Research on key technologies of ultra-high power density motor drive system for electric vehicles

The driving force behind the development of ultra-high power density motor drive systems is: with the same volume or mass, the output power is greater, the overtaking acceleration capability and high-speed continuous driving capability are stronger, and excellent power performance and driving experience are obtained; with the same output power, the miniaturized and lightweight design can achieve high performance in a given space, the layout is flexible, the vehicle's loadability is better, it is conducive to platform modularization and four-wheel drive layout, and it is suitable for native electric chassis architecture design, with less material consumption and lower cost.

1 Theoretical Analysis

The industry has not yet reached a consensus on the definition of power density. We have clarified the calculation methods for different indicator definitions and analyzed the connotations of the indicators, as shown in Table 1.

Table 1 Definition and connotation of power density index of motor drive system

Generally, electric drive systems are evaluated by mass power density indicators, the motor body is evaluated by effective specific power indicators, and the inverter is evaluated by volume power density indicators; generally, passenger car power systems are evaluated by power density indicators, while commercial vehicle power systems are evaluated by torque density indicators.

The evaluation of power density index needs to be carried out under certain prerequisites, and is closely related to factors such as index definition, evaluation object, operating voltage, operating temperature and cooling conditions, duration, constant power speed regulation range, etc. The quantitative index of power density varies greatly under different prerequisites. Due to the lack of unified standards, various companies currently tend to inflate indicators when promoting their products to improve market competitiveness. In response to this situation, during the preparation of the National Energy-Saving and New Energy Vehicle Technology Roadmap 2.0, a standardized definition of the motor effective specific power index was proposed:

Motor effective mass: the mass of the stator and rotor assembly, including insulation and curing materials, excluding shaft, housing, etc.;

Duration corresponding to peak power: 30 s;

Peak power definition: the maximum power that can be output within the range of base speed to 0.75 times the maximum operating speed for 30 seconds;

Current level: converted to 450 A;

Voltage level: converted to bus voltage 400 V;

Test environment: 85 ℃ environmental chamber, 65 ℃ coolant inlet temperature [1].

2 Technical Path

According to the above theoretical analysis, the power density of the motor drive system can be improved by increasing the peak output power, reducing the volume and mass, and improving the thermal design and thermal management, focusing on improving system integration and lean matching design, increasing speed and voltage, optimizing the design of new motors and electromagnetic performance, new power electronics and control technology, and innovating and upgrading materials and processes. The technical framework is shown in Figure 1.

Figure 1 High power density electric drive technology shelf

2.1 Increase output power

2.1.1 Electromagnetic performance lean design

Compared with other types of motors, permanent magnet synchronous motors have advantages in power density and efficiency and are suitable for traction drive of electric vehicles. Assuming that the main magnetic flux is the same, the permanent magnet torque is the same. The permanent magnet synchronous motor with built-in structure can further improve the total torque output capacity by using the newly added reluctance torque. The torque of the permanent magnet synchronous motor with surface-mounted structure is composed only of permanent magnet torque, as shown in formula (1). The torque of the permanent magnet synchronous motor with built-in structure is composed of permanent magnet torque and reluctance torque, as shown in formula (2) [2]. Based on the actual working conditions of the whole vehicle, the electromagnetic structure is refined, the electromagnetic load is reasonably distributed, and the motor pole pair number, permanent magnet flux, direct axis inductance, quadrature axis inductance, and phase resistance parameters are adjusted to obtain ideal power output characteristics.

(1)

(2)

Where: Te is the electromagnetic torque; p is the number of pole pairs; ψf is the magnetic flux generated by the permanent magnet; is is the stator current; β is the spatial electrical angle; Ld is the d-axis inductance; Lq is the q-axis inductance.

Mitsubishi Electric has achieved a motor output power density of 23 kW/L by adopting a comprehensive electromagnetic structure design of "asymmetric rotor + concentrated winding + unique magnet gap", especially maximizing the power density in one rotation direction, as shown in Figure 2.

