978-1-4673-4937-6/13/$31.00 ©2013IEEE

EM performance improvements through advanced winding techniques

Blaž Štefe Productionon department

Elaphe Propulsion Technologies Ltd Ljubljana, Slovenia

Gorazd Lampič

EM researcher & CEO Elaphe Propulsion Technologies Ltd

Ljubljana, Slovenia

Abstract— For the development of new electric motor winding techniques a detailed knowledge in the currently used techniques is essential; therefore a review of the most commonly used winding techniques is provided in this paper, focusing on the winding of PMSM AC winding. To compare several different winding topologies and production techniques the two key parameters are presented with the mathematical model. These two parameters are the length of the straight part of the wire and the conductor diameter, from which the efficiency peak and motor power are calculated. These calculations are based on the model of the newly developed high pole pair number three phase wave winding topology - Elaphe type winding. The results can be used also in other PMSM winding topologies since they show the key important facts of the winding detail. In addition, the methodology can be adopted for any winding type.

Keywords— Electric motor winding techniques, Electric motors, In-wheel motors, Mathematical model, Permanent magnet machines Introduction (HEADING 1)

I. INTRODUCTION Due to being withheld by electric motor producers, the field of electric motor winding is not extensively covered in the scientific literature, even though electric motors are one of the most commonly used professional and every day applications. The coil winding of middle size and large PMSM remains one of the key challenges for the coming decades and is in a global view still widely assembled by hand, as in such way better motor parameters can be achieved as there is no need for space for the tolling and also the probability for the scratches is reduced [1]. Due to the new coil topologies and increasing interest in electro mobility, new winding techniques need to be developed and the winding processes must reach a higher level of automation as handmade production faces slowness, high cost and in some cases also the scrap, as human can become sloppy during the tedious and continuously repeatable process. Besides the process done by men is significantly more expensive comparing to the fully automated process. Electric motor coil winding process can be defined as “joining of an iron core with conducting wire by a continued bending around the core or any other way of insertion the wire into the core” [2].

In the winding process the smallest possible gap between the winding overhangs among wires is required. Special challenge in automated winding of the motors is to achieve very small or no gap between a winding and a stator slot.

The most common challenges to in this field are the damages of the insulation or other scratches causing short circuits, low slot fill factor and layers of the wire climbing one over another.

II. PARAMETERS IN EM WINDING When designing a new type of winding it is of a great

importance that a high slot fill factor [4] is achieved and that a wire is properly fixed in its place – what is usually solved with a help of wedges [4], [5].

Slot fill factor can be in some cases increased by implying the pressure on the winding [6] or by piezoelectric excitation device [7].

Motors wound by hand usually boast the highest slot fill factor.

Shape of the back of the slot is also an important detail in this type of the winding and is generally found in two basic configurations: square bottom or a round bottom. The round bottom is the preferred shape.

During injection, the wire will come through the blade gap and toward the back of the slot.

With a round back slot, the wire has a tendency to fill the hack of the slot and “roll’’ into the comers (keeping the slot fill factor high).

Fig. 1. Round and square bottom of the slot [4]

Winding machines can be divided into two main groups: winding the coil onto the rotating armature (not for PMSM) and those which wind the coils onto stators [8]. The second group is furthermore divided into the direct method, where a coil is wound directly into the slots of the stator and the indirect method where the coil is first wound into a tool and is then slipped into a stator. Due to only one production step the direct methods are more time and cost efficient and are known for a higher level of automation. On the other hand the direct methods require more empty space between the walls of the slot and the wire, can achieve lower tolerances and produce longer winding overhangs, an inactive segment of the wire, increasing a total resistance of the wire. Thereafter the direct winding methods are appropriate for cheaper, less precise industrial applications, mass produced and smaller electric motors, with more loose technical and mechanical requirements. For the indirectly wound motors is typical to achieve higher slot fill factors, better wire positioning tolerances, shorter winding overhangs and higher tolerances. Thereafter indirect winding methods are used for higher performance electric motors and are known for efficiency and durability. Larger electric motors (5 to 50 kg), typically used as driving motors for the passenger vehicles and precise industrial applications, are typically wound by the indirect winding methods. The common property of the all the winding methods described so far is that they are not highly precise and they cannot produce controlled straight parts of the wire if such are demanded. Elaphe type motor, due to its design specifics, demands the straight parts to be straight within the tolerance of 0.05 mm and the winding overhangs to be precisely shaped and all equal, within the same tolerance, which is not a standard in EM industry.

