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Fractional slot permanent magnet (PM) brushless machines having concentrated non‐overlapping windings have been the subject of research over last few years. They have already been employed in the commercial hybrid electric vehicles (HEVs) due to high‐torque density, high efficiency, low‐torque ripple, good flux‐weakening and fault‐tolerance performance. The purpose of this paper is to overview recent development and research challenges in such machines in terms of various structural and design features for electric vehicle (EV)/HEV applications.

In the paper, fractional slot PM brushless machines are overviewed according to the following main and sub‐topics: first, machine topologies: slot and pole number combinations, all and alternate teeth wound (double‐ and single‐layer windings), unequal tooth structure, modular stator, interior magnet rotor; second, machine parameters and control performance: winding inductances, flux‐weakening capability, fault‐tolerant performance; and third, parasitic effects: cogging torque, iron loss, rotor eddy current loss, unbalanced magnetic force, acoustic noise and vibration.

Many fractional slot PM machine topologies exist. Owing to rich mmf harmonics, fractional slot PM brushless machines exhibit relatively high rotor eddy current loss, potentially high unbalanced magnetic force and acoustic noise and vibration, while the reluctance torque component is relatively low or even negligible when an interior PM rotor is employed.

This is the first overview paper which systematically reviews the recent development and research challenges in fractional‐slot PM machines. It summarizes their various structural and design features for EV/HEV applications.

The purpose of this paper is to analyze the phase coil connections and winding factors of flux‐switching permanent magnet (FSPM) brushless AC machines with all poles and alternate poles wound, and different combinations of stator and rotor pole numbers.

The coil‐emf vectors, which are widely used for analyzing the conventional fractional‐slot PM machines with non‐overlapping windings, are employed for FSPM machines.

Although the coil‐emf vectors have been employed to obtain coil connections in the conventional fractional‐slot PM machines, they are different in FSPM machines. It is mainly due to different polarities in the stator of FSPM machines. In addition, from the coil‐emf vectors it is able to predict whether the back‐emf waveforms are symmetrical or asymmetric.

This is the first time that coil‐emf vectors are used to determine the coil connections and winding factors in FSPM machines with different topologies and combination of stator and rotor pole numbers.

The purpose of this paper is to deal with several approach to recover the torque production capability of a five phase double-layer fractional-slot PM machine under faulty operation. The considered fault is an open-circuit coil in a given phase.

In a first step, the mean futures, such as the phase back-EMFs and the electromagnetic torque, are computed by finite element analysis under healthy operation, and are taken as references. Then, they are investigated, under a faulty coil, for different approaches to recover the torque production capability.

A comparison of the potentialities of the torque recovery approaches has clearly highlight the superiority of the approach consisting in the re-adjustment of the current initial phases, in an attempt to equilibrate the resulting air gap MMF.

This work should be extended by an experimental validation of the predicted results regarding the back-EMFs and the electromagnetic torque.

The investigation of the considered five phase fractional-slot PM machine under faulty operation should be extended to several faulty scenarios in order to fulfill the requirements of many applications such as the propulsion systems.

The paper proposes different approaches to recover the torque production capability of a five phase fractional-slot PM machine under faulty operation.

The paper is aimed at the investigation of the magnetic forces generated by fractional slot surface mounted PM machines, considering a comparative study between two topologies: a 9 slot/10 pole machine and a 12 slot/10 pole machine.

Following the distribution of the armature windings using the star of slots approach, an investigation of the magnetic forces developed by both machines under study, using 3D finite element analysis (FEA). Prior to such investigation, a 2D FEA based sizing procedure is carried out in order to select a set of suitable geometrical parameters. Then, the comparison between both machines is extended to the torque production capability.

It has been found that the 9 slot/10 pole machine has a pic value of the average magnetic force reaching almost 40N which is located in one side of the air gap. Such a peak does not exceed 7N in the 12 slot/10 pole machine and is located in two diametrically‐opposite areas of the air gap.

This work should be extended by an experimental validation of the FEA results regarding the magnetic force generation.

The list of the selection criteria of fractional slot PM machines should be extended to the magnetic force generation in order to fulfil the requirements of many applications such as the propulsion systems.

The paper proposes a combined electromagnetic‐mechanical approach to investigate the magnetic forces generated by fractional slot surface mounted PM machines using 2D and 3D finite element analysis.

