Application of grain-oriented electrical steel used in super-high speed electric machines

Electrical steel is one of the core magnetic materials in EV traction motor. In terms of high power density and torque density for the EV traction motors. Traditional Non Grain-Oriented (NGO) silicon steel is difficult to satisfy these requirements because of limited saturation flux density and magnetic permeability.^1,2 This paper mainly presents that a novel thin-anisotropic Grain-Oriented (GO) silicon steel is used in high-speed (up to 14500rpm) traction motors after deeply analysing magnetic saturation, power loss, thermal conductivity and mechanical property of a traction motor with high-frequency (50Hz-400Hz) and high-torque running status. It applies a combination design between NGO and GO steel in order to avoid the low magnetic permeability in the transverse direction.

The grain-oriented silicon steel is a soft magnetic material that is used as the core material in electrical transformers. It is characterized by a pronounced Goss texture, i.e a 110 <001> preferred crystal orientation.³ The grain structure of NGO silicon steel is disorderly, mixed with a large number of messy and different textures. At present, most of the core components of Permanent Magnet Synchronous Machine (PMSM) are laminated by NGO silicon steel sheets. Therefore, with the development trend of miniaturization and high density of electric motors system, the main requirements for iron-core silicon steel materials in the future will be high saturated flux density and low iron loss. However, due to the low silicon content of the NGO silicon steel itself (silicon content lower than 3.5%) and the limitations of metallographic structure, NGO silicon steel will become increasingly difficult to meet the design requirements of new motors.

Many researches have been done to solve the problem: Tao.F et al. made a 20kW PMSM using amorphous materials and NGO as iron core. The optimized amorphous motor has a volume reduction of 31% which lead to a increase in power density of 45% under the same condition of power rate. Experiments show that the use of amorphous materials as the stator core can greatly increase the power density.⁴ Yudai.M et al. proposed a new switched reluctance motor (SRM) with a rotor designed in a segmented structure of GO electrical steel. Compared with the performance of conventional SRM, the GO steel SRM is 4.1% higher in output torque, the efficiency is 2.8% higher, the eddy current loss is 7.0% lower, and the iron loss is 19.2% lower in total.⁵ These results demonstrate the superior performance of GO steel for iron core in new generation of motor system.

Thus, a novel stator design is proposed in this paper. The novel prototype of high-speed traction motor has been designed with 12 slots and 8 poles with centralized winding. The power under rated/peak condition is 16kW/32kW and speed demand is up to 14500rpm. The slot of the iron core is made by conventional NGO and the teeth is made by GO with 10% process allowance. The rated/peak power is 16/32kW, the grade of single silicon steel for NGO is B30AHV1500, whose thickness is 0.3mm and core loss at 1.5T 50Hz is 15W/kg, and the grade for GO is B30P105 whose thickness is also 0.3mm. The noise and magnetostriction of the electric machine can be reduced by applying a self-stick punching technique between GO steel sheets in a stator.⁶ The research on numerical modeling and experimental results has been done. The new prototype is being fabricated and it can be produced in mass production in next few years.

The silicon content of GO silicon steel (silicon content 3%-6.5%) is higher than that of NGO silicon steel, and the grain structure shows strong orientation along the rolling direction. The magnetic performance is significantly better than NGO in deviation from rolling direction between 0° to 20°. Therefore, GO in deviation angle between 40° to 60° from the rolling direction, the electromagnetic is a lot worse than NGO. To verify the conclusion, different comparisons under differents current excitation of GO and NGO core are compared. Different B-H-P curves are measured in the experiments. B-H curves are obtained by the formula Eq. (1) below:

Where: J_m stands for maximum current density; H_m stands for maximum magnetic field strength; μ₀ stands for permeability of vacuum. The iron loss can be than calculated by Eq. (2):

Where: 1 and 2 stands for two windings; ϕ stands for the difference of phase between voltage signal in X channel and Y channel.

