Search

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.

L’attività di ricerca si è focalizzata sulla determinazione di metodi analitici e analitico-numerici per la soluzione di alcuni problemi “aperti” di interesse nella progettazione e l'analisi di macchine elettriche. La tesi include una gamma piuttosto ampia di argomenti, ma è stata sviluppata seguendo una singola linea di indagine unitaria, che consiste nel tentativo di cercare e, dove possibile, definire e implementare, metodi matematici sufficientemente veloci ma accurati per eseguire calcoli su macchine elettriche in modo da evitare l'uso massiccio di tecniche basate su analisi agli elementi finiti. Queste infatti sono note per essere precise ed affidabili, ma allo stesso tempo richiedono ingenti risorse computazionali e sono quindi poco adatte quando è necessario esplorare un gran numero di varianti di progetto, come nel caso di programmi di ottimizzazione progettuale basati su algoritmi genetici. I metodi agli elementi finiti non sono stati esclusi, naturalmente, ma piuttosto utilizzati come “benchmark” per valutare la validità delle tecniche di calcolo alternative proposte, ove non fosse possibile effettuare un confronto diretto con dati sperimentali. I tipi di macchine elettriche a cui si sono applicati i metodi di calcolo sviluppati in questa tesi sono diversi e vanno dai motori a magneti permanenti superficiali con cave statoriche a motori a magneti permanenti superficiali con statore slotless ,dalle macchine sincrone a riluttanza alle macchine sincrone a rotore avvolto, per finire con motori a induzione a gabbia di scoiattolo. Tutti questi tipi di macchine sono sempre più importanti nelle applicazioni odierne, sia nel settore industriale che per i sistemi di trazione. Per affrontare lo studio di queste macchine si sono di volta in volta sviluppati approcci di calcolo analitico diversi, anche in funzione degli obiettivi da raggiungere. Esempi di questi approcci analitici impiegati sono i seguenti: la teoria della “winding function” applicata al calcolo della coppia di macchine a magneti superficiali; la teoria della “permeance function” applicata al calcolo delle macchine con eccentricità di rotore ; l'approccio mediante circuito equivalente magnetico o reti di riluttanze in combinazione a tecniche basate sulle mappe conformi per lo studio dei motori sincroni a riluttanza; la soluzione delle equazioni di Laplace e di Poisson per determinare il campo magnetico nelle macchine a magneti permanenti superficiali slotless e a gabbia di scoiattolo. Inoltre, l'applicazione degli algoritmi di calcolo proposti è illustrata mediante opportuni casi di studio ed i risultati ottenuti sono sempre convalidati mediante confronto con tecniche di calcolo alternative (in particolare l’analisi agli elementi finiti)

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 computation of end-coil leakage inductances of electric machines is a challenging task due to the complicated leakage flux 3-D distribution in the winding overhang region. In this paper, the problem of computing the field-circuit leakage inductance of round-rotor synchronous machines is addressed. The proposed method is fully analytical and descends from the symbolical solution of Neumann integrals applied to the computation of self-inductance and mutual inductance combined with the method of mirror images to account for core effects. With respect to existing analytical approaches, the methodology requires neither numerical integral solutions nor discretizing the end-coil geometry into small straight elements. The accuracy of the proposed technique for computing the mutual inductance between two single-end turns is assessed against measurements on a dedicated experimental setup. The extension of the method to the computation of the entire field-circuit end-coil leakage inductance is assessed by comparing with the 3-D finite-element analysis.

The study of electric machines subject to rotor eccentricity is usually performed using a winding and permeance function approach. In this paper, an alternative method is investigated based on conformal mapping. The geometry of a an eccentric round-rotor machine is mapped into a uniform air-gap topology by means of Moebius conformal map. The mathematical details of the method are provided along with explicit formulas to compute the air-gap field and inductances 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).

This paper investigates the hydraulic system drive used onboard ships to actuate stabilizing fins. The hydraulic drives are often characterized by low efficiencies, heavy maintenance, frequent oil leaks and large weight and size. To overcome these draw-backs, in this paper a concept design of a full-scale permanent magnet linear synchronous motor directly coupled to the shipboard fin stabilizer, is presented, used as a servo-motor. The innovative electric drive is based on prototype of high thrust linear motor built in a previous research. A complete Simulink model, where electric, mechanical and hydraulic domains can be connected, has been realized. Simulations has been performed to evaluate the hydraulic drive efficiency and in order to assess the servo-assistance capabilities. The results show the feasibility of the proposed solution for a future practical application. This research has been carried out in the framework of the Italian Defence Research National Program for the development of ‘dual use’ technologies.

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).

