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Abstract— The development of production methods, research

on material engineering and analysis tools for energy conversion processes establish a powerful design environment for any unconventional electromagnetic or electromechanical device. The design for manufacturability becomes the one of the main goals of the design apart from cost and performance improvement. The main focus of this paper lies on the development of production methods, the design of the product, a wave-winding, and the component assessment in an electrical machine. The production of non-complex coil with maximized performance capability of the molded core machine is the target for the design of the winding and also the design for the winding machine. Four different types of wave-windings are described and evaluated. One of them, wind and bend with conventional wire, has main focus in this work, and a prototype of the winding machine is built to produce the single-layer whole-lap wave-winding. The production method, manufacturing process and the processed component features are analyzed in this paper.

Index Terms— Alternative production, Design for manu-facturability, Winding, Wave winding.

I. INTRODUCTION he production technology and material engineering formulates a number of fundamental guidelines for the

design of electromagnetic or electromechanical energy converters. On the contrary, the design of electrical machines seeks for the rational and easily produced solutions with lower cost and improved performance. The research related to the powder technology and molding techniques has leaded us to new exciting solutions for electrical winding production [1]. Unconventionally, the coil or winding has to be produced prior to low pressure molding or in parallel to compression molding of the iron powder core [2][3]. More conventionally, the insertion of coils or the creation of a winding takes place in the slotted structure of the soft magnetic core. Therefore, special requirements are stressed when a coil is produced for a

Manuscript received August 15, 2012. The present work was carried out

with the generous support of the Swedish Foundation for Strategic Research (SSF), the Foundation for Strategic Environmental Research (Mistra), and the Sustainable Production Initiative (SPI). Their support is gratefully acknowledged.

L. Svensson, K. Frogner and M. Andersson is with the Division of Production and Materials Engineering, and A. Reinap, C. Högmark and M. Alaküla is with the Division of Industrial Electrical Engineering and Automation, Lund University, Sweden. Box 118, 221 00, Lund, Sweden.

(e-mail: leif.svensson@iprod.lth.se, Kenneth.frogner@iprod.lth.se, avo.reinap@iea.lth.se, conny.hogmark@iea.lth.se, mats.andersson@iprod.lth.se, mats.alakula@iea.lth.se).

powder core appliance. The main guidelines for the construction of powder core machines are: a) geometric shape determination of the least complex winding, b) specification of manufacturing methods and production steps, and c) actual realization of the coil and the fulfillment of the design and production requirements. The common target for the design and the production requirements of a coil are: a) net shape dimensions, rigidness of the constructed body and tolerances, b) maximized packing density (fill factor), and c) choice of production method and manufacturability aspects. As a matter of fact the electromagnetic design for manufacturability often compromises the performance to the practical realization. Therefore, the strength of machine design that in first hand solves the problem for performance is to consider deeply enough the manufacturability and the related material engineering. As the outcome, the material engineering and the production of molded powder cores for electrical machines has influenced the electromagnetic design to look towards wave-windings [4] as a possibility to produce high performance electrical machines, within the advantages and the limitations that the soft magnetic core has [5]. Thus, the goal of this article is to look towards manufacturability and automated production of the wave-windings. Traditional arrangements when manufacturing of coils for electric machines utilizes pre-manufactured magnetic cores with main insulation or parts of it as support during the winding process in a typical top-down approach. Separating the production of each part has the potential of streamlining the manufacturing by eliminating bottlenecks as well as reducing the number of difficult steps. The bottom-up methodology on the other hand will create new challenges regarding precision and assembling, setting hard demands on each process. This article will present a machine with the ability to shape a ring of round or rectangular copper wires into a finished wave coil, ready to be assembled into an electric machine. The requirements on the machine can be summarized into three important items; simple and limited number of fully automated steps, high geometric accuracy and repeatability, adjustable for different coil and wires dimensions.

The advantages of wave-windings have been used in many different ways even if it has been often considered to be impractical to most laminated electrical machines. The free formed magnetic core, which is made of soft magnetic powder, can merge the wave-winding into the magnetic core and therefore increase the utilization of the materials. In order

Alternative production process for electric machine windings

Leif Svensson, Kenneth Frogner, Avo Reinap, Conny Högmark, Mats Alaküla, Mats Andersson

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to be able to take advantage of these winding designs other materials than steel sheets are more suitable, even though the laminated electromagnetic steel has higher utilization factor than any powder core, which is seen from magnetic permeability point of view [6]. As an outcome, the material for soft magnetic cores must be possible to mount in an easy way in the winding. Therefore are we using a material called Soft Magnetic Moldable Composite (SM2C). This material can be molded around the coil and by doing that get a good magnetic linkage, even if the magnetic permeability is rather poor compared to other soft magnetic cores. This article looks to machine design more from wave-winding production point of view that, once again, can show the machine production related potential of SM2C due to the powder technology and molding techniques rather than the specific features related to SM2C ability to conduct magnetic flux.

