Air-gap Winding Research Articles

Optimum 7 MW HTS direct-drive wind turbine synchronous generator designs with different rotor and stator iron topologies

Partially superconducting direct-drive wind turbine generators with high-temperature superconducting excitation winding enable an increase of the rated unit power, higher efficiency, and a high, adjustable power factor. The high excitation ampere-turns allow for iron topologies that differ from conventional permanent magnet-excited generators. This study compares four different iron topologies for 7,mathrm{MW} rated power and 8.33 rpm with a slotted stator or stator air gap winding regarding technical key characteristics and economic aspects. The generator designs are numerically optimized based on 2D finite element simulations. Maximum efficiency and the most lightweight designs are obtained with the stator air gap winding, whereas all-iron topologies prove advantageous regarding the trade-off between small active mass and high-temperature superconductor material consumption. The costs of the cryogenic cooling system and of the full converter are both found to be less than 15% for a wide range of generator designs.

Litz wire loss performance and optimization for cryogenic windings

AbstractLitz wires operating in a cryogenic environment can potentially improve both the efficiency and power density of electrical machines and passive components. However, due to the low resistivity and high magnetic fields, eddy‐current losses may become significant in cryogenically cooled windings, especially in airgap winding arrangements or in the case of significant slot leakage fields, unless the litz wire parameters are carefully chosen. A framework for litz wire loss performance optimization and experimental characterisation at cryogenic temperatures is provided. An optimum operating temperature for minimum loss is derived based on analytical expressions, which highlights the role of litz wire parameters, current density and external field. The proximity loss model, used to calculate the optimum operating temperature, is validated experimentally. Two test rigs with different magnetic cores were designed and built. Copper and aluminium litz wires with a strand diameter down to 0.1 mm were tested in a liquid nitrogen bath with a uniform harmonic external magnetic field up to 0.5 T peak and a frequency up to 1 kHz. Measurements show good agreement with the theoretical results and confirm that the proposed model can be confidently used during the preliminary design of cryogenic windings.

Analytical Design and Optimization of Axial Flux Permanent Magnet Machines With Slotless Structure

The three-dimensional (3-D) finite element (FE) analysis is an extremely time-consuming task in the design and optimization of axial flux permanent magnet (AFPM) machines. The aim of this article is to present a fast and accurate design model of slotless AFPM machines. Thanks to the separation of active- and end-winding parts, the overhang structure, which has widely been used in the air gap winding, can be handled by the proposed method. The winding eddy current loss, a primary source of losses in the air-core structure, is incorporated into the analytical model. In addition to the high accuracy, this approach makes it possible to evaluate machine performances only once without needing to rotate the rotor position. As a result, the proposed analytical model can significantly reduce the computational time, preferred in applying an optimization algorithm. Finally, the accuracy of the proposed method is verified by both FE and experimental results.

Influence of Adhesive Tapes as Thermal Interface Materials on the Thermal Load of a Compact Electrical Machine

In this article, a novel form of thermal interface material (TIM), represented by three industrially manufactured pressure-sensitive adhesive (PSA) tapes with electrical insulating properties, is characterized regarding its applicability in an electric motor with air-gap winding. Firstly, the adhesion performances, in terms of the winding process, were investigated experimentally. Here, every TIM shows sufficient shear strength for the wire–TIM joints, as well as peel adhesion to the laminated iron core. Secondly, the thermal–physical properties of the TIMs are inspected experimentally via laser flash analysis (LFA) and differential scanning calorimetry (DSC). For every TIM, the value of the thermal resistance can double if the relatively smooth surface (Ra = 0.2 μm) of the adjacent layers is interchanged with a rougher one (Ra = 2.0–3.7 μm). Additionally, the TIM’s performance at the system level is examined. Therefore, a flat test section, according to the specifications of the original motor, is studied experimentally and numerically utilizing infrared (IR) thermography and the finite element method (FEM). The focus is set on the heat flow and temperature distribution in the test section under varying thermal loads, mass flow, and variety of TIMs.

