Search

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.

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.

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.

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.

A new linear topology is proposed in this paper for rotating electric machines. The design presented consists of an outer permanent-magnet rotor featuring the same structure as a flexible chain or track made of a sequence of ferromagnetic links connected together by hinges. Toothed wheels are used to convert the linear motion of the track-like rotor into a rotary torque to be transferred to the shaft. The stator portion is the same as for a double-sided linear electric machine. The proposed linear topology makes the electric machine suitable for installation in long thin compartments. A perspective application can be envisioned to direct drive wind generators: the linear machine topology makes the generator suitable for installation inside the wind tower, while conventional cylindrical generators need to be fit into the nacelle. The proposed design has been investigated by finite element analysis simulations and has been implemented into a technology demonstrator for experimental validation.

Salient-pole synchronous machines are conventional electric machines which have been widely used and studied over decades. Some aspects of their modeling and analysis, however, still constitute a challenge for designers and require state-of-the-art methodologies to be applied. As an example, this paper addresses a 14-MVA salient-pole shipboard alternator, of which full-scale prototypes are built and tested with different damper cage designs. A time-stepping finite-element (FE) analysis is assessed against test results as a method to predict generator performance for the various damper cage design alternatives. FE simulations are shown to give accurate results even without model tuning if continuous end rings are used to short circuit damper bars. For the design solution with partial end arcs, model tuning is required, instead. For this purpose, a calibration method is proposed and successfully validated, which only requires a set of impedance measurements to be taken on the machine at standstill

Slot leakage field accounts for an important quota of the total leakage flux in electric machines. The computation of unsaturated slot leakage inductances can rely on analytical formulas for simple open slot shapes, while the case of semiclosed slots is more critical and typically requires finite element analysis (FEA) or analytically solving the field over the whole machine domain. This paper sets forth an analytical procedure for computing the leakage field in semiclosed slots of unsaturated electric machines. The method is based on solving Maxwell's equations in the slot domain alone, which is made possible by appropriately defining boundary conditions in the slot opening region. The method proposed is assessed by comparison with FEA. Validations indicate that a good accuracy is achieved by the presented approach in predicting unsaturated leakage inductances, while FEA is required when magnetic saturation effects need to be taken into account.

The stator winding of multi-phase machines can be designed according to different schemes, which differ by either the number or the spatial arrangement of phases. For control and analysis purposes, it is often useful that the stator inductance matrix is reduced to a diagonal form. This allows for the multiphase machine to be split into independently-controllable systems and directly leads to stator harmonic impedance determination. Simple and compact diagonalization algorithms have been established in a companion paper covering all possible multiphase schemes of practical importance with a unified approach, under the hypothesis of uniform air-gap and negligible magnetic saturation. In this paper, the theory and algorithms proposed for inductance matrix diagonalizations are illustrated and validated by applying them on a on several multi-phase winding schemes implemented in a prototype machine with a reconfigurable stator winding.

State-of-the-art ships focus on enhancing vessel performance, lower emissions, optimize space on board and save fuel. This paper describes an innovative marine solution in which the same electric machine can be operated as a conventional shaft generator and also as a variable speed propulsion motors. The distinctive design (patent pending) consists of a special synchronous machine provided with two separate exciters (one AC and the second DC) each capable to energize the same main rotor field through two independent rotating rectifiers. Whilst the conventional PTO/PTI systems may use the synchronous machine in a very narrow speed range - the presence of the independent AC exciter allows to energize the main field from zero rpm. The propeller can therefore be fully electrically operated. The dual-excitation arrangement is described in the paper highlighting its benefits compared to the conventional design. Numerical simulations are presented to assess the possible electromagnetic interactions between the two coaxial rotating excitation systems. Extensive testing campaign validated the design expectations.

The ongoing gas industry expansion calls for quick and strong advances in turbomachinery-coupled highspeed electric motors. Very high efficiency, dynamic performance and reliability are among the main requirements to be met at competitive costs by electric machine suppliers. This paper reports on an industrial R&D project aimed at finding the best compromise between performance targets and production cost reduction through the realization of a 640-kW 10.000-rpm PM motor prototype. The preliminary and optimization design of the motor have been covered in a companion paper (Part I). Here the manufacturing and testing of the prototype are covered. The main technology challenges encountered throughout the manufacturing process are addressed. Furthermore, the paper reports on the testing campaign conducted on the motor under converter supply and in different load and speed conditions. The prototype testing was successful and proved the possibility to reach and exceed a 98% motor efficiency target with affordable costs and available technologies.