III. DIRECT WINDING METHODS The most widely used direct methods are the fly winding [4]

and winding by the needle/nozzle [8] [9] [10]. Besides winding a conventional laminated core joint lapped cores are also considered being wound directly.

Fly winding is most commonly used to wind armatures, but the principles can be also used for some versions of PMSM. It is a winding method where a flyer rotates around the stator or the armature of an electric machine, dispensing the wire into the slots of the laminated core. After the certain two, usually geometrically opposing, slots are fully wound, a laminated core rotates the way the wire can be wound into the next two slots. Fly winding boasts high speed and a possibility to wind several armatures on the same machine simultaneously whereas its disadvantage is usually expensive machining of the tools and poor precision with the coil insertion and when there are many slots, that require several circumferential layers of coils to be wound it is difficult to wind wire deeply into the slots.

Typically a thin wire can be wound as a conductor formed from smaller strands which are easier to bend into the required slot than one large conductor. Anyhow for certain applications

a large conductor cross-section is needed to achieve the desired ampacity, which makes fly winder less likely to be

used for that particular application.

Winding by the needle is based on a little nozzle/needle that guides the wire around the slots of the stator lamination, but due to needle gliding around the slots there are some physical constraints keeping it from achieving a high slot fill factor.

In the laminated core, coils are wound in the linear arrangement so that a nozzle glides between the stator teeth dispensing the wire. The space needed for a wire to glide is left empty and unused in the end, so that a slot-fill factor remains low.

It is estimated that curve of coils is caused when wires are drawn by the inner surface of the nozzle, and wound in such status that the residual distortion is not eliminated completely due to the tension. An estimated factor of bulge in wires is that wires are hardened by processing due to continuous plastic deformation on the inner surface of the nozzle.

The methods described so far are typically used for winding a rigid stator core, while the flexible laminated cores are also

Fig. 2. Fly winding method is a widely used winding method known forhigh speed, reliability and adjustability, but also for a low precision of the winding process [4].

Fig. 3. A needle (in the center) glides between the slots in a circulardirection disposing wire. An gap between the windings, needed for a needle,is a main cause for a low slot fill factor of needle wound motors.

Fig. 4. Joint-lapped core – stator is produced by several laminated segments that are joint together enabling higher slot fill factor while being wound by the needle or flyer.

known in the industry and a joint-lapped core is its typical example [11]. Joint-lapped core can be deformed into a suitable form to winding and a slot-fill factor about 75% can be achieved. The aim of the method is to have the wires stacked like straw-bags inside slots.

There are quite numerous advantages of a joint lapped core. A sufficient space around the coil means large radius during the winding is allowed, so the damage of the wire is less likely to happen and usage of a wider range of automated tooling is allowed. Because circular motions are made using the flyer, winding can be executed at higher speed compared with winding by the nozzle in rectangular orbit. Moreover as no space between the windings on the teeth is needed for the nozzle, the slot opening width and the number of slots can be designed arbitrarly.[10] Cable winding by the industrial robot is proposed in the paper [19]. The paper describes a fully automated winding process of a large scale electric machines, with a pure three phase wave winding, with a help of industrial robots. The winding topology is roughly similar to Elaphe type design of the winding, though, as it is on a larger scale, it is designed for a cable winding instead of the wire. The described winding is appropriate for high power, high voltage and direct drive applications and requires fewer manufacturing steps and is