Combined Optimal Torque Feedforward and Modal Current Feedback Control for Low Inductance PM Motors

Small sized electric motors providing high specific torque and power are required for many mobile applications. Air gap windings technology allows to create innovative lightweight and high-power electric motors that show low phase inductances. Low inductance leads to a small motor time constant, which enables fast current and torque control, but requires a high switching frequency and short sampling time to keep current ripples and losses in an acceptable range. This paper proposes an optimal torque feedforward control method, minimizing either torque ripples or motor losses, combined with a very robust and computation-efficient modal current feedback control. Compared to well-known control methods based on the Clarke-Park Transformations, the proposed strategy reduces torque ripples and motor losses significantly and offers a very fast implementation on standard microcontrollers with high robustness, e.g., against measurement errors of rotor angle. To verify the accuracy of the proposed control method, an experimental setup was used including a wheel hub motor built with a slotless air gap winding of low inductance, a standard microcontroller and GaN (Gallium Nitride) Power Devices allowing for high PWM switching frequencies. The proposed control method was validated first by correlation of simulation and experimental results and second by comparison to conventional field-oriented control.

Multilayer air gap winding designs for electric machines: theory, design, and characterisation

Here, the authors present a magnetic circuit design with a multilayer air gap winding. This design provides a new degree of freedom in the design process. The approach is based on a single layer air gap winding, which demonstrates the ability to scale, permitting application in small and large electric machines. A meandering structure with coils arranged parallel to the rotation axis are mounted in several layers on a slotless magnetic core. The authors consider a fixed ohmic power loss boundary condition to compare this approach with an existing validated machine design. Machine current is reduced with each winding layer, while self-inductance and induced voltage are increased. Since the air gap height is increased with every additional layer, the useable magnetic flux density decreases. Accordingly, an utilisable range in which optimal torque output is obtained can be described analytically. The authors provide comparison between a single-layer winding design, which achieves the global optimum torque, and a four-layer winding design, which gives a 16-fold increase in the phase self-inductance. Additional thermal simulations prove that the multilayer design give the same ohmic power losses as the single layer design, without having any thermal issues and hot spots.

Modular airgap windings for linear permanent magnet machines

This study proposes an offset concentrated wound air-core linear synchronous machine for launch applications where reduced supply volt-amp per newton (VA/N) and attraction force is essential. A winding factor model based on the conductor distribution is derived and used to analyse the winding factor of possible topologies. Two configurations, 3-coil/4-pole and 12-coil/14-pole, have high fundamental winding factors and are selected for the design comparisons. Machines using single- and double-sided offset windings are compared. The results obtained for these selected designs show that the 3-coil/4-pole design with offset winding provides the highest thrust due to its higher winding factor, but the 12-coil/14-pole design has lower supply VA/N. The implementation of the offset windings reduces VA/N. A significant reduction can be seen when the long stator short rotor arrangement is considered. A half-segment prototype is built and tested. The experimental results obtained validate both the algebraic and three-dimensional finite element analysis.

Electromagnetic Design of a Large-Scale Double-Stator Direct Driving HTS Wind Generator

High-temperature superconducting (HTS) direct driving wind generators have been extensively researched due to their inherent high torque density and small mass. In this paper, a double-stator HTS wind generator with rotating HTS field windings is proposed to further improve the torque density. The electromagnetic design of this generator is made and electromagnetic performances of different topologies with air-gap windings or iron teeth are compared and analyzed. In addition, two regular HTS wind generators and one double-stator permanent magnet wind generator with the same scale are put forward to make comparison with proposed double-stator HTS machines, and the feasibility and practical value of these five topologies are evaluated in terms of the weight, volume and cost of active materials. The results reveal double-stator HTS wind generator has great advantages in volume and mass reduction.

Parametric model of electric machines based on exponential Fourier approximations of magnetic air gap flux density and inductance

PurposeThis study aims to present a parametric model of a novel electrical machine, based on a slotless air gap winding, allowing for fast and precise magnetic circuit calculations.Design/methodology/approachApproximations of Fourier coefficients through an exponential function deliver the required nonlinear air gap flux density and inductance. Accordingly, major machine characteristics, such as back-EMF and torque, can be calculated analytically with high speed and precision. A physical model of the electrical machine with air gap windings is given. It is based on a finite element analysis of the air gap magnetic flux density and inductance. The air gap height and the permanent magnetic height are considered as magnetic circuit parameters.FindingsIn total, 11 Fourier coefficient matrixes with 65 sampling points each were generated. From each, matrix a two-dimensional surface function was approximated by using exponentials. Optimal parameters were calculated by the least-squares method. Comparison with the finite element model demonstrates a very low error of the analytical approximation for all Fourier coefficients considered. Finally, the dynamics of an electrical machine, modeled using the preceding magnetic flux density approximation, are analyzed in MATLAB Simulink. Required approximations of the phase self-inductance and mutual inductance were given. Accordingly, the effects of the two magnetic circuit parameters on the dynamics of electrical machine current as well as the electrical machine torque are explained.Originality/valueThe presented model offers high accuracy comparable to FE-models, needing only very limited computational complexity.