The ongoing gas industry expansion calls for quick and strong advances in turbomachinery-coupled highspeed electric motors. Very high efficiency, dynamic performance and reliability are among the main requirements to be met at competitive costs by electric machine suppliers. This paper reports on an industrial R&D project aimed at finding the best compromise between performance targets and production cost reduction. The development of a 640-kW 10.000-rpm PM motor prototype with slotless stator and Halbach-array magnetically-levitated PM rotor is discussed, including in particular the basic technology selection and design optimization processes. In a companion paper (Part II), prototype manufacturing and testing will be reported to assess the technology and design solutions adopted.

A promising solution to improve tomorrow's all-electric ship onboard power system design and performance relies on moving from 50-Hz or 60-Hz AC distribution to a Medium-Voltage DC (MVDC) grid. This paper provides a review of presently-available and under-development technologies for electric generators to be used in perspective MVDC shipboard systems. The inspiring reason for the survey lies in the fact that an MVDC grid design removes the constraints usually imposed on shipboard generation group design and opens the way for a wide variety of non-conventional machine alternatives. In combination of high-speed gas turbines, that can be employed as prime movers, several electric machine topologies, not yet used for ship applications, can be selected to feed MVDC onboard systems. Along with a review of possible generator types suitable for such use, the paper critically reviews some emerging technologies for their fault-tolerance enhancement through conventional and innovative split-phase architectures.

Many shipboard loads requiring high torques at very low speeds, like capstans, bow thrusters, rudders, stabilizing fins, etc., are presently driven by hydrostatic actuators. This paper presents an innovative inverter-fed electric linear motor prototype conceived to replace hydrostatic drives with large expected benefits in compactness, maintenance, efficiency and reliability. The linear motor prototype has been designed through special optimization algorithms based on finite element analysis to maximize the force density. The electromagnetic and mechanical structure has been also selected to achieve superior reliability by means of structural simplicity, robustness and redundancy. Along with design criteria, the paper outlines the prototype technology and the construction issues and provides some testing results collected during the experimental performance assessment.

Permanent magnet synchronous machines, especially if designed with open stator slots, may suffer from important cogging torque issues. A special magnetic wedge design has been presented in a companion paper to cope with this problem. The design of such wedge is herein processed through a genetic algorithm in order to find the optimal configuration, which leads to cogging torque minimization. An optimal design configuration is found and compared to non-optimal ones to quantify the benefits that can be achieved by genetic optimization.

Vehicular traction motors are usually designed to operate over a wide speed range and require important flux weakening capabilities at high speeds. This paper presents and innovative design intended to endow an interior permanent magnet (IPM) motor with an intrinsic capability of reducing its own rotor flux at high speeds. The design does not involve any additional current source nor control systems for rotor flux regulation. It relies on a purely mechanical device that establishes a partial magnetic short circuit between rotor permanent magnets. The device is able to self-activate due to centrifugal force when the speed exceeds a given threshold that can be fixed by the designer. The design features of the proposed IPM motor are illustrated in the paper and its static and dynamic performance prediction through finite element analysis (FEA) methods is presented. A prototype construction is presently in progress to experimentally validate the solution.

Inverter-fed Induction motor performance is known to be noticeably affected by the supply voltage waveform. Nevertheless, the prediction of how supply voltage harmonics impact on motor operation in terms of losses, noise and thermal behavior is still a problem that can be hardly approached through simulations and analysis, therefore experimental data constitute an important source for information. In this paper the test results of two high-power induction-motor drives are presented showing their performance under different voltage waveforms. Among the other things, it is shown how low-frequency distortions in the supply voltage may have considerably stronger impacts on motor behavior than PWM harmonics, although the overall THD under PWM supply is higher.

In electric permanent-magnet motors for traction applications, an open slot stator design is to be used when the winding is composed of flat-turn coils. The open slot design, combined with rotor permanent magnets, can produce remarkable cogging torque effects. This paper describes a magnetic wedge design which allows for a significant cogging torque reduction and, at the same time, allows for a fine adjustment of stator phase inductance values. The new wedge concept is applied in the paper to a 12-slot surface permanent-magnet motor. Finite-element analysis is used to study the wedge design effects on motor phase inductance and cogging torque amplitude.

It is known from the literature that in electric machines with open stator slots, the air-gap flux partly enters the slot openings and induces eddy currents in the conductors placed closest to the air gap, possibly causing local overheating issues. For the study of such phenomenon, time-stepping finite-element analysis (TSFEA) is employed by many authors. This paper presents an alternative approach based on a set of time-harmonic finite-element analysis (THFEA) simulations. The proposed method can be applied to surface permanent-magnet machines and offers the advantage of a shorter computation time with respect to THFEA. It is also more suitable for being integrated into automatic design optimization programs. The accuracy of the approach is assessed by comparing stator eddy-current losses independently computed by THFEA and TSFEA for different machine geometries and operating conditions. Results obtained in the two ways are shown to be in good accordance.

