Search

The use of permanent magnet synchronous machines (PMSMs) is becoming more popular [ 1 ] thanks to their compactness, high torque density and increased flux weakening performance [ 2 ]. Other advantages can be gained by equipping PMSM with concentrated winding in the stator. In fact, using concentrated winding can be beneficial regarding easy manufacturing and consequently lower production costs, modularity [ 3 ] and fault tolerance. More Fractional-Slot Concentrated Winding (FSCW) is an increasingly attractive option for the stator winding of synchronous permanent-magnet machines. Despite of the extensive studies conducted so far, there seem to be further interesting margins for expanding and improving such technology. A well-known drawback of their adoption is the occurrence of large magneto-motive force (MMF) harmonics, which produce eddycurrent losses in rotor permanent magnets. Moreover, it is commonly assumed in the existing literature that a symmetrical three-phase FSCW is feasible only on condition that the number of slots Z is an integer multiple of 3 times the maximum common divisor between Z and the number of pole pairs p. This thesis is intended to optimize the multi-layer winding design through a new general systematic methodology having the form of a multi-objective quadratic programming problem. The use of a multilayer design, with coils of different phases wound around the same tooth, is a possible countermeasure to mitigate problems as large magneto-motive force (MMF) harmonics, which produce eddy-current losses in rotor permanent magnets. In addition, this thesis explores the possibility to synthetize a symmetrical FSCWs having unconventional slot-pole combinations, i.e., with a number of slots and a number of poles that are commonly believed incompatible. It is shown that a multi-layer FSCW with (theoretically) arbitrary slot-pole combinations can be implemented and a methodology to synthetize it optimally is given. The performance of FSCW designs with conventional and unconventional pole-slot combinations is comparatively assessed. A study case is provided to illustrate the possible practical advantage of choosing an unconventional pole-slot combination. Fractional-Slot Concentrated Winding (FSCW) is an increasingly attractive option for the stator winding of synchronous permanent-magnet machines. Despite of the extensive studies conducted so far, there seem to be further interesting margins for expanding and improving such technology. A well-known drawback of their adoption is the occurrence of large magneto-motive force (MMF) harmonics, which produce eddycurrent losses in rotor permanent magnets. Moreover, it is commonly assumed in the existing literature that a symmetrical three-phase FSCW is feasible only on condition that the number of slots Z is an integer multiple of 3 times the maximum common divisor between Z and the number of pole pairs p. This thesis is intended to optimize the multi-layer winding design through a new general systematic methodology having the form of a multi-objective quadratic programming problem. The use of a multilayer design, with coils of different phases wound around the same tooth, is a possible countermeasure to mitigate problems as large magneto-motive force (MMF) harmonics, which produce eddy-current losses in rotor permanent magnets. In addition, this thesis explores the possibility to synthetize a symmetrical FSCWs having unconventional slot-pole combinations, i.e., with a number of slots and a number of poles that are commonly believed incompatible. It is shown that a multi-layer FSCW with (theoretically) arbitrary slot-pole combinations can be implemented and a methodology to synthetize it optimally is given. The performance of FSCW designs with conventional and unconventional pole-slot combinations is comparatively assessed. A study case is provided to illustrate the possible practical advantage of choosing an unconventional pole-slot combination.