suitable for automated production. More conventional indirect winding of large electric

machines enables preparing the windings outside the stator as it enables better packed and larger windings, with a higher slot fill-factor, but the windings must be inserted into the slots and the stator design must be adjusted for this. For the larger machines, mostly generators, insertion of the bars is mostly done by hand due to small production series and the large generator size. To push and pull a cable through the stator during winding, a cable feeder must be used. Preferably one cable feeder is used on each side of the stator so that the cable end can always be caught when being pushed through from the opposite side.

IV. INDIRECT WINDING METHODS When considering the indirect methods, the first step,

transformation of the shape of the wire, is normally represented by the winding of the wire onto the special iron

holder which can, during the second step of insertion of the wire into the laminated core, become an integral part of the stator or coil can be simply slid into the stator lamination. Its advantages make it commonly used especially in the high performance electric motors as it allows a mass production of the motors with a high slot fill factor and solid slot motors without a clearance between the wire and the stator teeth. The most commonly used indirect winding methods are the following ones: shed winding process [4], segmented stator with single-tooth segments [10] [12] and a spindle winding.

Shed winding and coil injection – the principle of the shed winder is that it wraps wire around a tapered coil form step. With each revolution of the rotating flyer, an additional turn of wire is added to the coil form. With the addition of each turn, the previous turns are pushed down the tapered form. When winding in production, the coils “shed” off the coil form at a relatively high rate of speed. The process is used for winding a thin coil.

Coil injection is the most economical process for the quick,

quality injection of the coil groups into the stator slots. It uses highly polished tool steel to cover the tooth edged stator slot opening from the incoming wires. As the wire is pushed into the slots, the slot closure wedge follows the wire up through the tooling.

When inserting coils into the stators that are used for light duty single phase applications, all of the coils can be injected at the same time.

For multi-phase/multi-layer applications and high slot fill requirements, multiple pass injection is typically employed. In this method, one layer of coils is injected into the stator core. These coils are drifted (intermediate formed) and the next layer of coils is injected. This method can produce a stator with slot fills exceeding 84% [20].

In the industry today the shed winding and coil insertion are only semi automated, as the manipulation of the parts, preparation of the coils, connections and ordering of the wound coils is still widely done by humans. Shed winding can be found in industry for armature and stator core winding.

Winding segmented stator core is from the conceptual point of view closely similar to the joint-lapped core method, only the segments are not connected among themselves and therefore need to be coupled by welding or another coupling method. In this method the wire can easily pass along the core

Fig. 5. A cable winding for a high power, large scale, direct drive electricalapplication, wound by typical industrial robots [19].

Fig. 6. A shed winding process. A winder is disposing wire on a tapered coilform step, from which the wound coil is injected onto the armature or statorcore.

back and around the tooth. However, with a stator having many teeth, the time to assemble and weld the stator is increased [10]. With such a method it is easy to place the pre-wound coils in the core and it can achieve a high slot-fill factor, but it causes an air gap between the segments, demands welding and is not convenient for a high number of teeth. The winding process is done by the flyer and for majority of applications thicker wire is allowed as for the conventional fly winding of a solid stator.

Spindle winding is used for winding small, axially symmetrical coils directly on a rotating bobbin. It is not appropriate for winding strongly asymmetrical bobbins as it does not allow a direct, unlimited accessibility to the winding area. This means that bobbins integrated into housings or placed very tight cannot be wound with this method [2].

Toroidal winding of a wire is a winding method where the winding apparatus elongates conductor into a coil. It comprises a bending roller assembly for bending roller assembly for bending the linear, elongated object into loops of the coil and leveling device for bringing the curvature of the coil to the desired level. In addition to using the spindle winding process and winding a coil onto the bobbin, the coil itself can also be wound by the toroidal process and furthermore inserted onto the stator's laminated tooth. The advantage of such winding is no necessity for a holder of the coil, because of the self sustaining shape of the coil.