Permanent-magnet synchronous motors are widely used in vehicular traction applications due to their high efficiency and torque density. A critical point of their operation is the flux weakening needed at high speed, which usually implies injecting a d-axis demagnetizing current by the inverter. This paper presents a critical survey of the alternative approaches found in the literature and patent databases to enhance flux-weakening performance by acting on motor mechanical and electromagnetic design.

The problem of accurately computing rotor eddy-current losses in synchronous machines subject to non-sinusoidal phase currents is usually approached with Time-Stepping Finite-Element Analysis (TSFEA), where rotor motion and armature current distortion effects are naturally included. The drawbacks of TSFEA are the long computation times and the need to wait for steady-state conditions to extract results. Such features make TSFEA unsuitable for integration into automatic design optimization programs. This paper illustrates how the problem can be alternatively solved by Time-Harmonic Finite-Element Analysis (THFEA) on a set of modified machine models conceived so that rotor motion and current distortion effects can be accounted for. As an illustrative example, the proposed method is applied to a Surface Permanent-Magnet (SPM) generator whose electromagnetic design is being processed through genetic optimization algorithms. Eddy-current losses are independently computed by both TSFEA and TFFEA in different generator operating conditions and design configurations showing that results are in good accordance. Furthermore, the proposed approach takes much smaller simulation times and can be easily incorporated into the overall optimization process.

Over the last few decades an increasing sensitivity has been spreading towards energy saving technologies. In particular, as far as electric drives are concerned, a strong demand for high efficiency motors has been raising in many industry sectors. This paper describes the development of an innovative high-efficiency solution for highspeed motors targeted for industrial turbomachinery applications in the 500–1000 kW power range at 10000–15000 rpm The technology presented features a slotless stator and a magnetically-levitated Halbach permanent-magnet rotor. A full-scale motor prototype is being manufactured for the assessment of the proposed technology. Design optimization techniques, based on genetic algorithms and multi-disciplinary analysis tools, have been employed in the prototype design stage to maximize motor efficiency and performance figures. A multilevel cascaded-cell converter with optimized voltage output is under parallel development for motor supply. Prototype full-power and full-speed qualification tests are planned to take place within the current year to demonstrate the effectiveness of the design and technology presented in this paper.

The on-load factory testing of high-power electric machines may be a challenge due to the large mechanical equipment required to drive or load them. In a companion paper, a regenerative full-load testing strategy has been proposed, where the power flows in a closed loop between the electric machine and a dual-stage converter so that only the power corresponding to system losses needs to be supplied by the test facility and no mechanical equipment is needed. In this paper, the implementation of such testing strategy on a 780-kW 14-r/min permanent-magnet alternator with fractional-slot concentrated stator winding is described. Test results are presented and compared with the predictions made in Part I, showing a satisfactory accordance between theoretical and experimental results.

Multiphase machines are AC machines characterized by a stator winding composed of a generic number n of phases. In today’s electric drive and power generation technology, multiphase machines play an important role for the benefits they bring compared to traditional three-phase ones (Levi, 2008; Levi et al., 2007). Such benefits have been widely highlighted by existing literature (Levi et al., 2007) and are mainly related to: an increased fault tolerance; higher power ratings achieved through power segmentation; enhanced performance in terms of efficiency and torque ripple. The use of multiphase machines is spreading both in small-power safety-critical applications as well as in very high-power industrial drives (Tessarolo et al., 2010), in electric-propulsion drives (Castellan et al., 2007) and power generation systems (Sulligoi et al., 2010). Regardless of whether they are used as motors or generators, multiphase electrical machines are almost always connected to power electronics systems (inverters for motors, rectifiers for generators), which interface them to the electric grid (Sulligoi et al., 2010; Castellan et al. 2008). Therefore, if the dynamic behaviour of a multiphase machine is to be predicted through simulations in the design and development stage, it is essential to do this by means of system-level simulations, where not only the electric machine is included, but also the power electronics and control systems that interact with it. Such a system-level simulation approach makes it difficult to use Finite-Element (FE) methods due to the complexity of the domain to be modelled and to the well-known computational heaviness of time-stepping FE simulations. Conversely, lumped-parameter models, to be implemented in the Matlab/Simulink environment, may provide designers with a powerful mean of analysis and investigation, provided that all the system components to be studied are modelled with an adequate level of accuracy and completeness. As concerns power electronics systems usually interfaced to multiphase machines, whether operating as motors or generators, the Matlab/Simulink environment offers wide and complete libraries where the designer can find reliable pre-defined blocks (for electronic switches, snubbers, diodes, etc.) to be used in building the application-related apparatus models. The same pertains to control and regulation blocks, which can be built up directly based on their transfer functions and logics. A possible criticality can be encountered when it comes to build the multiphase machine model. In fact, no predefined blocks are presently available in the Matlab/Simulink environment for this purpose. On the other side, building a dedicated user-defined machine