The design of the stator winding has direct influence on the electric machine performance. As technology has advanced, researchers and motor manufacturers have worked on possible ways to improve the stator winding layout to achieve several targets depending on the application which namely consist of minimization of harmonic losses for improved efficiency, torque pulsation reduction, acoustic noise and vibration reduction,... For this purpose, an extensive range of literature have presented several design strategies for both synchronous and induction motors acting on winding connection types, configurations, conductors type and shape..., to achieve the required electrical performance. It has been shown that - depending on the application - some winding configurations can be more problematic than others as they produce additional undesirable parasitic effects. However, in some cases, the choice of the winding layout is limited by the required application which leads to look for solutions to cope with the winding configuration drawbacks. The works presented in this thesis will deal with two different aspects of electrical machine windings that need to be further explored which consist of: fractional slot concentrated winding optimization in three-permanent magnet machines and study and improvement of the winding in terms of skin effect is large-medium voltage synchronous machines. Regarding the first topic, potentials and limits of the FSCW optimization through multi-layer arrangement will be presented to mitigate three problems: unbalanced magnetic pull (UMP), torque ripple and permanent-magnet eddy-current losses. As for the second topic, an extensive study of the eddy-currents losses in the stator equipped with form wound coils made of flat conductors embedded in open slots will be covered. Such study is important as it helps to identify the main causes of problems to avoid it through a suitable design. The design of the stator winding has direct influence on the electric machine performance. As technology has advanced, researchers and motor manufacturers have worked on possible ways to improve the stator winding layout to achieve several targets depending on the application which namely consist of minimization of harmonic losses for improved efficiency, torque pulsation reduction, acoustic noise and vibration reduction,... For this purpose, an extensive range of literature have presented several design strategies for both synchronous and induction motors acting on winding connection types, configurations, conductors type and shape..., to achieve the required electrical performance. It has been shown that - depending on the application - some winding configurations can be more problematic than others as they produce additional undesirable parasitic effects. However, in some cases, the choice of the winding layout is limited by the required application which leads to look for solutions to cope with the winding configuration drawbacks. The works presented in this thesis will deal with two different aspects of electrical machine windings that need to be further explored which consist of: fractional slot concentrated winding optimization in three-permanent magnet machines and study and improvement of the winding in terms of skin effect is large-medium voltage synchronous machines. Regarding the first topic, potentials and limits of the FSCW optimization through multi-layer arrangement will be presented to mitigate three problems: unbalanced magnetic pull (UMP), torque ripple and permanent-magnet eddy-current losses. As for the second topic, an extensive study of the eddy-currents losses in the stator equipped with form wound coils made of flat conductors embedded in open slots will be covered. Such study is important as it helps to identify the main causes of problems to avoid it through a suitable design.

Lo scopo di questa tesi è presentare alcuni metodi efficienti (dal punto di vista computazionale) per il calcolo degli effetti dovuti alle correnti parassite (eddy currents) in macchine elettriche rotanti in media tensione. Due applicazioni in particolare sono state considerate nel dettaglio. Inizialmente viene analizzato il fenomeno delle correnti parassite indotte nell'albero di motori asincroni a due poli e il conseguente effetto sulle prestazioni della macchina, focalizzandosi in particolare sul fattore di potenza. La seconda parte della tesi concentra la sua attenzione sullo studio dell'avviamento da rete di motori sincroni con rotore massiccio. Per ciascuna applicazione vengono introdotte alcune procedure di calcolo, facenti uso di opportuni modelli numerici basati sul metodo degli elementi finiti, per mezzo delle quali vengono adeguatamente calcolati i parametri dei circuiti equivalenti di macchina, tenendo conto degli effetti legati alle correnti parassite. I modelli numerici sono opportunamente definiti, in modo tale da ridurre al massimo la complessità delle geometrie e il conseguente onere computazionale. I risultati delle procedure innovative qui proposte sono confrontati con i dati provenienti da prove sperimentali sulle macchine oggetto di studio e con analoghi risultati di calcolo dedotti tramite le procedure comunemente utilizzate. Il confronto fra questi dati ha dimostrato che gli approcci di calcolo introdotti in questa tesi permettono di ottenere risultati con un elevato livello di accuratezza e una netta riduzione dell'onere computazionale. The efficient computation of eddy-current effects in medium voltage electric machines is discussed in this dissertation. Two particular cases are considered. Firstly, the effects of shaft eddy-currents on two-pole induction motor performance is addressed, with special focus on the power factor. In the second part of the thesis the start-up calculation of a large synchronous motor with solid rotor is analyzed. For each application a special calculation procedure is introduced. These procedures adopt a set of suitable finite-element models to properly compute the machine equivalent circuit parameters that are mainly influenced by eddy-current-related phenomena. By suitably choosing finite-element models boundary conditions and excitations their geometry is simplified to the maximum possible extent, in order to reduce the computational burden. The results of the new calculation methods are compared with experimental data and with analogous results obtained from commonly-adopted calculation procedures. The comparison proves that the proposed approaches can lead to high accuracy levels with very remarkable computational savings.