This paper presents the design techniques applied to a novel fractional-slot 6/4 pole permanent magnet brushless direct current (PMBLDC) motor, for cogging torque reduction. The notable feature of this motor is the simplicity of the design and low production cost. The purpose of this paper is to reduce the peak cogging torque of the motor. The focus is put on the stator topology tuning, and a new design for the stator poles is proposed. By determining the optimum stator pole arc length and the best pole shoe thickness, the cogging torque is significantly reduced. This new optimised motor design has been analysed in detail. The validation of the results is documented with respective figures and charts.

At the beginning, the design data for the 6/4 pole PMBLDC motor with concentrated three phase windings and asymmetric stator pole arcs is presented. In the study, this motor is taken as a reference model (A₀, T₀). A full performance finite element analysis of the reference motor has been carried out, and the weak points in the motor design have been identified. By simple design techniques, tuning the stator pole geometry, a two-stage design optimisation for cogging torque minimisation has been performed and the solution array has been derived. The optimised model is selected and proposed (A_opt, T_opt). The comparative analysis of the reference and optimised motors show the advantages of the proposed novel design and prove the methodology.

The results of the work demonstrate how simple design techniques can minimise the peak of the cogging torque profile, while maintaining the specified electromagnetic torque value. The sensitivity of the cogging torque profile because of changes of the stator pole design inside the prescribed constraints is apparent. The stator poles of the reference motor have an arc length of 85° and pole shoe thickness of 6 mm. The newly shaped stator poles have an arc length of 78.5° and pole shoe thickness 4.8 mm. The peak-cogging torque has been reduced from 0.158 Nm to a respectable value of 0.066 Nm. However, to reduce electromagnetic torque ripple and pulsations, further investigations are required.

The paper presents an approach to cogging torque reduction for a 6/4 PMBLDC motor. A two-step original design procedure is introduced and an optimised stator pole geometry is defined. The minimised cogging torque has been demonstrated with improved usage of the active materials. This work could serve as a good basis for further optimisation of the motor design.

The purpose of the paper is to investigate the performance of induction machines with fractional‐slot concentrated‐windings.

This paper examines induction machine performance with fractional‐slot concentrated windings using the standard distributed lap windings as reference. Four designs are compared and various performance tradeoffs highlighted. The first machine has integral‐slot distributed 2 slots/pole/phase lap winding and it serves as the reference winding. The second machine has a double‐layer 1/2 slot/pole/phase winding, a workhorse for brushless DC machines. The third machine has double‐layer 2/5 slot/pole/phase winding. Lastly, the fourth machine has single‐layer 2/5 slot/pole/phase windings. The comparison includes torque‐speed curves (including the effects of major space harmonic components), rotor bar losses, and ripple torque levels.

Based on the analysis results presented here, the traditional distributed lap winding is proven to be superior to FSCW in terms of torque production and rotor bar losses for induction machine applications. The 1/2 spp shows some promising results in terms of torque production, in addition to significant reduction and simplification of end turns with lower number of coils albeit with more turns/coil (12 slots vs 48 slots). The penalty is the additional rotor bar losses due to the 2nd and 4th harmonic mmf components. The 2/5 spp is not promising for torque production and should be avoided. The transient simulation results that simultaneously take into account the effects of all space harmonics and magnetic saturation showed comparable trends compared to the harmonic analysis results. It has also been shown that FSCW tend to have higher torque ripple compared to distributed windings.

To the best of the authors' knowledge, this paper for the first time attempts to quantitatively address the tradeoffs involved in using FSCW in induction machines.

The purpose of this paper is to apply a fast analytical model of the acoustic behaviour of pulse‐width modulation (PWM) controlled induction machines to a fractional‐slot winding machine, and to analytically clarify the interaction between space harmonics and time harmonics in audible electromagnetic noise spectrum.

A multilayer single‐phase equivalent circuit calculates the stator and rotor currents. Air‐gap radial flux density, which is supposed to be the only source of acoustic noise, is then computed with winding functions formalism. Mechanical and acoustic models are based on a 2D ring stator model. A method to analytically derive the orders and frequencies of most important vibration lines is detailed. The results are totally independent of the supply strategy and winding type of the machine. Some variable‐speed simulations and tests are run on a 700 W fractional‐slot induction machine in sinusoidal case as a first validation of theoretical results.

