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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.

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.

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.

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 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.

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.

In permanent-magnet motors used for vehicle traction application a well-known problem is the need to weaken the flux at high speeds. This is generally done by injecting a suitable demagnetizing current along the d axis. An alternative method has been recently proposed by the authors based on an Interior-Permanent-Magnet (IPM) synchronous motor equipped with a mechanical device which activates at high speeds by centrifugal force causing a significant reduction in the permanent magnet flux with no need for injecting d-axis stator currents. The performance of a road vehicle traction drive employing this machine under a maximum-torque-per-ampere control is investigated in this paper assuming a typical driving cycle. It is shown that remarkable benefits can be achieved in terms of energy saving, overload capability enhancement and speed range extension potential.

This work shows experimental evidences of abnormal end-ring torsional vibrations in fabricated cage induction motors and their role as root cause of bar breakages. A case-study of cage torsional resonances in railway traction motors is presented, which led to bar breakages with a peculiar failure pattern. A particular cage resonant mode (the second natural mode with both the rings vibrating in phase-opposition with respect to the rotor iron stack) is shown as responsible of excessive bar bending, fatigue, and bar fractures, through direct measurements of ring-stack oscillations in working motors. Cage torsions were excited by large harmonic torques due to inverter supply in the case considered. The mechanism of resonance-induced bar solicitations and fatigue is discussed, and the difference of failure pattern with respect to more common bar breakage mechanisms (overloads, heavy start-ups, etc.) is evidenced. Some theoretical models for modal frequency calculation are presented, which may help to identify the origin of vibrations and the fault root-cause in faulted drives. Dynamic simulations of the drive have been carried out for airgap torque calculation and bar length optimization.

Slotless permanent magnet machines are attractive in some modern drive and power generation fields, where the cogging torque and additional losses need to be minimized or removed. The stator slotless design can be combined with different surface permanent magnet (SPM) rotor topologies. In this paper, explicit analytical expressions are derived to analytically compute the slotless machine torque and no-load back Electro-Motive Force in the case of segmented SPM rotor with parallel, radial, or Halbach-array magnetization patterns. The expressions are found by solving the magnetic field due to the slotless stator winding and to the permanent magnet blocks; the latter modeled through equivalent surface current densities. The accuracy of the method is successfully assessed by comparison with the finite-element analys is (FEA). The proposed formulas are an effective alternative to the FEA to quickly compare different design solutions as well as to optimize them. Application examples are provided in which the presented method is adopted to define the machine cross section that maximizes the torque density.

The field circuit of wound-rotor synchronous machines can be supplied in various ways, including brushed and brushless excitation systems. Regardless of whether the synchronous machine is equipped with a rotating exciter or not, the external excitation power source necessarily injects some high-frequency harmonics into the field circuit. The frequency of these harmonics can be particularly large if Pulse-Width-Modulated (PWM) converters are used as excitation voltage supply. This paper investigates the possibility that the excitation supply harmonics can produce electrical resonance phenomena resulting in possibly harmful overvoltages. The investigation is carried out through different approaches: by experiments on a low-voltage salient-pole 20-kVA laboratory prototype; by experiments on a large (multi-MW) round-rotor synchronous motor; through an analytical model. The results obtained from the analytical model are shown to well match measurements provided that a resistive parameter (affected by eddy-current loss effects) is properly calibrated. Another important conclusion is that the possible overvoltages due to electrical resonance would occur between internal points of the field (not between its terminals) and would not be thereby damped by conventional field circuit protection devices.

The study of electric machines subject to rotor eccentricity is usually performed using a winding and permeance function approach. In a companion paper, an alternative method is investigated based on conformal mapping. The geometry of a an eccentric round-rotor machine has been mapped into a uniform air-gap topology by means of Moebius conformal map. In this paper, the approach is used to derive explicit formulas to compute the unbalanced magnetic pull of the eccentric machine. It shown how, unlike conventional winding-function theory, the proposed method yields closed-form formulas which do not involve series expansions nor require numerical integrations and are thus particularly fit for implementation into fast simulation programs. Such expressions are assessed by comparison with Finite Element Analysis (FEA).

