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This paper reviews recent developments in power loss analysis applicable, but not limited to, the thermal design of permanent-magnet machines. Accurate and computationally efficient loss prediction is an essential element in the thermal analysis of electrical machines and has become an increasingly important part of the machine design process. The continuous drive toward “more electric” technologies has resulted in a need for a more comprehensive and detailed design approach, where various multiphysics and multidisciplinary effects are accounted for. This “design for application” methodology relies strongly on the advancements and evolution of the existing theoretical and experimental design techniques to satisfy the evermore-demanding machine design requirements. The thermal behavior and efficiency of the power conversion are essential performance measures in the “design for application” approach. An overview of the challenges and limitations regarding power loss analysis in the context of the thermal design of electrical machines is provided in this paper. All of the major loss components associated with the active parts of a machine assembly are discussed. [ABSTRACT FROM AUTHOR]

This paper presents an alternative permanent magnet rotor containment technology. The use of a semipermeable material in a laminated structure is shown to result in a significant improvement in magnetic loading for a given thickness of containment compared to the use of magnetically inert materials such as carbon fiber or Inconel, while minimizing eddy current losses in the containment layer. An analytical model is presented for determining the air-gap field delivered from a permanent magnet array with a semipermeable containment. The analysis is validated through finite element analysis. The fabrication of a prototype machine is detailed and the results presented show good agreement with the analysis. The validated modeling process is used to assess the potential of the new technology. [ABSTRACT FROM AUTHOR]

This paper presents a thermal analysis of a segmented stator winding design. As the thermal performance is one of the main factors limiting a machine's output capability, a thermal test on a complete prototype machine is an essential part of the design process. However, for the segmented stator winding design, a test-informed thermal analysis on a single stator tooth can be performed prior to the manufacture of the full machine. This approach allows for a rapid and inexpensive assessment of the thermal performance of the complete machine and early identification of design modifications needed. The research has been applied to the design of a highly efficient and compact permanent-magnet traction motor. A thermal model for a single tooth was developed and supported by tests to identify key heat transfer coefficients. A number of winding assemblies were compared, and the most promising was selected for the final motor prototype. The results from the approach are compared with thermal test results from the complete machine. [ABSTRACT FROM AUTHOR]

This paper presents an alternative computationally efficient approach to the thermal design of compact wound components. The method is based on the use of anisotropic lumped regions within 3-D thermal finite-element analyses. The lumped regions replicate the multimaterial composites used in the construction of wound components. Material data for these lumped regions are obtained experimentally, accounting for the thermal anisotropy. Input loss data for the analysis were derived by combining electromagnetic finite-element iron loss calculations with experimental ac copper loss correlations. The technique is applied to a design of a high-energy-density filter inductor. Thermal measurements from prototype inductors are compared with the theoretical predictions showing a good agreement. [ABSTRACT FROM AUTHOR]

This paper describes the thermal analysis of an external-rotor brushless permanent-magnet motor used in a traction application. The machine has a nonstandard open-slot stator lamination and is wound with a single-layer concentrated "modular" winding. This structure potentially has several heat transfer advantages compared to the more traditional double-layer concentrated winding. An analytical lumped-parameter thermal model has been developed for the machine to gain an understanding of the main cooling mechanisms and to investigate the potential heat transfer advantages. The thermal advantage of the modular winding over the more conventional 1.5-slot-per-pole design is presented. Results from the developed thermal model have been validated experimentally on a prototype wheel hub motor. [ABSTRACT FROM AUTHOR]

This paper discusses the performance attributes of the open-slot, modular-wound, external-rotor, topology of electrical machine. Combinations of pole and slot numbers are presented for which the winding factor is maximal and torque ripple is minimal. An optimization of the magnetic circuit design of six promising pole-slot configurations is undertaken using a parametric finite element model (FEM) combined with a genetic algorithm (GA). These designs are benchmarked against a conventional 1.5 slots/pole external-rotor brushless dc machine. These candidate electrical machine versions are characterized by having the same external-rotor diameter, total slot area available for the winding and by equal volumes of permanent magnet (PM). Based on the analysis, the most promising motor structure was selected and a prototype wheel-hub motor has been built for application in a small electrical vehicle. Test data from the prototype is used to validate the findings of the initial analyses and practically demonstrate the attributes of the topology. [ABSTRACT FROM AUTHOR]

