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In the realm of electric machines, there has been an increasing interest in multiphase (greater than three-phase) and hybrid excited machines. The benefits of multiphase machines include improved power density, efficiency, reliability, and fault tolerance, while for hybrid electric machines, the literature offers a variety of topologies, each with its own advantages and disadvantages. In essence, the term hybrid for electric machines is used when there is more than one source of excitation, e.g., permanent magnet (PM) excitation combined with or assisted by wound field (WF) excitation. This paper presents an extensive review of the latest topologies in hybrid machines. It explores fundamental principles, multiphase winding, and the advantage of multiphase over three-phase, as well as a comparison of ripple in the DC link for different numbers of phase winding. Additionally, this review discusses applications across industries, including automotive, aerospace, marine, and renewable energy systems. This paper later studies the motoric and generator modes of hybrid machines while considering the machine characteristics in both of these modes. [ABSTRACT FROM AUTHOR]

Recently, multiphase drives have emerged as a notable alternative for three-phase drives in several domains, including electric marine propulsion, locomotive traction, and high-power industrial applications where fault tolerance and high reliability are required. This study uses a graphical approach to develop a new carrier-based Pulse-Width Modulation (PWM) that reduces the root-mean-square value of the output current ripple in two-level multiphase inverters with an odd number of phases. The optimization process calculates the optimal common-mode voltage for each switching period. Numerical models and experiments with five- and seven-phase inverters demonstrate the effectiveness of the developed PWM technique, which can reduce the number of commutations while obtaining the lowest output current ripple.

This paper presents a novel and simple procedure for the determination of a Fault-Tolerant Algorithm (namely FTA) for the adequate working operation of an electric machine equipped with a general $m$ -phase winding under possible open circuits occurring in some of its phases. The FTA is firstly theoretically derived from the analysis of the magnetic field distribution in polyphase systems, then simulated through finite-element analysis and finally implemented via software for real-time validation. The effectiveness of the proposed procedure is confirmed through experimental tests on a laboratory setup. Key contributions include the flexibility of the algorithm, which can be applied to any $m$ -phase electric machine with various winding configurations. Both the obtained FEA and experimental results demonstrate that the proposed control algorithm can be easily used and applied in electric drives supplied by $m$ -phase motors under fault conditions without significantly affecting the magnetic performance of the whole system and ensuring the continuity of operation even in the presence of faults.

Linear modulation techniques (LMTs) of an asymmetrical six-phase machine (ASPM) with two isolated neutral points synthesize the desired voltage vectors by applying at least five switching states. Different choices of applied voltage vectors, sequences in which they are used, distribution of dwell-times among the redundant switching states give rise to a large number of possible LMTs. It is desirable that these LMTs avoid more than two transitions of a particular inverter leg within a carrier period. Only a subset of existing LMTs of ASPM follows this rule. Through an innovative approach, this article finds a way to account for all possible infinitely many LMTs that follow the rule of at most two transitions per leg. Another important criterion for the selection of an LMT is its current-ripple performance. Therefore, through numerical optimization, this article finds optimal LMTs among the above infinite possible LMTs for all reference voltage vectors in the linear range and for the whole feasible range of a machine parameter. This parameter is related to the leakage inductance of the machine and impacts the current ripple performance of ASPM. An optimal hybrid strategy is proposed with these optimal techniques, which outperforms all existing techniques in terms of the current ripple. The theoretical analysis is validated through simulation in MATLAB and experiments performed up to 3.5 kW on a hardware prototype. [ABSTRACT FROM AUTHOR]

Two common defects in induction machines (IMs) are eccentricity and interturn faults, which should be diagnosed to prevent performance degradation and further damage. A popular fault-detection approach is the current signature analysis (CSA), because of its simplicity and nonintrusiveness. Under closed-loop control, it is combined with analogous voltage-reference (VR) signature analysis (VRSA). However, by using these methods in three-phase IMs, it is difficult to discriminate between these faults, which cause similar symptoms. Multiphase machines provide remarkable advantages such as inherent tolerance to open-phase faults. Six-phase IMs are particularly attractive since they allow adopting three-phase converters. Among them, those with symmetrical spatial arrangement of the stator phases offer superior fault tolerance. Nonetheless, the distinction between eccentricity and interturn failures in these IMs has not been addressed so far. This article studies the discrimination between eccentricity and interturn faults in symmetrical six-phase (S6) IMs by CSA or VRSA. It is shown that, conversely to three-phase IMs and most other multiphase IMs, in S6 ones, these two types of failures can be easily distinguished: interturn faults considerably alter the currents or VRs in the so-called $\boldsymbol{x}$ - $\boldsymbol{y}$ plane, whereas eccentricity leads to current/voltage symptoms only in the $\boldsymbol{\alpha} _{\mathbf{1}}$ - $\boldsymbol{\beta} _{\mathbf{1}}$ plane. Experimental results confirm the theory. [ABSTRACT FROM AUTHOR]

Multiphase drives (MPDs) have been the subject of research for the last two decades. Despite being a technology that is still in the process of development, a significant number of research studies and developments have been reported in scientific literature over the past few years. This article aims to collect and present a review of these recent contributions, providing a comprehensive and insightful state of the art on this topic and future technology trends. The elaborated aspects include the advantages of multiphase machines, a general introduction to five-phase and six-phase machines, and their modelling techniques. In addition, new promising MPD topologies are covered. Recent advances in modulation techniques and the control of multilevel converters are also discussed. Finally, future trends and challenges in further developing this technology are discussed. [ABSTRACT FROM AUTHOR]