The complex relative permeance function is a useful mathematical tool to study the magnetic field of electric machines with slotted air gap. In the literature, the complex permeance function is usually identified by means of numerical techniques based on conformal mapping. In this paper an alternative analytical approach is proposed. The method is based on solving the magnetostatic differential field equation over a portion of the slotted air gap by imposing suitable boundary conditions in the slot opening region. Such boundary conditions exploit the mathematical law which governs the divergence of the magnetic field in the neighborhood of corner-shaped ferromagnetic regions. Furthermore, the results of Carter theory on slot fringing effects on the air gap field are used. A simple easy-to-compute formula is obtained for the complex permeance function. Its accuracy is assessed by comparison with Finite Element Analysis (FEA).

Alternatives to hydraulic drives used on board ship are under study for the Italian Navy in order to reduce the use of the oil which cause environmental problems and high cost. The hydraulic system drive used onboard ship are affected by low efficiencies and generally require intensive maintenance, very high encumbrance. So that the Italian Navy has commissioned a research project for the construction and installation of an innovative oil-free electric drive based on a permanent magnet linear actuator. The new electric motor is more efficient, fault-tolerant, reduces the encumbrance and ensures a greater control.

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

Hydraulic drives used on board ship to empower rudders, stabilizing fins, anchor winches, etc., are often characterized by low efficiencies, heavy maintenance, frequent oil leaks and large weight and size. To overcome these drawbacks, oil-free inverter-fed alternative drives can be conceived. In this paper a concept design of a full-scale permanent magnet linear synchronous motor directly coupled to a twin rudder on board ship is presented, used as a servo-motor. The servo-assistance capabilities are simulated and evaluated by using a complete model of the electro-mechanic-hydraulic system in Simulink environment. The results show the effectiveness of the electromagnetic drive servo-assistance. This research has been carried out in the framework of the Italian Defence Research National Program for the development of `dual use' technologies.

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.

Squirrel-cage induction motors are the most widely used type of electric machinery in industrial applications thanks to their rugged, cheap and robust construction. For the fast prediction of induction motor steady-state performance without the need for time-consuming finite-element analyses, the equivalent circuit parameters of the machine need to be calculated, including rotor slot leakage inductances. This paper, in particular, proposes a new improved analytical formula to determine the rotor slot leakage inductance in squirrel-cage induction motors with rectangular-shaped bars. The formula is obtained by solving Poisson's equation in the slot domain. Results are assessed by comparison against Finite Element Analysis (FEA). The precision of the presented methodology is also compared to that of approximated analytical models available from the literature. The comparison shows that the accurate formula proposed is in excellent agreement with FEA with errors below 2%, while the simplified model generally leads to errors above 10% which increase as the height-to-width bar ratio decreases.

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.

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 that can facilitate 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 high performance levels. The proposed 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.

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.

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.

This paper reports the optimization process of a MW-size, medium voltage, squirrel cage induction motor for industrial applications. Starting from an existing construction, the aim of the optimization is to maximize the induction machine power density, while fulfilling high demanding, specific international Oil&Gas industry regulations. For every considered design, the motor performance is estimated by means of an algorithm based on T-type equivalent circuit. The circuit parameters are determined with a combination of magnetostatic and time harmonic Finite Elements Analysis (FEA), together with analytical formulas. This approach allows the evaluation of the precise parameters dependency on punctual current and frequency values, and allows good performance prediction ability. The multi-objective optimization process input are geometrical variables related to the motor electromagnetic configuration having direct impact on the output performance, is based on Genetic Algorithms (GA), and has been developed with the commercial software ModeFRONTIER®.

Transport industry is facing a paradigm shift towards the most promising and sustainable solution represented by the transportation electrification. This concept has not only been adopted for the propulsion powertrains, but also for the auxiliary systems. Indeed, in the last few decades electrification has come to smart actuators to improve both safety and comfort of the passengers. In this contest, multiphase electrical machines and drives represent a promising solution to satisfy the challenging requirements imposed by the applications, such as reduced weight and volume, high efficiency, low vibrations and noise, robustness and fault tolerance. With reference to the technical literature and applications, this paper presents an overview of the multiphase technology for the transportation electrification in road vehicle, rail vehicles, air transports and shipboard applications.

Fault tolerance is a basic requirement for modern electric motors and generators and can be achieved with a number of integrated approaches, like condition monitoring, post-fault control strategies and suitable system design architectures. In particular, this paper focuses on large synchronous machines (both permanent-magnet and wound-field ones) and discusses the main design provisions that can be adopted to improve their ability to withstand various kinds of fault. The fault-tolerant design solutions recommended for small-power machines are critically reviewed and their scalability to higher machine sizes is discussed also referring to practical industrial realizations. The most promising potential for fault-tolerance design is recognized in large low-speed high-pole-count permanent-magnet synchronous machines (e.g. for ship propulsion and wind power generation) thanks to their suitability for highly-modular multi-unit fractional-slot concentrated-winding architectures.