Negli ultimi anni, le macchine elettriche ad avvolgimenti concentrati hanno ricevuto sempre più attenzioni principalmente per le loro caratteristiche di un elevato rapporto coppia - peso e basso costo di produzione. Per contro, rispetto alle più diffuse macchine ad avvolgimenti distribuiti, sono note per avere un forte contenuto armonico spaziale che può dare origine a vari inconvenienti tra cui aumento delle perdite e forze radiali indesiderate tra rotore e statore (UMP). Nella presente tesi viene presentato un rigoroso metodo di valutazione del contenuto armonico di un avvolgimento concentrato. Sono analizzate in particolare l'efficacia nel produrre l'armonica fondamentale, le perdite prodotte nei magneti e le forze agenti tra rotore e statore. Infine viene proposto un metodo per l'ottimizzazione del contenuto armonico per ciascuno di questi aspetti e la sua applicazione ad una macchina prototipale. In the last years, fractional slot concentrated winding (FSCW) electric machines have received increasing attention due mainly to their characteristics of high torque-weight ratio and low manufacturing cost. Conversely, if compared with traditional distributed winding machines, FSCW machines are known to produce a high spatial harmonic content, which may lead to several inconveniences as higher losses and undesired unbalanced magnetic pull (UMP) between rotor and stator. In this thesis, a rigorous method for the evaluation of the harmonic content of an FSCW is presented. In particular, a sound analysis is focused on the effectiveness in producing the fundamental harmonic, the arising of magnet losses and UMP. Finally, a method to optimize the harmonic content for each of these aspects is presented along with the application to a prototype machine.

An accurate prediction of medium-voltage induction motor (IM) power factor in rated conditions is important in the design stage to verify the machine compliance with specifications according to international testing standards. Laboratory and industrial experiences suggest that significant errors in full-load power factor calculation can result for two-pole IMs in particular, due to the eddy currents induced in the solid-steel shaft at rated slip. Such eddy currents are responsible for rejecting the main flux into rotor laminated yokes causing an increase in their saturation and, hence, in the required magnetizing current with respect to no-load conditions. This article proposes a method to study the phenomenon through a combination of analytical and simplified Finite Element (FE) calculations as a computationally-efficient alternative to conventional FE simulations. The results of the proposed approach are experimentally assessed by comparison with measurements on a set of built and tested medium-voltage two-pole IMs of different sizes, showing very good accuracy and computational performance.

This paper presents a droop-based dis- tributed control strategy for multi-three-phase machines that provides augmented controllability during power shar- ing transients. The proposed strategy is able to mitigate the mutual interactions among different sets of windings without controlling any sub-space variable, also offering a modular and redundant design. On the contrary, in a centralised configuration sub-spaces would be controlled using the Vector Space Decomposition, but fault tolerance and reliability levels required by the stricter regulations and policies expected in future transportation systems would not be satisfied. The proposed method is analytically com- pared against the state-of-the-art power sharing technique and equivalent models and control design procedures have been derived and considered in the comparison. Uncon- trolled power sharing transients and their effects on mu- tual couplings among isolated sets of windings have been compared against the proposed regulated ones. Experi- mental results on a 22kW nine-phase multi-three-phase synchronous machine rig validate the design procedures showing good agreement with the expected performances.

Solid rotor synchronous motors are widely used in the Oil&Gas industry, because of their mechanical robustness and self-starting capability. The starting performance calculation for these machines is a challenging task. In this paper the specific case of a solid salient pole motor rated 17.5 MW at 1500 rpm is addressed. A simplified but accurate method is proposed and validated experimentally to predict the motor current and torque during direct-on-line (DOL) starting. The proposed methodology combines Finite Element Analysis (FEA) with a lumped-parameter machine model to obtain the best compromise between accuracy and computational efficiency.

The paper is aimed at an analytical prediction of the air gap flux density distribution of a spoke type interior permanent magnet machine, based on its magnetic equivalent circuit (MEC) incorporating the rotor position. The study is initiated by an elaboration of the machine MEC in two particular rotor positions corresponding to the maximum and null magnetic coupling between the stator and rotor which enable the prediction of the direct and quadrature components of the air gap flux density, respectively. The decomposition of the armature magnetic reaction into direct and quadrature components enables the generalization of the prediction of the air gap flux density waveform for any rotor position. The accuracy of the proposed approach is improved taking into account the stator and rotor slotting effects that are analytically incorporated considering complex permeance functions.