The influence of both winding space harmonics and PWM time harmonics on noise spectrum is exposed. Most dangerous orders and frequencies expressions are demonstrated in sinusoidal and PWM cases. For traditional integral windings, it is shown that vibration orders are necessarily even. When the stator slot number is not even, which is the case for fractional windings, some odd order deflections appear: the radial electromagnetic power can therefore dissipate as vibrations through all stator deformation modes, leading to a potentially lower noise level at resonance.

The analytical research does not consider saturation and eccentricity harmonics which can play a significant role in noise radiation.

The analytical model and theoretical results presented help in designing low‐noise induction machines, and diagnosing noise or vibration problems.

The paper details a fully analytical acoustic and electromagnetic model of a PWM fed induction machine, and demonstrate the theoretical expression of main noise spectrum lines combining both time and space harmonics. For the first time, a direct comparison between simulated and experimental vibration spectra is made.

This paper deals with the analysis, the modeling, the control and the fault‐tolerance capability of a three‐switch inverter (TSI, also known delta‐inverter) fed fractional‐slot six‐phase brushless DC motor (BDCM) drive.

Following the presentation of the advantages of multi‐phase fractional‐slot brushless machines and the possibility of their association to TSI, the analysis of the operating sequences as well as the modeling of a TSI fed six‐phase BDCM drive are developed. Then, a dedicated control strategy of such a drive is synthesized. Finally, a case study is simulated considering both transient behaviour during the start‐up of the BDCM as well as a steady‐state one under healthy and faulty operations.

It has been found that the 60‐electrical degree shift between the six phases of the BDCM makes it simple to achieve its operating sequences with its armature fed by a TSI, considering a suitable anti‐parallel connection of the six phases.

Crucial cost benefits associated with improved compactness, reliability, and fault‐tolerance capability could be gained thanks to the integration of TSI fed six‐phase BDCM drives in large‐scale production industries, such as the automotive one.

The paper proposes an analysis of the operating sequences as well as the fault‐tolerance capability of TSI fed six‐phase BDCM drives.

The purpose of this paper is to analyse and compare the functional parameters of three- and six-phase permanent magnet synchronous motors (PMSM) with fractional-slot concentrated windings (FSCW).

The investigations are focused on the comparison of the distortions of back electromotive force (emf) and magnetomotive force (mmf) waveforms, as well as torque ripples, radial force spatial harmonics and motor performance studies. The finite element models of the test machine and a personally developed computer code have been used to calculate motor characteristics and analyse and synthesise multiphase winding layouts, respectively.

Compared with the traditional three-phase PMSM designs, the proposed six-phase machines are characterized by a significantly lower content of sub-harmonics in mmf waveform distribution. Moreover, the investigated six-phase machines exhibited a higher average value of electromagnetic torque, significantly lower torque ripples and a reduced value of low-order harmonics of the radial component of the electromagnetic force in the air-gap of the machine.

The analyses presented in this paper show that six-phase PMSM with FSCWs are advantageous to their counterpart three-phase machines. Specifically, they are more suited to working with multiple drives supplying a segmented winding system while simultaneously offering higher performance. This suitability to the use of a multi-drive supply for one motor offers flexibility and cost reduction while increasing the fault tolerance of a power train system.

The purpose of this paper is to investigate advantages of multiphase permanent magnet synchronous motors (PMSM) with fractional slot concentrated windings (FSCW). The investigation is based on comparative analysis and assessment of FSCW PMSM wound as 3, 6, 9 and 12 phase machines suited for low speed applications.

The investigations are focussed on distortions of back electromotive (emf) and magnetomotive force (mmf) with the torque ripples and motors’ performance taken into account. The finite element models with the aid of customized computer code have been adopted for motor winding design and back emf, mmf and motor performance analyses.

The novel multiphase winding layouts were found to offer lower content of sub-harmonics in the mmf waveforms compared with the traditional three-phase machine designs. Moreover, the investigated multiphase machines exhibited higher average value of the electromagnetic torque, while the multiphase PMSM machines with FSCW were further characterized by significantly lower torque pulsations.

The analyses presented in this paper demonstrate that PMSM with FSCW are advantageous to their counterpart three-phase machines. Specifically, they offer higher performance and are more suitable to work with multiple drives supplying segmented winding system. This ability of using multi-drive supply for one motor offers flexibility and cost reduction while increasing fault tolerant power train system.