Electrical Machine Windings Research Articles

Prediction of Losses and Efficiency for Three-Phase Induction Machines Equipped With Combined Star–Delta Windings

Combined star–delta windings in electrical machines result in a higher fundamental winding factor and cause a smaller spatial harmonic content. This leads to lower I 2 R losses in the stator and the rotor windings, and thus to an increased efficiency. However, compared with an equivalent six-phase winding, additional spatial harmonics are generated due to the different magnetomotive forces in the star and the delta parts of the winding. In this paper, a complete theory and analysis method for the analytical calculation of the efficiency for induction motors equipped with combined star–delta windings is developed. The method takes into account the additional harmonic content due to the different magnetomotive forces in the star and delta parts. To check the analysis’ validity, an experimental test is reported both on a cage induction motor equipped with a combined star–delta winding in the stator and on a reference motor with the same core, but with a classical three-phase winding.

Topology Evaluation of a Slotless High-Speed Electrical Machine with Stator Core Made of an Amorphous Alloy for the Aerospace Industry

The paper presents the experimental research of different slotless high-speed electrical machine topologies for integration into the auxiliary power unit gearless. It was shown that the 6-poles and 8-poles with the concentrated windings and the wound stator core are the most effective for aerospace industry. Based on the experimental and computer research of different slotless high-speed electrical machine topologies. It was found that for use in the gearless auxiliary power unit of modern and perspective aircrafts, the use of the 6-poles and 8-poles slotless high-speed electrical machines with concentrated windings and wound stator core is the most effective. This topology allows providing the sufficient rigidity of the slotless high-speed electrical machine stator core, the minimum length of the frontal part and the minimum weight and size parameters of the slotless high-speed electrical machine at the maximum efficiency. In addition, in this case, the slotless high-speed electrical machine windings have a high inductance, which leads to limit the magnitude of short circuit currents.

Perturbation Finite Element Method for Efficient Copper Losses Calculation in Switched Reluctance Machines

Copper losses dissipated in the windings of electric machines are the sum of classical ohmic dc losses and additional ac eddy current losses. In fact, the level of eddy current losses is strongly correlated with the manner of disposition of coil conductors in machine slots. Then, to improve the efficiency in electric machines, the selection of an optimal winding configuration becomes substantial. Since eddy current losses derive from the strong electromagnetic coupling between current density and time-dependent magnetic field, which cannot be solved easily, numerical analyses, such as particularly the one using the finite element (FE) method, are often used. As for the FE modeling, it can employ moving band technique to perform the rotor motion and Newton-Raphson iterations to deal with the nonlinear behavior of magnetic circuits. It leads then to a substantial computational time that hinders any process of conception or optimization of winding geometries. To overcome this issue, a 2-D FE model reduction based on the perturbation method is proposed. It starts from one approximate FE solution of a simplified complete machine modeling to find fast but accurate solutions in slots subdomains when any variation of geometrical or physical data occurs. It allows adapting nonconforming meshes and provides clear advantages in repetitive analyses when we search the optimized winding configuration for a given number of turns.

Determination of Winding Lumped Parameter Equivalent Circuit by Means of Finite Element Method

In this paper, the finite element method is used in order to determine the electrical parameters of a machine winding lumped element model. Thanks to this model, it will be possible to determine the electrical behavior of electrical machine winding fed by a power converter and thus to study the distribution of the turn-to-turn maximum voltage stress as early as the design process. The suitability of the method is validated by comparison with measurements.

Thermal Conductivity Evaluation of Fractional-Slot Concentrated-Winding Machines

The use of fractional-slot concentrated windings (FSCW) in electrical machines allows more compact, efficient, and reliable design with respect to machines equipped with distributed windings. However, an electromagnetic design linked to a thermal analysis of the electrical machine is mandatory to achieve the desired performance, and to fulfill the requirements of efficiency and reliability. One of the most critical issues in thermal design of electrical machines is to assign fair values for the input parameters of the thermal simulation models, particularly those related to the stator winding insulation system. This paper deals with the assessment of the equivalent thermal conductivity of the insulation system of FSCW machines. For this purpose, three FSCW electrical machines for different applications were evaluated via an experimental method based on a dc thermal transient test. Whereas the investigated machines present different characteristics among themselves, different approaches were required to properly estimate the thermal conductivity.

