Towards Fully Additively-Manufactured Permanent Magnet Synchronous Machines: Opportunities and Challenges

With the growing interest in electrification and as hybrid and pure electric powertrains are adopted in more applications, electrical machine design is facing challenges in terms of meeting very demanding performance metrics for example high specific power, harsh environments, etc. This provides clear motivation to explore the impact of advanced materials and manufacturing on the performance of electrical machines. This paper provides an overview of additive manufacturing (AM) approaches that can be used for constructing permanent magnet (PM) machines, with a specific focus on additively-manufactured iron core, winding, insulation, PM as well as cooling systems. Since there has only been a few attempts so far to explore AM in electrical machines (especially when it comes to fully additively-manufactured machines), the benefits and challenges of AM have not been comprehensively understood. In this regard, this paper offers a detailed comparison of multiple multi-material AM methods, showing not only the possibility of fully additively-manufactured PM machines but also the potential significant improvements in their mechanical, electromagnetic and thermal properties. The paper will provide a comprehensive discussion of opportunities and challenges of AM in the context of electrical machines

Design Optimization and Experimental Study of Coreless Axial-Flux PM Machines with Wave Winding PCB Stators

This paper presents a systematic design optimization and experimental study of coreless axial-flux permanent magnet (AFPM) machines with wave winding printed circuit board (PCB) stators. Existing PCB winding configurations are briefly reviewed and compared to serve as a performance reference. A new concept –the “macro coil”– is proposed to model PCB stator windings in 3D finite element analysis (FEA) to facilitate large-scale multi-objective optimization with the winding eddy current losses evaluated in each design candidate. The detailed trace-by-trace model for a selected optimal design is then built to check the voltage difference between parallel paths and the resulting circulating currents. A prototype with two PM rotors and a central stator comprising multiple stacked PCBs has been fabricated and tested on a customized test fixture for experimental validation

Electric Power Systems and Components for Electric Aircraft

Electric aircraft have gained increasing attention in recent years due to their potential for environmental and economic benefits over conventional airplanes. In order to offer competitive flight times and payload capabilities, electric aircraft power systems (EAPS) must exhibit extremely high efficiencies and power densities. While advancements in enabling technologies have progressed the development of high performance EAPS, further research is required. One challenge in the design of EAPS is determining the best topology to be employed. This work proposes a new graph theory based method for the optimal design of EAPS. This method takes into account data surveyed from a large set of references on commonly seen components including electric machines, power electronics and jet engines. Thousands of design candidates are analyzed based on performance metrics such as end-to-end system efficiency, overall mass, and survivability. It is also shown that sensitivity analysis may be used to systematically evaluate the impact of components and their parameters on various aspects of the architecture design. Once an EAPS architecture has been selected, further, detailed, validation of the power system is required. In these EAPS, many subsystems exist with timescales varying from minutes to hours when considering the aerodynamics, to nanosecond dynamics in the power electronics. This dissertation presents a multiphysics co-simulation framework for the evaluation of EAPS with a unique decoupling method to reduce simulation time without sacrificing detail. The framework has been exemplified on a case study of a 500kW all-electric aircraft, including models for aerodynamics, energy storage, electric motors and power electronics. Electric machines for aviation propulsion must meet several performance requirements, including a constant power speed range (CPSR) of approximately thirty percent above rated speed. This operation is traditionally achieved through the flux weakening technique with an injection of negative d-axis current. However, the degree of CPSR achievable through flux weakening is a strong function of the back emf and d-axis inductance. This dissertation reviews alternative methods for CPSR operation in machines with low inductance. A new method of current weakening has been proposed to address this challenge, involving reducing the machine\u27s current inversely proportional to the operating speed, maintaining constant power through the extended speed range. One benefit of the proposed method is that all current is maintained in the q-axis, maintaining maximum torque per ampere operation. Coreless axial flux permanent magnet (AFPM) machines have recently gained significant attention due to their specific form factor, potentially higher power density and lower losses. Coreless machine designs promise high efficiency particularly at higher speeds, due to the absence of a ferromagnetic core. In this dissertation, coreless AFPM machines with PCB stators are investigated as candidates for propulsion in electric aircraft applications. Two PCB stator design variations are presented with both simulation and experimental results

Design Optimization of Coreless Axial-Flux PM Machines with Litz Wire and PCB Stator Windings

Coreless axial-flux permanent-magnet (AFPM) machines may be attractive options for high-speed and high-power-density applications due to the elimination of core losses. In order to make full use of the advantages offered by these machines and avoid excessive eddy current losses in windings, advanced technologies for winding conductors need to be employed to suppress the eddy effect, such as the Litz wire and printed circuit board (PCB). In this paper, the best practices for designing Litz wire/PCB windings are discussed and a brief survey of state of the art PCB winding technology is provided. Three coreless AFPM machines are mainly considered. A design optimization procedure based on the multi-objective differential evolution algorithm and 3-dimensional (3D) finite element analysis (FEA) is proposed to take into account the ac winding losses of Litz wires and PCB traces in the machine design stage. Selected designs are being prototyped and will be tested with a customized test fixture