II. MACHINE DESIGN A new design of an electrical machine can have at least two

intentions. The first is to create a motor with higher performance than earlier, and the second is to be able to manufacture it more easily. The goal for this section is to count for manufacturability aspects for the machine construction.

A. Manufacturability of wave-winding When looking into machine design from the winding design

point of view, and in particular to wave-winding, the following specific criteria are distinguished:

Profile of the conductor Production of the wave-winding Insulation system

The profile of conductor can be selected from the conventional round enameled wire towards to more specific profiled wire or even more extreme – laminated or foiled coils. When looking into the process of shaping the wave-winding, mainly two processes in any order can be distinguished: deform and wound. As an outcome, four processes of producing a wave-winding are distinguished:

Bend and wind with conventional or profiled wire [3]

Wind and bend with conventional, profiled wire, or laminated coil

Wind and cut a foil or laminated coil [8] Cut and wind a foil or laminated coil[7]

The main concern in this article is to provide a manufacturing method and investigate the prototype of the winding machine that is able to produce wave-windings made of conventional or profiled wire when using wind and bend approach.

B. Design features Depending on the conductor profile and the production

method the wave-winding obtain some special features. When investigating the different approaches, the main limitation and advantage between the foil or the laminated winding and the selection of number of turns and the achieved fill factor of the coil. The laminated winding have more limitation when it

comes to the selection of winding turns, where the maximum number of turns is related to the minimum thickness of the foil or conducting band. The minimum number of turns can easily be fixed with parallel strands or foils. When it comes to the fill factor then laminated coil can easily achieve 80% fill factor while wrapped coil need to have more concern to reach to 60%. With the relation to the fill factor, the thermal conductivity of the laminated winding is higher and can also dissipate heat in lateral direction due to the lamination direction, while the thermal properties of the conventional finding in lateral direction are isotropic and lower value due to lower fill factor. One specific feature that the wave-winding with wind and bend production type with conventional round or profiled wires has, is the cross-sectional area of the winding. The area is unchangeable. On the contrary, the laminated winding forms constant stator tooth area and the cross-sectional area is different in the end turns and in the stator slots and depends on the radius.

C. Design features and parameters The reason for choosing the wave coil design is because of

the possibility to use it in many applications and that it is possible to manufacture in a simple way. For the wave coil to be effective, from production point of view, the axial length cannot be to short compared to the thickness of the winding. This is because of the unused space that the shape creates when it turns. Anyhow, there are a set of parameters (Table I) where the design parameters meet the production parameters. From the electromagnetic design point of view the energy conversion point of view the important parameters is the physical size: inner radius, outer radius and the height of the coil as well as the number of waves or poles. From voltage and frequency conditioning point of view the importance of number of turns and parallel strands comes in. According to the selected production method for the wave-winding the initial inner and outer radius need to be specified for the desired processed length of the wave-winding after the deformation. All the design parameters have of target value for a stator segment of a 20-pole machine.

TABLE I PRODUCTION AND DESIGN PARAMETERS

Short description of parameter Symbol Value Unit Initial outer radius of the hoop coil Ro 157 mm Initial inner radius of the hoop coil Ri 177 mm Winding width or the width of initial coil Ww 8 mm Number of poles Np 20 - Processed outer radius of the wave-coil Rwo 69 mm Processed inner radius of the wave-coil Rwi 59 mm Processed axial height of the wave-coil Hw 20 mm Number of parallel strands Nw 1 strand Number of turns Nt 16-18 turns

Two different numbers of turns are specified (Table I) in

order to form the winding cross-sectional area either with 4x4 or 3x6 conventional or profiled wire.

D. Design for manufacturability When we are looking more general in the design of

electrical machines and orienting the electromagnet to be part

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of magnetic coupling and energy conversion in the presence of the electromagnetic field in an electromagnetic device then we are learning more about the different aspects that are behind the desired goals of the design for manufacturability. Most windings in modern electrical machines are based on radial flux configuration with distributed and overlapped coils or an array of concentrated coils (Fig. 1, CC). It is normally hard to manufacture these kind of windings with high copper density, when the coils consist of few winding turns and several stranded pairs and the coils has to be inserted into the soft magnetic core.