Nondestructive testing of the quality of winding impregnation in electric machines

The topicality of this work is accounted for by a need to upgrade the evaluation of the quality of winding impregnation in electric machines and to raise their safety margin and reliability. Our goal is to substantiate the possibility of using the measured capacitance of the winding of an electric machine (with respect to its magnetic core) for testing the impregnation of cavities in the major insulation and assessing metrological characteristics of the proposed testing technique. Methods of electric measurements for studying the capacitance characteristics of electric-machine windings, formulae for calculating the dielectric constant of binary mixtures, and techniques for assessing the errors of indirect measurements have been used.

The Impact of Thermal Degradation on Properties of Electrical Machine Winding Insulation Material

Interturn stator short circuits can develop quickly leading to serious damage of an electric machine. However, degradation mechanisms of winding insulation material are not yet fully understood. Therefore, the main contribution of this paper is the analysis of the impact of thermal ageing on the electrical properties of the thin-film winding insulation. The insulation samples have been aged thermally at 200 °C–275 °C and for 100–1600 h. After ageing, impedance spectroscopy measurements were undertaken on the samples and equivalent-circuit model (ECM) parameters fitted for each measurement. This allows the impact of thermal ageing on ECM parameters to be analyzed, giving insight into the changes in the electrical properties of the insulation. Finally, high voltage was applied to the samples aiming to identify the breakdown-voltage (BV) characteristics of the insulation material.

Fiber Bragg Grating Sensor for Electric Field Measurement in the End Windings of High-Voltage Electric Machines

This paper reports proof-of-principle of an optical fiber Bragg grating (FBG) operating as an electric field sensor. The system is based on a semiconductor diode laser, emitting around 1577.5 nm that interrogates the fiber sensor around its maximum reflectivity point. The application of a strong electric field causes a small variation of the optical path length of the waveguide due to electrostriction and thus perturbs the grating peak reflectivity wavelength. Calibration and dynamic range tests are performed using a parallel-plate capacitor to create a variable electric field (1–100 kV/cm). A great potential is pointed out in view of possible field applications for voltage graduation in power generators.

A New Software Tool for Design, Optimization, and Complete Analysis of Rotating Electrical Machines Windings

In this paper, a new software tool developed by the authors for ac winding design, optimization, and complete analysis is presented. In particular, this software can be used as a valid aid in design and analysis for a wide variety of motor and generator windings with generic number of phases (including the case of those machines in which some slots are left empty), pole pairs, slot number, and so on. The calculation of winding factors and the evaluation of their harmonic distribution is also accomplished. The software is implemented in the MATLAB programming environment. By means of some examples, covering the most relevant winding types, the capabilities of this software will be shown. In particular, integer and fractional slot, single- and double-layer winding, concentrated windings, imbrication, belt widening, and belt shifting techniques can be treated to reduce particular harmonics in the electromotive force (EMF) or in the magnetomotive force (MMF). Finally, the connections, the winding factor harmonic distribution, the total harmonic distortion (THD) of EMF and spatial distribution of both the MMF, the Görges polygon, and the flux density are shown together with an estimate of the differential leakage and the air-gap flux density spatial distribution. The software presented in this paper is very modular, highly structured, and flexible due to the conspicuous number of operations that it can fulfill. Clearly, for the sake of brevity and clarity, the authors limit to represent the main features of the software, without going into specific details of programming and coding. All the main mathematical relationships that are useful to solve the problem of design and optimization of the windings of rotating machines are hereafter presented.

Replacing Copper with New Carbon Nanomaterials in Electrical Machine Windings

Electrical machines have significant improvement potential. Nevertheless, the field is characterized by incremental innovations. Admittedly, steady improvement has been achieved, but no breakthrough development. Radical development in the field would require the introduction of new elements, such that may change the whole electrical machine industry system. Recent technological advancements in nanomaterials have opened up new horizons for the macroscopic application of carbon nanotube (CNT) fibres. With values of 100 MS/m measured on individual CNTs, CNT fibre materials hold promise for conductivities far beyond those of metals. Highly conductive, lightweight and strong CNT yarn is finally within reach; it could replace copper as a potentially better winding material. Although not yet providing low resistivity, the newest CNT yarn offers attractive perspectives for accelerated efficiency improvement of electrical machines. In this article, the potential for using new CNT materials to replace copper in machine windings is introduced. It does so, firstly, by describing the environment for a change that could revolutionize the industry and, secondly, by presenting the breakthrough results of a prototype construction. In the test motor, which is to our knowledge the first in its kind, the presently most electrically conductive carbon nanotube yarn replaces usual copper in the windings.

