Aircraft electrification is currently the best alternative to address the rising demand for more air transportation and deal with anticipated economic and environmental impacts. Although the all-electric-aircraft (AEA) concept is not yet a feasible solution, the more-electric aircraft (MEA) is gaining significant attention. Electrical systems either partially or entirely replace the large and inefficient hydraulic, pneumatic, and mechanical conventional aircraft actuating systems. The upgrade could also encompass the propulsion system, as in hybrid- and turbo-electric aircraft. This upgrade reduces the aircraft weight, reduces the usage of pollutant fluids, increases fuel efficiency, reduces carbon emissions, and increases aircraft controllability and reliability. This article reviews various application areas of electric machines in electrified aircraft, such as actuation, taxiing, propulsion, and generation. Moreover, it reviews the main types of currently/to be utilized electric machines and the critically required specifications. Finally, a comparison between the different considered machines and potential future research is discussed.

Review of Electric Machines in More-/Hybrid-/Turbo-Electric Aircraft

Six-phase induction machines have mostly shown promise in high-power electric drive applications as well as wind energy conversion systems. Different winding configurations for six-phase stators have been published, namely, dual three-phase (D3P), symmetrical six-phase (S6P), and asymmetrical six-phase (A6P) winding layouts. Although a body of research investigating six-phase machines and their control for different six-phase winding arrangements exists, a thorough comparative study between these different arrangements in terms of machine parameters and performance, has not been done so far. This paper employs a 12-phase stator with a configurable terminal box to compare different six-phase configurations by simply reconnecting the stator terminals of the twelve phases in different manners to obtain an equivalent six-terminal stator. This way, the same stator machine dimensions and copper volume will be assumed for all connections. The comparative study focuses on the effect of winding connection on machine parameters of the different subspaces, phase current quality and machine characteristic curves. Experimental validation has been carried out using a 1kW prototype system.

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Effect of Winding Configuration on Six-Phase Induction Machine Parameters and Performance

Multiphase drives offer enhanced fault-tolerant capabilities compared with conventional three-phase ones. Their phase redundancy makes them able to continue running in the event of faults (e.g., open/short-circuits) in certain phases. Moreover, their greater number of degrees of freedom permits improving diagnosis and performance, not only under faults affecting individual phases, but also under those affecting the machine/drive as a whole. That is the case of failures in the dc link, resolver/encoder, control unit, cooling system, etc. Accordingly, multiphase drives are becoming remarkable contenders for applications where high reliability is required, such as electric vehicles and standalone/off-shore generation. Actually, the literature on the subject has grown exponentially in recent years. Various review papers have been published, but none of them currently cover the state-of-the-art in a comprehensive and up-to-date fashion. This two-part paper presents an overview concerning fault tolerance in multiphase drives. Hundreds of citations are classified and critically discussed. Although the emphasis is put on fault tolerance, fault detection/diagnosis is also considered to some extent, because of its importance in fault-tolerant drives. The most important recent advances, emerging trends and open challenges are also identified. Part 1 provides a comprehensive survey considering numerous kinds of faults, whereas Part 2 is focused on phase/switch open-circuit failures.

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A Comprehensive Survey on Fault Tolerance in Multiphase AC Drives, Part 1: General Overview Considering Multiple Fault Types

This article proposes a modeling approach and an optimization strategy to exploit a third-harmonic current injection for the torque enhancement in multiphase isotropic permanent magnet synchronous machines with nonsinusoidal back electromotive forces. The modeling approach is based on a proper vector space decomposition and on the associated rotational transformation, aimed to properly select a set of stator current space vectors to be controlled. It is presented for a generic (i.e., asymmetrical, with an arbitrary angular shift) winding configuration. The injection strategy is related to the choice of a constant synchronous current set aimed at minimizing the average stator winding losses for a given reference torque by using the first and the third spatial harmonics of the air-gap flux density. The optimal solution has been found analytically and has been developed in detail for a selected set of asymmetrical winding configurations. Both the numerical and experimental results are in good agreement with the theoretical analysis.

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Optimal Third-Harmonic Current Injection for Asymmetrical Multiphase Permanent Magnet Synchronous Machines

This paper investigates multiphase drives in which winding configuration (symmetrical or asymmetrical) can be easily obtained by only rearranging voltage source inverter (VSI) power supply cables at the machine’s terminal box. The type of the machines where this is possible is identified and the examples of reconfiguration are given and explained. Following from the examples (for nine- and fifteen-phase cases), a general reconfiguration algorithm is introduced. As shown, changing asymmetrical to symmetrical winding configuration (and vice versa) means that just another mimic diagram needs to be placed over the existing one on the machine’s terminal box. Possible reconfigurations of a six-phase machine, which do not follow the same pattern, are also addressed. Differences caused by different winding configuration are identified and experimentally confirmed using a nine-phase surface mounted permanent magnet synchronous machine (PMSM) and a nine-phase induction machine (IM).

