presented and simulations were conducted to evaluate its design. The skewing rotor and Halbach magnet array are applied to the permanent surface-mounted magnet on the BLDC motor for eliminating torque ripples. In order to observe the skewing rotor effect, the rotor lamination layers were skewed with different angles and Halbach sinusoidal arrays. The determined skewing angle, the eliminated theoretically cogging torque, and the back electromotive force harmonics were also analyzed. Keywords-BLDC; permanent magnet; finite element method; Ansys Maxwell; SPEED; magnetic flux density

INTRODUCTION Permanent magnet (PM) Brushless DC (BLDC) motors are widely used due to features such as compactness, low weight, high efficiency, and easy assembly [1,2]. The reliability of BLDC motor is high since it is easy to mount the PM and has a robust structure. Different rotor configurations are available for the PM BLDC motor, e.g. the surface mounted PM design with the interior/exterior rotor, the interior PM design with buried magnets due to specific strengths and weaknesses [3][4][5]. Among these, there are the radial-flux motors, the surface mounted types with different magnet arrangement that have been used for electrical drives. This paper introduces a novel design of the BLDC35kW-Z36P12 rotor with high efficiency and low torque ripple. The electromagnetic performance of the PM BLDC with 36 stator slots and 12 rotor poles is discussed in this paper.

II. ELECTROMAGNETIC TORQUE ANALYSIS
The aim is to lower the cost with high overload efficiency and reliability. High efficiency and torque density for electric vehicle applications are the first priorities of this design program. The calculation process of the proposed BLDC motor conducted with the SPEED software is presented in Figure 1. The geometry specifications of the motor used for the analysis are given in Table I.  The geometry dimensions of the PM BLDC motor are saved in a database in matrix form. When the export command is generated, the drawing process will be executed. The program is developed with the MATLAB DXF library shown in Figure 2. For the drawing circle line, rotating object of stator slots, 2D modeling is implemented to get geometrical formulas of the circle lines. The algorithm needs to satisfy two requirements: the shape of these lines must be similar to the desired curves and the least possible points must be used. Using the minimum number of lines will help the system because it will not have to store a lot of data, which will slow down speed and present difficulties at exporting the drawings. On the other hand, rotation and mirroring are difficult tasks in programming. The strategy is to use a loop function to redraw several times and trigonometric function with angle steps was applied returning good results. MATLAB DXF drawing.
The system will export separate drawings of the motor, the rotor, and the stator. These drawings can be used in several simulation programs and in the design and manufacturing progress. The detail parameters are shown in Table II. Programs can support the material weight and volume calculation because the power and torque density are also important factors in each motor design. From the geometry parameters, the material weights of two PM BLDC motors were obtained and are shown in Table III. The two PM BLDC motors with the parallel magnet and the Halbach magnet array are presented in Figure 3. The magnet thickness of the conventional design is 4 mm and the electric angle is 150 0 while the Halbach magnet array has a thickness of 3.5 mm and 175 electric embraces. The total weight of a novel design of the Halbach magnet is lower with the same magnet weight. Electro Magnetic Force (EMF) and torque, calculated by a MATLAB analytical coupling program with the Finite Element Method (FEM) are expressed as [11,12]: where T ph is the turn per phase term, T e is the electromagnetic torque and E a is the back EMF.
The electromagnetic performances based on this design are shown in Table IV. The most important parameter is the efficiency of 93.792%. The efficiency of Halbach magnet and skewed rotor with 5 slices is optimized by the control current of 250A with 200VDC. In order to evaluate the maximum torque of the motor, a maximum current is applied to determine when the permanent magnetic is irrecoverable. The maximum torque is 350 Nm at the speed of 1500 rpm with a current of 500 A. Efficiency is calculated based on copper and iron losses. Those losses depend on the stator and rotor teeth dimensions. An efficiency map of the Halbach magnet array with rotor skewed slices is shown in Figure 4. The torque and speed operation areas have been plotted with different efficiency values. The maximum torque acquired is 350 Nm at the speed of 1000 rpm with low efficiency of 79%. A 2D BLDC motor model is solved and simulated by FEM [13][14][15]. After meshing the geometry model including the magnetic, the silicon steel, and the insolation materials, the flux density distribution of rotor and stator, is shown in Figure 5. The flux density of the air gaps is investigated in one pole. Many steps of rotor position, currents, torque, and flux density were recorded and saved in Matlab files to plot those characteristics. The leakage flux lines are enclosed to air gap areas, which also helps to increase the flux density and reduce the rotor yoke iron loss.

III. TORQUE RIPPLE ANALYSIS OF ROTOR SKEWED SLICES
Skewing slices of rotor are frequently used in PM BLDC motors for eliminating the cogging torque. For the optimum skew angle of those slices, the cogging torque can be eliminated theoretically. The skewed slots for the rotor slices are illustrated in Figure 6. Rotor skewing slices.
The torque ripple results are shown in Figure 7. The cogging torque can be calculated by the stored energy in the air gap. The variation of the co-energy given the cogging torque is expressed as [6][7][8]: where T c is the cogging torque, ∂θ is the displacement with mechanical degree, and ∂W is the stored co-energy in the air gap. Fig. 7.
Torque ripple analysis.
The cogging torque is periodic along the air gap. By using this periodicity feature, the Fourier series of the cogging torque can be obtained [6,7]: where K sk is the skew factor that is 1 for non-skewed motor laminations, C p is the least common multiple of the number of poles and number of stator slots, T i is the absolute value of the harmonic i, θ m is the mechanical angle between the stator and the rotor axis while the motor is rotating, and ߠ represents the phase angle. The skew factor K sk is defined by: where α sk is the skew angle and N s is the number of the slide.
The average values of load torques are nearly the same values for even one slot pitch skewed motor result in terms of average If the skew angle is increased, the torque ripple is reduced along with the average torque, so for optimal torque performance the ratio of magnetic pole/stator slots needs to be increased.
IV. CONCLUSION In this paper, a comprehensive design of the PM BLDC motor for electric vehicles has been presented. The design was computed by the analytical method, it was optimized by the SPEED software and the electromagnetic characteristics were evaluated by FEM. In particular, the parallel and Halbach PM array rotor with skewed slides have been compared in terms of efficiency, torque, and cogging torque. The torque ripple is minimum with a skew angle of 8 degrees. The best skewing angle is determined by a stator slot angle of 10 degrees. For the electrical drive, the PM surface mounted motor is easy to arrange Halbach and skewing structures.