In this chapter, high power synchronous motor drives will be addressed. First of all, an overview will be provided of the main technologies and design features which characterize large synchronous machines (section 4.1) and the relevant supplying converters (section 4.2), also taking into due account their field of application. Subsequently, the attention will be placed on the major strategies intended to improve high-power synchronous machine drive fault tolerance (section 4.3), acting on the system-level drive architecture as well as on the design and operation of the individual components (electric motor, converter, control system). Finally, the main diagnostics and condition monitoring techniques for high-power synchronous motor drives will be covered, describing the main methods to detect possible malfunctioning, anomalies and faults in drive operation before they result in serious damages or hazards.

importance in several application fields thanks to various factors. For example, it is often desirable to get rid of gear-boxes between high-speed turbines or compressors and the coupled electric machinery in favour of a direct-drive arrangement for better efficiency, higher reliability and easier maintenance. At the same time, raising the speed of the electric machine is an effective way to reduce its torque, and hence its size and weight, for any given power rating. This especially applies to electric motors and generators to be used in hybrid or electric vehicles and in moreelectric aircrafts, where room and weight restrictions make high power density a crucial design target. The field of high-speed electric machinery is very broad encompassing a large variety of technologies, applications, power ratings and performance requirements. In any case, the design of these machines is particularly delicate because materials and components in them are subject to extraordinary thermal, mechanical and electromagnetic stresses and tend to work close to their physical operating limits. For instance, high rated frequencies cause large magnetic losses in the stator core and eddy-current losses in stator conductors and rotor active parts, resulting in possibly dangerous temperatures; rotor surfaces may overheat also due to air friction losses at high rotational speeds. On the other hand, centrifugal forces induce mechanical stresses in rotating components causing wear, fatigue and possible early failures. Finally, the need to reach very high speeds may cause the rotor to temporarily cross or approach its critical speeds, resulting in possible vibrations and lateral dynamic instability of the whole shaft line. Accurately evaluating all of these aspects is mandatory for a safe design and must require a multi-physics approach due to the close interactions among electromagnetic, thermal, ventilation and mechanical phenomena. The design process is made ever more challenging by the frequent requirement to minimise the machine cost and maximise its power density together with other performance indices, leading to the need for a multi-objective constrained optimisation approach. This implies that hundreds or thousands of designs are to be comparatively explored in search for the optimal solutions and, to make such a wide exploration feasible, computationally-efficient methods need to be used for the analysis of each design. This Special Issue features thirteen peer-reviewed papers which provide some specific technical insights into the general topics and challenges mentioned above regarding the design, analysis and operation of high-speed electric motors and generators for state-ofthe-art and emerging applications. The first paper, ‘Maximisation of Power Density in Permanent Magnet Machines with the Aid of Optimisation Algorithms’, by F. Cupertino et al., clearly addresses the potentials and limits of power density maximisation in high-speed surface permanentmagnet machines for aeronautical use. It emphasises how, as the rated speed grows, the retaining sleeve thickness needed to secure the permanent magnet against centrifugal force grows as well, leading to larger air-gaps and thus posing a limit on the power density increase. An optimisation process, including both electromagnetic 2D finite-element analysis (FEA) simulations and analytical mechanical formulas, is proposed to identify the speed that produces the maximum achievable power density. The power density optimisation of a surface-permanent magnet machine for aeronautical use is also addressed in the second paper, ‘Optimisation Method to Maximise Torque Density of High-Speed Slotless Permanent Magnet Synchronous Machine in Aerospace Applications’, by D. Lee et al. Here the focus is on an outer-rotor machine topology with a slotless stator and a Halbach-array permanent-magnet arrangement. The optimisation approach is different as the speed is treated as a constraint, together with stator copper losses, rotor mechanical stress levels, inner and outer machine radii and core length. The internal machine dimensions, as well as the magnetisation directions of Halbach-array magnetic segments, are taken as design variables to maximise the power density through a 2D FEA-based optimisation. Design optimisation is covered again in the third paper, ‘Magnetic Circuit Designing and Structural Optimisation for a Three Degree-of-freedom Hybrid Magnetic Bearing’, by Z. Xu et al., but this time applied to magnetic bearings as a key component of many high-speed machines. A correct magnetic bearing design, targeting suitable load capacity and stiffness values, is essential to guarantee a satisfactory rotor-dynamics behavior of the high-speed shaft line. The magnetic bearing is analytically modeled through the magnetic equivalent circuit technique so as to speed-up the particle-swarm optimisation process. The optimal design finally selected is then investigated in more detail through 3D FEA simulations. An innovative approach to achieve magnetically-suspended rotors in high-speed machines as an alternative to conventional magnetic bearings is presented in the fourth paper, ‘1 kW/60,000 min−1 Bearingless PM Motor with Combined Winding for Torque and Rotor Suspension’, by D. Dietz et al. The high-speed motor is equipped with a six-phase stator winding. The multiple degrees of freedom offered by multiphase windings are exploited