The Fig. 1 demonstrated the conclusion of the experiments are matched with the conclusion we made above. Among the figure, the GO steel, when it deviated from the rolling direction by 52°, has the worst electromagnetic characteristics, and the core loss is the highest. Its saturated flux density at the lowest point are significantly lower than the NGO silicon steel of the same grade. At the maximum deviation from the rolling angle of 90°, the electromagnetic properties of the GO silicon steel slightly recovered, but still lower than the NGO silicon steel of the same grade.

In order to prevent the GO silicon steel from deviating from the rolling direction by more than 30° in a rotating magnetic field, we determined that the stator core is made of the GO teeth and the NGO yoke ring, which is shown in Fig. 2. The yoke of the stator is made of NGO silicon steel and the teeth were made of GO silicon steel. Two types of silicon steel are at the same thickness of 0.3 mm. In order to ensure that in the rotating magnetic field, the GO silicon steel always sustain the flux density in rolling direction.

We also simulated four points in the stator core. During one cycle of the mechanical angle, we simulated the magnetic density of the stator core. Where point 1 is at the top of the stator tooth, point 2 is at the end of the stator tooth, point 3 is in the center of the stator yoke; point 4 is at the outer side of the stator yoke. Furthermore, we performed two-dimensional decomposition of the magnetic density of the stator core at one cycle, as shown in Fig. 3. The horizontal axis and the vertical axis coordinate values are added to the magnetic field received by the stator core at different time. From the figure, we concluded that in rotating magnetic field, the tooth portion of the stator is always subjected to a magnetic field in one direction but opposite magnitude, and the yoke is subjected in a rotating magnetic field. Therefore, GO silicon steel can be applied to the iron core of the stator teeth in a motor. The specific grade of NGO silicon steel is B30AHV1500 which stands for the brand is Baosteel, and the thickness of a single sheet is 0.3mm, and the power loss under 1.5T 50Hz excitation is 1.5W/kg. The specific grade of GO silicon steel is B30P105 which stands for the brand is Baosteel, and the thickness of a single sheet is 0.3mm, and the power loss under 1.7T 50Hz excitation is 1.05W/kg in rolling direction. The same grade are applied to make iron cores in the experiment. In conventional project, the grade of NGO silicon steel is also B30AHV1500.

The prototype of this project is 16/32 kW and 14500 rpm maximum speed which possessed a liquid cooling system. The topology structure is modeled by FEM method. The rotor adopted a segmented skewing pole and the grade of rotor silicon sheet is also B30AHV1500. Through the centrifugal force simulation, which is shown in Fig. 4 under the condition of limit speed of 17400 rpm (120% maximum speed), the stress of rotor core through von Mises criterion is 285.66 MPa, the safety factor is 1.3233, which satisfied the yield limit of the material. The modal simulation and thermal simulation are also carried out. During the simulations, the gap problem caused by the splicing of the two is considered, leaving a margin of 10%.

Electromagnetic simulation is carried out, flux density, iron loss, output torque, and efficiency MAP are measured. The simulation results are shown in Fig. 5 and Fig. 6. As seen among the figures, under the same current excitation, two different cores show different reactions. In Fig. 6, the range above 1.96T in GO teeth portion is larger, which according to the result of experiment in Fig. 1, the GO silicon steel can reach saturation faster. GO steel sheet can reach the turning point at under 100A/m, while NGO steel sheet can reach the turning point at around 200A/m, and the maximum flux density of GO steel is higher. As it can be shown in Fig. 7, two core models are at the same excitation, the maximum iron loss point in GO type is lower. The region above 4.66W/m³ is larger, and is concentrated mostly in stator yoke made by NGO silicon steel. The average iron loss is measured at all working frequency, the results is shown that iron core of GO type motor achieved 12.1% lower than the conventional type.

Output torque where windings are sustained in the same voltage can be described in equation below:

where:

T_e: stands for output torque;

m: stands for the number of phase, which equals to 3;

p₀: stands for the number of poles, which equals to 8;

ω_s: stands for rotor mechanic speed divided by the number of pole;

E₀: stands for back EMF;

U_s: stands for winding voltage;

X_d: stands for reactance of d-axis;

X_q: stands for reactance of q-axis;

δ_{s f}: stands for the angle between permanent magnet linkage and stator linkage.