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Symmetrical/Asymmetrical Winding Reconfiguration in Multiphase Machines

This article investigates maximum possible torque improvement in a two-pole surface permanent magnet synchronous machine with a reduced magnet span, which causes production of highly nonsinusoidal back electromotive force (EMF). It contains a high third and a fifth harmonics, which can be used for the torque enhancement, using stator current harmonic injection. Optimal magnet span is studied first, and it is shown that with such a value, the machine would be able to develop an insignificantly lower maximum torque than with the full magnet span. Next, field-oriented control algorithm, which considers all nonfundamental EMF components lower than the machine phase number, is devised. Using maximum torque per ampere principles, optimal ratios between fundamental and all other injected components are calculated and then used in the drive control. The output torque can be in this way increased up to 45% with respect to the one obtainable with fundamental current only. Alternatively, for the same load torque, stator current rms value can be reduced by 45%. Last but not least, a method for position sensor fault mitigation is introduced. It is based on the alternative use of a back-EMF harmonic for rotor position estimation, instead of the torque enhancement. Experimental verification is provided throughout for all the relevant aspects.

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General Torque Enhancement Approach for a Nine-Phase Surface PMSM With Built-In Fault Tolerance

In this paper a two-pole surface permanent magnet synchronous machine (PMSM) with a symmetrical nine-phase winding configuration is investigated. Magnets on rotor are shortened which causes production of highly non-sinusoidal back-electromotive force (EMF) in the stator windings. Analysis of the EMF reveals a third-harmonic component, which is almost equal in magnitude to the fundamental. These harmonics have been used by researchers in the past to increase the torque density of multiphase machines. In this paper, a specific rotor structure is designed and tested under third-harmonic current injection in order to analyse torque improvement. It is shown that optimal injection ratio and torque improvement are dependent on the ratio of third-harmonic back-EMF to the fundamental one. The so-called ‘same RMS value’ constraint method is used for calculation of the optimal level of the third-harmonic current injection. It is found that the torque can be increased by up to 36% for the studied machine. Obtained torque increase results are validated using finite element method (FEM) simulations. Finally, the results acquired with the described rotor structure are compared with those obtained for rotor with full magnets and near-sinusoidal back-EMF. It is shown that for approximately 4 times less magnet material in non-sinusoidal configuration and with third-harmonic current injection, the total electromagnetic torque is only 22% lower.

Analysis of a Symmetrical Nine-phase Machine with Highly Non-Sinusoidal Back-Electromotive Force

This paper investigates an enhanced field-oriented control method for a two-pole surface permanent magnet synchronous machine (PMSM) with a symmetrical nine-phase winding configuration and a single neutral point. Magnets on the rotor are shortened, reducing cost of the machine, but cause production of highly non-sinusoidal back-electromotive force (EMF). FFT analysis of the EMF reveals a high third harmonic component, which is almost equal in magnitude to the fundamental. By investigating possible torque improvements using FEM software simulations, it was shown previously that, if controlled under optimal third harmonic current injection, the electromagnetic torque of the studied PMSM can be improved by up to 36%. The optimal current harmonic injection ratio between the fundamental and the third harmonic is determined first in this paper, using the maximum torque-per-ampere method (MTPA). An enhanced high-performance field-oriented control (FOC) algorithm to control a nine-phase non-sinusoidal back-EMF PM synchronous machine is devised next. The developed improved control algorithm is verified using simulation studies and further validated using an experimental prototype.

M. Slunjski

Papers

Optimal Third-Harmonic Current Injection for Asymmetrical Multiphase Permanent Magnet Synchronous Machines

Symmetrical/Asymmetrical Winding Reconfiguration in Multiphase Machines

General Torque Enhancement Approach for a Nine-Phase Surface PMSM With Built-In Fault Tolerance

Analysis of a Symmetrical Nine-phase Machine with Highly Non-Sinusoidal Back-Electromotive Force

Control of a Symmetrical Nine-phase PMSM with Highly Non-Sinusoidal Back-Electromotive Force Using Third harmonic Current Injection