This paper considers the operation and control of a brushless ac prototype embedded aircraft generator that has been designed for use as an alternative to the emergency run air turbine. To meet the wide operating speed requirements, the generator utilizes a hybrid rotor construction comprising permanent mag- net and variable reluctance sections. The constant current source characteristics of the design are exploited to derive the phase information required by the ac vector controller, thus avoiding the use of a shaft-mounted rotor position sensor. The methodology behind the controller operation is presented together with test results taken from a prototype generator system, and this is contrast with the results taken from the conventionally controlled machine with resolver position feedback. [ABSTRACT FROM AUTHOR]

This paper investigates the optimal design of a brushless machine with an internal rotor comprising a permanent magnet (PM) array that is made up from discrete anisotropic arc segments. The particular case where the magnet array is constructed from two arc segments per pole is considered, where the magnets are alternately magnetized normally and tangentially to the rotor surface. The influence of the relative magnetic airgap and magnet thickness on the optimal PM array arrangement is analyzed, and it is shown that the optimal arrangement is largely independent of the airgap width. Both analytical and numerical approaches to the optimization problem are presented, the latter using a 2-D parametric finite-element model. [ABSTRACT FROM AUTHOR]

This paper presents recent research into the use of an embedded generation system as an alternative to the emergency ram air turbine on aircraft, the proposal being to utilize the wind- mill effect of the low-pressure turbine of the main aircraft engine for emergency power generation. A novel topology of a high specific output ac permanent-magnet generator is described that has been designed to be driven directly from the low-pressure turbine via an integral fixed-ratio gearbox, the requirement being to generate a regulated 270-V dc 0-20-kW emergency supply over a 12:1 3000-36 000-r/min generator shaft speed range. The methodologies behind the machine design and operation are described together with test results taken from a prototype generator system. [ABSTRACT FROM AUTHOR]

This paper presents the results of a brushless permanent magnet (PM) starter-generator design, which caters for low-cost manufacture and a highly constrained mechanical space envelope. The starter-generator design addresses the low-cost requirement through the use of aluminium winding conductors and ferrite PM. This presents several challenges which include, but are not limited to, the selection of an appropriate machine topology to realise a high specific output with the lower performance materials, minimizing the power losses associated with the higher resistivity of aluminium, and the enhancement of thermal performance. The problem is further exacerbated by the demanding space envelope, operating requirements, and the necessity of “design for manufacture.” The selection of an appropriate machine topology is paramount in the present application as the limited mechanical space-envelope results in a “pancake” like geometry in which the aspect ratio of the stator outer diameter to the machine active length is high. To provide a solution satisfying all these challenging design requirements, an approach combining the theoretical electromagnetic and thermal analyses together with tests on machine subassemblies has been employed here. Such a method allows for a more informed design process, where the manufacture and assembly nuances affecting the starter-generator's performance are identified and accounted for prior to prototyping of the complete machine assembly. This paper discusses the employed design methodology in detail. A number of machine designs with alternative winding constructions have been considered providing an insight into challenges and limitations for the cost-effective winding construction utilizing aluminium conductors. The results from analysis of the starter generator suggest that the proposed machine design is capable of achieving the design requirements for both continuous and transient operating modes. [ABSTRACT FROM PUBLISHER]

This paper presents a coupled electromagnetic and thermal design methodology which addresses the problem of balancing accuracy against computation time. A case study on a short-duty permanent-magnet (PM) linear actuator is used to illustrate the approach. The proposed method employs a two-dimensional electromagnetic finite-element (FE) model coupled with a detailed thermal equivalent circuit (TEC) model which is automatically constructed and parameterized using geometric and material data. A numerical method of estimating the equivalent thermal properties of the winding amalgam is used along with published empirically derived convection and radiation heat transfer correlations. The relatively high number of network nodes and more accurate thermal material properties minimizes model calibration and allows improved temperature prediction, including winding hot spots, while maintaining a low computational cost for both steady-state and transient analyses. A comparison between experimental and theoretical actuator performance shows that the design methodology provides good accuracy electromagnetic and transient thermal performance predictions without the need for direct model calibration and can yield an optimized design within an acceptable time frame. [ABSTRACT FROM PUBLISHER]