Выравнивание напряженности электриче- ского поля в изоляции пазовых частей обмоток электрических машин

This paper describes application of the study methods based on the solution of inverse problems of mathematical physics to improve the reliability of electrical systems with multi-layer insulation. Techniques of alignment of intensity of electric field and alignment of margin of safety in isolation of angular zones of slotted area of windings of electrical machines are developed. The technique is based on the determination of the field strength using the finite element method for the solution of the direct problem of electrostatics and the gradient method for minimizing the target functional. Depending on the objectives of the research, three options are functional, taking into account the permissible maximum, the average field intensity for all layers of insulation and different maximum permissible field strength for the individual layers. Example of the techniques application is given. The results show that the alignment of intensity of electric field and alignment of margin of safety in isolation in order to increase the reliability of electrical systems can be effectively on a basis of the solution of inverse problems of electrostatics.

Winding Factor of Permanent Magnet Synchronous Machines with Fractional Slot Concentrated Windings

The use of fractional slot concentrated windings in electrical machines has a variety of advantages. A winding factor is one of the most important parameter which is necessary for electrical machines designing and modeling.

Design of a High-Force-Density Tubular Motor

This paper deals with the design, construction and experimental verification of a high force density, tubular, linear, permanent magnet motor, driven from a high power density matrix converter for an aerospace application. The work also describes the implementation and experimental verification of a novel, thermal management technique for the phase windings of electrical machines. The technique introduces a higher thermal conductivity path between the centre of the slot and the cooling arrangement, thus increasing the heat flow away from the slot centre. An introduction to the design of the motor is first given, after which an introduction to the technique is presented. A study of how the implementation of the technique affects motor performance is then presented. A detailed overview of the construction aspects is highlighted and finally, experimental validation is used to illustrate the comparison between the predicted results and the measured results, obtained from an instrumented, test rig.

A cable feeder tool for robotized cable winding

Cable winding is an alternative technology to create stator windings in large electrical machines. Today such cable winding is performed manually, which is very repetitive, time-consuming and therefore also expensive. This paper presents the design, function and control system of a developed cable feeder tool for robotized stator cable winding. The presented tool was able to catch a cable inside a cable guiding system and to grab the cable between two wheels. One of these wheels was used to feed cable through the feeder. A control system was integrated in the tool to detect feeding slippage and to supervise the feeding force on the cable. Functions to calculate the cable feed length, to release the cable from the tool and for positional calibration of the stator to be wound were also integrated in the tool. In validating the function of the cable feeder tool, the stator of the linear generator used in the Wave Energy Converter generator developed at Uppsala University was used as an example. Through these experiments, it was shown that the developed robot tool design could be used to achieve automated robotized cable winding. These results also complied with the cycle time assumptions for automated cable winding from earlier research. Hence, it was theoretically indicated that the total winding cycle time for one Uppsala University Wave Energy Converter stator could be reduced from about 80h for manual winding with four personnel to less than 20h in a fully developed cable winding robot cell. The same robot tool and winding automation could also be used, with minor adjustments, for other stator designs.

Interaction of conductor with polymeric materials (XLPE/EPR) at partial discharges

A novel investigation into the behavior of the interface between an insulating material and a conductor subjected to sinusoidal stimulus is presented. Both copper and aluminum wires are used in the windings of electrical machines and also as power cable conductors. During in-service operation, partial discharges (PD) may be initiated in the insulation systems. The paper presents experimental results showing the interaction of different kinds of conductors and dielectric materials on the mechanism of partial discharges in the insulation system. The dedicated test setup is designed to perform experiments with various electrodes, with special focus on copper. Two kinds of specially prepared insulation samples were tested, ethylene propylene rubber (EPR), cross-linked polyethylene (XLPE). These insulating materials were subjected to a sinusoidal stimulus at predefined time intervals and were exposed to PD action. The paper comprises the results of PD measurements and microstructural observations performed by means of scanning electron microscope (SEM) coupled with energy dispersive analysis (EDS). The morphological and elemental analysis reveals the interaction of conductor on polymeric material degradation. Two zones were distinguished, migration and erosion. The migration of atoms of electrode material into the dielectric substrate due to PD activity was observed.

