High speed solid rotor permanent magnet machines: concept and design

This paper proposes a novel solid rotor topology for an Interior Permanent Magnet (IPM) machine, adopted in this case for an aircraft starter-generator design. The key challenge in the design is to satisfy two operating conditions which are: a high torque at start and a high speed at cruise. Conventional IPM topologies which are highly capable of extended field weakening are found to be limited at high speed due to structural constraints associated with the rotor material. To adopt the IPM concept for high speed operation, it is proposed to adopt a rotor constructed from semi-magnetic stainless steel, which has a higher yield strength than laminated silicon steel. To maintain minimal stress levels and also minimize the resultant eddy current losses due to the lack of laminations, different approaches are considered and studied. Finally, to achieve a better tradeoff between the structural and electromagnetic constraints, a novel slitted approach is implemented on the rotor. The proposed rotor topology is verified using electromagnetic, static structural and dynamic structural Finite Element (FE) analyses. An experiment is performed to confirm the feasibility of the proposed rotor. It is shown that the proposed solid rotor concept for an IPM fulfils the design requirements whilst satisfying the structural, thermal and magnetic limitations

Extension of Virtual-Signal-Injection-Based MTPA Control for Interior Permanent-Magnet Synchronous Machine Drives Into the Field-Weakening Region

This paper presents a field-weakening control scheme to expand the speed operating region of the recently reported virtual signal injection control (VSIC) method for interior permanent-magnet synchronous machine (IPMSM) drives. Because of voltage saturation, the VSIC for IPMSM drives is not effective in the field-weakening region. A new control scheme is developed to guarantee that the torque can be controlled with minimum current amplitude. The proposed method realizes fast dynamic response and efficient operation of IPMSM drives in both constant torque and field-weakening regions by controlling the d-axis current through virtual signal injection and detection of the voltage saturation. The proposed method can track maximum torque per ampere (MTPA) points in the constant torque region and voltage-constrained MTPA points in the field-weakening region accurately without prior knowledge of accurate machine parameters. The proposed control method is demonstrated by both simulations and experiments under various operating conditions on a prototype IPMSM drive system

Direct Torque Control for Silicon Carbide Motor Drives

Direct torque control (DTC) is an extensively used control method for motor drives due to its unique advantages, e.g., the fast dynamic response and the robustness against motor parameters variations, uncertainties, and external disturbances. Using higher switching frequency is generally required by DTC to reduce the torque ripples and decrease stator current total harmonic distortion (THD), which however can lower the drive efficiency. Through the use of the emerging silicon carbide (SiC) devices, which have lower switching losses compared to their silicon counterparts, it is feasible to achieve high efficiency and low torque ripple simultaneously for DTC drives. To overcome the above challenges, a SiC T-type neutral point clamped (NPC) inverter is studied in this work to significantly reduce the torque and flux ripples which also effectively reduce the stator current ripples, while retaining the fast-dynamic response as the conventional DTC. The unbalanced DC-link is an intrinsic issue of the T-type inverter, which may also lead to higher torque ripple. To address this issue, a novel DTC algorithm, which only utilizes the real voltage space vectors and the virtual space vectors (VSVs) that do not contribute to the neutral point current, is proposed to achieve inherent dc-link capacitor voltage balancing without using any DC-link voltage controls or additional DC-link capacitor voltages and/or neutral point current sensors. Both dynamic performance and efficiency are critical for the interior permanent-magnet (IPM) motor drives for transportation applications. It is critical to determine the optimal reference stator flux linkage to improve the efficiency further of DTC drives and maintain the stability of the drive system, which usually obtained by tuning offline and storing in a look-up table or calculated online using machine models and parameters. In this work, the relationship between the stator flux linkage and the magnitude of stator current is analyzed mathematically. Then, based on this relationship, a perturb and observe (P&O) method is proposed to determine the optimal flux for the motor which does not need any prior knowledge of the machine parameters and offline tuning. However, due to the fixed amplitude of the injected signal the P&O algorithm suffers from large oscillations at the steady state conditions. To mitigate the drawback of the P&O method, an adaptive high frequency signal injection based extremum seeking control (ESC) algorithm is proposed to determine the optimal reference flux in real-time, leading to a maximum torque per ampere (MTPA) like approach for DTC drives. The stability analysis and key parameters selection for the proposed ESC algorithm are studied. The proposed method can effectively reduce the motor copper loss and at the same time eliminate the time consuming offline tuning effort. Furthermore, since the ESC is a model-free approach, it is robust against motor parameters variations, which is desirable for IPM motors

