Multiphase Permanent Magnet Synchronous Machines Research Articles

Open-Phase-Tolerant Online Current References for Maximum Torque Range and Minimum Loss With Current and Torque-Ripple Limits for n-Phase Nonsalient PMSMs With Nonsinusoidal Back EMF

Multiphase permanent-magnet synchronous machines (PMSMs) with nonsinusoidal back-electromotive force (back-EMF) offer high fault tolerance and torque density for electric vehicles. Most current-reference generation methods either minimize stator copper loss (SCL) or maximize achievable torque. Optimization of both goals is accomplished by full-torque-range minimum-loss (FRML) strategies, but so far just for sinusoidal back-EMF. Thus, FRML for nonsinusoidal back-EMF should be sought. Moreover, many methods are only suitable for healthy conditions or specific machines, harmonics, or open-phase-fault (OPF) scenarios. Additionally, the torque range may be extended by permitting torque ripple or (transiently) greater rms current, but this approach is not general nor FRML yet. This paper proposes online FRML current-reference generation for multiphase PMSMs with nonsinusoidal back-EMF: nonsinusoidal-back-EMF FRML (NSBE-FRML). When the torque reference is feasible, minimum SCL is attained while maximizing the achievable torque (i.e., FRML). For higher torque references, the instantaneous torque deviation is minimized, and the torque reference is saturated in consecutive samples limiting the torque ripple to a pre-specified threshold. Furthermore, the rms current is limited after transient overload by automatically decreasing the torque reference. The NSBE-FRML is suitable for any harmonics, healthy/OPF conditions, and multiphase PMSMs with negligible saliency ratio. Experiments are performed with a six-phase PMSM.

Optimal torque control with radial force compensation for multiphase PMSMs under an open-circuit fault

Open-circuit (OC) faults of permanent magnet synchronous machines (PMSM) yield deviations from the desired torque and result in undesired radial forces, which cause vibrations and noise. Multiphase PMSMs give additional degrees of freedom for the control of the machine, which allow to (partially) mitigate the impacts of an OC fault. Therefore, a fault-tolerant control strategy with radial force compensation under single open-circuit (OC) faults is developed in this paper for multiphase PMSMs. It is based on a magnetic equivalent circuit (MEC) model, which ensures a high model accuracy even for motors with non-fundamental wave behavior and magnetic saturation, and which is applicable to arbitrary multiphase PMSM designs both for the healthy-circuit (HC) and the OC fault case. The proposed method allows to minimize the torque tracking error, the power losses and the radial forces. The results of the proposed fault-tolerant control strategy, which are presented for an experimentally validated model, prove that almost complete mitigation of an OC fault is possible.

Enhancement of Sensorless Control for Non-Sinusoidal Multiphase Drives-Part I: Operation in Medium and High-Speed Range

This two-part study proposes a new sensorless control strategy for non-sinusoidal multiphase permanent magnet synchronous machines (PMSMs), especially integrated motor drives (IMDs). Based on the Sliding Mode Observer (SMO), the proposed sensorless control strategy uses the signals (currents and voltages) of all fictitious machines of the multiphase PMSMs. It can estimate the high-accuracy rotor positions that are required in vector control. This proposed strategy is compared to the conventional sensorless control strategy that applies only current and voltage signals of the main fictitious machine, including the fundamental component of back electromotive force (back EMF) of non-sinusoidal multiphase PMSMs. Therefore, in order to choose an appropriate sensorless control strategy for the non-sinusoidal multiphase PMSMs, these two sensorless control strategies will be highlighted in terms of precision with respect to rotor position and speed estimation. Simulations and the experimental results obtained with a non-sinusoidal seven-phase PMSM will be shown to verify and compare the two sensorless control strategies. In this part of the study (part I), only sensorless control in the medium and high-speed range is considered. Sensorless control at the zero and low-speed range will be treated in the second part of this study (part II).

Advances in Dual-Three-Phase Permanent Magnet Synchronous Machines and Control Techniques

Multiphase electrical machines are advantageous for many industrial applications that require a high power rating, smooth torque, power/torque sharing capability, and fault-tolerant capability, compared with conventional single three-phase electrical machines. Consequently, a significant number of studies of multiphase machines has been published in recent years. This paper presents an overview of the recent advances in multiphase permanent magnet synchronous machines (PMSMs) and drive control techniques, with a focus on dual-three-phase PMSMs. It includes an extensive overview of the machine topologies, as well as their modelling methods, pulse-width-modulation techniques, field-oriented control, direct torque control, model predictive control, sensorless control, and fault-tolerant control, together with the newest control strategies for suppressing current harmonics and torque ripples, as well as carrier phase shift techniques, all with worked examples.