This paper presents a novel procedure for the emergency sensorless operation of a six-phase electric drive. More degrees of freedom provided by multiphase technology have been utilized to determine the rotor flux position. The proposed technique is based on the direct measurement of the back EMF generated on a disconnected three-phase winding of a six-phase permanent-magnet synchronous machine (PMSM). The method is suitable for the emergency operation of an electric drive in the case of speed/position sensor failure. The proposed solution has been verified utilizing a six-phase permanent-magnet synchronous machine with a displacement of 30° between two three-phase sets of the stator winding and a power of 1 kW. [ABSTRACT FROM AUTHOR]

Six-phase machines are increasingly used in safety-critical applications due to their inherent fault-tolerant capabilities. Due to the greater complexity of controlling six-phase machines and the fast dynamics required in safety-critical applications, finite control set model predictive control (FCS-MPC) emerged as an ideal candidate for the control of six-phase machines. However, most of the available FCS-MPC strategies only apply to six-phase machines where the two sets of three-phase windings are star-connected with isolated neutral points (2N). Nevertheless, the 2N configuration does not take full advantage of the machine's capabilities in terms of fault tolerance. Hence, this paper proposes a predictive current control strategy based on virtual vectors for six-phase permanent magnet synchronous (PMSM) drives with a single isolated neutral point (1N) configuration. The proposed method reduces the current harmonic distortion, decreases the copper losses, and is suitable to operate the six-phase drive in fault-tolerant conditions. The included simulation and experimental results demonstrate the good performance obtained with the proposed strategy. [ABSTRACT FROM AUTHOR]

Multiphase winding configurations have gained significant attention in high-performance variable speed drives and wind energy conversion systems (WECS) owing to their myriad advantages. In the available literature, various multiphase winding layouts have been designed aiming at boosting the machine performance to meet the requirements of the proposed applications. Ultimately, this paper surveyed the state-of-the-art in the available multiphase winding layouts proposed for various innovative applications. Various types of windings were discussed, while investigating their advantages and limitations. This typically considers the winding layouts employed in multiphase induction motors (IMs) and permanent magnet (PM) machines with prime phase and multiple three-phase orders. This study extensively provides innovative winding arrangements that offer better machine characteristics in terms of torque density, efficiency, and fault-tolerance capability. Moreover, the construction of multiphase machines with general $n$ -phase using standard three-phase stator frames has been elaborated. This latter technique obviates the basic necessity of special stator frames with a prime number of phases. Finally, this paper sheds light on the commercial applications that include multiphase winding layouts.

This article presents a maximum torque per ampere strategy for a five-phase synchronous reluctance drive. The approach is developed considering general machine parameters and is formalized as a constrained optimization problem. The optimal solution is found analytically by using Lagrange’s multipliers method and is based on the computation of the eigenvalues and eigenvectors of the inductance derivatives matrix. The proposed approach is evaluated both numerically and experimentally. It is also compared with other current references control strategies, effectively showing a reduction of the machine rms currents for the same developed torque. The same approach can also be extended to machines with a different number of phases. [ABSTRACT FROM AUTHOR]

Fault-tolerant capability is one of the dominant advantage of multiphase drives. In case of open-phase fault occurrence, the control strategy needs to be modified in order to maintain smooth operation, resulting in larger amplitude of the phase current. Consequently, the winding losses are increased in comparison with healthy operation. Also, the torque operation range (TOR) is reduced inevitably. This article proposed a unified fault-tolerant strategy that can ensure the widest TOR while minimizing losses. Compared with state-of-the-art literature, the execution of the fault-tolerant algorithm does not require any online or offline global optimization process. Instead, simple closed-form expressions are adopted, which could be considered a feasible simplification. Furthermore, the proposed strategy can be adopted for symmetrical or asymmetrical multiphase machines with any number of phases. Experimental results validate the effectiveness of the proposed fault-tolerant method. [ABSTRACT FROM AUTHOR]

As the fault localization for multiphase machines is critical for fault isolation and active fault-tolerant strategies, reliable fault diagnosis strategy is essential to be proposed. After open-circuit fault occurs, harmonic current vectors will follow different trajectories according to different faulty phases. However, this event can only be evident in case of single phase fault occurrence. Thus, existing technique can only identify limited types of open-switch fault. This article proposes a simple and efficient open-switch fault detection strategy to extract fault features. Proposed strategy is able to locate any combinations of open-switch faults, which cannot be realized in the previous literature. The faulty phases are detected first to reduce computational burden and for avoidance of false alarms. Then the faulty switches are determined on the basis of the direction of current vectors in the harmonic plane. Taking account of open-switch faults in different phases, normal current vectors are reconstructed with newly defined auxiliary current variables, which can be utilized to detect the failure of multiple switches and result in enhanced robustness. Experimental results validate the effectiveness of the proposed multiple open-switch fault detection method in a seven-phase machine at 5.5 kW power level. [ABSTRACT FROM AUTHOR]