When seeking for the least complex shape as simplest target for the production and a selection of the manufacturing process then the simplest coil to manufacture is a transversal flux configuration (Fig. 1, HC). It has the advantage of simplicity, due to its simple geometry and that it can be pre manufactured to its final shape with high accuracy, high fill factor and then mounted.

Somewhere in between these two types of coil configurations are the wave coil (Fig. 1, SW). It has a simple geometry, though not as simple as the transversal flux configuration, but not as complicated as the radial flux configuration, Fig. 1.

Fig. 1. Transversal flux configuration with a hoop coil (HC) and claw-pole structure, radial-to-transversal flux configuration with a single layer wave-winding (SW) or double layer wave-winding (DW) and semi claw-pole structure, and radial flux configuration with an array of concentrated coils (CC).The axially distributed single layer wave-windings and circumferentially distribute wave-winding segments (that preferably are double layer wave-windings) are shown on left.

The wave-winding can compose a whole-lap single-layer

wave-winding of several turns and parallel strands in each turn. Alternatively, the wave-winding can have a returning path so that it builds up a double-layer wave-winding (Fig. 1, DW) [6]. It is possible to place the coil in different ways so that it works as a single-phase, a three-phase axially distributed (Fig. 1 above right) or as a three-phase circumferentially distributed wave winding, (Fig. 1, bottom

right). It can be concluded that in the last alternative where the coils are divided circumferentially into several sections, the structure of the pre-made winding for molding the core is less stable. The stability of the coil is important because the geometry will in some cases define the teeth’s placement. Therefore we are focusing on coils that are continuous for a whole turn. A single-layer wave coil (Fig. 1, SW) can be wound as a transversal flux configuration (Fig. 1, HC) and then pressed to its final shape. It is this characteristic that makes it suitable for easy manufacturing.

The soft magnetic material must be mounted as two bodies from two separate directions. The coil will otherwise interfere with teeth at the assembling operation. It is therefore advantageous if the soft magnetic body can be built directly as a true 3D structure, and not only in 2D and then “extruded” into a 3D body as with laminated steel packages. In other words SMC is a suitable flux conductor material as it has the same characteristics in all 3 dimensions. The disadvantage with SMC is that the size of the produced specimen is limited by the maximum compression pressure. The SM2C on the other hand, being a castable compound of powder and epoxy binder, can be manufactured in any size and easily molded around the coil as well [1].

III. FORMING OF WAVE WINDINGS The main target for the winding machine is to develop an

efficient manufacturing method, possible to be integrated into a fully automated production line. The process can be divided into a winding step, followed by a bend and pressing operation, creating a wave-winding with high geometric accuracy and superior fill factor compared to traditional production methods. The machine supports both conventional and profiled wire and the outcome of the machine is compared to handmade versions, one of them shown in Fig. 2.

Fig. 2. Prototype wave-winding.

A. Specification The specification for the single-layer wave-winding is

according to the outer-stator phase-segment needed for a three-phase synchronous machine (Table I). The inner radius of the processed winding must not be smaller than the specified inner radius in the table. The outer radius of the processed wave segment is not that critical when the core is molded around the winding. Also the regular arrangement of

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poles can vary as the salient structure along the inner periphery is well defined inside the stator mold. On the other hand the axial height of the winding is more critical as an oversized winding would not fit completely into the mold which leads to unfinished edge-surface of the stator segment. On the contrary, an undersized winding would cause an axial space between the winding and the mold, which would be filled with cast material during the molding process.

B. Winding forming machine development The principle of the machine is illustrated in Fig. 3. The

dies can move towards each other when a force is applied axially, and by doing that the coil will never have to slide against any surface. This procedure avoids unnecessary scratch marks on the isolation, as well as making it possible to get a good physical definition of the coil. The number of waves necessary in a winding then defines the setup of the complete wave forming tool.

In order to initially analyze press forces and compression ratios, a simplified single tooth press tool was utilized. The simplified press tool has fixed die, somewhat different to the full scale machine (Fig. 4). The disadvantage with the single tooth tool is that the coil will have to slide on the die during pressing, similar to a deep drawing process. This will of course have an influence on surface damage on the winding material as well as the contact forces between tool and work material, but it will nevertheless give a good idea of press forces in the actual wave forming machine. Experiments with different pressures were performed in order to analyze copper fill factor and the geometrical shape of the coil. The thickness of the coil was measured at the bottom of the die and at the top.

