Loss analysis of magnetization annealed amorphous alloy replacement stator teeth

Amorphous is widely used in electric motors, power electronics, sensors, etc. because of its low loss and high strength advantages. Amorphous is obtained by ultra-rapid cold solidification. The quenched state alloy strips have varying degrees of internal stresses and defects, so annealing treatment is required to remove the internal stresses to enhance the properties. To further enhance the electromagnetic properties of amorphous alloys, a magnetic field can be added to the annealing process in amorphous alloys.

There are two types of magnetization annealing: longitudinal and transverse magnetization annealing. In the case of amorphous, both methods increase the saturation flux density and reduce the coercivity and losses,¹ but increase the magnetostriction coefficient of the material. Compared to longitudinal annealing, transverse magnetic annealing increases the magnetostriction coefficient of the material to a lesser extent,² which will increase the vibration noise in motors using this material. Zhang et al.³ analyzed the differences in the properties of amorphous alloys with different annealing magnetic field strengths at the same temperature and found that by applying a magnetic field strength of 0.000 25 T, the electromagnetic properties were similar to those at 1 T. Zhang et al.⁴ considered the effect of different P content, annealing temperature and annealing rate on the electromagnetic properties. Kwon et al.⁵ obtained the optimum conditions for electromagnetic properties by varying Si content, annealing temperature and annealing rate. Li et al.,⁶ Zhao et al.⁷ compared the magnetic properties of amorphous alloys with different Fe content after magnetization annealing treatment, and the magnetic properties of such material’s structure was characterized to reveal the mechanism of performance improvement. Wang et al.⁸ investigated the effects of different magnetization annealing conditions on the magnetic properties, mechanical properties and microstructure of Fe₈₀Si₉B₁₁ amorphous alloy, which provided a basis for improving the soft magnetic properties of the amorphous alloy.

Magnetization annealing of amorphous alloys was first applied in the 1970s,⁹ but little research has been done to apply it to electric motors. Therefore, in this paper, a cross-sectional comparison of the electromagnetic properties of UA, AA and AALM is performed. The obtained data are then applied to different structures for simulation and analysis. Finally, Grain oriented silicon steel (GO) is used to make a longitudinal comparison of the electric motor.

According to the national standard GB/T 3658-2008 for the measurement of magnetic properties of ring specimens, the following three specimens are set up, which materials are UA, AA and AALM, respectively. The test specimen is shown in Fig. 1. Although the dimensions of the samples are different, the B-H curves and specific losses of the materials are measured and the results are not affected. The magnetic properties of GO are tested along the rolling direction by the Epstein square circle method. The thickness of all three amorphous alloy materials is 0.026 mm and 0.08 mm for GO.

Figure 2 shows the magnetic properties of three amorphous alloys with different heat treatments and GO along the rolling direction (GO 0 °C) at f = 400 Hz. Figures 2(a) and 2(b) show the B-H and B-P curves for the four materials respectively.

From Fig. 2, it can be seen that the saturation magnetic density of AALM is about 1.54 T, which is about 6% higher than that of AA at 1.46 T and 15% higher than that of UA at 1.34 T, while the loss of AALM at 1 T is about 80% of that of AA, which indicates that longitudinal magnetic annealing can make the amorphous with higher saturation magnetic flux density and lower iron loss and improve the soft magnetic performance.

GO and AALM have similar characteristics and both have the advantage of high saturation magnetic flux density and low loss along a specific direction. The magnetic properties of GO along the rolling direction at f = 400 Hz are also shown in the figure. It can be seen that the saturation magnetic density of GO in the 0° direction is 1.85 T. Its initial magnetic permeability is also significantly higher than that of the amorphous alloy, but at 1.5 T, its loss is higher than that of the amorphous alloy.

Annealing is one way to enhance material properties by improving or eliminating various tissue defects. Amorphous strips prepared by ultra-rapid cold solidification can generate internal stresses during solidification to impair the properties of thin strips, so annealing can be used to eliminate internal stresses and improve soft magnetic properties, including higher saturation magnetic flux density, higher relative permeability and lower losses. The XRD pattern of AALM is shown in Fig. 3. There is no obvious crystallization peak and the alloy is still in the amorphous state.

Unlike annealing heat treatment, AALM shows regular and orderly arrangement of magnetic domains due to the addition of magnetic field on the basis of annealing. This is because the magnetic moment within the magnetic domains is more active during heating and will turn in the direction of the magnetic field. After magnetization annealing, the magnetic domain of the amorphous alloy becomes wide, and the direction of the magnetic domain wall is parallel to the length of the strip. As the annealing temperature decreases, the resultant rotation of the magnetic moment is maintained in the material, affecting the internal microstructure of the material and causing an improvement in the macroscopic properties of the material.

The above experimental results show that AALM has lower loss and higher magnetic flux density, and the amorphous alloy treated by longitudinal magnetic annealing has significantly higher ease of magnetization along the longitudinal direction and significantly improved soft magnetic properties. Further analysis of the magnetic field annealing mechanism shows that this advantage exists only in the direction of the applied magnetic field during annealing and does not exist in other directions.