Transfer learning‐based surrogate‐assisted design optimisation of a five‐phase magnet‐shaping PMSM

Multi-phase permanent-magnet synchronous machines (MPMSMs) with high reliability due to sufficient fault-tolerant capability have considerable potential for transportation electrification applications. Here, an efficient surrogate-assisted design optimisation method is proposed based on analytical model transfer learning for torque characteristic optimisation of a five-phase magnet-shaping PMSM. By employing transfer learning of the source domain analytical model data and the target domain finite element analysis (FEA) data in surrogate model training, the proposed method can achieve both high accuracy and high efficiency from the merits of FEA- and analytical-based optimisations, respectively. The studied machine with five-phases and harmonic injected surface-mounted PMs to enable harmonic injection for torque capability improvement is introduced and the analytical model is built based on the segmented PM and the complex conformal mapping methods. Besides, the optimal Latin hypercube design (LHD) and Taguchi methods are used to form the source and target domain datasets, respectively, so that data features can be efficiently captured over a wide range of optimisation variables. An optimal design is obtained by multi-objective optimisation using the trained surrogate model, which is prototyped and measured to validate the proposed method.

A Novel Matrix Transformation for Decoupled Control of Modular Multiphase PMSM Drives

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.

Experimental assessment of winding inter‐turn short‐circuits fault signatures in six‐phase AC permanent magnet synchronous motors

The interest for multiphase permanent magnet synchronous machines, in modern renewable power generation systems, is increasing rapidly due principally to their high efficiency and fault-tolerant capability. To meet the high-performance requirements, monitoring the stator winding state is a key item. The detectability of a stator winding inter-turn short-circuit (ITSC), for an asymmetrical six-phase surface-mounted permanent magnet synchronous motor, is analysed in this study. The impact of an ITSC on the stator back-emfs, stator currents and voltages is examined by spectral analysis of the corresponding space-vectors, in different α–β planes. In particular, the detectability of a stator ITSC, in closed-loop operating conditions, is investigated using voltage and current signature analysis. The spectral fault signature is identified through 2D finite element analysis, and then confirmed by laboratory experimental tests.

Feedforward Compensation of Torque Ripples in Dual Three-Phase PMSM Fed From Separate DC Links With Different Voltage Levels

Multiphase permanent magnet synchronous machines (PMSMs) with concentrated windings are gaining popularity in the development of fault-tolerant drives. The 12-slot/8-pole PMSM offers the possibility of forming two spatially separated three-phase winding sets, where each spans half of the extent. Additionally, it is attractive in terms of the acoustic noise that the radial magnetic stress is balanced for each quarter of the circumference. Unfortunately, there is no phase shift between the winding sets, which leads to additional torque ripple. A compensation using further construction measures is only partially possible. This article discusses causes for the torque ripples in this type of PMSM. Besides magnetic saturation, additional torque ripples appear when the winding sets are fed by different dc links, which is necessary in some applications in order to increase the availability of the drive in case of a failure. For complete cancellation of the dominant torque harmonics a method for identification of harmonic voltages, and currents is proposed. Using the measured angular acceleration as error signal, an adaptive narrowband filter is adjusted continuously to drive the harmonics to zero. The proposed method does neither suffer from nonlinearities due to magnetic saturation nor from model complexity due to the increased phase number.

Rotor magnet demagnetisation diagnosis in asymmetrical six‐phase surface‐mounted AC PMSM drives

Multiphase permanent-magnet synchronous machines (PMSMs) are receiving more and more interest in safety-critical modern industries owing to their higher reliability when compared with conventional three-phase PMSMs. Rotor magnets are critical components, which, in case of fault, directly affect the performance of the PMSMs. Thus, monitoring the rotor magnets status is essential to ensure both high level of efficiency and service continuity. The present study focuses on the investigation of a new approach for the detection of rotor magnet demagnetisation in a vector-controlled asymmetrical six-phase surface-mounted AC PMSM. The main contribution of the proposed technique is the assessment of a rotor demagnetisation fault index derived from the fifth and seventh harmonics of the stator voltage space vector evaluated in the α 5–β 5-plane, and already available in the control system platform. The performance of the proposed approach is evaluated using finite element analysis and numerical simulations, both validated by experimental tests.

