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One of the attractive benefits of slotless machines is low losses at high speeds, which could be emphasized by a careful stator core loss assessment, potentially available already at the predesign stage. Unfortunately, mainstream iron loss estimation methods are typically implemented in the finite element analysis environment with a constant coefficients dummy model, leading to weak extrapolations with huge errors. In this article, an analytical method for the iron loss prediction in the stator core of slotless PM machines is derived. It is based on the extension of the 2-D field solution over the entire machine geometry. Then, the analytical solution is combined with variable coefficient loss models (VARCO) or constant coefficient loss models, which can be efficiently computed by vectorized postprocessing. VARCO loss models are shown to be preferred at a general level. Moreover, this article proposes a lookup-table-based solution as an alternative approach. The main contribution lies in the numerical link between the analytical field solution and the iron loss estimate, with the aid of a code implementation of the proposed methodology. First, the models are compared against a sufficiently dense dataset available from laminations manufacturer for validation purposes. Then, all the methods are compared for the slotless machine case. Finally, the models are applied to a real case study and validated experimentally. [ABSTRACT FROM AUTHOR]

This paper presents a novel design concept for Synchronous Reluctance (SynRel) machines aimed at reducing the torque ripple. Two general sizing approaches based on the homothetic scaling principle are defined and compared. An in depth analysis on the torque ripple, for a wide range of scaled geometries, evaluated by finite element, has been carried out at different operating conditions. A further analysis is performed on 4 scaled geometries that have been optimized starting from 4 random rotor geometries. It is shown that the main rotor geometrical variables converge to similar values for all scaled machines. The accuracy of the proposed model is then validated by comparing the FE simulated torque ripple waveforms with the experimental data carried out, for a range of operating conditions, on a machine prototype. The outcome of this work is a fast and accurate scaling technique for the preliminary design of SynRel machines with reduced torque ripple. [ABSTRACT FROM AUTHOR]

Large medium-voltage electric machine stators are usually equipped with form wound coils made of flat conductors (strands) and embedded in open (rectangular) slots. Air-gap magnetic flux lines can enter the slot and, sweeping the strands placed nearest the slot opening, induce eddy currents in them. Such eddy currents cause additional losses which can be much higher than usual skin-effect and proximity losses. In order to avoid dangerous overheating and hot spots, the additional losses in question need to be carefully predicted in the design stage. Time-stepping finite-element analysis (TSFEA) can be used for the purpose, which however implies a large computational burden and requires the machine geometry to be modeled in detail. This article proposes alternative methods based on time-harmonic finite-element analysis (THFEA) simulations performed on highly simplified machine models and with no need to take rotor motion into account. The proposed methods are shown to produce very accurate results, compared to TSFEA, but with very significant time and computational savings. [ABSTRACT FROM AUTHOR]

The rotor structure of synchronous reluctance motor (SynRm) is usually provided with redial ribs to improve structural ruggedness for high‐speed operation. While these radial ribs contribute to mechanical robustness, they also reduce the average torque and limit the flux‐weakening capability due to an increase in inductance along the q ‐axis. To address this issue, the rotor design is refined by adopting non‐magnetic radial ribs in this study. The introduction of titanium as non‐magnetic material for radial ribs prevents magnetic leakage flux from flowing along the q ‐axis, and thereby improves the average torque, saliency ratio, efficiency and power factor of the SynRm. Furthermore, along with the electromagnetic analysis, the mechanical integrity of the proposed rotor design is also investigated. Two prototypes rated 1 kW at 1500 rpm are fabricated using both the conventional and the proposed rotor design, while the stator and other parameters are maintained constant in the two cases. The conventional design uses magnetic radial ribs whereas the proposed design uses non‐magnetic radial ribs. Finally, a thorough comparative study, including inductance tests on the prototypes, is performed between the proposed and the conventional SynRm, confirming the strengths of the technology solution being set forth. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Fractional-slot concentrated windings (FSCWs) are an attractive option for the design of synchronous permanent-magnet machines. It is commonly assumed in the existing literature that a symmetrical three-phase FSCW is feasible only on a condition that the number of slots Z is an integer multiple of three times the maximum common divisor between Z and the number of pole pairs p. Slot-pole combinations satisfying this rule can be defined conventionally, the others unconventionally. In contrast to the common belief, this paper shows that, using a multi-layer arrangement, it is possible to synthetize a symmetrical FSCW having unconventional slot-pole combinations. A general design methodology for this purpose is presented and validated by finite element analysis. The pros and contras of FSCWs with unconventional slot-pole combinations are examined. Finally, the application of an unconventional FSCW to a shipboard surface permanent-magnet machine prototype is presented to illustrate the possible practical convenience of this kind of winding and tests on the prototype are reported for experimental validation. [ABSTRACT FROM AUTHOR]