The topic of this paper is the determination of rotor harmonic losses in multiphase machines. Specifically, harmonic losses occur in the rotor winding and core due to higher-order spatial harmonics of the flux density. This phenomenon influences machine parameters and overall performance and increases temperature rise in parts of the rotor. The flux density distribution is determined by the stator magnetomotive force harmonic content, which is directly related to the winding distribution. A cage-rotor asymmetrical six-phase induction machine is selected for this case study. An analysis of different stator winding topologies and their influence on harmonic losses is presented. A finite element-based method for calculating the contribution of individual stator magnetomotive force harmonics to the rotor losses is developed and described in the paper. The analysis includes scenarios with different phase current waveforms to emphasize the issues specific to the asymmetrical six-phase machine. It is found that the magnetomotive force components generated by non-torque-producing current components contribute significantly to harmonic losses. The obtained results can represent a foundation for optimal stator winding topology selection. This work is intended to motivate the development of new and the modification of existing models to properly include rotor harmonic losses during the design, performance prediction, and control of multiphase machines. [ABSTRACT FROM AUTHOR]

Multiphase machines are very convenient for applications that require high reliability. In this two-part survey, the state of the art about fault tolerance in multiphase drives is reviewed. In Part 1, an overview including numerous fault types was presented, along with fundamental notions about multiphase drives. Here, in Part 2, the focus is placed on phase/switch open-circuit (OC) faults in particular, which have received the most attention in the literature. Phase OC failures involve OCs in stator phases or in converter-machine connections, and switch/diode OCs are frequently dealt with similarly or identically. Thanks to the phase redundancy of multiphase drives, their operation can be satisfactorily continued under a certain number of OCs. Nonetheless, the procedure to follow for this purpose is far from unique. For given OC fault conditions, numerous fault-tolerant possibilities can be found in the literature, each of them with different advantages and disadvantages. Moreover, a great variety of methods have also been devised to detect and diagnose phase/switch OC failures so that, as soon as possible, the most appropriate fault-tolerance measures are applied. Thus, given the broad literature about tolerance to phase/switch OC faults in multiphase drives, the survey presented here is expected to be of great interest for the research community and industry. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

When targeting mission critical applications, the design of the electronic actuation systems needs to consider many requirements and constraints not typical in standard industrial applications. One of these is tolerance to faults, as the unplanned shutdown of a critical subsystem, if not handled correctly, could lead to financial harm, environmental disaster, or even loss of life. One way this can be avoided is through the design of an electric drive systems based on multi-phase machines that can keep operating, albeit with degraded performance, in a partial configuration under fault conditions. Distributed architectures are uniquely suited to meet these challenges, by providing a large degree of isolation between the various components. This paper presents a system architecture suitable for scalable and high-performance fault tolerant machine drive systems. the effectiveness of this system is demonstrated through theoretical analysis and experimental verification on a six-phase machine.

Even though multiphase machines have inherent fault-tolerant capabilities due to their additional degrees of freedom, fault diagnosis still plays an important role to continuously monitor these machines and, thus, prevent critical failures. High-resistance connection (HRC) is one of the most common electric faults in electrical machines and is caused by damaged power connections. These faults create a current unbalance, increasing both the machine losses and torque pulsations, and can eventually lead to open-phase faults (OPFs). This article proposes an online fault diagnostic method to detect HRCs and OPFs in six-phase permanent magnet machine drives. The proposed method is intended for application in electric drives controlled by a predictive current control strategy and does not require the installation of extra hardware. Two variants of the fault diagnostic algorithm are presented in this work: one based on the analysis of the secondary components of the reference current errors and the other variant based on the prediction current errors. Experimental results demonstrate the effectiveness and sensitivity of the proposed diagnostic method for the detection of these types of fault. [ABSTRACT FROM AUTHOR]

Multiphase machines, associated with state-of-the-art control strategies like finite control set model predictive control (FCS-MPC) are suitable for applications where reliability and disturbance-free operation are key factors. Although FCS-MPC provides good transient performance and straightforward design, the performance of the drive system is affected by model inaccuracies due to parameter mismatch and unmodeled dynamics, such as deadtime effects in the power converters and back-electromotive force harmonics. In this context, this article proposes a novel and robust disturbance observer based predictive current control strategy for six-phase permanent magnet synchronous machine drives that minimizes the steady-state errors due to parameter mismatch errors, while simultaneously reducing the amplitude of the fifth- and seventh-order current harmonics due to unmodeled dynamics, which are non-negligible in six-phase machines. Simulation and experimental results demonstrate the very good performance of the proposed strategy even when considering significant errors in the system parameters and different values of the deadtime in the power converters. [ABSTRACT FROM AUTHOR]

Integrated on-board battery chargers (OBCs) have been recently introduced as an optimal/elegant solution to increase electric vehicle (EV) market penetration as well as minimize overall EV cost. Unlike conventional off-board and on-board battery chargers, integrated OBCs exploit the existing propulsion equipment for battery charging without extra bulky components and/or dedicated infrastructure. OBCs are broadly categorized into three-phase and single-phase types with unidirectional or bidirectional power flow. This paper starts with surveying the main topologies introduced in the recent literature employing either induction or permanent magnet motors to realize fully integrated slow (single-phase) and fast (three-phase) on-board EV battery charging systems, with emphasis on topologies that entail no or minimum hardware reconfiguration. Although, permanent magnet (PM) motors with conventional double-layer distributed winding layouts have been deployed in most commercial EV motors, the non-overlapped fractional slot concentrated winding (FSCW) has been the prevailing choice in the most recent permanent magnet motor designs due to its outstanding operational merits. Hence, a thorough investigation of the impact different FSCW stator winding designs have on machine performance under the charging process is presented in this paper. To this end, the induced magnet losses, which represent a challenging demerit of the FSCW, have been used to compare different topologies under both propulsion and charging operation modes. Based on the introduced comparative study, the optimal slot/pole combinations that correspond to the best compromise under both operational modes have been highlighted.