Current Harmonic Suppression in Dual Three-Phase Permanent Magnet Synchronous Machine With Extended State Observer

Dual three-phase (DTP) permanent magnet synchronous machines (PMSM) have been utilized in many applications due to their outstanding performance. However, large stator current harmonics limit the further application of the DTP-PMSM due to the low impedance in the harmonic subspace. To solve this problem, this article proposes a current harmonic suppression strategy based on an extended state observer (ESO). A detailed analysis is carried out to demonstrate the disturbance rejection ability and robustness of the proposed method. The theoretical analysis also shows that the ESO strategy outstands the conventional proportional-integral controller and advanced proportional resonance (PR) controller in terms of harmonic reduction. The advantages are verified by simulation and experimental results under different operating conditions. The proposed strategy still works well when considering other factors that may cause harmonic distortion, such as magnetic circuit saturation, saliency ratio, over-modulation of the inverter, the voltage drop caused by electric devices, and dead time. Meanwhile, some limitations are also pointed out. The proposed strategy can also be easily applied to other multiphase PMSM types.

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

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.

Investigation of Volumic Permanent-Magnet Eddy-Current Losses in Multi-Phase Synchronous Machines from Hybrid Multi-Layer Model

This paper investigates the permanent-magnet (PM) eddy-current losses in multi-phase PM synchronous machines (PMSM) with concentric winding and surface-mounted PMs. A hybrid multi-layer model, combining a two-dimensional (2-D) generic magnetic equivalent circuit (MEC) with a 2-D analytical model based on the Maxwell–Fourier method (i.e., the formal resolution of Maxwell’s equations by using the separation of variables method and the Fourier’s series), performs the eddy-current loss calculations. First, the magnetic flux density was obtained from the 2-D generic MEC and then subjected to the Fast Fourier Transform (FFT). The semi-analytical model includes the automatic mesh of static/moving zones, the saturation effect and zones connection in accordance with rotor motion based on a new approach called “Air-gap sliding line technic”. The results of the hybrid multi-layer model were compared with those obtained by three-dimensional (3-D) nonlinear finite-element analysis (FEA). The PM eddy-current losses were estimated on different paths for different segmentations as follow: (i) one segment (no segmentation), (ii) five axial segments, and (iii) two circumferential segments, where the non-uniformity loss distribution is shown. The top of PMs presents a higher quantity of losses compared to the bottom.

Influence of Gear Ratio on the Performance of Fractional Slot Concentrated Winding Permanent Magnet Machines

Fractional slot concentrated winding (FSCW) permanent magnet synchronous machines (PMSMs) have been a research hotspot over the past few decades. Recently, the magnetic gearing effect in FSCW PMSMs is revealed along with its gear ratio, which is a function of slot/pole number combination. At the design stage of FSCW PMSMs, one of the key issues is the selection of the slot/pole number combination. This paper shows that the gear ratio can contribute to a proper slot/pole number selection in any multiphase FSCW PMSMs by acting as a unified index for a quick overall performance comparison. In this paper, first, the magnetic gearing effect in FSCW PMSMs is explained and the gear ratio is further discussed. The advantages of adopting gear ratio as the overall performance index over other indices are revealed. The influence of the gear ratio on the winding factor, torque output, cogging torque, inductance, and rotor losses of three- and six-phase FSCW PMSMs is analyzed and validated by experiments, thus proving the analyses.

Control strategies for non‐sinusoidal multiphase PMSM drives in faulty modes under constraints on copper losses and peak phase voltage

In the context of future permanent magnet synchronous machines (PMSMs) with a high number of phases (>7) in integrated drives, this study proposes several control strategies when multiphase PMSMs with non-sinusoidal back electromotive forces (back-EMFs) operate in healthy and open-circuit faults. In all operation modes, the considered constraint on current is related to the maximum root mean square current allowable in one phase of the machine. The constraint on voltage limits the maximum peak value of the phase voltage determined by the DC-bus voltage of the converter. When one or two phases are open-circuited, to maximise torque and respect the constraints, new current references obtained by several proposed methods in rotating and natural frames are imposed to the machine. Owing to the non-sinusoidal waveform of back-EMFs and the considered constraints, numerical computations based on analytical formulations are required to obtain maximal torque-speed characteristics, including the flux-weakening operation. The usefulness of the proposed strategies is verified by numerical and experimental results.

Inverse-Based Current Harmonic Controller for Multiphase PMSMs

Multiphase electric machines can provide significant advantages over conventional three-phase machines. Unfortunately, multiphase machines also have a notable disadvantage that even a small voltage excitation of certain frequency components can cause large stator current harmonics. This paper presents an inverse-based current harmonic controller to eliminate the current harmonics. A detailed theoretical analysis of the proposed harmonic controller is given including a comprehensive set of analytical design principles. The robustness of the method is studied with a multi-input multi-output approach based on a structured singular value and H∞ norm analyses. In addition to the theoretical work, the performance of the harmonic controller is investigated with experimental results from a dual three-phase permanent magnet synchronous machine (PMSM). The analysis and results of this paper show that the inverse-based current harmonic controller provides a robust and high-performance method to eliminate the current harmonics in multiphase PMSMs.