High-Frequency Litz “Air-Gap” Windings for High-Power Density Electrical Machines

ABSTRACTThis paper describes the development of high-frequency air-gap windings for a 1 MW, 96% efficient permanent magnet (PM) motor, targeted at 13 kW/kg. Due to the power level and high-specific power requirements, special attention has to be paid to the electromagnetic, thermal, and mechanical performance for the high-frequency coils. Specifically, AC losses have to be minimized while fill factor and thermal conductivity of the assembly are maximized. The paper presents analytical and finite element-based AC loss studies along with experimental validation by indirect balanced calorimetry, and shares the results which should be scalable for other large machines. Hardware development and corresponding manufacturing process are also described.

Winding Machine for Automated Production of an Innovative Air-Gap Winding for Lightweight Electric Machines

This paper presents a newly developed winding machine, which enables an automated production of stator-mounted air-gap windings with meander structure. This structure has very high accuracy requirements. Therefore, automation is realized by the interaction of 15 actuators and a compound construction with 13 degrees of freedom. The programming works with discrete open-loop motion control to generate the kinematics. Above all, a flexible prototype of the winding machine is developed, manufactured, and tested for a motor with external rotor. Finally, experimental results of the developed automation for air-gap windings with meander structure are presented.

Boosting Power Density of Electric Machines by Combining Two Different Winding Types

Abstract: This paper presents a novel magnetic circuit and winding design for extreme lightweight and high torque permanent magnet electric machines. Therefore, two different types of winding are combined to boost torque sharing the same magnetic circuit. One is an air gap winding, which is mounted directly on the back iron of the stator surface using the magnetic flux field in the air gap to generate Lorentz forces acting on the winding and providing one part of the torque. The other one is a slot winding, which is fixed inside small slots of the stator back iron generating magnetic forces and contributing the second part of the torque. Overall, only a small amount of iron is necessary for this novel magnetic circuit design. This results in an extreme lightweight, efficient, dynamic and high torque electrical machines for a wide range of use in mechatronics. The paper gives a design example of a wheel-hub-motor for a passenger vehicle with an extraordinary power-to-weight-ratio.

Theoretical and Experimental Investigation of Flex-PCB Air-Gap Windings in Slotless BLDC Machines

Slotless brushless dc motors find more and more applications due to their high performance and their low production cost. This paper focuses on the windings inserted in the air gap of these motors and, in particular, to an original production technique that consists in printing them on a flexible printed circuit board. It theoretically shows that this technique, when coupled with an optimization of the winding shape, can improve the power density of about 23% compared with basic skewed and rhombic windings made of round wire. It also presents a first prototype of a winding realized using this technique and an experimental characterization aimed at identifying the importance and the origin of the differences between theory and practice.

Comparative Analysis on Superconducting Direct-Drive Wind Generators With Iron Teeth and Air-Gap Winding

Superconducting direct-drive wind generators have been a research focus, since needs for wind energy development are more and more urgent. Generator stator can be with air-gap winding or iron teeth. Different stator concepts could yield different mechanical, thermal, and electromagnetic performances. This paper compares different stator configurations for 12-MW superconducting direct-drive wind generators with the help of finite element analysis software. Ferromagnetic materials in superconducting machines can help reduce reluctance in magnetic circuits and save costs of superconducting coils in rotor. Moreover, iron teeth could bring cogging torque and vibrations in generators, which can be greatly reduced with air-gap windings. In addition, iron teeth can lead to different losses, including damping shell, copper, and iron losses.

