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An Analytical Approach for the Design of Innovative Hairpin Winding Layouts
... The requirement for maximizing the electric motors' power and torque density has led to a revolution in winding technology [7]. The key enabler point is to find a way to increase the slot-filling factor. ...
... The key enabler point is to find a way to increase the slot-filling factor. In earlier investigations, research studies mainly concentrated on increasing the traditional round-wound wire-filling factor by proposing an orthocyclic, layered winding methodology and the pressing tooth-wound [7]. However, none of the aforementioned methods achieved the desired result; the orthocyclic and layered winding methodology requires specific and expensive equipment and machinery types, and the pressing tooth-wound can only be implemented for electric motors with concentrated windings [7]. ...
... In earlier investigations, research studies mainly concentrated on increasing the traditional round-wound wire-filling factor by proposing an orthocyclic, layered winding methodology and the pressing tooth-wound [7]. However, none of the aforementioned methods achieved the desired result; the orthocyclic and layered winding methodology requires specific and expensive equipment and machinery types, and the pressing tooth-wound can only be implemented for electric motors with concentrated windings [7]. ...
The hairpin winding configuration has been attracting attention as a solution to increase the power density of electric vehicle motors by enhancing the slot-filling factor. However, this winding configuration brings high AC losses during high-speed operation and we require new approaches to tackle this challenge. This paper considers reducing AC losses by proposing two main methods: correct transposition of conductors in parallel paths, and enhancing the number of conductor layers in a slot. First, the proper connection of conductors in parallel paths is considered, and the essential rules for this purpose are described. Next, the paper uses a numerical approach to deal with the effect of incorrect conductor transposition in winding paths on generating additional AC losses due to circulating currents. Finally, the impact of the number of conductor layers in the mit-igation of AC losses is also discussed in detail. According to the results, by increasing the number of layers, ohmic losses in the layer near the slot opening dramatically decrease. For instance, ohmic losses in the layer near the slot opening of the eight-layer setup were 82% less than the two-layer layout.
... In high power density traction applications, hairpin windings are widely spreading and currently seeing an ever-increasing interest in several documents [2][3][4][5]. In comparison to windings with round conductors, the end-winding length is shortened and, consequently, the DC copper loss is reduced [6]. ...
... However, the number of hairpin layers in the slot was kept fixed, thus limiting the degrees of freedom of the design optimization. Additional work has been recently published on hairpin windings, but they focus either on modelling aspects (e.g., AC loss estimation [2]) or preliminary calculations [6] or sensitivity analyses [16]. ...
... Having preliminarily selected the D/L ratio, the starting point for the motor sizing is the torque expression given in (2). In (2) B is the RMS value of the fundamental harmonic B max of the airgap flux density, which is obtained from (3) using the Fourier series decomposition of a square wave waveform that has amplitude B ag . ...
Nowadays, interest in electric propulsion is increasing due to the need to decarbonize society. Electric drives and their components play a key role in this electrification trend. The electrical machine, in particular, is seeing an ever-increasing development and extensive research is currently being dedicated to the improvement of its efficiency and torque/power density. Among the winding methods, hairpin technologies are gaining extensive attention due to their inherently high slot fill factor, good heat dissipation, strong rigidity, and short end-winding length. These features make hairpin windings a potential candidate for some traction applications which require high power and/or torque densities. However, they also have some drawbacks, such as high losses at high frequency operations due to skin and proximity effects. In this paper, a multi-objective design optimization is proposed aiming to provide a fast and useful tool to enhance the exploitation of the hairpin technology in electrical machines. Efficiency and volume power density are considered as main design objectives. Analytical and finite element evaluations are performed to support the proposed methodology.
... There are various ways to enhance the power density of electric motors [5]. The most effective way is to increase the slot filling factor (increasing the amount of copper inside the slot). ...
... Several different methods have been considered to achieve this target. Hence, several research studies investigated enhancing the conventional strand winding filling factor of the electric motors for traction application [5]. An orthocylic and layered winding methodology is proposed in [5] to reach the slot filling factor in the range of 0.65 to 0.7. ...