A five-phase 20-slot/18-pole PMSM for electric vehicles

Purpose – The purpose of this paper is to investigate a five-phase permanent-magnet synchronous machine (PMSM) that features high-power density and high-fault-tolerant capability for electric vehicles (EVs). Design/methodology/approach – The five-phase 20-slot/18-pole PMSM is designed by finite-element method. Two typical rotor structures which include Halbach array and rotor eccentricity are compared to achieve sinusoidal back electromotive force (EMF). The influence of slot dimensions on leakage inductance and short-circuit current is analyzed. The method to reduce eddy current loss of permanent magnets (PMs) is investigated. The machine performances under both healthy and fault conditions are evaluated. Finally, thermal behavior of the machine is studied by Ansys. Findings – With both no-load and load performances considered, rotor eccentricity is proposed to reduce the harmonic contents of EMF. Increasing slot leakage inductance is an effective way to limit the short-circuit current. By segmenting PMs in circumferential direction, the PM eddy current loss is reduced and the machine efficiency is improved. With proper fault-tolerant control strategy, acceptable torque performance can be achieved under fault conditions. The proposed machine can safely operate under Class F insulation. Originality/value – So far, many researches focus on multiphase PMSMs used in aviation fields, such as fuel pump and electric actuator. Differing from PMSMs used in aviation applications, machines for EVs require characteristics like wide speed ranges and variable operating conditions. Hence, this paper proposes a five-phase 20-slot/18-pole PMSM for EVs. The proposed design methodology is applicable to multiphase PMSMs with different slot/pole combinations.

Research on a 20-Slot/22-Pole Five-Phase Fault-Tolerant PMSM Used for Four-Wheel-Drive Electric Vehicles

This paper presents a five-phase fault-tolerant permanent-magnet synchronous machine (PMSM) used for electric vehicles. In multiphase fault-tolerant PMSMs equipped with fractional-slot concentrated windings, excessive magneto-motive force (MMF) harmonics can lead to thermal demagnetization of the permanent magnets (PMs). In order to reduce the lower-order harmonics, the origins of the 2-pole harmonic in conventional winding configurations are investigated, and an unequal-turn winding configuration is applied to cancel the lower-order harmonics. The main electromagnetic performances of the unequal-turn winding configuration are investigated and compared with conventional winding topologies. Based on the principle of maintaining constant instantaneous power, the fault-tolerant control strategies for open-circuits of up to two phases are developed. All of the investigations are verified by finite element analysis (FEA) results.

Real and complex models of multi-phase permanent magnet synchronous motors

This paper deals with the modelling of multi-phase permanent magnet synchronous machines (PMSM) with an arbitrary number of phases and arbitrary shape of the rotor flux. The aim of the paper is to propose new dynamic models for the multi-phase electrical machines based on the use of real and complex state space transformations which are invariant with respect to power. The dynamic models of the motor are obtained using a Lagrangian approach in the frame of the power-oriented graphs technique. The obtained dynamic models can directly be implemented in Simulink whatever the number of phase is. The state space transformations presented in the paper (both real and complex) are compared to the main other transformations know in the literature. Some simulation results are finally presented.

Complex Dynamic Models of Multi-phase Permanent Magnet Synchronous Motors

AbstractIn the paper two power-invariant real and complex state space transformations for modeling multi-phase electrical machines in a compact and general form are proposed. In particular the paper deals with the modeling of multi-phase permanent magnet synchronous machines with an arbitrary number of phases and an arbitrary shape of the rotor flux. The dynamic model of the motor is obtained using a Lagrangian approach and in the frame of the Power-Oriented Graphs technique. The obtained models are equivalent from a mathematical point of view and can be directly implemented in Simulink. The complex transformed model is quite compact and uses a reduced order state vector. Some simulation results end the paper.

Saturated Vectorial Control of Multi-phase Synchronous Motors

This paper deals with the torque control of multi-phase permanent magnet synchronous machines. The dynamic model of these motors is obtained using the Power-Oriented Graphs technique. This graphical modeling technique allows to write the dynamic equations of the system in a very compact form. Starting from the dynamic equations, a torque control law with saturated input voltages is proposed based on a new current vector approach for describing the voltage constraint. Some simulation results validate the proposed control law.