This paper describes a computationally efficient approach for mapping rotor power loss in permanent magnet (PM) machines. The PM loss mapping methodology discussed here utilizes a small number of time-step finite-element analyses (FEAs) to determine the parameters of a functional representation of loss variation with speed (frequency) and stator current and is intended for a rapid evaluation of machine performance over the entire torque–speed envelope. The research focus is placed on field-oriented-controlled brushless AC PM machines with surface-mounted PM rotor construction, although the method could be adapted for other rotor formats. The loss mapping procedure accounts for the axial segmentation of the PM array through the use of an equivalent electrical resistivity of the segmented PM array, which is obtained from three-dimensional (3-D) FEA. The PM loss can be accurately mapped across the full operational envelope, including the field-weakened mode, through a single 3-D and four two-dimensional time-step FEAs. The proposed methodology is validated on an 18-slot 16-pole surface-mounted brushless AC PM machine design. The loss mapping procedure results closely agree with the computationally demanding alternative of direct 3-D finite-element prediction of the PM power loss undertaken at each of the machine's operating points. [ABSTRACT FROM PUBLISHER]

Leaf-spring crossed flexure pivots are used in an increasingly large number of applications, many of which require the fatigue life and/or the rotational capability of the pivots to be maximized. Since these two qualities are determined by the endurance limit or the yield stress of the leaf-springs’ material, maximizing them will necessarily require reducing the stress levels arising in the aforementioned springs during operation. Partly for the sake of simplicity, partly due to manufacturing constraints, constant thickness leaf-springs have traditionally been used in crossed flexure pivots. However, minimizing stress simply by increasing the length or by decreasing the thickness of the leaf-springs is often not viable, either due to spatial constraints or to the attendant degradation in spring performance. This paper investigates the scope for stress reduction through shape optimization of the leaf-springs. To this end, a procedure combining a linear strain energy formulation, a parametric thickness profile definition and a series of optimization algorithms is employed. The resulting optimized thickness profiles are proven to be not only independent of the angular rotation at which the pivot operates, but also linearly scalable to leaf-springs of any length, minimum thickness and width. Validated using non-linear finite element analysis, the results show very significant reductions in relative maximum stress, of up to as much as 24% in the case of some pivot configurations. The optimized profiles and their corresponding constant thickness counterparts are also compared in terms of stiffness, strain energy and parasitic motion characteristics. It is concluded that shape optimization offers great potential for extending the fatigue life and/or rotational range of crossed flexure pivots. [ABSTRACT FROM AUTHOR]

This paper presents a method of automatically constructing and parameterizing accurate lumped parameter thermal equivalent circuits (TECs) with nodes arranged in a regular mesh pattern. The approach exhibits a number of advantages over conventional lumped parameter TECs including superior thermal field resolution, reduced model construction and setup times and more accurate identification of hot-spot temperatures and their location. The method serves as a desirable compromise between the fine detail and high computational cost of a full finite element analysis and the coarse detail and short solution times afforded by lumped parameter TECs. The benefits of the method are demonstrated by the analysis and experimental test of a demonstration linear actuator. [ABSTRACT FROM AUTHOR]

This paper demonstrates the benefits of a generalized arc-segment lumped parameter thermal equivalent circuit representation that can be used in the 3-D thermal modeling of transformers, wound passive components, and electrical machines. The arc-segment is particularly suited to modeling stator/rotor teeth and end-windings. Commonly used two-resistor lumped parameter thermal networks do not accurately account for internal heat generation, which is essential for accurate temperature prediction. In contrast, the use of a T-network formulation can accurately cater for internal heat generation along with material anisotropy. The element formulation is verified numerically and experimentally using 2-D and 3-D finite element analysis and through the thermal analysis of a power inductor. [ABSTRACT FROM AUTHOR]