to generate the torque through a conventional field-oriented control and, at the same time, to produce the radial force required for rotor magnetic levitation. The solution is implemented into a prototype and successfully validated through various tests. The multi-disciplinary nature of high-speed motor design is illustrated in the fifth paper, ‘Design of High Speed Interior Permanent Magnet Motor Based on Multi-Physics Fields’, by F. Zhang et al., which presents the design process for a high-speed interior permanent-magnet motor. The need for a multi-physics approach is emphasised, stressing the importance and interdependence of the electromagnetic, structural, rotor-dynamics, heat-transfer and fluid-dynamics analyses which need to be performed in the design of a high-speed machine. An insight into the rotor-dynamics analysis in high-speed machine design is given in the sixth paper, ‘Rotor-Dynamics Modelling and Analysis of High-Speed Permanent Magnet Electrical Machine Rotors’, by Z. Huang and Y. Le. Predicting the natural frequencies associated with the rotor bending modes (especially the first two) is, in fact, essential to ensure that all steady-state operating points are sufficiently far from critical speeds and avoid the occurrence of dangerous vibrations and mechanical failures. The integration of mechanical and electromagnetic calculations in the design of high-speed synchronous reluctance motors is addressed in the seventh paper, ‘Design Methodology for HighSpeed Synchronous Reluctance Machines’, by C. Babetto et al. IET Electr. Power Appl., 2018, Vol. 12 Iss. 8, pp. 1065-1066 © The Institution of Engineering and Technology 2018 1065 The paper proposes a semi-analytical procedure for the optimal design of high-speed synchronous reluctance machines intended to achieve the maximum power density and the minimum torque ripple while keeping centrifugal force stresses within safe margins in all rotor regions. This is mainly achieved by an appropriate dimensioning of rotor flux barriers and tangential rib thickness based on both electromagnetic and mechanical calculations. Synchronous reluctance machines are an example of magnetfree solutions for high-speed motors and generators. The benefits of removing permanent magnets are significant in terms of cost reduction and improved reliability above all, at the expense of a lower power density. However, other kinds of magnet-less topologies are emerging as discussed in the eighth paper, ‘Overview of Magnetless Brushless Machines’, by C.H.T. Lee et al. Some of them are suitable for low-speed high-torque applications only, while others may have a potential in the field of high-speed drives and generation systems. Examples are switched reluctance and flux-switching DC machines thanks to their simple and robust rotor structure. The ninth paper, ‘High-Speed Solid Rotor Induction Motor Design with Improved Efficiency and Decreased Harmonic Effect’, by M.O. Gulbahce and D.A. Kocabas considers what is probably the most common and traditional high-speed machine; the solidrotor induction motor. A new asymmetrical stator winding design, including slots and coil sides of different sizes, is presented to reduce the air-gap armature field space harmonics, which are responsible for eddy-current losses in the solid rotor surface. The solution may be helpful to increase motor efficiency and reduce rotor surface overheating risks. A high-speed induction motor is also considered in the tenth paper, ‘Nine-phase IM for Hybridisation of a Compact Vehicle by Parallel TTR Architecture’, by R.Á. Silva et al. The paper discusses the high-speed motor use for the hybridisation of an existing car to reduce its fuel consumption and polluting emissions. A simple hybrid parallel arrangement is proposed for the car refitting with the original combustion engine driving front wheels and the electric motor used for rear axle propulsion and regenerative breaking. Showing the results of driving test campaigns, the authors prove that significant improvements can be obtained with the simple proposed refitting scheme in terms of vehicle efficiency and reduced pollution. The eleventh paper, ‘Influence of Rotor Endcaps on the Electromagnetic Performance of High-Speed PM Machine’, by A. Al-Timimy et al., investigates high-speed surface-permanent magnet machines focusing on the impact of rotor ferromagnetic endcaps used to guarantee rotor mechanical integrity. The authors show that the presence of endcaps significantly reduces the output torque due to an increase in end leakage fluxes. It is also shown how this performance degradation cannot be predicted using 2D FEA, while it is in excellent agreement with 3D FEA simulation results. The previous paper stresses the importance of using accurate 3D FEA models to capture some parasitic effects that may affect high-speed motor performance. However, for the purpose of design optimisation, the use of 3D FEA models would lead to inacceptable computational burden and simplified (analytical or semi-analytical) models are needed for a fast first-attempt performance prediction. To this end, the twelfth paper, ‘Calculation of Rotor Losses in PM Machines with Retaining Sleeves Using Transfer Matrices’, by J. R. Anglada et al., presents an analytical approach for the fast prediction of eddy-current losses in high-speed surface permanentmagnet machines with a partly-conductive retaining sleeve. The paper proposes a computationally-improved approach to the solution of the Helmholtz's equation in the rotor domain under the hypotheses of neglecting end effects and magnetic saturation. Finally, the last paper, ‘Parameters and Performance Analysis of a Dual Stator Composite Rotor Axial Flux Induction Motor by an Analytical Method’, by C. Hong et al., proposes an analytical method to rapidly compute the performance of high-speed axialflux induction machines to be used, for example, as flywheels for kinetic energy storage devices. As in the previous paper, the Helmholtz's equation is solved in the rotor domain which is modelled according to a multi-layer, multi-slice scheme. The solution is particularly interesting as it takes into account the magnetic saturation as well as end effects. The reliability of the proposed analytical calculation method is assessed by comparison with both 3D FEA and experimental results.