Disadvantages are the tolerances of the diameter of loops, as they differ in value range from +-5 mm to +- 10 mm. Therefore the loops extend alternatively to right and left in zigzag form what makes it impossible to produce a smooth cylindrical coil and to tight wind another layer of wire on it [21]. Thick wire can be wound by this method, as the method is basically a machine for spring production.

In all indirect methods Care must be excercised when inserting the wire, so that conductors are not scraped, wrappings are not disturbed and insulations are kept intact.

V. ELAPHE TYPE MOTOR

On the contrary to the spinning tools for the conventional winding methods, the Elaphe type motor requires a completely different approach to the wire shape transformation and its insertion into the stator. By definition it is a motor with a high pole pair number and consequently a high number of teeth. In Elaphe type winding [3] with a high pole pair number three phase wave winding, a rotational movement between the core and the wire is not feasible in any of conventional ways described so far, therefore several techniques still being in a research phase have been developed for wire transformation into a demanded shape.

VI. MATHEMATICAL MODEL – EFFECT OF THE QUALITY PRODUCTION ON THE MOTOR PERFORMANCE

Over two decades of experience in Elaphe type coil winding have shown several winding parameters influence characteristics of every motor design, where the winding overhang length and the slot fill factor are two of the most important, since they strongly influence key parameters such as winding resistivity and thermal conductivity. The winding overhang length is parameterized by the length of the straight part of the conductor. Its major part is the active part, generating torque, but there can be additional straight segments at both axial sides which are in-between the curved part of the winding overhang and the active part. A wire resistivity and partly a slot fill factor as well as motor compactness are represented by the conductor diameter D in a slot of the fixed width.

Naturally the shorter the straight segments and the wider the wire, the better the motor performance you get. On the other hand in 90% of cases the motor failure during the production and assembly or during operation is caused by the scratch of the wire varnish insulation or the break of the paper insulation isolating the wire and the laminated core and consequently the short circuit, especially on the edge of the laminated core.

Thereafter the longer straight part of the wire and thinner wire can obviously decrease pressure of the wire onto the edges of the core and reduce the failure rate. There need to be taken into account the precautions, for a lower failure rate, have their own electro-magnetic disadvantages which turn out as a decrease in power and efficiency.

Fig. 9. Elaphe type winding consists of a number of coils which are formed from the meandrically preshaped wires. Depending on a transformationmethod the wires can be of limited lengths or endless, as described in section V.

Fig. 7. Spindle winding process. Bobbin on which the couil is wound can beeither used as a guide for the coil to be injected into the stator core or used asa functional element of the stator.

Fig. 8. Toroidal winding of the coil – basically the method equals the springproduction method.

In order to analyze the influence of winding overhang length and slot fill factor the analytical mathematical model, called POINT, of electric motor parameters and characteristics is used [22]. The complete description of the POINT goes beyond the scope of this article, but the following general description can argument its use for the purpose of this article.

POINT is a computer program developed for PMSM motors with Elaphe type winding, but the results can be qualitatively generalized for other electric motor winding types. Taking into account electromagnetic forces, geometry relations and material data, several equations defining motor torque, induced voltages, efficiencies and other values are written in relation to basic motor dimensions, such as active length, air gap radius, magnet thickness as well as some winding parameters, such as magnetic period number, number of conductors connected in parallel or in series. The results of motor characteristics are accurate to the level of around 5% and even more precise when comparing the parameters of several motors with similar parameters. The results of the simulation of straight conductor segment length (L) and conductor diameter (D) for a specific Smart2 in-wheel motor in relation to three key characteristics are presented. These three characteristics which we focused on are: motor efficiency and the peak motor power.