Real Time Torque Control of IPMM under Flux Variations

학위논문 (박사)-- 서울대학교 대학원 : 공과대학 전기·컴퓨터공학부, 2019. 2. 설승기.For IPMSM drives, accurate torque control and loss-minimizing operation are important issues to maintain the excellent features such as high efficiency and wide speed operating range. Thus, a current magnitude should be minimized while maintaining a torque accuracy under the base speed, i.e., in a maximum torque per ampere (MTPA) region. On the other hand, a current angle as well as the current magnitude should be adjusted above the base speed due to the limit of a dc-link voltage, i.e., in a flux-weakening region. For efficient operations, current references should be carefully decided by considering current and voltage constraints. However, it is difficult to find optimal current references due to the influence of magnetic saturation, cross-coupling effects and their variation according to operating conditions. In this thesis, an online minimum-copper-loss torque control is proposed to satisfy both a torque accuracy and high-efficiency operation in consideration of flux linkage variations due to magnetic saturation. Firstly, the minimum-copper-loss torque control can be dealt as a constrained optimization problem to satisfy both torque reference tracking and loss-minimizing operation. Nonlinear simultaneous equations can be derived from Lagrange multiplier method, which could be solved by numerical algorithms. Among them, Levenberg-Marquardt algorithm is employed as the numerical algorithm to guarantee a robust calculation of current references in a command optimizer. It can be optimized to alleviate calculation burden while maintaining the stability of the proposed algorithm. In addition, a torque reference limiter is implemented to satisfy the current and voltage constraints in real time. Stator flux linkages and dynamic inductances should be estimated to calculate the current references in the command optimizer. However, not only a dc offset but also low-order harmonics are contained in the estimated flux linkages due to various reasons. Thus, a distortion-minimizing flux observer is proposed to suppress the dc offset and harmonic flux errors by adopting frequency-adaptive observers. A proposed frequency-adaptive flux observer is appropriate to extract the fundamental flux linakges while keeping a simple structure even if a rotating speed of IPMSM varies. The fundamental flux linkages could be estimated quickly by the proposed current- and voltage- model based flux observer, which is implemented with not only the frequency-adaptive flux observer but also harmonic extractors. In addition, a dynamic inductance estimator is proposed to estimate the dynamic inductances by using a high-frequency signal injection method. The self- and mutual- inductance informations can be derived from a high-freqeuncy current response in case of a rotating vector voltage injection method. Thus, the voltage injection and corresponding signal processing method are derived based on the high-freqeuncy impedance modeling, where the effects of cross-coupling terms are included. As a result, the proposed estimator with sine-wave or square-wave injection method improves the precision of estimated dyanamic inductances in high speed operations. The feasibility of the proposed methods has been verified under various operating conditions by simulation and experimental results. Through the proposed command opimizer, the copper-loss minimizing torque control has been achieved under not only unsaturated but also highly-saturated operating conditions. In case of the tested IPMSM, the robust calculation of current references could be guaranteed even when the torque reference is over 80 % of a peak torque by applying the proposed Levenberg-Marquardt algorithm unlike the conventional Gauss-Newton algorithm. Furthermore, the proposed fundamental flux observer and dynamic inductance estimator have maintained high dynamic performance over a wide operating range. In case of the tested IPMSM, the fundamental stator flux linkages and dynamic inductances could be estimated even under 10 pu/s torque refenence variation and 3.75 pu/s load speed variation.