Cogging torque is a known disadvantage of surface permanent magnet (SPM) machines, especially if equipped with open stator slots. A possible strategy to reduce the cogging torque is to use suitably designed non-isotropic magnetic wedges. The design of such wedges for cogging torque minimization requires a genetic optimization approach. This has been done in previous works using Finite Element Analysis (FEA) simulations to predict the cogging torque for each machine design being explored, leading to a very time consuming optimization process. In this paper, the same optimization is performed using an analytical method to compute the cogging torque. It is shown that the optimization process, based on the analytical formula for cogging torque prediction, leads to the same result as the FEA-based procedure, but with a significant reduction in the computational burden.

Fractional slot windings based on concentrated tooth coils are frequently used in today's electric machines. A drawback of these windings is the occurrence of large space harmonics in the air gap, which can cause torque ripple issues and rotor additional losses. Various provisions have been proposed in the literature to improve the air-gap harmonic content due to concentrated-winding armature reaction. This paper proposes an enhancement based on a multiple-layer design characterized by tooth coils with different number of turns. The improvement is proposed for 12-slot 10-pole permanent magnet machines but can be generalized to other possible topologies. It is shown that the proposed approach leads to the same air-gap field harmonic content as for a dual-three-phase dual-layer winding with all space harmonics reduced by the same coefficient (equal to 0.928). Pros and contras of the proposed design with respect to other existing improved solutions are highlighted in the paper. Comparisons and assessments are provided based on Finite Element Analysis (FEA).

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

Round conductive bars embedded in semi-closed slots are frequently used in the construction of electric machinery, for instance in the squirrel cage of induction motors. Frequently, such as in the study of induction motor steady-state performance at rated slip, it is useful to estimate the slot leakage inductance of these conductors under the hypothesis of no eddy currents and no magnetic saturation. This is usually done through simple approximated analytical formulas available in the literature. In this paper, an improved explicit ready-to-use leakage inductance expression for circular bars embedded in semi-closed slots is derived by solving Poisson's equation in the slot domain. The precision of the proposed formulation is assessed against Finite Element Analysis (FEA) for various slot geometries and is shown to always give very accurate results, with errors below 2%. Conversely, approximated simplified formulas available from the literature are demonstrated to possibly give large errors, which exceed 20% for some explored slot geometries.

Split-phase synchronous motors equipped with multiple-stator three-phase windings, each supplied by a load-commutated inverter, play an important role in today's very high power electrical-drive applications. A criticality of these systems is the possibility that commutations occur in different motor windings simultaneously. The resulting electromagnetic transient depends on the magnetic coupling of motor phases among them and with rotor circuits. In this paper, a model to describe this phenomenon is presented along with some dedicated tests, conducted on various split-phase configurations, to assess the model validity.

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 are established in this paper covering all possible multi-phase schemes of practical importance with a unified approach, under the hypothesis of uniform air-gap and negligible magnetic saturation. In a companion paper, All the proposed equations are experimentally assessed though dedicated tests on several multiphase winding schemes.

This paper (Part II), together with a companion one (Part I), deals with the fault-tolerant operation of a 5-phase permanent magnet (PM) brushless DC (BLDC) drive. In particular, this paper takes advantage of the drive model formulated in Part I to determine automatically the motor current supply, namely scheduling and magnitude of the currents for the surviving phases, in the cases of a drive with two and three open phase faults. Maximum magnitude of the supply currents is also determined by equating the rms value of current in the most solicited phase to the nominal rms value Open faults of adjacent and non-adjacent phases are studied and for each fault location, besides determining the motor current supply, the torque developed by the motor is calculated. The results indicate that the reduction in the motor torque as well as the extent of the torque ripple is depending on the relative location of the faults.

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.