The relation between full motor mass and the other two parameters is:

(1)

Where L is the factor of straight part vs. active part and D is conductor diameter. The relation between the motor efficiency at 100 Nm and 600 RPM can be calculated as:

) (2)

The relation between L and D and the peak continuous

motor power can be expressed as a relation larger than ten pages and is not a subject of this article. In addition, peak values are selected by a special computer function, not analytically. The reason for the length of the equation is a combination of thermal and electrical set of equations which have to be combined in order to get the final result.

The effect of the length of the straight part and the

conductor diameter on the electric motor efficiency is presented in the table below:

The results are clearly in consistence with the expectations of the highest power at the thickest wire and shortest straight parts and the lowest power when the thinnest wire and the longest straight parts lengths are used. The real value of the table is to provide the data about how much efficiency can be expected to be lost when easier production method (thinner wire and longer straight parts) is used.

The effect of the length L of the straight part and the conductor diameter D on the maximum power of the motor:

From the table above we can conclude the max power difference for the same parameters of the motor weighting all in all around 17 kg, varying only the two parameters (the wire length L and thickness D) with a purpose of easier and less advanced assembly system, is almost 6 kW or 20%.

Anyhow, there is not only manufacturability, material utilization and tolerances that are attractive but also the choice of conductor materials, the technique that establishes dielectric insulation system and the possibility to directly cool the component where the heat is produced in the winding [23].

Not only the winding technique but also the winding type has an influence on efficiency of an induction motor. The paper [24] describes effects of concentric single layer winding and concentric double layer.

VII. ANALYSIS OF THE RESULTS From the obtained results we can deduct from the

perspective of the motor performance the most promising are those winding techniques that allow us to produce shortest winding overhangs and a high slot fill factor, while in the same time presenting a low failure rate due the broken insulation on either wire or the slot insulation. We have to be aware that the technique chosen strongly depends on the winding topology, so the table of winding methods and their advantages can serve as a guideline for the engineers choosing the most appropriate method for their application.

VIII. CONCLUSION The results calculated in this paper provide a great tool for

the engineers to estimate the value and the functionality of the more precise and more demanding winding techniques and assembly of the winding in relation to the efficiency and performance of the electric motor. The ongoing research is focused on the feasibility of the high speed and massive production of the tight slots of the Elaphe type winding with as short straight parts as possible and a low scrap rate. The

TABLE I EFFICIENCY OF THE ELECTRIC MOTOR [%] FOR DIFFERENT VALUES OF THE

WIRE LENGTH [mm] AND CONDUCTOR DIAMETER [mm] L & D 1.9 2.0 2.1 2.2 2.3

1. 92.66 93.02 93.33 93.60 93.83 1.02 92.61 92.97 93.29 93.56 93.80 1.04 92.57 92.93 93.25 93.53 93.77 1.06 92.52 92.89 93.21 93.49 93.74 1.08 92.47 92.85 93.17 93.45 93.70 1.1 92.43 92.80 93.13 93.42 93.67

TABLE II

POWER [kW] OF THE SPECIFIC PMSM FOR DIFFERENT VALUES OF THE WIRE LENGTH [mm] AND CONDUCTOR DIAMETER [mm]

L & D 1.9 2.0 2.1 2.2 2.3 1. 30083 31609 33048 34438 35815

1.02 29879 31412 32862 34263 35649 1.04 29669 31210 32676 34087 35481 1.06 29455 31003 32486 33904 35297 1.08 29237 30795 32282 33708 35123 1.1 29028 30597 32085 33528 34944

paper has clearly shown that the precise and high quality assembly can return up 20% better performance of the motors, and the presented winding methods can be of a great help when developing a winding technique for a new kind of electric motor.

REFERENCES

[1] S. Lan, "Optimization of Electric Motor Assembly Operation with Work Study".

[2] J. Franke, A. Dobroschke, J. Tremel and A. Kühl, "Innovative Processes and Systems for the Automated Manufacture, Assembly and Test of Magnetic Components for Electric Motors," 2011.