On the Design of Coreless Permanent Magnet Machines for Electric Aircraft Propulsion

This paper presents design and prototyping studies for coreless and slotless permanent magnet (PM) machines, which have the potential for high power density and efficiency, and discusses their feasibility for electric aircraft propulsion. The emphasis is on axial flux permanent magnet (AFPM) machines with printed circuit board (PCB) stators that have advantages over their wired counterparts in terms of design flexibility, coil accuracy, manufacturing process reliability, and heat dissipation. Detailed electromagnetic finite element analysis models were developed and employed alongside analytical sizing equations to evaluate the performance of two dual-rotor single-stator coreless AFPM designs employing wave and spiral PCB winding patterns. Design considerations for a 10kW 2,600rpm rating similar to the NASA X-57 electric aircraft propulsor motors are included. A 26-pole prototype machine has been developed and experimental testing results are presented

Development of an axial flux MEMS BLDC micromotor with increased efficiency and power density

This paper presents a rigorous design and optimization of an axial flux microelectromechanical systems (MEMS) brushless dc (BLDC) micromotor with dual rotor improving both efficiency and power density with an external diameter of only around 10 mm. The stator is made of two layers of windings by MEMS technology. The rotor is developed by film permanent magnets assembled over the rotor yoke. The characteristics of the MEMS micromotor are analyzed and modeled through a 3-D magnetic equivalent circuit (MEC) taking the leakage flux and fringing effect into account. Such a model yields a relatively accurate prediction of the flux in the air gap, back electromotive force (EMF) and electromagnetic torque, whilst being computationally efficient. Based on 3-D MEC model the multi-objective firefly algorithm (MOFA) is developed for the optimal design of this special machine. Both 3-D finite element (FE) simulation and experiments are employed to validate the MEC model and MOFA optimization design

Special Power Electronics Converters and Machine Drives with Wide Band-Gap Devices

Power electronic converters play a key role in power generation, storage, and consumption. The major portion of power losses in the converters is dissipated in the semiconductor switching devices. In recent years, new power semiconductors based on wide band-gap (WBG) devices have been increasingly developed and employed in terms of promising merits including the lower on-state resistance, lower turn-on/off energy, higher capable switching frequency, higher temperature tolerance than conventional Si devices. However, WBG devices also brought new challenges including lower fault tolerance, higher system cost, gate driver challenges, and high dv/dt and resulting increased bearing current in electric machines. This work first proposed a hybrid Si IGBTs + SiC MOSFETs five-level transistor clamped H-bridge (TCHB) inverter which required significantly fewer number of semiconductor switches and fewer isolated DC sources than the conventional cascaded H-bridge inverter. As a result, system cost was largely reduced considering the high price of WBG devices in the present market. The semiconductor switches operated at carrier frequency were configured as Silicon Carbide (SiC) devices to improve the inverter efficiency, while the switches operated at fundamental output frequency (i.e., grid frequency) were constituted by Silicon (Si) IGBT devices. Different modulation strategies and control methods were developed and compared. In other words, this proposed SiC+Si hybrid TCHB inverter provided a solution to ride through a load short-circuit fault. Another special power electronic, multiport converter, was designed for EV charging station integrated with PV power generation and battery energy storage system. The control scheme for different charging modes was carefully developed to improve stabilization including power gap balancing, peak shaving, and valley filling, and voltage sag compensation. As a result, the influence on the power grid was reduced due to the matching between daily charging demand and adequate daytime PV generation. For special machine drives, such as slotless and coreless machines with low inductance, low core losses, typical drive implementations using conventional silicon-based devices are performance limited and also produce large current and torque ripples. In this research, WBG devices were employed to increase inverter switching frequency, reduce current ripple, reduce filter size, and as a result reduce drive system cost. Two inverter drive configurations were proposed and implemented with WBG devices in order to mitigate such issues for 2-phase very low inductance machines. Two inverter topologies, i.e., a dual H-bridge inverter with maximum redundancy and survivability and a 3-leg inverter for reduced cost, were considered. Simulation and experimental results validated the drive configurations in this dissertation. An integrated AC/AC converter was developed for 2-phase motor drives. Additionally, the proposed integrated AC/AC converter was systematically compared with commonly used topologies including AC/DC/AC converter and matrix converters, in terms of the output voltage/current capability, total harmonics distortion (THD), and system cost. Furthermore, closed-loop speed controllers were developed for the three topologies, and the maximum operating range and output phase currents were investigated. The proposed integrated AC/AC converter with a single-phase input and a 2-phase output reduced the switch count to six and resulting in minimized system cost and size for low power applications. In contrast, AC/DC/AC pulse width modulation (PWM) converters contained twelve active power semiconductor switches and a common DC link. Furthermore, a modulation scheme and filters for the proposed converter were developed and modeled in detail. For the significantly increased bearing current caused by the transition from Si devices to WBG devices, advanced modeling and analysis approach was proposed by using coupled field-circuit electromagnetic finite element analysis (FEA) to model bearing voltage and current in electric machines, which took into account the influence of distributed winding conductors and frequency-dependent winding RL parameters. Possible bearing current issues in axial-flux machines, and possibilities of computation time reduction, were also discussed. Two experimental validation approaches were proposed: the time-domain analysis approach to accurately capture the time transient, the stationary testing approach to measure bearing capacitance without complex control development or loading condition limitations. In addition, two types of motors were employed for experimental validation: an inside-out N-type PMSM was used for rotating testing and stationary testing, and an N-type BLDC was used for stationary testing. Possible solutions for the increased CMV and bearing currents caused by the implementation of WGB devices were discussed and developed in simulation validation, including multi-carrier SPWM modulation and H-8 converter topology