Spoke-type interior permanent magnet (IPM) machines are an attractive topology for high performance electric motors, especially designed for vehicle traction applications. In this paper, a special design for a spoke-type IPM motor is presented to enhance motor flux-weakening capability in the operation over a wide speed range. The proposed design consists of a simple and robust mechanical device that includes radially-displaceable rotor yokes, connected to the shaft by means of springs. At high speed, the centrifugal force prevails over the elastic one due to springs, causing the mobile yokes to displace radially and to establish a partial magnetic short circuit between permanent magnets. This increases permanent magnet leakage flux and consequently reduces the air-gap field. As a result, a mechanical flux weakening effect is achieved at high speed, which helps significantly reduce the demagnetizing d-axis current to be injected by the inverter, along with the related copper losses and demagnetization issues. The proposed design is investigated in the paper using an analytical model whose parameters are computed by finite-element analysis (FEA). The effectiveness of the solution being set forth is successfully proven by some testing on a laboratory prototype. Experimental results are compared to analytical predictions showing a satisfactory accordance.

The accurate determination of stator leakage inductances is presently an open issue in the analysis and testing of multi- phase electric machines. Calculation methods are available, which involve complicated and often poorly precise three-dimensional (3-D) analyses. Experimental determination techniques, using measurements on the wound stator with the rotor removed, are also possible, but quite impractical, as they need to be performed during machine manufacturing or require rotor withdrawal. In this paper, a new approach is proposed to determine all the stator self- and mutual leakage inductances of a nine-phase synchronous machine based on a minimal set (a couple) of magnetostatic finite- element (FE) simulations, and on the measurements taken during no-load and short-circuit routine tests. The procedure is applied to a wound-field salient pole nine-phase synchronous generator for validation, showing good accordance with the results obtained from measurements on the machine with the rotor removed. A discussion is also proposed on the possibility to extend the presented procedure to other multiphase topologies

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.

Induction motors are presently the most widespread kind of electric machinery used for industrial and general purpose applications. The parameters of their equivalent circuit model can be determined with a combination of magneto-static and time-harmonic Finite Element Analysis (FEA) simulations. This paper investigates how equivalent circuit parameters depend on current and frequency variations. Calculation results obtained from FEA combined with analytical formulas are experimentally assessed by measurements on a small sample motor used for household appliances.

One of the key tasks in the modeling of electric machines is the analytical expression of self and mutual phase inductances as a function of the rotor position. Winding and permeance functions are a powerful tool for this purpose. This paper investigates the expression of the air-gap permeance function for a round rotor machine subject to pronounced eccentricity. An expression is sought in the form of a Fourier series expansion in order to make it suitable for modeling purposes. Although derived through non trivial algebraic passages, the final expression proposed is proved to be accurate and computationally efficient as the theoretically infinite sums involved in it exhibit a very rapid convergence to their asymptotical values.

Load-commutated inverters (LCIs) in conjunction with the wound-field synchronous motors (WFSMs) are one of the most applicable drive topologies, and in some cases the only choice, in high-power medium-voltage applications. In this paper, a novel strategy is presented for field oriented control (FOC) of these drives in stator flux coordinates. Considering rotor saliency, independent flux and speed control is carried out to achieve maximum torque to ampere ratio. Furthermore, the proposed strategy provides a simple way to design the PI controllers' parameters according to the phase-controlled DC machine and WFSM drive analogy. Simulation studies are presented to verify the performance of the total proposed control scheme. Comparisons show the superiority of the proposed method over the conventional drive technologies for LCI-fed WFSM drives.

DC power generation for MVDC grids is in the easiest way realized by a cascade connection of a synchronous generator and a diode rectifier. Due to large peak fault currents, including oscillating transient components, it is important to know in depth the short circuit behaviour of diode rectifiers, mainly during the first moments after the fault. This is helpful to be able to perform a suitable sizing of the system and its protections. In order to obtain an accurate estimation of the transient short circuit current, models for software simulation of 6-pulse and 12-pulse DC generator systems have been implemented. This paper shows the validation of such models by the comparison of simulation results with experimental tests.

In many applications, electric machines are designed for operation over a wide speed range and require important flux weakening capabilities at high speeds. This paper presents and innovative rotor mechanical design that endows an interior permanent magnet (IPM) machine with an intrinsic capability of reducing its own rotor flux at high speeds. Unlike most state-of-the-art flux-weakening techniques, the design does not involve any additional current source nor demagnetization currents to be injected into stator circuits. 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 by centrifugal force when the speed exceeds a given threshold that can be fixed by design. The paper presents criteria for dimensioning the new IPM rotor and a mathematical model to predict its performance. The proposed solution is implemented into a motor prototype whose manufacturing and testing are discussed for experimental validation.