This paper investigates the performance of 3-phase synchronous reluctance machines (SynRMs) when upgrading their stator to 5-phase, keeping the same rotor. The design of the 5-phase stator has been optimized to select the optimal dimensions of the slots and teeth. Moreover, a combined star-pentagon winding is employed to further improve the machine performance. Different winding configurations have been studied and compared using 2D Ansys Maxwell transient simulations. It is observed that at optimal current angle and rated current, the average torque is increased by 17.41% when changing the 3-phase stator with the proposed 5-phase one. In addition, the efficiency of the 5-phase SynRM is increased by about 0.8% compared to 3-phase SynRM. At 3 times the rated speed, the torque and efficiency are significantly increased by around 33% and 3.5% respectively. Moreover, the 5-phase SynRM shows a superior performance in the faulty case with one phase opened. It works at 98.84% of the rated torque of the healthy 3-phase machine, whereas the 3-phase machine works at only 43.35% with huge torque ripple (228%). Finally, an experimental validation using the reference 3-phase machine has been done.

This paper presents a novel procedure for the emergency sensorless operation of a six-phase electric drive. More degrees of freedom provided by multiphase technology have been utilized to determine the rotor flux position. The proposed technique is based on the direct measurement of the back EMF generated on a disconnected three-phase winding of a six-phase permanent-magnet synchronous machine (PMSM). The method is suitable for the emergency operation of an electric drive in the case of speed/position sensor failure. The proposed solution has been verified utilizing a six-phase permanent-magnet synchronous machine with a displacement of 30° between two three-phase sets of the stator winding and a power of 1 kW.

Multiphase drives (MPDs) have been the subject of research for the last two decades. Despite being a technology that is still in the process of development, a significant number of research studies and developments have been reported in scientific literature over the past few years. This article aims to collect and present a review of these recent contributions, providing a comprehensive and insightful state of the art on this topic and future technology trends. The elaborated aspects include the advantages of multiphase machines, a general introduction to five-phase and six-phase machines, and their modelling techniques. In addition, new promising MPD topologies are covered. Recent advances in modulation techniques and the control of multilevel converters are also discussed. Finally, future trends and challenges in further developing this technology are discussed.

Six-phase machines are increasingly used in safety-critical applications due to their inherent fault-tolerant capabilities. Due to the greater complexity of controlling six-phase machines and the fast dynamics required in safety-critical applications, finite control set model predictive control (FCS-MPC) emerged as an ideal candidate for the control of six-phase machines. However, most of the available FCS-MPC strategies only apply to six-phase machines where the two sets of three-phase windings are star-connected with isolated neutral points (2N). Nevertheless, the 2N configuration does not take full advantage of the machine’s capabilities in terms of fault tolerance. Hence, this paper proposes a predictive current control strategy based on virtual vectors for six-phase permanent magnet synchronous (PMSM) drives with a single isolated neutral point (1N) configuration. The proposed method reduces the current harmonic distortion, decreases the copper losses, and is suitable to operate the six-phase drive in fault-tolerant conditions. The included simulation and experimental results demonstrate the good performance obtained with the proposed strategy.

This article presents a current limitation technique for a multiphase bearingless machine featuring a combined winding system. This winding structure allows the machine to produce both suspension force and motoring torque. The main challenges with the combined winding configuration consist of decoupling the suspension force and torque generation and designing a proper current limitation algorithm. The former topic has been already tackled and presented in previous publications, while the latter will be addressed in this article. In particular, the proposed suspension force control technique will allow to prioritize either suspension force or torque generation. In this article, the priority is given to the rotor levitation, hence the suspension force rather than the torque is essential. Finally, simulation results and experiment validation on a $\text{1.5}\,\text{kW}/3000\,\text{r/min}$ prototype machine are provided. [ABSTRACT FROM AUTHOR]

Commonly, healthy five-phase permanent magnet assisted synchronous reluctance machines (PMaSynRMs) are modeled in harmonic fixed rotating (dq1dq) reference frames. Such approaches feature several levels of detail, namely, constant parameters or saturation in four axes, i.e., 4-D saturation or cross-saturation effects in conjunction with space harmonics in 5-D models. In this article, a 6-D model is proposed that comprises all aforementioned nonlinearities not just in faultless but also in open-phase fault conditions. Moreover, magnetic axial asymmetry introduced by discrete skew is taken into account and included in the investigated modeling. The proposed 6-D model is based on flux maps obtained from static Finite-Element program posteriori modified to comprise the skewing effect. Very close to finite-element precision is guaranteed and experimental validations have been performed to confirm the authenticity and contrast the effectiveness of the open-phase model with skew. [ABSTRACT FROM AUTHOR]