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.

The use of linear machines (LMs) is increasing in research and industry fields. All kinds of LMs are studied: synchronous, induction, flux switching. Research is leading to overcome the known drawbacks of all these machines. Anyway the use of innovative direct-drive solutions are under study to obtain more efficient energy conversion, such as for marine field. As alternative to classical systems, linear force can be used to obtain a more efficient plant. Hydraulic drives used on board ship to empower rudders, stabilizing fins, anchor winches, etc., are often characterized by low efficiencies, heavy maintenance, frequent oil leaks and large weight and size. To overcome these drawbacks, an oil-free inverter-fed alternative drives can be conceived. In this paper a concept design of full-scale permanent magnet linear synchronous motor directly coupled to a twin rudder on board ship is presented, used as a servo-motor. A complete model of the electro-mechanic-hydraulic system in Simulink environment was built. Several simulations are performed to evaluate the servo-assistance capabilities. The results show the effectiveness of the electromagnetic drive servo-assistance and an increased overall efficiency of the steering gear. This research has been carried out in the framework of the Italian Defence Research National Program for the development of `dual use' technologies.

Hydrostatic drives used on board ships are often characterized by low efficiencies, heavy maintenance, frequent oil leaks and large weight and size. To overcome these drawbacks, oil-free inverter-fed alternative drives can be proposed. This paper analyzes the ship rudder hydraulic drive performances, and a proposal for improvements is shown. A full-scale permanent magnet linear synchronous motor directly coupled to a twin rudder is presented, used as a servo-motor. Both servo-assistance features and performance and efficiency improvements are evaluated by using a complete physical model in Simulink environment.

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.

This paper (Part I), together with a companion one (Part II), deals with the fault-tolerant operation of a 5-phase permanent magnet (PM) brushless DC (BLDC) drive. In particular, this paper proposes an algebraic approach to set up a model of the motor current supply in healthy conditions. Afterwards, a faulty drive is considered with one or more open phases and, by constraining the proposed model with fault-specific conditions, scheduling and magnitude of the currents in the surviving phases are determined automatically as well as the maximum magnitude of the currents and the maximum torque developed by the motor. At last, the paper applies the model to the case of a drive with one open phase fault. In the companion paper the model is applied to the cases of more-than-one open phase faults.

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

Electric drives with a three-phase structure are commonly utilized in road vehicles for propulsion and in aeronautics as control-surface actuators. In recent times, some concerns have emerged on the use of three-phase drives in such applications in regard to power capability, efficiency and fault-tolerance. A way to effectively cope with these concerns is the adoption of multiphase drives. In this sense, a five-phase drive is a promising solution as it does not imply an excessively complicated structure. In this paper a five-phase PM BLDC drive is considered and its torque capabilities are investigated for square-wave current supply. Specifically, the torque developed by a five-phase drive is first compared to the three-phase counterpart; torque comparison is carried out by keeping motor size and loading unchanged and by considering two hypotheses: equal phase back-emf and equal phase rms current. Then the torque available from a five-phase drive is determined under various supply modes, characterized by the simultaneous conduction of a reduced number of phases; the torque available is determined by imposing an equal rms phase current in the supply modes.

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 finite reluctance approach is very well suited for fast modeling and dynamic simulation of permanent magnet synchronous machines with inverter feeding. This paper shows validating simulations, in both steady-state and transient conditions, carried out for a permanent magnet linear motor designed for on board ship load actuation. The basics of the method are resumed. The simulations obtained by using the finite reluctance approach are compared with FEM simulations, showing good agreement. Keywords—Dynamic model, dynamic simulation, linear machine, magnetic network, reluctance.

The finite reluctance approach is based on a simple synergy between a basic magnetic domain decomposition rule and the voltage balance at the terminals of any winding acting in the domain. The finite reluctance approach is thought as a tool for fast dynamic simulations of any kind of machine, but it is particularly adapt for irregular flux geometries as in fractional-slot surface-mounted permanent magnet machines with large slots and high slot flux leakages, for which only the finite element method has found good usage. This approach is based on the definition of a complementary mesh interlaced with the reluctance mesh, on the concept of material cell, and on unique rules to carry out both reluctances and magneto-motive forces. At this scope, definitions of mesh nodes, co-nodes, and auxiliary nodes are shown. The iron saturation can be easily taken in account, as well as motion effects. The circuit flux linkages are elected as state variables for integration, and they are directly linked to the loop fluxes in the magnetic network. The finite reluctance approach has been applied in this paper for dynamic modeling of an inverter-fed permanent-magnet linear synchronous motor.