[3] A. Detela and G. Lampič, "Compact Multiphase Wave Winding of a High Specific Torque Electric Machine". Patent WO 2012/138303 A2, 11 October 2012.

[4] J. Kirkhoff, "Process and Design Considerations for Automatic Assembly of Electric Motor Stators," Vols. Alliance Winding Equipment, Inc, 2003.

[5] "Slot Wedges," SPIndustries, [Online]. Available: http://spindustries.at/products-slotwedges.html. [Accessed 20 Jan 2013].

[6] 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 Prepressed Windings," Transactions on industry applications, vol. 36, no. 4, 2000.

[7] J. Savage, P. Micheli and J. Suriano, "Achievement of higher motor winding slot fill through the use of piezoelectroc excitation device".

[8] B. Mahawan and Z.-H. Luo, "High-Speed high-precision tracking control for electronically controlled winding machines," 2001.

[9] D. Mumford, "In-slot stator winding of DC brushless motors," in Electrical Insulation Conference and Electrical Manufacturing & Coil Winding Technology Conference, 2003. Proceedings, Tipp City, OH, USA, 2003.

[10] H. Akita, N. Miyake, Y. Nakahara and T. Oikawa, "New Core Structure and Manufacturing Method for High Efficiency of Permanent Magnet Motors," in Industry Applications Conference, 2003. 38th IAS Annual Meeting, Amagasaki, Japan , 12-16 Oct. 2003.

[11] T. Oikawa, T. Tajima, H. Akita, H. Kawaguchi and H. Kometani, "Development Of High Efficiency Brushless DC Motor With New Manufacturing Method Of Stator For Compressors," 2002.

[12] F. Meier, "Permanent-Magnet Synchronous Machines with Non-Overlapping Concentrated Windings for Low-Speed Direct-Drive Applications," 2008.

[13] Ø. Krøvel, R. Nilssen, S. E. Skaar, E. Løvli and N. Sandøy, "Design of an Integrated 100kW Permanent Magnet Synchronous Machine in a Prototype Thruster for Ship Propulsion," Trondheim, Norway.

[14] R. G. Kelley, "Toor for forming controlled bends in wire". US Patent 5,520,227, 28 May 1996.

[15] D. J. Benes, D. D. Kosch and V. C. Kieffer, "Wire bending apparatus". NE, US Patent US 6,341,517, 29 Jan. 2002.

[16] Y. Latour, "Wire bending device". TX, US Patent US 2005/0076694 A1, 14 Apr. 2005.

[17] "Overview of KSC Wire Insulation Repair Development," 2005. [18] N. J. F. K. S. Center, "Self-healing Wire Insulation". FL 32899 (US)

Patent US2008/0234463 A1, 25 Sept. 2008. [19] E. Hultman and M. Leijon, "Utilizing cable winding and industrial robots

to facilitate the manufacturing," Robotics and Computer - Integrated Manufacturing, vol. 28, no. 12, pp. 246-256, 2013.

[20] Kirkhoff Jim, "Processes and design considerations for automatic assembly of electric motor stators", Alliance equipment Inc., Electrical Insulation Conference and Electrical Manufacturing & Coil Winding Technology Conference, 2003. Proceedings

[21] Miura Yoshio, Kobayashi Kazuhiro and Yusa Koue, "Coil winding machine", 797,691 (JP) Patent B21F 3/02, 17 May 1977

[22] Lampič, Gorazd, Analiza uvajanja elektricnih pogonov v razlicne vrste vozil in zasnova pogona za sodobni mestni elektricni hibridni avto, , Magistrsko delo, UDK 629(043.3), 2006

[23] Hogmark, C.; Andersson, R; Reinap, A.; Alakula, M.; "Electrical machines with laminated winding for hybrid vehicle applications", Electric Drives Production Conference (EDPC), 2012 2nd International, Oct. 2012