Recent developments in power electronics are making the multiphase machines a competitive alternative to the conventional three-phase counterparts. Due to their fault-tolerant features, multiphase drives represent a robust technology in high-power/high-current, safety-critical applications. Besides, their introduction to transportation electrification is gaining importance. Among the multiphase solutions, the multi-three-phase machines are receiving a lot of attention by the industry since they use the well-consolidated three-phase technology, thus reducing the design time and also the cost. Therefore, this article proposes a modular vector control scheme for multi-three-phase permanent magnet synchronous motors. The proposed solution uses a modular modeling approach for the independent and decoupled torque control of each three-phase unit, allowing the implementation of torque-sharing strategies among the three-phase sets of the machine. The developed modular control has been validated on a nine-phase permanent magnet machine. [ABSTRACT FROM AUTHOR]

When multiphase drives are used for specific applications, the modular solutions are preferred as they use consolidated power electronics technologies. The literature reports two modeling approaches for multiphase machines having a modular configuration of the stator winding. The first approach is the vector space decomposition (VSD) that models the energy conversion as for an equivalent three-phase machine. The main alternative to the VSD is the multistator (MS) modeling that emphasizes machine modularity in terms of torque production. Both approaches have advantages and disadvantages for multiphase machines with a modular structure. Therefore, this article aims to combine the VSD and MS approaches, defining a new matrix transformation and, hence, developing a new modeling approach for multiphase machines with a modular structure. The proposed transformation allows a decoupled and independent torque control of the sets composing the machine, preserving the torque regulation's modularity. Together with a new vector control scheme, it has been applied to a modular permanent magnet synchronous machine (PMSM) with a nonstandard spatial shift between windings. Experimental results are presented for a nine-phase PMSM prototype with a triple-three-phase stator winding configuration. [ABSTRACT FROM AUTHOR]

The most recent finite control set model predictive control (FCS-MPC) strategies for six-phase electric drives avoid the excitation of the secondary subspace by using virtual vectors to minimize the current ripple. However, these strategies can only control the fundamental current components, which adjust the machine flux and torque and leave the secondary current components unregulated, which should be zero during regular operation in order to reduce the losses in the machine. Due to this limitation and also due to the low impedance of the secondary subspace in machines with distributed windings, the stator currents might contain non-negligible low-frequency harmonics caused by small asymmetries in the machine, non-linear dynamics of the semiconductors in the power converters and back-EMF harmonics. This article proposes a novel multistage predictive current control strategy based on virtual vectors (MVV-PCC) for six-phase drives based on permanent magnet synchronous machines (PMSMs), which contains a fundamental and a secondary control stages that are responsible for tracking the reference currents in both subspaces separately. The obtained experimental results validate the effectiveness of the proposed strategy in the minimization of the current harmonics and the reduction of the torque ripple for different operating conditions of the six-phase drive. [ABSTRACT FROM AUTHOR]

Overmodulation techniques of asymmetrical six-phase machine achieve higher dc-bus utilization by applying voltage in the nonenergy transfer plane. This results into unwanted current and associated copper loss. Existing overmodulation technique minimizes this voltage from space-vector perspective with predefined set of four active vectors. To find the best technique, one needs to perform the above minimization problem with all possible sets of active vectors with which higher voltage gain can be attained. Therefore, this requires evaluation of large number of cases. This article formulates the above minimization problem in terms of average voltage vectors of two three-phase inverters, where active vectors need not to be specified beforehand and, thus, the analysis is more general. Sixteen possible techniques with different set of active vectors are derived following the above analysis, which attain minimum voltage injection in the nonenergy transfer plane. This article also identifies one of these 16 techniques, which can be implemented without involving complex six-dimensional transformation. The above technique is validated through experiments on six-phase induction motor and simulations in MATLAB at 3.5-kW power level. [ABSTRACT FROM AUTHOR]

This paper presents independent vector control of six-phase two-motor drive machine series-connected fed by a three-level six-phase inverter. Via appropriate phase transposition during the series connection of the stator windings, the fully decoupled control of the two machines is possible. The control system multi machines classic based on vector control with conventional inverters comprise various problems are related to low power quality, pressure on motor bearing, etc. However, decoupling control of a series- six-phase two-motor drive machine by a three-level six-phase inverter is developed. A simulations results clearly shows the possibility of independent vector control of the two machines, although a single three-level six-phase inverter is used as the supply.