높은 효율과 넓은 운전 영역을 갖는 매입형 영구자석 전동기의 효과적인 활용을 위해 정확한 토크 제어와 손실 최소화 운전은 중요하게 다뤄져 왔다. 따라서, 기저 속도 이하에서는 출력 토크를 유지하면서 전류 크기를 최소화하도록 하는 단위 전류 당 최대 토크(MTPA) 운전 방식이 사용된다. 반면, 기저 속도 이상에서는 직류단 전압의 제한으로 인해 전류 크기와 전류 각을 변동하는 약자속(flux-weaking) 운전 방식이 사용된다. 효율적인 운전을 위해 전류 지령은 전류 및 전압 제한 조건을 고려하여 주의 깊게 설정되어야 한다. 하지만, 전동기의 자기포화, 교차결합 현상 및 운전 조건 변동의 영향으로 인해 최적의 전류 지령을 계산하는 것은 쉽지 않다. 본 논문에서는 자기포화에 의한 쇄교자속 변동을 고려하여 토크 정밀도와 고효율 운전을 동시에 만족하는 실시간 최소 동손 토크 운전을 제안하였다. 우선적으로, 최소 동손 토크 운전은 토크 지령 추종과 손실 최소화 운전을 만족하는 제한 최적화 문제로 접근할 수 있다. 이는 라그랑즈 승수법을 통해 비선형 연립 방정식 형태로 유도될 수 있으며 수치 해석 알고리즘을 통해 풀어낼 수 있다. 여러 수치 해석 알고리즘을 비교하여 전류 지령 계산기의 안정적인 계산을 보장할 수 있는 레벤버그-마쿼트법(Levenberg-Marquardt algorithm)을 수치 해석 알고리즘으로 선택하였다. 이를 통해 제안된 전류 지령 계산기의 안정성을 보장하면서 동시에 계산 부담을 최소화할 수 있다. 또한 전류 및 전압 제한을 실시간으로 반영하기 위해 토크 지령 제한기를 구현하였다. 제안된 전류 지령 계산기의 동작을 위해 영구자석 전동기의 쇄교자속 및 동적 인덕턴스의 추정이 필요하다. 그러나 추정 쇄교자속은 다양한 이유로 인해 DC 오프셋 및 저차 고조파를 포함하게 된다. 따라서, 이러한 DC 오프셋 및 고조파 자속 오차를 억제하기 위하여 주파수 적응 관측기를 기반으로 한 외란 최소화 자속 관측기를 제안하였다. 제안된 주파수 적응 자속 관측기는 회전 속도 변동 시에도 단순한 구조에도 불구하고 기본파 쇄교자속만을 추출해 낼 수 있다. 주파수 적응 자속 관측기와 고조파 추출기로 구현한 제안된 전류 모델 및 전압 모델 자속 관측기를 통해 기본파 쇄교자속을 빠르게 추정하는 것이 가능하다. 또한 고주파 신호 주입에 기반하여 동적 인덕턴스를 추정하기 위한 동적 인덕턴스 추정기를 제안하였다. 회전 벡터 전압 주입 방법의 경우, 유기된 고주파 전류를 통해 자기 및 상호 인덕턴스를 추출할 수 있다. 따라서, 상호 교차항의 영향을 포함한 고주파 임피던스 모델링에 기반하여 전압 주입 및 전류 신호 처리 방안을 제안하였다. 결과적으로 정현파 혹은 구형파 주입에 기반한 제안된 추정기를 통해 고속 운전 시 동적 인덕턴스 추정 정밀도를 향상시킬 수 있다. 제안된 알고리즘들의 타당성은 다양한 조건에서의 시뮬레이션 및 실험을 통해 검증되었다. 우선적으로 제안된 전류 지령 계산기를 통해 자기포화가 거의 없는 영역 뿐만 아니라 포화가 심한 영역에서도 실시간 최소 동손 토크 운전이 가능함을 확인하였다. 시험용 전동기의 경우, 기존 가우스-뉴튼법과 달리 제안된 레벤버그-마쿼트법을 통해 0.8 pu 이상의 토크 지령에 대해서도 최소 동손 토크 운전이 가능하였다. 또한 제안된 기본파 자속 관측기 및 동적 인덕턴스 추정기가 넓은 운전 영역에서 충분한 동특성을 가지는 것을 확인할 수 있었다. 시험용 전동기의 경우, 10 pu/s 토크 지령 변동 및 3.75 pu/s 부하 속도 변동에서도 기본파 자속 및 동적 인덕턴스를 잘 추정할 수 있었다.제 1장 서론 １ 1.1 연구의 배경 １ 1.2 연구의 목적 ７ 1.3 논문의 구성 ８ 제 2장 영구자석 전동기의 특성 및 운전 방안 ９ 2.1 영구자석 전동기의 특성 １０ 2.1.1 자기포화 및 교차결합 현상 １０ 2.1.2 공간 고조파 １６ 2.1.3 제정수 변동에 따른 토크 영향 １９ 2.1.4 영구자석 전동기의 모델링 ２３ 2.2 기존의 손실 최소화 운전 방안 ２５ 2.2.1 섭동 기반 탐색 방법 ２６ 2.2.2 수학적 모델 기반 계산 방법 ２８ 2.2.3 기존 운전 방안의 특성 비교 ３２ 2.3 제안된 손실 최소화 운전 방안 ３４ 제 3장 최소 동손 전류 지령 계산 알고리즘 ３６ 3.1 최소 동손 운전 방정식 ３７ 3.1.1 MTPA 영역 ３８ 3.1.2 약자속 영역 ４０ 3.2 제안된 전류 지령 계산기 [49] ４２ 3.2.1 수치 해석 알고리즘의 검토 ４２ 3.2.2 레벤버그-마쿼트법의 감쇠비 영향 고찰 ５４ 3.2.3 계산 알고리즘의 구현 ６０ 3.3 제안된 토크 지령 제한기 ６２ 3.3.1 최대 토크 운전 방정식 ６３ 3.3.2 계산 알고리즘의 구현 ６５ 3.4 운전 영역 판별 및 절환 방안 ６８ 3.5 시뮬레이션 및 실험 결과 ７５ 3.5.1 시뮬레이션 결과 ７６ 3.5.2 실험 결과 ８２ 제 4장 외란 최소화 기본파 자속 관측기 ８８ 4.1 기존 자속 관측기의 특성 ８９ 4.2 제안된 주파수 적응 자속 관측기 [68] ９３ 4.3 제안된 기본파 자속 관측기 ９９ 4.3.1 전압 모델 기반 자속 관측기 ９９ 4.3.2 전류 모델 기반 자속 관측기 １０４ 4.4 자속 관측기 구현 시 고려 사항 １０７ 4.4.1 전동기 제정수 오차 및 오프셋 영향 분석 １０７ 4.4.2 자속 관측기 간 절환 방안 １０８ 4.4.3 디지털 시지연을 고려한 보상 방안 １１０ 4.5 시뮬레이션 및 실험 결과 １１２ 4.5.1 시뮬레이션 결과 １１３ 4.5.2 실험 결과 １２１ 제 5장 신호 주입 기반 동적 인덕턴스 추정기 １２９ 5.1 신호 주입 알고리즘 특성 １３０ 5.2 고주파 임피던스 모델링 １３５ 5.3 제안된 동적 인덕턴스 추정기 １３７ 5.3.1 정현파 전압 고주파 주입 １３７ 5.3.2 구형파 전압 고주파 주입 １４１ 5.4 고주파 신호 주입 시 고려 사항 １４６ 5.4.1 공간 고조파 영향 １４６ 5.4.2 부차적인 손실 및 오차 영향 １４７ 5.5 시뮬레이션 및 실험 결과 １４９ 5.5.1 시뮬레이션 결과 １５０ 5.5.2 실험 결과 １５６ 제 6장 시뮬레이션 및 실험 １６２ 6.1 시뮬레이션 결과 １６３ 6.2 실험 결과 １６７ 제 7장 결론 １７２ 7.1 연구 결과 １７２ 7.2 향후 과제 １７４ 부 록 １７７ A. 제안된 기본파 자속 관측기의 타 전동기 적용 결과 １７７ B. 시험용 영구자석 전동기의 실험 제정수 １９１ 참고 문헌 １９４ Abstract １９９Docto

