The electric revolution is underway in the transportation sector, and the aviation industry is poised to embrace fundamental disruption. Moving to electric aircraft brings undeniable benefits in terms of environmental impact, cost savings, maintenance, noise pollution, and safety. Nevertheless, several technical challenges are yet to be overcome to build electric airplanes that meet public needs while gaining acceptance and trust. From urban air mobility to long-haul flight applications, hundreds of projects are under research to push toward more electrification. At the heart of each aircraft architecture, power electronics plays a crucial role in the new era of transportation. This article aims to provide a comprehensive analysis of state-of-the-art power electronics in electric aircraft. A review of the current status of aircraft electrification will be provided, and technology surveys of power electronic converters will be detailed. Challenges for forthcoming power electronics in response to the future trends of the electrical network will be explained. Finally, emerging technologies regarding wide bandgap devices, advanced topologies and control, thermal management, passive components, and system integration will be discussed.

Power Electronic Converters in Electric Aircraft: Current Status, Challenges, and Emerging Technologies

This article investigates the design of a 250 kW permanent magnet (PM) machine used for aircraft propulsion. The focus is to pursue high specific power (>20 kW/kg), high efficiency (>95%), and low thermal resistance direct-cooling design for the demanding aerospace applications. The proposed key-enabling technology is additively manufactured hollow conductors integrated with heat pipes. Compared to the existing high specific power PM machines for aerospace applications, the proposed design features a much lower speed – 5000 r/min without any torque amplifier, which helps promote the specific power of the overall system. Based on parametric finite-element analysis, tradeoff studies on a variety of winding configurations as well as geometrical parameters have been carried out, highlighting their impact on specific power, winding ac losses, and efficiency. AlSi10Mg coil samples printed by direct metal laser sintering as well as thermal management system have been demonstrated to show the feasibility of the concepts.

Additively Manufactured Hollow Conductors Integrated With Heat Pipes: Design Tradeoffs and Hardware Demonstration

Global transportation has shifted toward electromobility to achieve net-zero emission, and in the next few decades, commercial electric aircraft is likely to become a reality. This transition has embarked on through the existing more electric aircraft (MEA), and the ultimate goal will be potentially achieved by hybrid-electric and all-electric airliners, along with green fuels, such as green hydrogen or supercritical CO2 (sCO2) and its potential Gg CO2 equivalent elimination—with or without combustion. Electric propulsion replaces conventional jet propulsors with electric fans powered by electric generators rotated by an engine, a combination of generators and energy storage, or just energy storage. An appealing idea is to distribute the electric fans along the aircraft wings or tails to improve aerodynamics, boost energy efficiency, and reduce carbon emissions and acoustic noise. Focusing on distributed electric propulsion (DEP) systems, this article reviews the state-of-the-art advancements in aircraft electrification. Three major DEP categories, i.e., turboelectric, hybrid-electric, and all-electric propulsion technologies, are investigated. Although all of them utilize electric fans as propulsors, their system structures and power generation stages are different. Hence, comprehensive considerations are required to optimize the DEP system designs. Starting with the multifarious electrical system architectures proposed in the literature, a thorough review is conducted including the system parametric specifications, design considerations of power converters, the power electronics devices’ characteristics in cryogenic conditions, and various energy storage systems. This review aims to provide a reference to researchers, engineers, and policy-makers in aviation to accelerate the progress toward future net-zero emissions. 

Aircraft Distributed Electric Propulsion Technologies—A Review

A technology revolution is unfolding around us as every major mode of transportation is in some stage of electrification, whether on land, sea, or in the air. All of these transportation modes depend on electric machines that play critical roles in both propulsion systems as well as in a wide variety of accessory equipment. Fuel economy and reduced emissions of greenhouse gases and other pollutants are major driving forces for electrified transportation. Despite the significant differences between these varied transportation modes, they all share strong motivations to make their electric machines and adjustable-speed drives smaller, lighter, and more efficient. An excellent example of this trend for the power electronic drives is provided by comparing the drive units in the Tesla S (2012) and Tesla 3 (2017). As shown in the side-by-side views provided in Figure 1, the drive unit shrank in size from an enclosure that matched the size of its main traction motor to a much smaller unit that, at first sight, appears to be a non-descript end-bell attached to one end of the motor. Rather than focusing here on how this impressive reduction in mass and volume was achieved (addressed later in this article), the key point here is that similar technology trends are under way in many of the other transportation modes as well, making electrified transportation increasingly appealing and competitive in the process.