This article presents three-phase and single-phase open-circuit fault-tolerant control strategies of radial suspension force and torque generation for bearingless multisector surface mounted permanent magnet synchronous machines. A combined winding topology is installed in the stator so that each phase contributes to both suspension force and motoring torque production. Indeed, a triple sectored three-phase winding is employed. Each three-phase winding is supplied by a power converter to independently control the suspension force vector and torque over each sector. An analytical formulation of the force and torque generation is described considering both three-phase open-circuit and single-phase open-circuit fault conditions. The analytical models of the machine are presented in a generalized matrix form so that they can be applied to any machine with a multisector winding structure. The analytical models are then used to derive the control algorithms that allow us to control the bearingless machine when a whole three-phase winding or one phase of one or two sectors are open circuited. The theoretical analysis is verified with both finite elements analysis and experiment tests. This system shows a good open-circuit fault-tolerant capability. [ABSTRACT FROM AUTHOR]

Overmodulation (OVM) techniques of asymmetrical six-phase induction machine (ASIM) achieve higher dc-bus utilization by applying nonzero average voltage in nonenergy transfer plane. This results into unwanted current and associated copper loss. Existing OVM techniques reduce this loss with pulsewidth modulation (PWM) techniques from six-dimensional space vector perspective, which is both conceptually difficult and computationally challenging. The computational complexity of space vector based techniques is reduced by few existing PWM techniques in linear region by splitting the six-phase inverter as two three-phase inverters and modulate these inverters with linear PWM techniques in 30 $^\circ$ phase shifted. This is referred as three-phase inverter based technique in the title. But, three-phase inverter based OVM technique does not exist in the literature due to lack of knowledge of the relationship between six-dimensional space vectors and space vectors of two three-phase inverters. This article first establishes this relation. Based on this relation, an OVM technique is proposed where the reference voltage vectors of two inverters are phase shifted by 30 $^\circ$ but of different magnitudes. The proposed technique achieves THD and WTHD performances similar to space vector based best technique of ASIM with reduced computational complexity, as shown in details. The proposed strategy is validated through experiments and simulations on six-phase induction machine at 4.2 kW. [ABSTRACT FROM AUTHOR]

This article presents a postfault control algorithm that minimizes the stator Joule losses in multiphase induction machines under an open-phase fault and for different star connection layouts. The key novelty is that the algorithm can be applied to any configuration of a multi-n-phase machine, independently of the connection of the neutral points. The latter is analytically derived and is based on the space vector representation of the machine model. In addition, it is shown that a low number of neutral points helps to reduce the winding losses in case of an open-phase fault but requires additional control regulators and computational efforts. The theory is applied to an asymmetrical quadruple-three-phase induction machine, which is configured to represent five different motor layouts. Finally, experimental results are presented to validate the control algorithm. The optimal solution that is given in this article can be employed for the control of symmetrical or asymmetrical multiphase machines with different star connection layouts and in any open-phase postfault operation. [ABSTRACT FROM AUTHOR]

The rare-earth (RE) permanent magnets (PM) have been increasingly adopted in traction motor application. However, the RE PM is expensive, less abundant, and has cost uncertainties due to limited market suppliers. This paper presents a new design of a RE-free five-phase ferrite permanent magnet-assisted synchronous reluctance motor (Fe-PMaSynRM) with the external rotor architecture with a high saliency ratio. In such architecture, the low magnetic coercivity and demagnetization risk of the ferrite PM is the challenge. This limits the number of flux barriers, saliency ratio, and reluctance torque. A precise analytical design procedure of rotor and stator configuration is presented with differential evolution numerical optimizations by utilizing a lumped parameter model. A 3.7 kW prototype is fabricated to validate the proposed idea. [ABSTRACT FROM AUTHOR]

The topic of this paper is the determination of rotor harmonic losses in multiphase machines. Specifically, harmonic losses occur in the rotor winding and core due to higher-order spatial harmonics of the flux density. This phenomenon influences machine parameters and overall performance and increases temperature rise in parts of the rotor. The flux density distribution is determined by the stator magnetomotive force harmonic content, which is directly related to the winding distribution. A cage-rotor asymmetrical six-phase induction machine is selected for this case study. An analysis of different stator winding topologies and their influence on harmonic losses is presented. A finite element-based method for calculating the contribution of individual stator magnetomotive force harmonics to the rotor losses is developed and described in the paper. The analysis includes scenarios with different phase current waveforms to emphasize the issues specific to the asymmetrical six-phase machine. It is found that the magnetomotive force components generated by non-torque-producing current components contribute significantly to harmonic losses. The obtained results can represent a foundation for optimal stator winding topology selection. This work is intended to motivate the development of new and the modification of existing models to properly include rotor harmonic losses during the design, performance prediction, and control of multiphase machines.

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.

Refurbishing electrical machines to have a higher performance and/or reliability is a clear trend in the circular economy. This paper investigates the gain in efficiency, torque density and reliability with replacing the windings and the iron of the stator of existing three-phase synchronous reluctance machines (SynRMs), resulting in a multiphase machine. The stator housing, shaft, bearings and the rotor are not changed, to keep the cost of refurbishing low. As the housing and rotor are kept, logical constraints are to have the same inner and/or outer stator diameters, air gap length and axial length. In the new windings, an identical copper volume is considered as a constraint. An optimization technique is coupled to 2D finite element method to select the optimal slot dimensions. Case studies showed that it is possible to replace the stator of existing three-phase SynRMs with a better performance multiphase one. It is found that the average torque, efficiency and power factor of the five-phase SynRM are increased by 11.78%, 0.72% and 5.54%, respectively, compared to a three-phase SynRM at rated conditions. At higher speeds (three times rated value), the efficiency and average torque are greatly improved by about 3.64% and 33.67%, respectively, compared to the three-phase SynRM. Moreover, a faulty case of one phase opened is also investigated. The five-phase SynRM works at 75.45% of the healthy rated torque of the three-phase SynRM, whereas the three-phase SynRM works at only 43%. Measurements on an existing 5.5-kW, three-phase SynRM confirm the observed results. [ABSTRACT FROM AUTHOR]