Comparison of High Pole Number Ultra-Low Speed Generator Designs Using Slotted and Air-Gap Windings

This paper describes the simple design rules that can be used to design direct-drive brushless generators for an ocean wave device. This is a very low speed device so the pole number and diameter is very large. While these machines may be large, the pole pitch and axial length is low. The application is described and air-gap wound and slotted designs are developed and simulated using analytical and finite element analysis techniques. A 248-pole design with surface rotor magnets is developed. The machines use a substantial amount of permanent magnet material and a more compact version of the slotted machine is also developed.

Stator Design for a 1000 kW HTSC Motor With Air-gap Winding

An air-gap winding (air-core) stator for a 1000 kW high temperature superconducting synchronous motor is designed by the Global R&D Center, TECO-Westinghouse Motor Company. This motor uses HTSC wire in the rotor field coils and Litz-wire cable in the stator coils. The rotor's cryogenic system cools the superconducting field coils to low temperature. Stator winding and back iron both are in a `warm stator' state. The stator coil chooses Roebel type Litz-wire with to eliminate the harmonics, cogging torque caused by slot opening, and to reduce vibration/noise. A quick algorithm is developed to design the stator. Electromagnetic and mechanical analyses are based on 3D FEA and thermal analysis is based on 2D FEA. Manufacture techniques for Litz wires are being developed, e.g., coil winding, crimping, testing and installation. The stator coil is in the manufacturing stage currently, and the complete machine will be tested by the end of 2010.

Design Optimization of Reluctance-Synchronous Linear Machines for Electromagnetic Aircraft Launch System

This paper describes the design optimization and analysis of reluctance-synchronous linear motor with a solid or laminated moving part (secondary). The calculation of the motor performance is described using analytical formulation and numerical method. The main advantage of this system is the removal of permanent-magnet secondary and replacing it with a reluctance-type secondary. The first purpose of this paper is designing a reluctance-synchronous linear motor with the maximum-output thrust force and optimum power factor. In this model, a long primary (armature) and a short secondary are evaluated. Distributed winding is considered for design and analysis. Finally, air-gap winding configuration is evaluated.

Power density comparison for various types of non-slotted double-sided axial flux PM motors

Abstract:There are two topologies for non-slotted double-sided axial flux PM (AFPM) motors.The stator of the non-slotted AFPM machine is realized by non-slotted tape wound corewith AC polyphase air gap windings and the rotor structure is formed by axiallymagnetized fan-shaped surface mounted Neodymium Iron Boron (NdFeB) permanentmagnets. Selecting an AFPM motors with high power density is an important parameterin applications. So, comparison of power density between different topologies ofdouble-sided AFPM motors seems to be necessary.In this paper, the sizing equations of axial flux non-slotted one-stator-two-rotor(TORUS) and two-stator-one-rotor (AFIR) type PM motors is presented andcomparison of the TORUS and AFIR topologies in terms of power density is illustrated.Finally a high power non-slotted double-sided AFPM motor is introduced in the paper.

The Inductance Computation for the Passive Compulsator

In order to decrease the internal impedance of the passively compensated pulsed generator (or passive compulsator) and enhance its power density, air gap windings are generally selected as the armature winding of the passive compulsator, for its inductance is much lower than that of the general embedded windings in the machine slot. Due to that the peak power of an electromagnetic launcher system driven by a compulsator is simply determined by system impedance and the output voltage of the compulsator, it becomes necessary to gauge the total inductance of the air gap windings accurately with straight forward calculation, while neglecting the easily obtained resistance parameters, particularly in its design period for electromagnetic launchers. By taking into account the flux compression effects generated by the eddy current of the continuous conductive shield in the compulsator, the magnetic field of the armature coils in the air gap of generator is analyzed by Ampere's law. The analytical solution to the inductance of armature air gap coil is deduced. This one equation solution is used for re-designing a 25 MW compulsator with the final calculation result: 4.04 muH. The measurement on the compulsator (4.05 muH) verified the accuracy of the analysis equation.

Eddy Current Effects in a Switched Reluctance Motor

This paper investigates the losses that may occur in the windings of a switched reluctance machine due to additional eddy currents. Essentially, turns on the surface of the leading-edge coil side of the stator pole will experience a magnetic field during the conducting cycle in a similar manner to an airgap winding. A finite element model is developed that splits the coils into individual turns so that the losses in each turn can be calculated individually