Load-commutated inverters (LCIs) are reliable and efficient drives to supply synchronous motors in high power industrial applications. However, inadequate attention is dedicated to this type of drive especially for higher phase numbers and in the case of faults. It is a proven fact that the fault tolerance of the voltage-source inverter (VSI)-fed multiphase machines is enhanced as the number of phases increases. However, the same principle is not necessarily applicable to the LCI-fed multiphase synchronous machine drives. As it will be shown in this paper, a thyristor failure may cause the drive instability and the situation can be more critical for higher phase numbers. In this paper, open-circuit faults and their consequences are investigated and compared for symmetrical and asymmetrical multiphase LCI-fed drives. Also, a new control strategy is proposed to restore the drive stability in the case of open-circuit faults in LCI-fed drives. Simulation results for a five-phase drive show that the control system can efficiently keep the machine synchronism in the event of phase open-circuit faults.

The complex relative permeance function is a suitable tool to predict the air gap magnetic field in slotted stator electrical machines. In the literature, the complex permeance function is usually identified by means of numerical techniques based on complex techniques such as conformal mapping or subdomain method. In this paper, an alternative approach is proposed. This method is based on solving the magnetostatic Laplace equation over a portion of the slotted air gap by imposing appropriate boundary conditions in the slot opening region. Such boundary condition tries to represent the theoretical trend of the magnetic field divergence near the corner-shaped ferromagnetic regions. Furthermore, the Carter theory for the slot opening consideration is used. A fully analytical formulation for the complex relative permeance function is obtained, and its accuracy is assessed using finite element analysis.

In the literature, the effect of the mismatch due to manufacturing tolerances on PV plant productivity has been investigated under the hypothesis of plant operation in Standard Test Conditions (STC). In this paper, mismatch impacts are evaluated in more realistic terms taking into account various possible operating conditions. Results are illustrated through the study case of a 1 MWp solar park for which module datasheets as well as flash test data are available. The plant production is evaluated assuming operating conditions that comply with the European efficiency standards. It is shown how the effect of a given mismatch on the annual productivity estimation can significantly change depending on the operating conditions.

Synchronous Reluctance Machines (SRM’s) are gaining more and more importance in various fields of application thanks to their known merits like rugged construction, high efficiency, absence of field windings, no or reduced need for permanent magnets. Out of the possible design variants, in this paper SynRel motors with uniform mechanical air-gap and circularly-shaped flux barriers are taken into account and a conformal-mapping approach to their analytical modeling and simulation is proposed. A suitable conformal transformation is introduced to compute the reluctance of each rotor circularly-shaped flux barrier and the result is then used to analytically determine the air-gap flux density distribution and the electromagnetic torque of the machine in arbitrary operating conditions. The accuracy of the methodology proposed is assessed against finite element analysis.