In this study, a saturated model of an asymmetrical six‐phase induction machine is presented. The model is based on the vector space decomposition approach, and it includes main and leakage flux saturation, as well as the mutual coupling between orthogonal planes while using the Gamma equivalent circuit approach. The accuracy of the proposed model in unbalanced operating modes that are characteristic for multiphase machines, such as post‐fault and power‐sharing operation, makes it advantageous compared to existing models. The model is developed assuming that the machine operates in asymmetrical conditions and that, therefore, there is fundamental frequency excitation in both planes. The inter‐plane coupling effect is examined using finite element analysis and an experimental procedure for its quantification is developed. The model is verified by comparison with the experimental results obtained from a prototype asymmetrical six‐phase induction machine, and its advantages compared to existing models are emphasised. [ABSTRACT FROM AUTHOR]

In this paper we discuss magnetomotive force (MMF) in the multiphase machines during fast charging of batteries in electric vehicles (EV). Analytical considerations show that using an adequate topology of power converter and additional levels of freedom one can achieve a fast charging without creating any revolving magnetic field within the machine. They also show that this regime can be achieved by using only the elements of the traction system and without changing the configuration between charging and traction regimes. The simulation models have been developed for the relevant configurations of the stator asymmetrical winding in six-phase and nine- phase integrated electrical machines within the charging system of an EV. As the result, the time-space distribution of the magnetic field in charging mode is obtained. Based on the results, suggestions are given for the design of an integrated traction / charging system within electric vehicles.

Metal industries combine several conditions that match with the advantages of multiphase motor drives, such as high power and high reliability. High power usually means cumbersome multilevel converters, with very expensive high-blocking voltage semiconductors. By adding more phases, it is possible to reduce the motor drive voltage levels and to use two-level converters with more common and less expensive low-blocking voltage power modules. High reliability means lower probability of unexpected production interruption, which is achieved by the fault-tolerant characteristic of the multiphase motor drive. In this article, a wire rod plant is depicted and the replacement of a traditional three-phase motor drive by a nine-phase one is discussed. Experimental results showing the effects of faults in multiphase drive on torque and current waveforms are presented, including their mitigation by the use of a post fault control method. This article also includes reliability calculations for the nine-phase motor drive in order to compare its lifetime with that of the three-phase conventional alternative. [ABSTRACT FROM AUTHOR]

This paper describes the characteristic investigation of the Novel Dual-stator V-Shape Magnetic Pole Six-Phase Permanent Magnet Synchronous Generator (NDSVSMPSP-PMSG) for wind power application. The proposed generator has V-shaped embedded magnetic pole in the rotor and six-phase winding in both the stator. The proposed generator has high-power density and high efficiency. For emphasizing the significance of the proposed generator, the characteristics of the NDSVSMPSP generator are analyzed and compared to one of the traditional generators, i.e., the Dual-stator Surface Mounted Six-Phase Permanent Magnet Synchronous Generator (DSSMSP-PMSG). For the design and characteristic investigation of both the generator, Finite Element Method (FEM) is chosen because of its high accuracy. Two modes of FEM analysis are considered, namely magneto-statics and transients. The magneto static analysis is used for the study of flux line and flux density distribution, while the transient analysis is considered for the generator's characteristic investigation. The performance characteristics such as generated Electromotive Force (EMF) for both inner and outer stator, Percentage Total Harmonic Distortion (THD) of developed voltage, Developed EMF vs rpm, terminal voltage vs. current, developed rotor torque vs time, percent (%) ripple content in torque, and percent efficiency vs current for both generators are investigated. As a result, it can be stated that the power density and reliability of the proposed generator is higher than that of the traditional generator. [ABSTRACT FROM AUTHOR]

Multiphase machines are interesting options for high‐reliability applications due to their inherent fault tolerance against open‐circuit faults (OCFs). Moreover, if the regulation of the x–y currents is realised in open‐loop mode using virtual voltage vectors (VVs), the mandatory post‐fault control reconfiguration is avoided. The new reconfigurationless approach was recently defined as a natural/passive fault‐tolerant strategy, offering good prospects for industry applications. This work extends the idea to the fault detection procedure and suggests new settings for the current imbalance localisation method. The proposal is based on the vector space decomposition approach that allows the joint detection of OCFs and stator resistances dissymmetry. Experimental results in a five‐phase induction motor drive using VVs confirm the viability of the technique. [ABSTRACT FROM AUTHOR]

The industrial application of electric machines in variable‐speed drives has recently grown thanks to the development of microprocessors and power converters. Although three‐phase machines constitute the most common case, the interest of the research community is also focused on multiphase machines with more than three phases. The principal reason lies in the exploitation of their advantages such as reliability, better current distribution among phases or lower current harmonic in the converter, to name a few. Nevertheless, multiphase drive applications require the development of complex controllers to regulate the torque (or speed) and flux of the machine. In this regard, predictive current controllers have appeared as a viable alternative due to an easy formulation and high flexibility to incorporate different control objectives. It is found, however, that these controllers face some peculiarities and limitations in their use that require attention. This work attempts to tackle the predictive current control technique as a viable alternative for the regulation of multiphase drives, paying special attention to the development of the control technique and the discussion of the benefits and limitations. Case examples with experimental results using a symmetrical five‐phase induction machine with distributed windings are included to this end. [ABSTRACT FROM AUTHOR]