A permanent magnet synchronous motor with a rated power of 3.6 kW is used as a comparison model in this paper. Its specific parameters are shown in Table I. The motor model is shown in Fig. 4. The three marked points A, B and C, the three phase windings and the permanent magnets have also been marked in Fig. 4. Figure 5 shows the magnetic density cloud of the motor at rated operating conditions, with the highest flux density at the magnetic barrier bridge. In the stator, the highest magnetic density is 0.66 T in the tooth and 0.88 T in the yoke.

The magnetic field on the stator teeth is a pulsating magnetic field with basically radial flux density and little tangential component. Figure 6 shows the magnitude of the radial and tangential flux density at three different locations marked points A, B and C on the tooth of the motor.

The difference in the radial magnetic densities of marker points A and B is not significant, both are about 1.5 T, and the tangential magnetic densities are basically less than 0.1 T, so the magnetic fields are approximately parallel to the teeth. The tangential magnetic density of marker point C is 0.36 T, and the magnetic field forms a large angle with the tooth, which is not conducive to the characteristics of the material.

According to the magnetic properties of AALM, this paper splices AALM as the teeth, and compares the influence of different splicing lengths on iron loss and efficiency. The specific splicing length is from the marking points A and B to the inner diameter of the stator. According to different splicing length, it can be defined as long splicing (LS) and short splicing (SS). The splicing model is shown in Fig. 7, and the splicing shape used is circular. The material used for the rest of the stator is B27AVH1500.

The total iron loss and iron loss per unit area in tooth in one cycle at rated speed for SS, LS, and no splice structure (NS) were calculated by finite element. The calculation results are shown in Fig. 8. Compared with SS, the spliced structure of SS and LS can reduce the iron loss by 45.14% and 41.25%. Although the total iron loss of LS is smaller than that of SS, the tooth iron loss is larger than that of SS type. This is because the LS is positioned at the root of the tooth, closer to the yoke, which will lead to excessive flux density and its magnetic field is not along the optimal direction, which will also lead to increased iron loss.

The use of high-performance soft magnetic materials can reduce losses, increase efficiency, and thus reduce heat generation, solving the problem of demagnetization of permanent magnets and destruction of insulators. As mentioned above, both splicing structures can reduce iron loss, and their efficiency comparisons for application in motors are shown in Table II. At maximum efficiency and efficiency ratios greater than 90%, the difference between the three structures is not significant, but when the efficiency ratio is greater than 95%, the advantage of SS and LS is more pronounced, approaching 2.5 times that of NS. This indicates that the iron loss can be reduced and thus the advantage of high efficiency can be achieved by a suitable splicing structure.

The magnetic properties of GO are similar to those of AALM, with the highest saturation magnetic flux density and lowest losses along the rolling direction. The study of GO tooth and yoke splicing structure has also been conducted in other papers. With the above analysis, this paper compares the motor models using both GO and AALM based on the LS structure. Figure 9 shows the efficiency comparison with the specific efficiency percentages in Table III.

The simulation results are shown in Table III. The maximum efficiency difference between GO and AALM is less than 1%. However, by calculating the percentage of the area of the efficiency cloud with efficiency greater than 95%, AALM is 1.5 times more than GO. AALM is more suitable for splicing than GO in this structure. The saturation magnetic flux density of AALM is about 1.55 T, while the saturation flux density of GO along the rolling direction from 0 is about 1.8 T. Therefore, AALM is suitable for motors with low flux density in the teeth but high operating frequency, which can effectively reduce iron loss and improve efficiency.

In this paper, it is verified that AALM can increase magnetic density and reduce losses, but this property exists only in the direction of the magnetic field applied during annealing. The following conclusions are obtained by comparative simulations.

1. AALM cores have the advantages of high flux density and low iron loss, which are conducive to improving efficiency.

2. For tooth and yoke splicing of anisotropic materials, the calculated splice length should be as parallel as possible to the magnetic field direction and the best magnetic energy direction so that the advantages of the materials can be fully utilized and the best performance of the motor can be obtained.

3. Under the same splicing scheme, the efficiency of using AALM as motor teeth is significantly higher than that of GO.

This research was funded by programme of Scholars of the Xingliao Plan (No. XLYC2002113) and Shenyang University of Technology Interdisciplinary Team Project (No. 100600453).

The authors have no conflicts to disclose.

Haiyang Hu: Conceptualization (equal); Data curation (equal); Methodology (equal); Software (lead); Validation (equal); Writing – original draft (equal); Writing – review & editing (equal). Jiabao Wang: Conceptualization (lead); Data curation (equal); Methodology (equal); Software (supporting); Writing – original draft (supporting). Shoushuai Huang: Data curation (equal); Investigation (equal); Resources (equal); Writing – original draft (supporting). Baozhi Tian: Data curation (equal); Investigation (equal); Resources (equal); Supervision (equal); Writing – original draft (supporting). Lihui Wang: Conceptualization (equal); Resources (equal); Supervision (equal). Ruilin Pei: Conceptualization (equal); Data curation (equal); Resources (equal); Supervision (equal).

The data that support the findings of this study are available from the corresponding author upon reasonable request.