It is common practice to represent a composite electrical winding as an equivalent lumped anisotropic material as this greatly simplifies a thermal model and reduces computation times. Existing techniques for estimating the bulk thermal properties of such composite materials use either analytical, numerical, or experimental approaches; however, these methods exhibit a number of drawbacks and limitations regarding their applicability. In this paper, a numerical thermal conductivity and analytical specific heat capacity estimation technique is proposed. The method is validated experimentally against three winding samples with differing configuration. A procedure is presented which enables bulk thermal properties to be estimated with a minimal need for experimental measurement, thereby accelerating the thermal modeling process. The proposed procedure is illustrated by the modeling of three coil exemplars with differing windings. Experimental thermal transients obtained by dc test of the coils show close agreement with a lumped-parameter thermal model utilizing estimated material data. [ABSTRACT FROM PUBLISHER]

This paper evaluates a three-phase ac main exciter (ME) for a wound-field synchronous aircraft starter–generator capable of operating in both starting and generating modes. The research has been based around a 225-kVA generator utilizing the existing single-phase ME hardware. Initially, a reconfiguration of the existing single-phase unit into a three-phase one has been carried out. Subsequently, an optimization of the ME stator has been performed. The hardware evaluations have been performed in parallel to the development of design methodologies suitable for future starter/generator design. The three-phase ac ME design needs to satisfy the required current output at both start and generate modes with minimum input voltampere rating of the inverter required to drive the ME. The ME's output capabilities have been predicted in all operating conditions using a computationally efficient combination of 3-D finite-element models together with a lumped-parameter circuit approach. The theoretical findings from the analyses have been validated on a prototype ME, confirming that the three-phase ac ME is capable of generating the required current output in both operating modes. The developed modeling system allowed for the identification of the ME's optimal operating points with regard to the minimum input apparent power (in voltamperes) at the required output power and a methodology for the accurate and computationally efficient evaluation of future starter/generator designs. [ABSTRACT FROM AUTHOR]

This paper presents an investigation into proximity losses in end-windings informed from 3-D finite-element analysis of an ac power inductor. The proximity effect in winding conductors is a result of circulating ac currents caused by magnetic fields generated by nearby conductors. The calculated results confirm that the effects within the end-winding while significant are lower compared to those with the active length of the conductors. The common approach of predicting proximity losses using 2-D field analyses accounts for active length of the conductors only, and therefore, an end-winding correction factor is needed to obtain a more accurate estimate of loss. The theoretical prediction of losses within the inductor has been validated experimentally on a prototype inductor. A simple method to account for the interdependence of ac loss and temperature is presented and is shown to differ significantly from the well-known dc variation of resistance with temperature. [ABSTRACT FROM PUBLISHER]

This paper presents a methodology for analyzing the thermal performance of compact planar wound components. A high-energy-density prebiased choke is used to demonstrate and validate the proposed approach. Loss predictions from electro. magnetic finite-element analyses are coupled to an equivalent lumped-circuit thermal model and used to determine the operating thermal envelope for the wound component. Results from the proposed method are directly compared with test measurements taken from the prototype choke and are shown to be in good agreement. A sensitivity analysis indicates that copper loss is the dominant component in such devices and that ac resistance effects are more prominent than core loss. [ABSTRACT FROM AUTHOR]

This paper reports the application of silicon bipolar junction transistor (BJT) modeling techniques to the modeling of dynamic behavior of high-voltage 4H-SiC BJTs, and the experimental validation thereof. High-voltage silicon BJTs are impractical due to their low current gain that requires a bulky base driver. Emergence of high-voltage 4H-SiC vertical NPN BJTs with a tenfold higher gain enables the application of efficient drivers, with ratings close to those of IGBTs. This paper demonstrates the advantages offered by 4H-SiC BJTs by means of wide-scale measurements at 800 V and 10 A in a range of temperatures up to 175 °C and adjusted base driver switching rates. This paper shows that the turn-off storage delay in the SiC BJT is two orders of magnitude lower than that of the silicon device. It also shows that the turn-on switching transients of SiC device are by an order of magnitude and the turn-off transients are by two orders of magnitude faster than that of its silicon counterpart, resulting in a tenfold reduction of the switching energy. It also demonstrates the temperature dependence of switching transients of the silicon BJT, and the relative temperature-invariance of the SiC device's performance. This paper concludes with validation of the transient models for the 4H-SiC NPN BJT, showing that the model is sufficiently accurate for transient switching and loss calculations. [ABSTRACT FROM AUTHOR]