DC distribution is being considered as an attractive alternative to its traditional AC counterpart in many fields of applications and, in particular, for shipboard power systems. The adoption of a DC distribution network makes it possible and sometimes necessary to radically revise the technology of the generation systems in many regards. Not only frequency-related constraints are removed, allowing for high-speed smaller generators to be used, but new fault-tolerant configurations, involving power electronics, generation control and management too, become feasible as a natural alternative to the traditional arrangement of AC power plants. In this paper some of these new technological scenarios are outlined, referring to state-of-the-art realizations as well as to novel concept designs presently under development and investigation.

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.

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.

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.

This work deals with the description of the Photovoltaic Laboratory at the Department of Engineering and Architecture of the University of Trieste. The description of the main facilities involved, such as three grid-connected photovoltaic plants and a test facility for the characterization of photovoltaic modules, is being proposed. Moreover, an overview of the recent and future research activities is given.

A fault-tolerant machine used for safety-critical tasks must a) guarantee at least a reduced-performance operation in case of partial machine fault and b) guarantee avoidance of drive mechanical jam/stall in case of total machine fault, to allow for the intervention of the back-up systems. Classical hydrostatic transmissions used on board ships for critical tasks such as rudder and stabilizing fin steering gears fulfill both the requirements a), b) above, but recent proposals for substitution with full-electric drives (rotary motors coupled with multistage reduction gears) usually do not. Especially the requirement b) needs particular attention and increased complexity when dealing with geared drives. This paper proposes a linear permanent-magnet direct drive fulfilling both the requirements above, for (but not limited to) rudder/fin steering gears. The absence of gears grants the requirement b), whereas the full-modular structure satisfies a), with independently fed stator modules and multiple inverters. This paper addresses some fault scenarios including electrical failures, in the machine winding (short-circuited coils) and in the inverters (trip of one or more units). The performance degradation is studied and assessed for the cases considered by both simulations and measurements on a prototype.

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.

Load-Commutated Inverters (LCIs) are today one of the main converter topologies used for very-high-power drive applications, although new topologies, based on PWM technology and employing IGBT or IGCT as power switches instead of SCR's, have been recently gaining attractiveness also for increasing power ratings. The paper presents an overview of today's LCI technology and its applications, highlighting its strengths and weaknesses. Furthermore, the most promising lines of development are drawn along which LCI drive technology is being improved to overcome its major drawbacks and maintain competitiveness with Voltage-Source-Inverter (VSI) based counterparts.

This paper analyzes a different approach for actuation of shipboard loads such as rudders, stabilizing fins, or other linear drives, which are usually powered by oil-fed actuators. Hydrostatic transmissions are classically used when the highest torques/forces with low speeds are demanded. However, the oil used as force-transmission fluid is troublesome due to environmental issues. Oil production, use, and final disposal is costly and presents environmental risks. Also the maintenance onboard ship is intensive and hazardous for the operators. A full electric solutions is ulteriorly studied in this paper, based on a prototyped linear machine using permanent magnets and inverter feeding. Copper, friction, and iron losses are the main limiting factors in developing the required thrust at low speed, which is, ultimately, the main obstacle to a gear-less or direct-drive full-electric application. This paper shows incremental work on the prototype, through a dedicated circuit model accounting for all the machine losses and providing an affordable model of machine overall efficiency. The model will be used in further works to asses the machine performances in typical onboard service duties.

An approach to machine modeling by using finite reluctances is proposed. The finite reluctance approach is well suited for fast modeling and dynamic simulation of electrical machines in general and of fractional-slot surface-mounted permanent magnet machines in particular. The latter usually have very large air gap and slots, with relevant leakage and large tangential components of flux density in air gap, slot, and magnets as well. Modeling tools as winding functions are not accurate enough in this case, and thus time-stepping finite elements are used, which are slow and with limited time-step resolution (i.e. low simulation frequency). The approach used in this paper uses magnetic networks built on the principle of the cell theory. Network reluctances and magnet MMFs are defined by using unique rules. This paper shows that, besides faster dynamic simulations, the finite reluctance approach allows the graphical representation of the magnetic vector and scalar fields inside the machine, in a way very similar to the more cumbersome finite element method, but with a much smaller computational effort. The results from both the methods are presented and compared for a case study of permanent magnet linear synchronous motor.

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.

Integrated Power Systems (IPS) with DC distribution require the massive use of power electronics for many tasks: 1) the output voltage rectification of generators and the connection of energy storage systems to the IPS, 2) the supply of medium voltage high power users (propulsion drives, bow thrusters, compressors for air conditioning), 3) the adjustment of voltage levels, which is performed by power transformers in AC distribution systems and by DC/DC power converters called Solid-State Transformers (SST) in DC distribution systems. In this paper, an overview is given of the power converters found in the literature and on the market that could be suitable for employment in the mentioned subsystems as parts of a future MVDC IPS. Finally authors propose a structure for an SST to be employed in three-wire MVDC power systems.