Circulating and Eddy Current Losses in Coreless Axial Flux PM Machine Stators with PCB Windings

Printed circuit board (PCB) stators in coreless axial flux permanent magnet (AFPM) machines have been proposed, designed, and studied for use in multiple industries due to their design flexibility and reduction of manufacturing costs, volume, and weight compared to conventional stators. This paper investigates mechanisms and methods of approximating open circuit losses in PCB stators within example wave and spiral winding topologies for a dual rotor, single stator configuration using 3D FEA, analytical hybrid techniques and experiments. The effect of rotor magnet shape, end winding, and active conductor geometry on eddy currents is studied, and some mitigation techniques are proposed. Through stator equivalent circuit analysis, circulating current losses caused by mechanical abnormalities and magnetic circuit asymmetry are assessed. Possible strategies and schemes to minimize circulating current losses are also described. The tradeoff between stator loss components and some practical design considerations are outlined in detail. The open circuit power losses of a prototype coreless AFPM motor were experimentally tested using multiple example PCB stators and emulated rotor asymmetries, with the findings being comparable to the FEA and hybrid analytical methods results

WAVED: A Coreless Axial Flux PM Motor for Drive Systems with Constant Power Operation

In this paper, a two-phase coreless AFPM machine with wave winding, 2-stators, and 3-rotors for traction applications is studied. A highly general optimization method, employing 3D FEA as the computational engine is employed. Number of poles in addition to other geometrical variables are included as independent optimization variables. A method for extending the speed range by rotating one of the stator discs with respect to the other at constant power operation is proposed. An inverter configuration including Si and wide band gap devices is proposed to be employed in conjunction with the machine under study. The study includes a comparison of the optimally designed coreless machine with a commercial yokeless and segmented armature electric motor

Optimal Design of Special High Torque Density Electric Machines based on Electromagnetic FEA

Electric machines with high torque density are essential for many low-speed direct-drive systems, such as wind turbines, electric vehicles, and industrial automation. Permanent magnet (PM) machines that incorporate a magnetic gearing effect are particularly useful for these applications due to their potential for achieving extremely high torque density. However, when the number of rotor polarities is increased, there is a corresponding need to increase the number of stator slots and coils proportionally. This can result in manufacturing challenges. A new topology of an axial-flux vernier-type machine of MAGNUS type has been presented to address the mentioned limitation. These machines can attain high electrical frequency using only a few stator coils and teeth, which can simplify construction and manufacturing under certain conditions. Additionally, the inclusion of auxiliary small teeth within the stator main teeth can generate a noteworthy increase in output torque, making it a unique characteristic of this motor. By analyzing the operating principle of the proposed VTFM PM machine, possible pole-slot combinations have been derived. The process of designing an electric machine is complicated and involves several variables and factors that must be balanced by the designer, such as efficiency, cost, and performance requirements. To achieve a successful design, it is crucial to employ multi-objective optimization. Using a 3D FEA model can consider the impact of magnetic saturation, leakage flux, and end effects, which are not accounted for in 2D. Optimization using a 3D parametric model can offer a more precise analysis. Validating the machine\u27s performance requires prototyping a model and testing it under different operating conditions, such as speed and load, which is a crucial step. This approach provides valuable insights into the machine\u27s behavior, allowing the identification of any areas for improvement or weaknesses. A large-scale multi-objective optimization study has been conducted for an axial-flux vernier-type PM machine with a 3-dimensional (3D) finite element analysis (FEA) to minimize the material cost and maximize the electromagnetic efficiency. A detailed study for torque contribution has indicated that auxiliary teeth on each stator main teeth amplify net torque production. A prototype of optimal design has been built and tested