Metal additive manufacturing enables production of copper and aluminium parts exhibiting unmatched geometric freedom, giving rise to new possibilities in the winding design of electrical machines and wound passive components. Early adopters of additive manufacturing are primarily concerned with improving power-density or efficiency by increasing slot fill-factor, optimizing end-winding topology or incorporating cooling channels into the conductors. In this article, a method of minimizing the elevated ac loss present in open slot electrical machine topologies is presented, in which individual conductor profiles are shaped according to their magnetic environment. Such shaped profile windings are shown to exhibit operating mode dependent loss behavior giving them potential for efficiency improvement in applications with a dominant operating mode such as electric vehicle traction, aerospace propulsion fans, and generators. For the first time, the conductor shaping method and associated benefits are demonstrated by the retrospective design, additive manufacture, and experimental test of a three-phase set of shaped profile windings for a concentrated wound, open slot, permanent magnet electrical machine. [ABSTRACT FROM AUTHOR]

A highly efficient 5-kW bidirectional dc–dc converter power stage operating from a 400-V supply implementing super-junction (SJ) metal–oxide–semiconductor field-effect transistor (MOSFETs) is presented. SJ MOSFETs have low on-state resistances and low switching losses. However, their application in voltage-source converters can be compromised by the reverse recovery behavior of their intrinsic diodes and their highly nonlinear output capacitances. A series switching-aid circuit is used to control the output capacitance charging current. The dead times between switching transitions are assessed and optimized in order to deactivate the intrinsic diodes. The combination of these techniques enables very high efficiencies to be attained. Calorimetric measurements indicate a full-load efficiency of 99.1% for the prototype 5-kW dc–dc converter power stage. A loss reduction of approximately 50% is achieved with the prototype converter power stage when compared to an equivalent insulated-gate bipolar transistor (IGBT)-based power stage. Lastly, a loss versus duty cycle function is experimentally determined, which can be used to inform the design of a maximum efficiency point tracking system. [ABSTRACT FROM AUTHOR]

This paper presents an approach to the thermal design of an ac machine where the application requires low-duty transient operation. To provide accurate temperature predictions, the design process has been informed with experimental data from tests on a stator-winding sector (motorette). These have been shown to be a time and cost-effective means of calibrating the thermal model of a full machine assembly, prior to manufacture of the final design. Such an approach is usually adopted in design analysis of machines with a concentrated winding topology. Here, the motorette testing has been extended to a machine with a distributed winding. Several alternate slot liner and impregnating materials have been evaluated to maximize the heat transfer from the winding body into the machine periphery, and a total of nine stator section samples have been manufactured and tested. The performance tradeoffs between the various combinations are discussed in detail alongside their ability to satisfy the design requirements. Based upon these experimental results, the most promising designs have been selected for transient duty analysis against the design specification. A lumped parameter thermal model has been used for this purpose and calibrated using the data from the subassembly testing. Experimental results on the full machine assembly are presented, showing good correlation with the motorette calibrated thermal model. [ABSTRACT FROM AUTHOR]

This paper presents results from an analysis of thermal behavior for alternative slot liner insulation and varnish impregnation materials used in the construction of electrical machines. These materials are typically characterized by the individual material properties altered to suit a particular application. However, the manufacturer provided material data are usually inadequate when comparing the complete insulation systems. This research is focused on the conductive heat transfer from the winding body into the machine periphery in context of the complete insulation system. An experimental approach using the stator-winding segment subassembly has been adopted here, for the representative in situ heat transfer measurements. The effects of impregnation “goodness,” in volume manufacture repeatability, and individual versus in situ material physical properties on the stator-winding thermal behavior are discussed in detail. The results suggest that the use of a particular slot liner insulation and varnish impregnation has a prominent impact on the winding heat transfer, as well as appropriate manufacture and assembly processes used. The experimental work has been supplemented with theoretical analysis to provide a more comprehensive insight into the winding heat transfer phenomena; in particular the winding-to-slot contact thermal resistance. [ABSTRACT FROM AUTHOR]