This paper deals with the optimal fault-tolerant operation of a 5-phase permanent magnet (PM) brushless DC (BLDC) drive. In particular, this paper (Part I) proposes the modelization used to optimize the motor current supply to obtain the maximum power in healthy conditions and its analytical solution is presented. In the companion paper (Part II) the drive is considered both in healthy and faulty conditions and the optimization problem is solved numerically. In all cases the solution is compared with the one obtained with a “Maximum Torque Per Ampere” objective.

Nowadays, the pollution caused by the massive use of fossil fuel is a well-known critical issue makes the design of the electrical machines a crucial task because the vast majority of the industrial or household applications are integrated with this kind of technology; hence improving the efficiency of electric motors is expected to result in an extraordinary benefit in terms of energy saving. On the other side, the increasing use of renewable energies (like wind energy, hydropower, tidal energy), combined with the distributed generation concept, call for new electric machine technology and for modern design approaches. This doctoral thesis has been mainly focused on finding analytical procedures to model and analyze some types of electrical machines which are of interest for renewable and distributed generation. The use of analytical approaches is, in some cases, fundamental because numerical methods, mostly based on Finite Element Analysis (FEA), are very inefficient in terms of required time and computation resources. The types of electrical machines considered in this work are various. The attention is first placed on the slotless surface permanent magnet (SPM) topology (with different types of rotor magnetization). Wound-field synchronous generators with both three phase or multiphase stator are then considered. For the various kinds of the machines taken into account, some modeling, design and analysis studies have been conducted in the attempt to fill some gaps in the existing technical literature. In the first part of the thesis the purely analytical modeling of slotless SPM machines is addressed. In the work, it has been considered how the stator slotless design can be combined with different surface permanent magnet rotor topologies. The main efforts of the study have been addressed to the purpose of finding an explicit analytical expression to compute slotless machine torque and no-load back-EMF, covering all the SPM rotor topologies of interest. The subsequent part of the thesis, is about the analytical computation of end-coil leakage inductance in concentric windings. For a good dynamic modeling of machines equipping this kind of winding, it is useful that their mathematical model is implemented and that model parameters are identified with good accuracy. The proposed technique shows a very good accordance compared with a 3D FEA simulation and is also validated though tests conducted on a dedicated experimental set-up. Multiphase machines are attractive for many energy-saving fault-tolerant applications thanks to their higher efficiency and intrinsic resilience to faults. A challenging task in the modeling of multiphase machines for design and simulation is identifying the self and mutual inductances due to leakage fluxes. In this thesis is also presented a novel approach for the leakage inductance determination in multiphase machines based on routine tests combined with very simple 2D magnetostatic FEA simulations. The subsequent part of the thesis is about the design of wound-field synchronous generators specifically required to operate in a distributed generation system where significant fluctuations are known to frequently occur in both voltage and frequency. Design provisions are investigated to improve the generator resilience to these grid disturbances. In the last part of this thesis the attention is focused on multiphase alternators interfaced to DC distribution systems through multiple rectifiers. As a new finding presented as a part of this investigation, it is shown that a short circuit fault occurring on AC/DC rectifier terminals generates a strongly oscillatory behavior with much larger current peaks than could be predicted with conventional models regarding short circuit transients in DC networks.

Direct-drive generators to be used in off-shore wind farms are very large low-speed electric machines which pose remarkable design and manufacturing challenges. Demand forecasts for this kind of machines are urging manufacturers to work out design and technological solutions capable of facilitating series production at competitive costs. This paper presents the development of an interior-permanent magnet generator design and technology aimed at reducing series manufacturing costs while preserving good performance levels. The focus is placed on two of the most critical issues in the machine design and analysis, namely the prediction and reduction of eddy-current losses in stator conductors and in permanent magnets. The proposed design solutions are validated through the construction and testing of a 780 kVA generator prototype conceived for easy scalability to higher power ratings (up to around 2.5 MVA) by core length increase.

This paper deals with the optimal fault-tolerant operation of a 5-phase permanent magnet (PM) brushless DC (BLDC) drive. In particular, the companion paper (Part I) proposes the optimization of the current supply to obtain the maximum power in healthy conditions and its analytical solution. In the present paper (Part II), by constraining the proposed model with healthy and fault specific conditions, the magnitude schedule of the currents in the surviving phases is determined in order to obtain the maximum average power and the optimization problem is solved through a numerical approach. In all cases the solution is compared with the one obtained with a “Maximum Torque Per Ampere” criterion.

