Novel structure to mitigate shaft voltage in IPMSM for EV propulsion based on electric blocking effect

다국어 초록 (Multilingual Abstract)

In this paper, a stator and rotor structure for reducing shaft voltage in electric vehicle (EV) traction motors is proposed, focusing on shape variables that control the electric-field distribution. The current trend in motor design emphasizes higher power density achieved through high-voltage operation. However, this approach inevitably increases the common-mode voltage and consequently exacerbates bearing electrical erosion caused by elevated shaft voltage. Therefore, a design-oriented solution that mitigates shaft voltage without additional components or cost is required.
First, the equivalent circuit of the motor was derived by modeling the parasitic capacitances generated inside the machine, establishing the design direction for shaft voltage reduction. It was then confirmed that conventional stator and rotor geometries have a limitation in that back-yoke saturation occurs when reducing shaft voltage, resulting in output degradation. To overcome this issue, a wedge-less stator structure is proposed, which blocks the electric field between the winding and the rotor, thereby reducing shaft voltage while mechanically fixing the coils and improving manufacturability by eliminating wedges. Furthermore, structural asymmetry was introduced to enhance the electric-field blocking effect.
In addition, a cut-off rotor is proposed to reduce the electric field between the end-winding region and the rotor. Since the slot opening alone cannot effectively reduce the end-winding electric field, the upper and lower parts of the rotor core were trimmed to achieve field reduction. The theoretical background of these geometries is explained from the viewpoint of electric-field distribution, and their effectiveness is verified through finite element analysis (FEA). Sensitivity analysis was also performed to investigate the influence of each shape variable on the electric-field and electromagnetic performance.
Finally, the proposed geometries were applied to an 80-kW class interior permanent magnet synchronous motor (IPMSM) for EV traction. Electro–electromagnetic optimization was carried out to meet the target performance requirements, and the optimal design was derived. The optimized design was then refined considering mechanical stiffness and irreversible demagnetization robustness, and the fabricated prototype was experimentally validated to confirm its practical applicability.

국문 초록 (Abstract)

본 논문에서는 전기자동차 구동모터의 축전압을 저감하기 위해 전계 분포를 제어하는 형상변수를 기반으로 한 고정자 및 회전자 구조를 제안하였다. 최근 모터 설계의 주요 트렌드는 고전압화를 통한 고출력밀도 향상에 초점이 맞추어져 있다. 그러나 이러한 고전압 구동은 공통모드 전압을 증가시켜 축전압을 상승시키며, 이에 따른 베어링 전식을 유발하여 신뢰성 저하를 초래한다. 따라서 추가적인 부품이나 비용 증가 없이 형상 설계만으로 축전압을 저감할 수 있는 설계 기반 해법이 필요하다.
우선, 모터 내부에서 발생하는 기생 커패시턴스를 모델링하여 모터의 등가회로를 도출하고, 이를 통해 축전압 저감의 설계 방향성을 수립하였다. 그 결과 기존 고정자 및 회전자 형상에서는 축전압을 줄이기 위해 백요크가 포화되어 출력이 저하되는 한계가 있음을 확인하였다. 이러한 문제를 극복하기 위하여, 권선과 회전자 사이의 전계를 차단함으로써 축전압을 저감하고, 동시에 웨지를 제거하여 제작성을 향상시키는 웨지리스 고정자 구조를 제안하였다. 또한, 구조적 비대칭성을 적용하여 전계 차단 효과를 극대화하였다.
또한 엔드권선 영역과 회전자 사이의 전계를 줄이기 위해 컷오프 회전자 구조를 제안하였다. 슬롯 오프닝 형상만으로는 엔드권선 전계를 충분히 저감하기 어렵기 때문에 회전자 철심의 상하단을 절삭하여 전계 저감 효과를 확보하였다. 제안된 형상들의 이론적 배경은 전계 분포 관점에서 설명하였으며, 유한요소해석을 통해 그 유효성을 검증하였다. 또한 민감도 해석을 수행하여 각 형상변수가 전계 및 전자계 성능에 미치는 영향을 분석하였다.
마지막으로, 제안된 형상을 80 kW급 전기자동차용 매입형 영구자석 동기모터에 적용하여 전계–전자계 연동 최적설계를 수행하였으며, 목표 성능을 만족하는 최적 설계안을 도출하였다. 도출된 최적 설계안은 기계적 강도 및 불가역 감자에 대한 강건성을 고려하여 수정하였고, 이를 기반으로 제작된 시제품을 시험 검증함으로써 제안 형상의 실질적 유효성을 입증하였다.

목차 (Table of Contents)

• 1. Introduction 1
• 1.1 Research Background 1
• 1.2 Research Objective 7
• 1.3 Overview of Research 10
• 2. Permanent Magnet Synchronous Motor 12
• 2.1 Introduction to PMSM 12
• 2.2 Surface Mounted PMSM 14
• 2.2.1 a-b-c phase equation 15
• 2.2.2 d-q axis equation 16
• 2.3 Interior PMSM 22
• 2.3.1 Operating characteristics of IPMSMs 23
• 3. Study of Shaft Voltage 32
• 3.1 Generation Mechanism of Shaft Voltage 32
• 3.2 PWM Inverter and Common-Mode Voltage 35
• 3.3 Parasitic Capacitance 43
• 3.3.1 Capacitive Equivalent Circuit 45
• 3.3.2 Winding to Rotor Capacitance 46
• 3.3.3 Stator to Rotor Capacitance 48
• 4. Shaft Voltage Mitigation Method 49
• 4.1 Mitigation Strategy Through Motor Design 49
• 4.1.1 Winding Position Modification 50
• 4.1.2 Slot opening modification 52
• 4.1.3 Novel structure for SV reduction 52
• 5. Proposal of Novel Structure to Reduce Shaft Voltage 54
• 5.1 Novel Stator Structure: Wedge-Less Stator 54
• 5.1.1 Working Principle 54
• 5.1.2 FEA Validation of the Proposed WLS Geometry 59
• 5.1.3 Sensitivity Analysis according to the Design Variables 64
• 5.2 Novel Rotor Structure: Cut-Off Rotor 85
• 5.2.1 Working Principle 85
• 5.2.2 FEA Validation of the Proposed Rotor Structure 86
• 5.2.3 Sensitivity Analysis according to the Design Variables 90
• 6. Optimal Design for Shaft Voltage Mitigation 104
• 6.1 Motor Specification for the EV Propulsion 104
• 6.2 Design Variables and Objective Function 106
• 6.2.1 Design Variables and Boundary Condition 106
• 6.2.2 Objective Function for Optimization 109
• 6.3 Optimization Results 111
• 6.3.1 No-load characteristics 115
• 6.3.2 Load characteristics 117
• 6.3.3 Shaft voltage characteristics 122
• 6.4 Performance Analysis of the Manufactured Model 123
• 7. Experimental Verification 131
• 7.1 Experimental Set-up and Manufactured IPMSM Model 131
• 7.2 Comparison of FEA and Measured Results for Validation 133
• 8. Conclusion 136