Multiphase drives are convenient for high-power/high-current applications as they allow the reduction of the phase current for given rated power and phase voltage. Due to their redundant structure, the multiphase drives have intrinsic open-phase fault-tolerant operation capability. This situation may happen when one or more power electronic units are turned off after a fault event, and the drive configuration allows phase disconnection. In this case, the healthy machine phases can be overloaded to keep the torque constant and without any pulsations. The goal of the work is the evaluation of the thermal parameters of the stator windings of multiphase machines to be used in the analysis of both short thermal transients and steady-state operation, during normal and open-phase faults. The approach has general validity and can be applied to any ac multiphase machine having a distributed winding configuration. The prototype used for the experimental tests is an asymmetrical 12-phase induction machine, having four 3-phase stator sets with isolated neutral points. The stator windings thermal model is obtained experimentally, by considering the mutual heat exchange phenomena among the windings when one or more winding sets are disconnected. This thermal model allows at evaluating the proper machine overloading for short thermal transients and steady-state operation in case of open-phase faults. [ABSTRACT FROM AUTHOR]

Short-time thermal transient identification method was successfully adopted to evaluate the winding thermal parameters of induction motors for industry applications. In this work, the modeling approach and the identification methodology are extended to the more sophisticated case of multiple three-phase machines. The generalized model takes into consideration the mutual heat exchange between the windings as well as the possible causes of temperature mismatch. A complete procedure to evaluate the parameters of the modified model is provided, supported by experimental validation on a 7.5 kW machine with two three-phase windings in contact at slot level. The method covers any type of multiple three-phase machines, whatever the thermal promiscuity of the winding sets: From deep coupling as the ones presented, to the case where only the end-turns are in contact, to the completely decoupled case. The proposed technique can be useful for the machine design and for real-time temperature monitoring during operation. [ABSTRACT FROM AUTHOR]

In this article, a novel post fault control algorithm based on the space vector method is presented and applied to a bearingless multisector permanent magnet synchronous machine with a triple three-phase winding. First, the expressions of the current space vectors, as a function of the electromagnetic suspension force and motoring torque, are introduced. Then, the open-circuit fault is described and a new post-fault algorithm is introduced. The introduced post-fault algorithm, based on the minimization of the stator copper losses, is compared at simulation level with an existing one, where the above-mentioned loss minimization was not considered. The simulation results show a remarkable improvement in the performance when the novel post-fault algorithm is employed. Finally, the developed control algorithm is experimentally validated on a prototype of a bearingless multisector permanent magnet synchronous machine. [ABSTRACT FROM AUTHOR]

Multiphase machines have several benefits compared with three-phase machines. Their inherently higher reliability makes them attractive in several applications. For instance, the nine-phase motor can operate with up to six open phases in its stator but with torque ripple production. However, the modeling and control schemes for a balanced motor cannot be used since now the stator is unbalanced. In order to allow the multiphase machine to work magnetically balanced even in open-phase conditions, an adequate dq current control should be employed. To achieve this control scheme, it is necessary to have a synchronous-frame modeling of a nine-phase induction motor under open-phase faults, which is developed here. Using it, a synchronous-frame control is derived, still using proportional controllers in the dq current loops. This is possible due to the use of an appropriate voltage decoupling considering the fault condition. Hence, the unbalanced multiphase induction motor can be controlled very similarly as a balanced one, additionally resulting in the torque ripple minimization. Experimental results characterize the operation with open phases. Simulation and finite-element results show the success of the proposed control scheme. [ABSTRACT FROM AUTHOR]

Testing the high-power machines in general is not an easy task. One of the standard tests is the full-load test. This test typically requires another machine, of the same or higher power rating, to be coupled to the tested one. For multiphase machines, which are commonly designed for high-power applications, this test can be conducted in a different way. In order to simplify full-load test, this paper proposes a new method that is applicable for multiple three-phase machines with even number of neutral points. The method is based on indirect rotor-field oriented control. It enables evaluation of the efficiency and the thermal design in the case of synchronous machines and the segregation of the constant and stator variable losses for induction machines, without the need for coupling another machine as a load. In the presented method, the full-load test conditions on the stator are obtained by circulating the rated active power flow in a closed loop from one winding set to another. The only power used during the test is to cover machine and converter losses. The proposed control scheme is unique and is based around y-component from the vector space decomposition subspace. It is validated through the simulation and experimental results. [ABSTRACT FROM AUTHOR]

The usage of multiphase electrical drives expands the operation possibilities of electrical machines and opens new directions of research on inverter-fed electrical machines. With an increasing number of phases, the standard approach of the electromagnetic design of machines has to be generalized to m-phase systems, which is not usually respected in the literature focused on electric machine design, and it is rarely published. This paper summarizes the specific problems linked with the design of machines with different numbers of phases, focusing on the winding design and the calculation of equivalent circuit parameters. In addition to the direct effect of different numbers of phases, the impact of injecting higher order time harmonic components on the electromagnetic design of electric machines is analyzed. The obtained analytical results are verified by the measurement of a nine-phase experimental induction motor.