The detection of latent defects in the insulation of generators and large electrical machines

It is shown that, for standard tests of the windings of electrical machines subjected to the action of a test voltage, the insulation of the frontal parts and frontal windings inside the phases, and also the positions where the lead rods are connected to the busbars and the winding retaining rings, in practice are not subjected to the action of the test voltage. Moreover, when testing the insulation of the windings with a test voltage, charring of the breakdown channel with destructive lamination of the dielectric barrier of the insulation and the penetration of soot particles along the layers formed occurs. These drawbacks will be completely eliminated by using the KVIS-40 equipment, which enables nondestructive testing of the insulation to be effectively carried out using an increased rectified voltage.

A Thermal Improvement Technique for the Phase Windings of Electrical Machines

In electrical machines, a higher torque/force density can usually be achieved by increasing the current density in the windings. However, the resulting increase in copper losses leads to higher temperatures in the coils, particularly in the center of the slots where the thermal resistance to the ambient/cooling surfaces is highest. In this paper, a novel, simple technique is presented in which a higher thermal conductivity path between the center of the slot and the cooling arrangement is created, thus increasing the heat flow away from the slot center. A lumped-parameter thermal model is presented and used along with finite-element analysis to investigate the effectiveness of the proposed technique. The lumped-parameter model is also used for optimizing the high conductivity path for maximum air-gap shear stress and to obtain a compromise between the reduced slot area and the improved temperature distribution. Experimental validation is then presented to compare the predicted results with the measured results on a purposely built instrumented setup.

Thermal Model With Winding Homogenization and FIT Discretization for Stator Slot

The aim of the method detailed in this paper is to get an equivalent thermal model of a stator slot, in order to simplify the calculation of desired temperatures in an electrical machine winding. This study is divided in two steps: First, the equivalent thermal conductivity is deduced from a homogenization of the winding, and next, a discretization is achieved using the finite integration technique considering transient analysis. In order to evaluate the method, results from the equivalent model are compared with finite element simulations considering two slot geometries.

AC Losses of Superconducting Racetrack Coil in Various Magnetic Conditions

High-temperature superconductors have created the opportunity for a step change in the technology of power applications. Racetrack superconducting coils made from YBCO coated conductors have been used in several engineering applications including SMES, rotor or stator windings of electric machines. AC loss is one of the most important factors that determine the design and performance of superconducting devices. In this paper, a numerical model is developed to calculate the AC losses in superconducting racetrack coils in different magnetic conditions. This paper first discusses the AC losses of the coils in self-field or external field only. It then goes to investigate the AC losses of the coils being exposed to AC ripple field and a DC background field. Finally, the AC losses of the coils carrying DC current and being exposed to AC field are calculated. These two scenarios correspond to using superconducting coils as the rotor field winding of an electric machine.

Homogenization of Form-Wound Windings in Frequency and Time Domain Finite-Element Modeling of Electrical Machines

In this paper, the authors deal with the finite-element (FE) modeling of eddy-current effects in form-wound windings of electrical machines using a previously proposed general frequency- and time-domain homogenization method. By way of demonstration and validation, a real-life 1250-kW induction machine with double-layer stator winding is considered. The skin and proximity effects in one stator conductor (copper bar) are first quantified by means of a simple low-cost FE model, leading to complex and frequency-dependent coefficients for the homogenized winding (reluctivity for proximity effect and conductivity or resistance for skin effect). These complex coefficients are subsequently translated into real-valued and constant coefficients that allow for time-domain homogenization when introducing a limited number of additional degrees of freedom in the FE model. All results obtained with the homogenized model (considering one conductor or a complete slot) agree well with those produced by a brute-force approach (modeling and finely discretizing each conductor).