Interior permanent magnet (IPM) machines with spoke-type design are very attractive for several applications, including vehicle traction, which requires a good flux weakening performance to achieve high speed. For this purpose, a high demagnetizing current is usually injected to reduce the machine flux at high speed, with detrimental effect in terms of efficiency. This problem can be mitigated by equipping the rotor with a mechanical flux weakening device, which activates during high-speed operation by centrifugal force. This paper addresses the analytical modeling of this special kind of IPM motor through a simplified magnetic equivalent circuit (MEC) approach, which incorporates saturation and slotting effects as well as the flux weakening device operation. It is shown how the MEC model can effectively describe the IPM machine behavior despite its simplicity. Results are successfully validated by comparison both with finite element analysis, which requires much more computational resources, and with measurements from laboratory prototype testing. [ABSTRACT FROM AUTHOR]

This paper presents a droop-based distributed control strategy for multithree-phase machines that provides augmented controllability during power sharing transients. The proposed strategy is able to mitigate the mutual interactions among different sets of windings without controlling any subspace variable, also offering a modular and redundant design. On the contrary, in a centralized configuration, subspaces would be controlled using the vector space decomposition, but fault tolerance and reliability levels required by the stricter regulations and policies expected in future transportation systems would not be satisfied. The proposed method is analytically compared against the state-of-the-art power sharing technique and equivalent models and control design procedures have been derived and considered in the comparison. Uncontrolled power sharing transients and their effects on mutual couplings among isolated sets of windings have been compared against the proposed regulated ones. Experimental results on a 22-kW nine-phase multithree-phase synchronous machine rig validate the design procedures showing good agreement with the expected performances. [ABSTRACT FROM AUTHOR]

Load-commutated inverters (LCIs), combined with wound-field synchronous machines (WFSMs), can be an excellent solution for high power drives, but their present technology suffers from important drawbacks related to low power factor, large torque pulsations, and poor starting performance. This paper presents a new LCI design intended to overcome the mentioned limitations. An SCR-based forced-commutation circuit is added to the common inverter topology to obtain a five-level waveform for the stator current. This leads to significantly reduced current harmonics and torque pulsations, in addition to bringing benefits in terms of lower additional losses. As a further advantage, the proposed design allows for a significant power factor enhancement. Finally, it enables the WFSM to be started with a much smoother torque compared to the traditional pulsed operating mode of conventional LCI drives. Simulation studies are conducted on a high-power drive scheme to show the aforementioned improvements. Also, a reduced-scale laboratory prototype of a WFSM drive system is tested to verify the feasibility of the proposed converter. [ABSTRACT FROM AUTHOR]

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 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. [ABSTRACT FROM AUTHOR]

Wound-field synchronous motors (WFSMs) fed by load-commutated inverters (LCIs) are widely used for high-power applications in many fields like ship propulsion, oil and gas industry, and pumped-storage hydropower generation. Several design architectures exist for LCI drives, depending on the number of LCIs and their dc-link connection as well as on the number of WFSM phase count. The prediction of LCI drive performance at steady state is important in the design stage, especially in regard to the prediction of the torque pulsations, which can give rise to serious mechanical resonance issues. This paper proposes an algebraic method to simulate the steady-state behavior of LCI drives in all their configurations of practical interest. Compared to conventional dynamic simulation approaches based on differential equation solution, the method is much more computationally efficient and requires a very limited knowledge of system parameters. Its accuracy is experimentally assessed by comparison against measurements taken on a real LCI drive arranged according to various possible schemes. Furthermore, the advantages of the proposed algebraic method over the dynamic simulations are highlighted by comparison against the simulation results on a high-power LCI-fed WFSM drive in MATLAB/Simulink environment. [ABSTRACT FROM AUTHOR]