... Hence, several research studies investigated enhancing the conventional strand winding filling factor of the electric motors for traction application [5]. An orthocylic and layered winding methodology is proposed in [5] to reach the slot filling factor in the range of 0.65 to 0.7. However, the disadvantages of implementing these methods are the specific and expensive equipment and types of machinery [5]. ...
This paper investigates the hairpin winding configuration as a solution to increase the power density of electric vehicle motors by enhancing the slot filling factor. Moreover, the paper presents various manufacturing techniques of this winding and describes it in detail. Further, it provides a novel additive manufacturing method as an alternative option for conventional production methods to mitigate AC losses using flat rectangular conductors in hairpin winding rods. Following the proper connection of conductors in parallel paths is considered, and the essential rules for this purpose are described. Finally, the analytical and numerical approaches for calculating AC copper losses and the AC loss factor are presented in detail.
... In [18], a comparison between random and hairpin windings is carried out always in terms of AC losses. In [19], innovative winding patterns that can significantly reduce AC losses are introduced. Always in [19], modifications to the classical analytical models are proposed for predicting AC losses in such new winding concepts. ...
... In [19], innovative winding patterns that can significantly reduce AC losses are introduced. Always in [19], modifications to the classical analytical models are proposed for predicting AC losses in such new winding concepts. ...
... The estimation of the AC winding losses can be a very challenging task, so different approaches have been proposed and investigated to improve accuracy and reduce the computation times [6,[11][12][13][14][15][16][17][18][19][20][21]. However, the advantages of reducing these losses at earlier stages of the electrical machine design are of critical importance for a comprehensive machine design practice [22][23][24][25]. ...
In this paper, the AC copper losses in classical random windings are investigated and mitigated using several techniques across a range of permanent magnet synchronous motor designs. At high operating frequencies, AC copper losses can represent a substantial share of the total loss in electrical machines, thus, reducing the machine’s overall performance, and increasing the thermal loading. Recently, different approaches for modelling AC copper losses have been proposed. This paper utilises simulation software to quantify the expected AC losses in six different propulsion motor designs. The motor designs are then modified to reduce the AC winding losses through the implementation of five different methods. Using two-dimensional finite element analysis, the magnetisation direction, magnet to airgap ratio, copper stranding, magnetic wedges and the motor slot openings are modified to reduce AC losses. The paper considers distributed, fractional, slot and concentrated windings, and the results show promising reductions across these different winding configurations.
... The round copper wire winding has low slot fill factor and poor heat dissipation capacity. The hairpin winding can effectively improve the slot fill factor [5], [6]. But the hairpin winding has a large eddy current loss due to its large cross-sectional area [7][8][9][10]. ...
Permanent magnet synchronous motors with the formed winding can not only improve the slot fill factor and heat dissipation capacity but also effectively reduce the additional loss in the winding. Formed winding permanent magnet synchronous motors will become the future development tendency. Due to inherent features of the formed winding, the circulating current loss is a non-negligible issue. This paper proposes using the formed winding in permanent magnet synchronous motors and analyzes the circulating current loss in the formed winding. First, the finite element models of integer slot and fractional slot motors are established, and the magnetic field analysis are carried out. Then two commonly used methods of calculating circulating current loss are introduced in detail, and their calculation results are compared. Moreover, the high-precision field-circuit coupled finite element method is used to calculate the circulating current loss of integer and fractional motors. And circulating currents of the hairpin winding and the formed winding are compared. The analysis indicates that the circulating current in the formed winding cannot be ignored. Finally, comprehensive and in-depth research on the influencing factors of circulating current loss in the formed winding has been conducted. The results clarify that the number of parallel strands, the width of the slot opening, and the stator current have a considerable influence on the circulating current loss.
... 3) adopting new winding concepts, such as segmented [8] or asymmetric [9] bar layouts. However, also these solutions increase the manufacturing complexity. ...