For several reasons, relating to reliability, power segmentation and performance enhancement, it may be convenient for an induction motor to be supplied by two (if not more) three-phase inverters. The most common design solution for this purpose is the split-phase one, where the stator wining is split into two three-phase sections displaced by 30 electrical degrees apart, each fed by an inverter. An alternative possibility is explored in this paper where the two motor windings are not displaced and they are suitable for being fed by in-phase supply voltage systems. The solution has been implemented in a 13 MW induction motor drive. Its performance is reported in this paper based on measurements taken during system testing.

Over the last few decades, the production of liquefied natural gas (LNG) has been pushing the development of electric drives with increasingly high power ratings, up to several tens of megawatts. A consolidated technology in this field entails dual-star two-pole synchronous motors fed by load-commutated inverters with supply frequencies between 50 and 80 Hz. This paper presents a novel drive concept for very high-power and high-performance LNG applications based on a 45-MW four-pole 100-Hz quadruple-star synchronous motor supplied by four pulsewidth modulation multilevel voltage source inverters. The genesis, development, and industrial implementation of this new design concept are outlined. Full-load drive system testing and commissioning results are presented which successfully validate the proposed solution.

Split-phase machines are multi-phase machines whose stator phases are split into two or more three-phase windings. When all windings are equally loaded, the split-phase topology produces a better air-gap field than the usual three-phase one, with beneficial effects on rotor additional losses and torque quality. This does not occur when the windings are differently loaded, as typically occurs is split-phase motors with one or more out-of-service supplying inverters. This paper presents an analytical approach to accurately predict the air-gap field distortion in split-phase mechines with unequally-loaded windings and in presence of possible phase current distortion. Results obtained with the proposed method are assessed by comparison with FE simulations.

In the design of electric machinery concentric coil windings are often used where each end-turn consists of straight conductors connected by circular arcs. This paper proposes an analytical approach to the evaluation of end-turn leakage inductance for this winding geometry. The technique employs an asymptotical series expansion of Neumann integrals and, unlike existing methods, does not require to approximate the end-turn shape by a succession of elementary straight filaments. The computation formulas obtained have the advantage of a fast convergence speed and are easy to implement for practical use. Their correctness is assessed against direct numerical integration results.

The adoption of multi-phase stator topologies in electrical machine design brings well-known advantages in terms of power segmentation, reliability and performance. In addition to these functional benefits, which are independent of machine size, this paper highlights how the use of more than three phases can lead to construction advantages when applied to electric machines of large size. The advantages relate to the winding manufacturing technology, which can be retained similar to that of small machines in presence of a multi-phase solution, while a traditional three-phase one would imply a more costly Roebel bar construction. The point is justified in terms of machine sizing equations and is assessed referring to some built high-power multi-phase machines for industrial and ship propulsion use.

DC distribution networks are often employed in isolated power systems like in marine and aeronautical applications. An interesting alternative to DC machines for the supply of DC grids is constituted by a three-phase alternator feeding an AC/DC rectifier. In this paper, alternative solutions are investigated where the AC alternator is equipped with more (N) three-phase windings and feeds the DC load through more (N) cascade-connected rectifiers. The advantages of this solution with respect to the conventional three-phase topology are highlighted along with their dependence on the number (N) of the alternator windings. For this purpose, test results are reported and discussed on a prototype generator whose stator coils can be differently connected to obtained one, two, three or four windings. The major benefits (in terms of DC output current quality and field current ripple) are found when passing from the three-phase solution to the dual winding topology; conversely, relatively little advantage is gained by increasing the number of stator windings above two.

In high-power electric drives the use of multiple three-phase stator windings, supplied by voltage-source PWM inverters, is a design strategy for power rating, performance and reliability enhancement. Possible drawbacks, highlighted in the literature, come from the high-frequency circulation currents that originate in motor phases due to PWM switching. When a synchronous machine is used as the motor, low-frequency current harmonics can arise too, as a consequence of the non-uniform air-gap permeance. This paper discusses the point focusing on a PWM-fed 22-MW salient-pole synchronous motor with dual three-phase stator arrangement conceived for electric ship propulsion. The machine behavior under voltage-source supply is studied through time-stepping Finite-Element Analysis and predicts high internally-generated circulating currents, with a dominant 5th harmonic component. Before its application to the 22-MW motor study, the time-stepping FE methodology for current harmonic prediction is assessed against measurements on a real synchronous machine and on a laboratory prototype.