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 analysis (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. [ABSTRACT FROM PUBLISHER]

Load-commutated inverters (LCI's) are often used to supply dual-three-phase synchronous motors in high-power variable-speed applications. A pair of LCIs is used in this arrangement to feed the two motor three-phase winding sets. In order to cope with inter-harmonic issues, a drive configuration with an interconnection of the two LCI dc-links has been proposed. In this paper, such a drive design is shown to produce an increased voltage stress on motor windings compared with traditional configurations. The problem is investigated in the paper by proposing an algebraic algorithm capable of predicting the steady-state voltage waveform applied to the motor terminals and arising between the star points of the two winding sets. Unlike conventional dynamic simulations, the proposed approach gives practically instantaneous results, making it possible to quickly investigate a wide number of possible operating conditions. Furthermore, it requires a limited knowledge of system parameters, which are often hardly available. Its reliability and accuracy are assessed by comparison with measurements on a test drive system and examples are given of the method application to the sizing of motor insulation system. [ABSTRACT FROM PUBLISHER]

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. [ABSTRACT FROM AUTHOR]

The accurate determination of stator leakage inductances is presently an open issue in the analysis and testing of multiphase 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. [ABSTRACT FROM AUTHOR]

Slot leakage field and inductance computation are, in general, nontrivial tasks in the analysis of electric machines equipped with semiclosed slots, even under the hypothesis of neglecting magnetic saturation. This paper proposes an analytical method to evaluate the slot leakage field and inductances in dual-layer distributed winding machines, extending the results of a previous work where the single-layer winding design was addressed. A direct solution to Poisson's differential equation in the slot domain is found by suitably defining boundary conditions in the slot opening area. Boundary conditions are defined exploiting the analytical form taken by the magnetic field in the neighborhood of ferromagnetic corner-shaped regions. The presented approach is successfully validated against finite element simulations. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Synchronous reluctance (SynRel) machines 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, and 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 considered 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. [ABSTRACT FROM AUTHOR]

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 establish a partial magnetic short circuit between PMs. This increases PM 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 to reduce the demagnetizing \boldsymbol d -axis current to be injected by the inverter, along with the related copper losses and demagnetization issues. The proposed design is investigated in this paper using an analytical model whose parameters are computed by finite-element analysis. The effectiveness of the solution being set forth is successfully proven by some testing on a laboratory prototype. Experimental results are compared with analytical predictions showing a satisfactory accordance. [ABSTRACT FROM AUTHOR]

Permanent-magnet synchronous linear motors (PMSLMs) are more and more frequently used as all-electric direct-drive actuators in those applications where a force needs to be developed along a fixed direction. In this paper, an accurate 3-D thermal model of a PMSLM is derived through a lumped-parameter network approach which exploits all the symmetries in the actuator structure to maximize the spatial density of nodes. Numerically efficient techniques are then proposed to solve the thermal network analytically. Some experimental validations are finally presented based on the thermal testing of a laboratory prototype. [ABSTRACT FROM PUBLISHER]

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. [ABSTRACT FROM PUBLISHER]

An electric machine equipped with n stator phases is characterized by a set of n \times 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. [ABSTRACT FROM AUTHOR]

Some high-torque electric machines, such as low-speed wind generators, may be very difficult to test on load because of the large and expensive mechanical equipment required to load them (motors) or to drive them (generators). In this paper, a full-load regenerative testing methodology is described which does not require any rotating equipment to be coupled to the machine where the rated active power flows in a closed loop between the machine and a suitably connected dual-stage converter. The successful application of this methodology to a 780-kW 14-r/min wind generator prototype is addressed as a study case. Relevant experimental results are reported in a companion paper. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM PUBLISHER]

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

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]