Increasing current density is a common way to improve the power density of electrical machines (EMs) which also boosts the winding temperature. Higher winding temperature accelerates the thermal degradation of insulation and also increases the risk of partial discharge (PD). Commonly, the winding hotspot temperature constraint is decided based on an over-engineering approach (i.e., the winding hotspot temperature is 30°C below the wire thermal class) constraining the EM’s performance. In this paper, the margin of winding hotspot temperature is precisely determined focusing mainly on the insulation degradation issues through experimental investigation. The partial discharge inception voltage and partial discharge extinction voltage have been measured under different temperature conditions for samples with different thermal aging statuses. Rectangular wires which find application in automotive motors are selected as test samples. The results link the temperature with the PD risk and the thermal lifetime of the insulation, and the winding hotspot temperature margin is accordingly determined. An automotive case study is presented to prove the applicability of the investigation outcomes.
In the last years, hairpin windings are amongst the most studied and implemented winding topologies to increase torque and power densities of electrical machines, especially in the traction field. However, their main drawback remains the elevated ohmic losses generated at high frequency operations. Recently, hairpin windings with segmented conductors have been proposed to overcome this challenge, but an analytical model to predict the losses in such unconventional winding is missing, as well as a critical analysis on the extra manufacturing complexities needed. In this paper, first a simple analytical formulation for the estimation of AC losses in segmented hairpin conductors is proposed, developed and validated at various operating frequencies, both through finite element analyses and experimental measurements. Then, an in-depth analysis of the main advantages and disadvantages of this winding topology is proposed with the aim at providing guidelines and recommendations for their practical realization, adopting standard methodologies for their connection and transposition.
Interior permanent magnet (IPM) machines with hairpin windings have attracted significant attention in EV applications owing to their low DC resistance and excellent thermal capabilities. In this paper, we present a comprehensive investigation of AC winding losses in IPM machines for traction applications, including analytical modeling, the influence of design parameters, and finite element (FE) verification. The proposed analytical model can predict the trends in AC winding losses for any number of bar conductors and slot/pole combinations. The results of the parametric study, obtained via the analytical model, are presented to examine the effects of key design parameters, such as conductor width and height, phase arrangement, and slot-per-pole-per-phase (SPP). To incorporate more practical issues into the analysis of IPM machines with hairpin windings, extensive FE simulations were conducted. The results indicated that the AC winding losses decrease with an increasing number of conductor layers and phases inside the slot.
Advances in electrical machines and power electronics (PEs) are helping to achieve the power density and efficiency required by the transportation sector. However, the reliability of components and production processes is a challenge. This is especially true for electrical machines, whose winding processes are far from the high levels of automation, programmability, and repeatability that are required. This article looks into hairpin windings and outlines a number of future actions to address challenges and eventually enable the complete penetration of hairpin windings in transportation.
Nowadays, one of the key challenges in transport electrification is the reduction of components’ size and weight. The electrical machine plays a relevant role in this regard. Designing machines with higher rotational speeds and excitation frequencies is one of the most effective solutions to increase power densities, but this comes at the cost of increased losses in cores and windings. This challenge is even more pronounced in preformed windings, such as hairpins, which enable higher slot fill factors and shorten manufacturing cycle times. In this work an improved hairpin winding concept is proposed, aiming to minimize high-frequency losses while maintaining the benefits deriving from the implementation of hairpin windings onto electrical machines. Analytical and finite element models are first used to assess the high-frequency losses in the proposed winding concept, namely the segmented hairpin, proving the benefits compared to conventional layouts. Experimental tests are also performed on a number of motorettes comprising both conventional and proposed segmented hairpin configurations. Finally, these experimental results are compared against those collected from motorettes equipped with random windings, demonstrating the competitiveness of the segmented hairpin layout even at high-frequency operations.