Salient-pole synchronous generators are conventional electric machines which have been widely studied over decades. Nevertheless, some aspects still constitute a challenge for designers and need to be approached with state-of-the-art modeling and computation techniques. An example is illustrated in this paper dealing with a 14 MVA synchronous generator. Different design solutions for its damper cage are studied and implemented on full-scale machine prototypes. Transient finite-element analyses are used to predict the influence of damper design on machine sub-transient reactances. Comparisons are presented between experimental results and finite element simulations and a physical interpretation is provided of the relationship found between damper structure and machine sub-transient reactances.

Electric motors equipped with multiple three-phase windings are often used for the advantages they offer in terms of reliability, performance, and inverter power segmentation. When the windings are fed by independent voltage-source inverters (VSIs), circulation harmonic currents can occur in stator phases. If the motor is an induction machine, circulation currents are known to originate form transient imbalances in the applied voltages due to inverter switching. This paper investigates the problem when a synchronous machine is used and shows how additional (and possibly major) sources for circulation currents can arise in this case (even with a round-rotor design) due to the nonsinusoidal air-gap flux distribution. The phenomenon is illustrated for a 45-MW quadruple three-phase synchronous motor supplied by four medium-voltage (MV) multilevel VSIs. Its circulation currents are predicted with two alternative methods, i.e., analytically and from time-stepping finite-element simulation. The results obtained in both ways are shown to well match measurement results collected on the actual motor during full-load system testing.

Multi-phase machines are of increasing importance in today’s electric drives and generation systems. While in three-phase machines stator leakage flux phenomena are usually described with a single parameter, a higher number of independent leakage inductances is required as the number of phases increases. The impact of leakage inductances on multi-phase machine performance, especially in PWM inverter-supplied motors, is proved by many studies. Hence the need originates for the electromagnetic designer to have easy-to-use but sufficiently accurate calculation formulas giving the complete set of mutual inductance in a multi-phase machine. This paper aims at providing such formulas making abstraction of the particular phase arrangement in the stator winding and under the hypothesis of equal phase belts. The computation algorithms provided are assessed against measurements on actual machines wherever possible; otherwise accurate Finite Element (FE) analyses are used for validation.

In the field of poly-phase electric machines, a variety of stator winding alternatives is possible in terms of phase physical arrangements. The specific winding scheme, in addition to the number of phases, needs to be taken into account as far as the machine dynamic modeling is concerned. In particular, the Vector-Space Decomposition (VSD) method, which is widely used for n-phase machine analysis, requires different transformation matrices to be used depending on the displacement angle between successive phases, as discussed in a companion work. The explicit forms of such matrices are illustrated in this paper, taking the VSD of a five-phase motor, for different possible phase arrangements, as a numeric case study and validation example.

The possibility of equipping the stator winding of an electric machine with more than three phases is an increasingly important strategy in today's drive design for its beneficial implications in terms of power rating, reliability enhancement and design degrees-of-freedom. Vector-Space Decomposition (VSD) is an important technique recently introduced for poly-phase machine modeling and analysis. A consolidated VSD technique exists covering symmetrical poly-phase machines, where the n stator phases are displaced by 360/n electrical degrees apart; conversely, other poly-phase configurations cannot be reduced to such a scheme and require some more dedicated treatment. Particularly, this paper shows how the VSD method can be effectively extended to poly-phase windings where a half phase progression (equal to 180/n electrical degrees) is used, as typically occurs in split-phase machines.

In this paper an analytical-numeric method for stator end-coil leakage inductance computation is presented based on the solution of Neumann integrals. The method extends an approach already introduced for three-phase machines to the case of an arbitrary phase number and arrangement. Moreover, some original contributions are proposed to enhance the calculation of end-coil self inductances, for which Neumann integrals cannot be directly used. Measurements on three machines with different numbers of poles and phases are presented to assess the accuracy of the method.

In conventional three-phase electric machines, coil pitch ratio is usually selected around 5/6 so as to minimize the 5 th and 7 th flux linkage harmonics. In the electromagnetic deign of inverter-fed multi-phase motors different criteria should be adopted, depending on motor and inverter typology and possibly on the number of phases too. In this paper the main design issues involved in this choice are discussed showing that the final decision has to be the best compromise among various design needs and objectives.