Figure 2 Asymmetric rotor electromagnetic structure of Mitsubishi high power density motor

2.1.2 High-speed motor design

According to the motor design formula (3), under the premise of equal power, the higher the speed, the smaller the torque, the smaller the motor size D2L, the lower the material usage and the lower the cost, the higher the specific power can be achieved.

(3)

Where: CA is the motor constant; D is the inner diameter of the stator; n is the speed; αδ is the pole arc coefficient; lδ is the effective length of the core; kB is the waveform factor; kW is the winding factor; Bδ is the air gap flux density (magnetic load); A is the line load (electric load),

is the number of turns per phase; m is the number of phases; I is the current value.

The key technologies for high-speed motors are: to ensure stable control, higher control frequency and computing power are required, which requires faster hardware execution speed of the main control chip and optimized software function design; high speed leads to increased back electromotive force of the motor, which requires increased device withstand voltage, and system protection functions are designed, such as active short circuit, to improve system safety; the operating frequency of high-speed motors is increased, and ultra-thin silicon steel sheets and magnetic steel segmentation designs are required to suppress iron losses; high-speed motors need to adopt high-strength rotor electromagnetic structures, high-speed bearings, high-strength silicon steel and other designs to achieve this, as shown in Figure 3.

Fig. 3 SKF’s new high-speed ball bearing HSBB 1.8[4]

2.1.3 New multiphase motor design

A multi-phase motor refers to a motor with more than three power supply phases. Under the same bus supply voltage, it improves the current output capacity and thus the power output capacity. It is particularly suitable for application scenarios where the supply voltage is limited and the power demand is relatively large. By increasing the number of phases, the motor input torque pulsation is reduced and the NVH characteristics are improved. At the same time, it can avoid problems such as dynamic and static voltage balancing in two-level inverters and improve the reliability of the electric drive system [4]. Compared with traditional three-phase motors, the advantages of multi-phase motors are small torque pulsation, high torque density, low voltage and high power, and high fault tolerance and reliability [4-6]. Figure 4 shows the stator structure comparison between a multi-phase motor and a traditional three-phase motor.

Figure 4. The actual slot winding distribution of a surface-mounted 12-slot 10-pole permanent magnet motor.

2.1.4 New axial flux motor design

Axial flux motors, also known as disc motors, have a flat air gap and the direction of the excitation magnetic field is parallel to the motor shaft. Compared with ordinary radial motors, the rotor of axial flux motors has a larger diameter. It can be seen from the torque formula that under the same force, an increase in rotor diameter can obtain a greater torque, which also means that when the permanent magnet material and copper wire material are the same, the axial flux motor has a stronger torque output capacity [7]. Generally, the new axial motor structure can bring a 30% increase in torque capacity compared to the traditional radial motor structure [7]. Due to its structural characteristics, the axial flux motor has the characteristics of compact axial structure, flat appearance, small size and high power density. In recent years, after continuous improvement and perfection in the industry, it has gradually been applied to new energy electric vehicles [7-8]. Figure 5 shows the structure and magnetic circuit comparison between the traditional radial flux motor and the new axial flux motor.

Figure 5 Comparison between traditional radial flux motor and new axial flux motor

2.1.5 Voltage vector overmodulation control

Compared with the torque control method based on current vector, voltage vector control does not need to reserve the margin of voltage closed-loop regulator, has natural weak magnetic capability, and can achieve deeper weak magnetic depth with the same bus voltage, fully tapping the maximum output capacity of the motor. The comparison of various voltage vector control schemes is shown in Table 2.

Table 2 Brief introduction to voltage vector control scheme characteristics [10]

The operating range of SVPWM is extended to the hexagonal area through the overmodulation PWM strategy, as shown in Figure 6. Combined with the voltage vector control method, the utilization rate of the DC bus voltage is increased from 1 to 1.15, and the bus voltage is maintained unchanged. The output torque and power of the motor system can be greatly improved [9-11].

Figure 6 PWM overmodulation strategy