Objective of this paper is to provide design recommendations to improve the performance and power density of a 400kVA salient-pole synchronous generator, without exceeding the limitations due to critical parameters such as total harmonic distortion of the no-load voltage. Preliminarily, a finite-element analysis of the considered machine is carried out, aiming at validating the inherent model against experiment measurements. An in-detailed sensitivity analysis of the machine’s design parameters that mostly affect the performance of the platform under investigation is performed. It has been found that the position and the shape of axial ventilation ducts, as well as the stator slot shape do not affect the electromagnetic performance in a significant way and it can be highly beneficial from a thermal point of view, resulting in reduced rotor temperatures. Additionally, the shape of the salient poles and the damper winding arrangement can produce positive effects on the general performance, particularly allowing for an improved voltage THD and power density of the machine being studied in this paper.
In a world of growing electrification, the demand for high-quality, well-optimized electric motors continues to rise. The hairpin winding is one such optimization, improving the slot-fill ratio and handling during production. As this winding technology leads to a high amount of contact points, special attention is drawn to contacting processes, with laser welding being one promising choice. The challenge now is to make the process more stable by means of advanced methods for quality monitoring. Therefore, this paper proposes a novel, cost-efficient quality monitoring system for the laser welding process using a machine learning architecture. The investigated data sources are machine parameters as well as visual information acquired by a CCD camera. Firstly, the usage of machine parameters to predict weld defects and the overall quality of a weld seam before contacting is investigated. In the case of hairpin windings, not only the mechanical but also the electrical properties of each contact point contribute to the overall quality. Secondly, it is illustrated that convolutional neural networks are well suited to analyze image data. Thereby, different network architectures for directly assessing the weld quality as well as for classifying visible weld defects by their severity in a post-process manner are presented. Thirdly, these results are compared to a more explainable two-stage approach which detects weld defects in a first step and uses this information for weld quality prediction in a second step. Finally, these applications are combined into a quality monitoring system consisting of a pre-process plausibility test as well as a post-process quality assessment and defect classification. The proposed system architecture is not only applicable to the contacting of hairpin windings but also to other applications of laser welding.
The worldwide demand for electric vehicles is on the rise and production capacities for electric powertrains need to keep pace with this development. The innovative hairpin stator technology, in which solid copper bars are used instead of flexible round wire for coil production, promises valuable unique selling points on both the product and process side, but also confronts automotive manufacturers and plant manufacturers with major challenges. Due to the cost-intensive and only slightly variant flexible production lines, the optimum product and process concepts must be developed and defined at an early stage. The key for industrialization of hairpin stators is an effective process development and technology selection, especially for the necessary bending and contacting technologies. To systematically break down the technological challenges of process development, a process failure mode and effects analysis (FMEA) is used in this paper with the aim of a scalable, ramp-up-optimized and economical production line. Therefore, the FMEA-method is applied to the partially unknown process technologies of hairpin stators through the investigation of results from early prototypical production concepts especially for the necessary bending and contacting technologies. The analysis lists processes with possibly insufficient process capabilities due to process interdependencies that have been derived using early production prototypes for hairpin stators. In conclusion, this paper presents a method that applies the FMEA-method to early prototypical production concepts of hairpin stators and makes it possible to integrate the results into the final process development and technology selection.
The electrification of the drive train of passenger cars is currently undergoing an evolution from windings previously drawn in, which are known from industrially used electrical machines, to windings specially tailored to the requirements of mobile electric traction. A main trend is the use of pluggable shape windings, so-called hairpins. In order to fully exploit the economic advantages of this winding, elaborated production concepts and strategies are required. Due to the novelty and the currently ongoing innovation of these type of windings, the differentiated manufacturing possibilities per process step have not yet been fully defined. In view of the economic efficiency, high process speeds with increasing process reliability play a major role. The aim of this paper is to show a process chain of hairpin technology that is as universally valid as possible, to name the different alternatives for each process step and to explain their characteristics.
This paper focuses on the improved thermal performance of electrical machines by increasing the conductor fill factor. A comparative design study of integrated starter generator (ISG) is used as a case study to investigate the mechanical, electrical and thermal aspects of pressing coils. By performing quasi-static explicit dynamic simulations, deformation of insulation has been investigated. 1.25 mm diameter magnet wires are simulated and demonstrated to be compressible up to a 0.73 fill factor without observing insulation failure. Thermal conductivity enhancement of the stator windings at improved slot fill factor is investigated by performing steady state FEA thermal simulations and short time thermal transient tests.