This paper investigates components of mechanical loss together with heat transfer effects in an axial-flux permanent-magnet motor. The mechanical loss components generated within electrical machines are well known; however, their prediction or derivation has not been widely reported in the literature. These, together with the electromagnetic loss sources and heat transfer effects, are crucial and must be accounted for when considering high-power-density, high-speed, and/or compact machine designs. [ABSTRACT FROM PUBLISHER]

Accurate prediction of temperature-dependent ac winding loss effects is crucial in the design of electrical machinery. Average ac winding loss as a function of operating frequency is commonly characterized by the ratio of the equivalent ac and dc resistances (Rac/Rdc). However, as the ac and dc components of the winding loss scale differently with temperature, a single value of Rac/Rdc derived for one temperature can be inadequate when used in thermal modeling. In this paper, methods of deriving the Rac/Rdc ratio, together with scaling techniques of the ac winding loss accounting for thermal effects, are discussed. As an alternative to computationally intensive 3-D finite-element analysis, an experimental approach based on tests on full-scale stator assemblies is proposed. A previously proposed scaling technique of the ac winding loss is discussed and developed further. The proposed techniques of deriving the ac winding loss accounting for temperature variation are illustrated using both theoretical analysis and experimental data. [ABSTRACT FROM PUBLISHER]

Initial sizing of a brushless radial-flux permanent-magnet (PM) machine is typically based on assumptions regarding the electric and magnetic loadings together with the motor's active volume. Additional design assumptions include the winding current density and magnetic flux density within the core pack. These initial design decisions are crucial as they largely define the resultant electromagnetic and thermal behaviors of the motor. The general rules regarding the choice of the initial design parameters are well known. However, the effects of the initial sizing on the final motor performance are not widely reported. This paper presents an analysis of the design considerations for a brushless open-slot radial-flux PM hub motor. A number of alternative motor designs are compared to demonstrate the effect of the initial design decisions on the final motor performance. Both electromagnetic and thermal aspects of the motor design are considered. The design variants are characterized by the same electric and magnetic loadings together with active volume while the winding rated current density and no-load/open-circuit magnetic flux density within the core pack are varied. The employed sizing methodology combines the classical approach with a nonlinear magnetostatic finite-element solver and an optimization routine. A prototype hub motor has been manufactured to validate the theoretical findings from the design process. The experimental data show good agreement with the theoretical findings. [ABSTRACT FROM PUBLISHER]

The present paper describes an angle and speed estimation algorithm suitable for the control of a wound-field synchronous starter/generator (S/G) without position sensors for aircraft engine starting applications. The proposed control strategy is based on the injection of a high-frequency voltage signal and the subsequent demodulation of the resulting high-frequency stator current components. The coupling due to high-speed operation that degrades the estimation accuracy of similar algorithms based on signal injection is taken into account. Modeling of the machine response to a high-frequency voltage signal injected into the estimated d-axis is presented, also considering the influence of damping circuits both on the d- and q-axis. Local asymptotic stability of the proposed observer is also demonstrated. Through extensive experimental results, the proposed control technique is shown capable of guaranteeing full-torque sensorless operation at zero speed and sustained operation up to engine ignition speed. [ABSTRACT FROM AUTHOR]

A high-frequency injection sensorless rotor position estimation algorithm is incorporated directly into the finite-element design process to realize a permanent-magnet (PM) machine that is suited to zero- and low-speed sensorless control. The machine design is tightly constrained by an existing stator assembly, only enabling the redesign of the replacement PM rotor, and by the requirement that the manufacture of the resulting machine must be simple while meeting aerospace standards. Experimental results confirm that the resulting machine matches the original performance specification and that it operates under position sensorless control. [ABSTRACT FROM AUTHOR]

This paper investigates the application of reset voltage feedback for reducing errors due to droop in the signal derived from a current transformer sensing unidirectional current pulses in switched-mode power converter applications. Droop is minimised by applying a correcting voltage in series with the transformer's output terminals during the current pulse. The magnitude of the correcting voltage is based on the transformer's reset voltage sensed during the power converter's previous switching cycles. A model of the scheme is presented and a circuit is implemented. Reductions in both peak and average current droop are demonstrated. [ABSTRACT FROM AUTHOR]