This article reports on the design, development, and validation of advanced prototype 2 MVA generation equipment [i.e., naval package (NP)] for a shipboard medium-voltage dc integrated power system (MVDC IPS). The generation equipment is based on an ultrahigh-speed 22,500-r/min 12-phase alternator, which feeds an ac/dc power electronics converter comprising four diode rectifiers and four insulated-gate bipolar transistor (IGBT) choppers. The prototype realization constitutes a follow-up of a previous NP version employing a wound-field 6,300-r/min alternator feeding noncontrolled ac/dc converters. The major technical challenges faced in the design and development of the advanced NP prototype are outlined in this article, taking the previous lower-speed version as a technology reference. The system performance, as determined by the testing campaign, and lessons learned for future studies are addressed.

Load commutated inverters (LCIs) are still widely used for their robustness and reliability in high-power synchronous motor drives, in either single or multiple three-phase configurations. A restriction to their use in high-speed applications is due to the criticalities of thyristor operation at high switching frequencies. The upper frequency limits of LCIs are usually addressed in the existing literature as something independent of the drive architecture. On the contrary, this paper highlights how the maximum frequency that can be safely attained closely relates to the number of LCIs that are used to supply the synchronous motor. In fact, moving from a single to multiple three-phase arrangements is proved to introduce more stringent frequency constraints due to the mutual interaction between stator windings during commutations. Frequency limits for safe operation of single- and multiple-LCI drives are derived in quantitative terms and experimentally assessed on a 2-MW synchronous motor fed by a couple of LCIs. Practical implications of such limits in the design of high-power high-speed drives are finally discussed.

An electric machine equipped with n stator phases is characterized by a set of n × n phase inductances as concerns the stator section. In the case of nonuniform air gap, these inductances vary as the rotor moves. For their numerical determination as functions of the rotor position an accurate computation algorithm is proposed in this paper based on winding function theory. For permeance function identification, a numerically efficient method is employed based on the magnetostatic finite-element analysis of only three appropriately simplified machine models. In the presence of a field circuit, computation of mutual inductances among it and stator phases is also covered with the same approach. An extension of the method to permanent-magnet machines is also presented. Results are assessed on a six-phase synchronous generator prototype equipped with either a salient-pole wound-field rotor or with an interior permanent-magnet rotor.

Among challenges and requirements of on-going electrification process and future transportation systems there is demand for arrangements with both increased fault tolerance and reliability. Next aerospace, power-train and automotive systems exploiting new technologies are delving for new features and functionalities. Multi-three phase arrangements are one of these novel approaches where future implementation of aforementioned applications will benefit from. This paper presents and analyses distributed current control design for asymmetrical split-phase schemes composed by symmetrical three phase sections with even number of phases. The proposed design within the dq0 reference frame in nominal, open and short circuit condition of one three-phase system is compared with the vector space decomposition technique and further validated by mean of Matlab/Simulink ~R simulations.

In questo lavoro di tesi vengono rispettivamente trattati: Un metodo di dimensionamento preliminare basato sul criterio di omotetia dedicato a macchine elettriche asincrone caratterizzate da diverse polarità e dal medesimo sistema di raffreddamento. Un algoritmo ibrido basato sul calcolo agli elementi finiti e analitico per la verifica prestazionale di macchine elettriche a induzione alettate ed esposizione del metodo di ottimizzazione e delle analisi specifiche agli elementi finiti transitori per la definizione delle perdite addizionali negli avvolgimenti statorici e rotorici. Descrizione delle scelte progettuali, del processo produttivo e delle analisi agli elementi finiti di un di un generatore eolico a magneti permanenti interno. In this thesis are respectively treated: A preliminary dimensioning method based on homothetic criteria devoted to induction machines that are characterized by the same polarity and cooling system. A hybrid algorithm based on the finite elements analysis and the analytical formulas that is able to estimate the performance of electrical induction machines furthermore there is described the optimization process and the transient finite elements analysis used to define the additional losses in the stator and rotor windings. Description of the design choices, the production process and the finite element analysis of a wind generator with internal permanent magnets.