The Incredible Shrinking Motor Drive: Accelerating the Transition to Integrated Motor Drives

Global transportation has shifted toward electromobility to achieve net-zero emission, and in the next few decades, commercial electric aircraft is likely to become a reality. This transition has embarked on through the existing more electric aircraft (MEA), and the ultimate goal will be potentially achieved by hybrid-electric and all-electric airliners, along with green fuels, such as green hydrogen or supercritical CO2 (sCO2) and its potential Gg CO2 equivalent elimination—with or without combustion. Electric propulsion replaces conventional jet propulsors with electric fans powered by electric generators rotated by an engine, a combination of generators and energy storage, or just energy storage. An appealing idea is to distribute the electric fans along the aircraft wings or tails to improve aerodynamics, boost energy efficiency, and reduce carbon emissions and acoustic noise. Focusing on distributed electric propulsion (DEP) systems, this article reviews the state-of-the-art advancements in aircraft electrification. Three major DEP categories, i.e., turboelectric, hybrid-electric, and all-electric propulsion technologies, are investigated. Although all of them utilize electric fans as propulsors, their system structures and power generation stages are different. Hence, comprehensive considerations are required to optimize the DEP system designs. Starting with the multifarious electrical system architectures proposed in the literature, a thorough review is conducted including the system parametric specifications, design considerations of power converters, the power electronics devices’ characteristics in cryogenic conditions, and various energy storage systems. This review aims to provide a reference to researchers, engineers, and policy-makers in aviation to accelerate the progress toward future net-zero emissions.

Development of megawatt-scale high-power-density electric machines is critical to the feasibility of large commercial electrified aircraft. Permanent magnet (PM) synchronous machines are often adopted in high-power-density and high-efficiency applications and are serious candidates for aircraft electrified propulsion systems. The design of a 1 MW high-speed surface permanent magnet (SPM) machine with an active material power density of 23.7 kW/kg is presented. A 200 kW machine with identical dimensions is also under development in order to address key technical risks. Both machines use a modular stator with the intention to integrating power electronics around the machine's outer circumference. The 200 kW machine is being used to address key structural and thermal challenges before finalizing the design of the 1 MW high-speed prototype machine.

Design of High-Speed Permanent Magnet Machine for Aerospace Propulsion

Electrification of aircraft propulsion will require significant improvements in the power density of electric machines beyond the current state-of-the-art. Permanent magnet (PM)synchronous machines are capable of achieving high power density values, but selection of the rotor topology is sensitive to the application. This paper presents and compares four 1 MW highspeed PM machine designs, including variations of surface and interior PM machines. Each design meets identical specifications and is compatible with stator modularization, which offers multiple drive system benefits. The performance and features of the developed machines are compared, and a preliminary PM machine design using an inner rotor surface PM machine topology is presented for further investigation as a candidate for a future aircraft propulsion system.

Comparison of Modular PM Propulsion Machines for High Power Density

Permanent magnet (PM) modular motor drives (MMDs) have been recognized as promising candidate machine drive propulsion units in electrified aircraft due to their high power density, high efficiency, and high fault tolerance. Heterarchical, distributed, and modular control schemes are preferred to enhance the system reliability by eliminating single points of failure (SPOFs) from controllers. To facilitate modular machine control design and analysis, a generalized module-level machine analysis and modeling framework has been developed. Module-level machine analysis reveals intra-module unbalanced inductances and inter-module unbalanced cross-coupled fluxes that differ from conventional machines due to asymmetric mutual slot leakage flux couplings among the modules. An asymmetric module-level machine model has been developed by using complex vectors and conjugate complex vectors. Due to the asymmetry, balanced three-phase module voltage excitation induces both positive and negative sequence module current responses when the modules are not evenly loaded. This feature deteriorates the inter-module independence of MMDs, especially under fault conditions where maximum inter-module independence is desired. A modular control with a negative sequence regulator is proposed to eliminate the undesired negative sequence component under uneven loading and fault conditions in order to achieve the desired high level of inter-module independence.

Modular Modeling and Distributed Control of Permanent-Magnet Modular Motor Drives (MMDs) for Electric Aircraft Propulsion

High-speed permanent magnet machines are drawing attention in applications such as aircraft electric propulsion due to their high power density and high efficiency. However, the fundamental electrical frequency becomes higher because of high speed and high number of poles, which poses challenges for torque control. Issues caused by discrete time control implementation such as delay-induced input cross-coupling, incorrect complex pole cancellation, and phase lag introduced by update delay are the key factors that limit the control performance at high electrical frequencies. A control framework is presented that allows the machine to operate at high fundamental frequencies with good command tracking and disturbance rejection performance. After analyzing the effect of delay-induced input cross-coupling, the input decoupling including disturbance input decoupling (DID) and manipulated input decoupling (MID) is designed and evaluated based on an accurate discrete time model. This leads to derivation of a direct synthesized complex vector current regulator (CVCR) that is compared with the baseline PI-based CVCR. The direct synthesized CVCR exhibits excellent complex pole cancellation even at high speed with good command tracking and disturbance rejection capabilities.

Discrete-Time Torque Control of High-Speed SPM Machine For Aircraft Electric Propulsion

Multi-Physics Based Analysis and Design of Stator Coil in High Power Density PMSM for Aircraft Propulsion Applications

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