In the design of compact, power dense electrical machines found in automotive and aerospace applications it may be preferable to accept a degree of eddy current loss within the winding to realise a low cost high integrity winding. Commonly adopted techniques for analysing AC effects in electrical machine windings tend only to be applicable where the conductor dimensions are smaller than the skin depth and for ideally transposed windings. Further the temperature variation of AC losses is often overlooked. In this paper a more general approach is proposed based on an established analytical model and is equally applicable to windings where the conductor size greatly exceeds the skin depth. The model is validated against test measurements and FE analysis on representative single layer winding arrangements. A case study is used to illustrate the application of the proposed analytical method to the coupled electromagnetic and thermal modelling of a typical slot winding.
A stator structure with rectangular wire distributed winding exhibiting high productivity and high design flexibility was designed for HEV motors. We devised a method of forming thick rectangular wire diamond coils and determined the best conditions for high density coil forming. A terminal connection structure was also devised to achieve a short coil end. The slot-fill rate of a prototype stator was 80.5% and its total coil end was 45 mm, which is roughly the same motor performance of a stator with segmented coil wave winding. The key advantage here is that the proposed stator has fewer coils and welding points than the stator with segmented coil wave winding and that the possible number of conductors per slot is higher than four, which is the limit of the stator with segmented coil wave winding.
Recently, according to Eco-technology accelerating, new types of traction motor with the high efficiency and power are demanded. This paper presents that the Hairpin winding is applied to a design of the electric vehicle traction motor for the high efficiency and power density. Generally, the winding method using the toroidal coil has the restriction which is the unnecessary spaces between coils. However the Hairpin winding method can reduce the gap between coils, therefore the space factor is higher than that of winding methods. However, because of using the full pitch winding and wave winding, this method has the disadvantages of increasing the THD of back electromotive force and torque ripple. Thus, to minimize the THD and torque ripple, the full factorial method, Response surface method and the step skew to the rotor are applied. The paper presents the motor design using the Hairpin winding for the high space factor, and the analysis result by using RSM, the step skew and full factorial method shows the reduced space harmonics.
In this letter, a method to calculate the ac losses, including skin effect and proximity losses, in planar windings with rectangular cross-sectional conductors is proposed. The aim is proposing proper ac losses expressions similar to the formulas available for round cross-sectional wires, to be used for the calculation of the ac losses and the optimization of planar magnetic windings implemented in the printed circuit board. The proposed model is based on the decomposition into conduction and proximity losses. Conduction losses only depend on the properties of the conductor, whereas proximity losses are calculated by using the orthogonal decomposition of the magnetic fields in which the conductors are immersed. Functions including the frequency and geometrical dependences of the both types of losses are extracted by means of finite element method simulation. Finally, several prototypes are used to verify the proposed expressions and some design considerations are also outlined.
Chapter 4 concentrates on discussing leakage flux components, their nature, and calculation resulting in leakage inductances, which have a significant impact on machine performance. Expressions for the permeance factor of single-layer and double-layer windings in connection with the shape of the slots are derived and applied in some examples. Correction of the permeance factor because of the skin effect is shown, and the skewing factor and skew leakage inductance are defined. All components of leakage inductance are discussed in detail.
The paper deals with the Roebel bar losses computation. It was considered the most general Roebel bar structure, including both solid and hollow elementary conductors. One proposes an analytical method for the estimation of the currents in the elementary conductors, as function on the geometrical dimensions and material data. It is possible to emphasize also the current density distribution on the cross section of each elementary conductor. The radial field in the slot area and that in the end coil area were taken in to account. One can analyze the influence of any quantities (machine and material data) on the Roebel bar losses. Therefore, the proposed method is very useful to optimize the bar structure, for the machine efficiency improvement.