EFFICIENCY OPTIMISED CONTROL OF INTERIOR MOUNTED PERMANENT MAGNET MACHINES FOR ELECTRIC VEHICLE TRACTION

Electric vehicles (EV) are playing a momentous role to the wide society as they facilitate the use of clean energy technologies. Interior mounted permanent magnet (IPM) synchronous machines are commonly employed in EVs owing to their superior characteristics such as high efficiency and power density and a wide field weakening operating range. High efficiency motor operation extends EVs drive range with the same amount of energy. Advanced control techniques to achieve high efficiency operation and smooth output torque production are, therefore, highly important areas to be researched. This thesis deals with the state-of-art motor drives and further develops advanced control strategies for minimum loss operation with good torque control quality. Modern AC drives can be classified in two groups, viz., field oriented control (FOC) and direct torque control (DTC). Whilst the former controls the phase currents for torque realization, the latter controls the torque directly. This thesis researches both and the novel advanced techniques are underpinned by extensive simulations and supported by experimental validations on a prototype motor designed for a specific class of EVs. The biggest challenge associated with the FOC drives is to improve the efficiency due to highly nonlinear characteristics of IPM machines. It has been discovered that even if the machine parameters are accurately modelled and stored in controllers to achieve optimal efficiency operation in a great number of FOC based IPM drives, there is still much deviation from the ideal operating points. A novel approach for online efficiency optimisation is proposed and comprehensively analysed in this thesis. The challenges pertinent to the DTC based IPM drives are to improve the observer quality and to reduce the strong coupling and the nonlinearity in the control loops. Novel observer structures, and the decoupled and linearized control techniques are among the novel contributions for DTC drives in this thesis. In addition, a comprehensive analysis of the relationship between stator flux vector and the torque has not been performed in the literature. The detailed analysis is made in this thesis and the maximum torque per voltage (MTPV) control theory for DTC drives is introduced. It is noteworthy that this thesis is based on comparative studies between the state-of-art and the proposed techniques throughout, and hence offers an insightful understanding for modern IPM drives