The ships have long been an easy play-ground for hydraulic technologies. When high torques and low speeds are demanded on board ship, both technical and traditional reasons usually lead to hydraulic solutions. In fact, the conventional electrical machines are better suited for low-torque, high-speed applications, and coupling gears must be used to reduce the speed. However, oil-free direct-drive alternatives can be conceived, by new concepts of low-speed permanent-magnet actuators fed by low-frequency inverters. This paper shows the experimental results obtained with a linear permanent-magnet motor designed for direct coupling to rudder stocks or stabilizing fins. The prototype weights 700kg and features a thrust-to-weight ratio of 5. Even better results can be obtained by using composite materials for the machine frame. The machine features a full structural redundancy and inherent fault tolerance, thanks to the modular construction. The present research has been carried out in the framework of the Italian Defence's Research National Program.

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.

Synchronous reluctance (SynRel) motors with rotor flux barriers are gaining increasing attractiveness in automotive applications thanks to their cheap, rugged and magnet-free rotor construction. When equipped with a split-phase stator winding and supplied from multiple inverters, these machines can exhibit further merits as traction motors in regard to enhanced fault tolerance compared to conventional three-phase solutions. Since SynRel motors are usually designed through iterative optimization techniques, it is highly desirable to have accurate and fast methods to predict their performance without the need for time-consuming finite element analysis (FEA) simulations. An analytical procedure is set forth in this paper to analytically model and simulate a SynRel motor with a split-phase stator winding through a magnetic equivalent circuit (MEC) technique. MEC parameters are computed from analytical formulas describing the air-gap magneto-motive force distribution and the magnetic field inside flux barriers. As an output, the air-gap flux density of the SynRel motor can be computed through the presented technique at any operating point. Results are positively assessed by comparison with FEA simulation on a sample SynRel motor including magnetic saturation effects.

The increasing complexity of electric grids including renewable sources causes significant impacts in terms of power quality. This implies that new generators are to be designed so that they can withstand larger and larger disturbances in the amplitude and frequency of the grid to which they are connected. This purpose can be attained by appropriate provisions in the generator electromagnetic design. In this paper, some case studies will be presented comparing the transient stability performance of possible alternative electromagnetic designs of a 4.7-MVA 6-kV for power station based on an Organic Rankine Cycle. The fault-tolerance capabilities of the alternative design solutions are assessed through lumped-parameter simulations in the Matlab-Simulink environment used to predict machine compliance with typical transient functional requirements.

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.

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.

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.

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.

This paper reports on the design, development and validation of an advanced prototype 2MVA generation equipment (Naval Package) for a shipboard Medium-Voltage DC integrated power system. The generation equipment is based on a ultra-high-speed 22500-rpm 12-phase alternator which feeds an AC/DC power electronics converter composed of four diode rectifiers and four IGBT choppers. The prototype realization constitutes a follow-up of a previous Naval Package version employing a wound-field 6300-rpm alternator feeding non-controlled AC/DC converters. The major technical challenges faced in the design and development of the advanced Naval Package prototype are outlined in this paper taking the previous lower-speed version as a technology reference. The system performance highlighted by the testing campaign are finally addressed along with the lesson learnt to be capitalized for future developments.

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.

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.

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.

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.

In this paper, a demonstrative technological implementation of a shipboard 2.15 MVA generation system is described along with its modeling, numeric simulation and some factory tests. The system, named Naval Package (NP), is part of a demonstrative programme commissioned by Italian Navy for preliminary evaluations of Medium-Voltage DC (MVDC) supply technologies aimed at equipping her future vessels. The NP is made by a dual-star alternator, whose two stator three-phase windings respectively feed two diode-rectifier full-bridges. The alternator (rated speed 6300 rpm) is designed to be moved by a 22000 rpm gas turbine through a gearbox. This paper will present NP innovative issues, with reference to design solutions, operative requirements, dynamic modeling under normal and faulty conditions and some factory tests.