The GO silicon steel has higher magnetostriction compared with NGO silicon steel, from the mechanism of magnetostriction of ferromagnetic materials, the movement of domain walls and the rotation of magnetic moment during magnetization are the causes of magnetostriction. The magnetostriction of the GO silicon steel sheet not only has a strong nonlinear relationship with the magnetic field, but also is affected by various complicated factors such as system temperature and condition of pre-stress, which means that the equivalent magnetostrictive force can not be solved simultaneously with magnetic flux density. Thus, structural field is required, and the weak coupling model can be used to meet the requirements. The weak coupling model for solving the magnetic field and structure field of ferromagnetic materials by FEM method is as follows:

Where: S is the magnetic field stiffness matrix; A represents the magnetic potential loss to be determined in vector; J is the current density in vector; K is the mechanical stiffness matrix; U is the displacement deformation in vector; F is the magnetic material stress vector (including mechanical force, electromagnetic force and equivalent magnetostrictive force);

Fig. 7 has shown the comparison of efficiency MAP of two motors at two working conditions. The GO type motor has lower core loss than the conventional one, especially in the high efficiency range, which demonstrates that GO type motor has obvious advantages in reducing the core loss. From the comparison, we observed that the maximum power has increased from 95.2% to 96.8%, about 1.6%. Meanwhile, the range of high efficiency has increased among all working zone of the motor.

Under the rated conditions, the efficiency of both motors increases with the speed. The efficiency augments and then decreases. The efficiency of the GO type motor reaches the maximum at 5120 rpm, and then decreases slowly. The efficiency at 5120 rpm is as high as 96.2%. The efficiency of the conventional type motor reaches the highest point at 6800rpm, and the efficiency decreases gradually at above 10000rpm.

Under the peak conditions, the efficiency increases with the speed. The efficiency augments and then decreases. The efficiency of the GO type motor reaches the maximum at 8300rpm, and decreases slowly along with the speed. The efficiency at 8300 rpm is as high as 96.8%. The efficiency of the conventional type motor reaches the highest point at 11000rpm, and the efficiency decreases along with the speed.

This project applies GO silicon steel to a high-speed PMSM. The stator yoke and rotor adopt traditional NGO silicon steel, the stator tooth is anisotropic GO silicon steel, and the permanent magnet material is rare earth neodymium iron boron.

Compared with the traditional motor processing technology, the production of GO iron core requires new technology in the manufacturing process due to the particularity of the GO silicon steel material. Among different process in iron core manufacturing, punching and lamination process, assembly process, coating material manufacturing processes have an important impact on the performance of GO silicon steel motors.

The NGO silicon steel yoke ring adopts a single stamping die method, and then the single piece is formed into a ring-shaped cylinder by lamination and welding. The GO steel sheet adopts a single stamping die method similarly, and is first stamped into teeth shapes and then bonded by self-bonding process to form GO silicon steel blocks having the same shape and the same direction which is in the rolling direction. The inner side of the NGO stator yoke is provided with a plurality of tenons, and each GO stator tooth is provided with mortises. Each stator core is joined by NGO stator yoke and GO stator teeth as can be seen in Fig. 8.

When GO silicon steel is laminated, it is quite different from the traditional punching sheet. Because of the GO punching sheet is in tooth structure, the self-bonding coated structure can be stable, and the vibration is relatively small. This process has the performance of high insulation, corrosion resistance and high strength. It has concluded that the self-bonding process has positive influence on overall performance of the motor by reducing the noise and vibration due to stronger magnetostriction with the application of GO silicon steel in the rotor.

This article described a 12 slots 8 poles motor in which the core is made of B30AHV1500. The conventional NGO stator core was partially replaced with a tooth made of GO silicon steel, wherein the GO silicon steel grade was B30P105. The maximum efficiency of the motor increased from 95.2% to 96.8%, an increase of 1.6%. The excitation current at maximum output torque droped by 8%, which means that at the same current, the maximum torque will increase by about 8%. At the same time, the overall iron loss of the motor decreased by 12.2%, and the above results all considered the calculation margin due to the splicing process, which is about 10%. The test results are consistent with our assumptions. With this stator core design, GO silicon steel does exhibit significant performance advantages.