Fig. 3. Principle of winding forming process, illustrated as 3 interacting dies (grey) forming part of a winding (orange).

Fig. 4. Forming of a single wave using a simplified tool with a sliding action.

When a coil with the geometry as a toroid are compressed

on 10 points on each side axially against each other a wave coil is created. Because of the axially forces the diameter will shrink as the wave pattern amerces. Therefore is it necessary

to allow the dies to move radially. Otherwise there would be scratch marks on the coil isolation.

C. Design The device is built around two robust quadratic steel plates,

assembled parallel to each other using linear ball bearings in each corner to allow a relative movement. On the plates 20 rails are symmetrically arranged around the center axis, 10 on each one, with independent linear motion bearings allowed to travel radially in and out. Each linear unit is equipped with a die, having a slot just large enough for a pre-wound coil to fit. Positioning all the dies at the outer ends and the plates away from each other allows the coil to be entered. The forming is done by applying a force on each plate, pushing them together and making the blocks to move towards the center, transforming a hoop coil into a wave coil with significantly smaller diameter than the original one, Fig. 5. To facilitate a more controlled radial movement, e.g. small pneumatic cylinders can be mounted together with each rail.

Fig. 5. CAD-model of the winding forming machine with pneumatic cylinders.

IV. FORMING PROCESS Knowing how the coil will react is important when

choosing components and materials to this kind of machines. The combination of press force and type of material in the dies will balance the risk for damages to the electric isolation, which in turn is a critical issue in the production of windings since it may dramatically reduce the lifetime of the finished product.

The first step of the forming process is to place a hoop coil in the dies on the lower plate. The dies are now placed at their outer diameter position. Secondly the upper plate will be lowered slowly to make sure the coil will fit in the upper dies. From this step the displacement and the forces will be measured during the whole process. This is done so that it will be possible to evaluate the forces compared to the displacement as well as the density and the geometrical accuracy.

V. RESULTS AND DISCUSSION In the single tooth press test it is possible to measure the

thickness of the winding at different forces, as well as the displacement vs force. The thickness is measured to see how

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much the copper will deform depending on the pressure, so that the copper density may be calculated. The results are shown in Fig. 6.

Fig. 6. Measured thicknesses on lower surface and upper surface respectively at different press forces.

In the test-coil a wire with rectangular cross-section has been used. This gives a fill factor of about 80 %, Fig. 7.

Fig. 7. Left: Microscope photo of a cross-section from test coil pressed with a force of 60 kN. Right: Microscope photo of a cross-section of a non-pressed copper wire.

It can be seen in Fig. 6 that the thickness is affected by the force, but as Fig. 7 shows the copper wire have not been deformed significantly. The difference that can be measured in Fig. 6 is likely an effect of the copper wires non uniformity before pressing. When the copper wire then even out small dents, the thickness measured will also shrink without any actual compression of the wires. This can also be seen in Fig. 8 where photos of single tooth coils are presented. On the photo to the left there is no noticeable flat surface on the right surface. In the middle photo a flat surface can be noticed, yet small. On the right photo there is on the other hand a significant flat surface. This shows that the geometry will change with increased pressure force, but just marginally.

Fig. 8. Single teeth pressed with 15, 37, 60 kN respectively from left to right.

In the single tooth tests the axial displacement vs the press

force were measured. These results are presented in Fig. 9 it can be seen that ca 7,8 kN is needed to get acceptable dimensions, at higher pressures the displacement is negligible.

When the force is increased above this level it will only make a marginal difference as seen in Fig. 6.

Fig. 9. Axial displacement vs press force.

The coil in Fig. 2 is handmade and is therefore the closest a perfect coil achievable, when it comes to the rectangular wave shape. Pressed in a machine, the copper will get a larger radius, especially the outer radius. This phenomenon can be seen in Fig. 8.

The initial tests in the wave forming machine were performed using the same wire configuration as for the single tooth tests. According to single tooth tests a press force of 3,9 kN would be needed to form one wave, which would be equivalent to total force of 78 kN for the 20 waves in the wave forming machine. This is however an overestimation, since the wave forming process includes no sliding on tool surfaces, thus implying a much lower total press force. However, the first tests in the wave forming machine reveals that a higher force than expected is needed to get the final shape. The bearings move smoothly in the beginning, but closer to the center they move slower and slower. At 60 kN total press force the forming process is stopped, due to the allowed force on the bearings. It can be seen in Fig. 12, that the force doesn’t have much impact on the displacement at that time.

Fig. 10. Coil pressed at 60 kN.

The coil has not been pressed completely to its correct axially height, which is evident due to the sides that are 45 degrees instead of 0 degrees, related to the center axis. Some radial springback on each top of the coil (Fig. 11) is evident, as well as in the axial direction (Fig. 10), which makes it loose some of its form stability, but this can easily be fixated at mounting.

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Fig. 11. Radial springback on each coil top.

It is evident that it will not sufficient only to apply force axially, it is necessary to apply it radially as well. The force can be applied in several ways such as using screws on each die and push it in to the middle, or design the dies in an angle so that they move both axially and radially when an axial force are applied. On this coil applying a radial force will not work satisfactory due to the small height. This is due to a very small lever effect, which can be seen in Fig. 10. A way around this problem would be to make a coil with bigger start diameter.

Two modifications were therefore made on the machine; the dies were elevated from a press height of 15 mm to 35 mm, and screws were mounted so that it is possible to exert a force in radial direction on each die. The screw solution is not ideal because it will not be possible to apply the same force on all dies at the same time, but it will work satisfactory for this prototype.

Fig. 12. Axial displacement vs press force. Coil with 15 mm height represented by the dashed and 35 mm height by the solid line.

In Fig. 12 are the force vs displacement for both the 15 mm and the 35 mm coil plotted. They both have approximately 3 mm left on the displacement, but the 35 mm coil needs less force radially to push it to its final position. The finished coil is shown in Fig. 13. The geometrical accuracy is adequate for use in a motor, but some final adjustments would be possible to perform in a correction operation succeeding the winding forming process.

Fig. 13. Coil with 35 mm press height.

VI. CONCLUSIONS The design for manufacturability is essential and can be

achieved when the production methodology is closely analyzed and accounted in the electromagnetic design of the machine. Different types of wave-windings that form a single-layer whole-lap coil is compared, since this type of winding is still relatively easy to produce and, as it is shown previously, makes improvements in machine performance possible when taking advantage of molding techniques that establishes low permeability core. As the manufacturing of magnetic core is simple and fast, so does the wave-winding machine, the production takes a favor of the process even if the performance is not outstanding due to lower torque capability of SM2C core machines.

The results from the initial testing clearly shows the geometric difference compared to the handmade winding, having big radiuses at the outer corners. Nevertheless, the produced geometry represents a nearly optimal design, having a high packing density and also having less air pockets limiting the cooling need of the winding. The production of self-supporting winding with very accurate teeth positioning will put high demands on the repeatability and accuracy of the production process. This method has shown the potential of providing these features, but still needs some more development.

VII. REFERENCES [1] L.Svensson, K. Frogner, P. Jeppsson, T. Cedell, M. Andersson, “Soft

magnetic moldable composites: Properties and applications”, J. Magn. Magn. Mater 324 (2012) 2717-2722.

[2] A.G. Jack, B.C. Mecrow, P.G. Dickinson, D. Stephenson, J.S. Burdess, N. Fawcett, and J.T. Evans, “Permanent magnet machines with powdered iron cores and pre-pressed windings”, IAS’99, Vol 1, pp. 97-103

[3] A. Reinap, D Hagstedt, C. Högmark, M. Alaküla, “Evaluation of a Semi Claw-Pole Machine with SM2C Core” – International Electrical Machines and Drives Conference – IEMDC, 2011

[4] US Patent Ernst F.W. Alexanderson, High frequency alternator, 1911 [5] A. Reinap, D. Hagstedt, C. Högmark, M. Alaküla, “Sub-optimization of

a claw-pole structure according to material properties of soft magnetic materials”. IEEE Trans Magn Vol 48, n 4, 2012, pp. 1681-1684

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[6] A. Reinap, M. Alaküla, “Impact of soft magnetic material on construction of radial flux electrical machines”, IEEE Trans Magn, Vol 48, n 4, 2012, pp. 1613-1616.

[7] C. Högmark, A. Reinap, K. Frogner, M. Alaküla, ”Laminated winding with rapid cooling capability”, International Coil Winding, Insulation and Electrical Manufacturing Exhibition and conference – CWIEME, 2012

[8] US Patent 4398112A, A. W. van Gils, Laminated windings for electric machines, 1983

[9] L. Svensson, M. Andersson, T. Cedell, P. Jeppsson, “Electrical isolation of coils in Soft Magnetic Composite applications” 4th Swedish Production Symposium, 3-5 May 2011, Lund