Hairpin Winding Research Articles

Segmented Hairpin Topology for Reduced Losses at High-Frequency Operations

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.

Electromagnetic and Thermal Analysis of Interior Permanent Magnet Motors Using Filled Slots and Hairpin Windings

This paper analyzes the electromagnetic and thermal design of interior permanent magnet motors using filled slots and hairpin windings for electric vehicle applications. Two models of ∇ shape of the interior permanent magnet motors have been proposed to evaluate the temperature distribution and cogging torque performance. A narrow opening slot of the interior permanent magnet of 48 slots/8 poles with the filled winding has been designed to investigate the electromagnetic torque because the cogging torque depends on opening stator slots. A parallel–rectangle slot of the interior permanent magnet with the hairpin winding has been also implemented with finite element analysis to evaluate their performances. Normally, the slot opening of the interior permanent magnet stator equals the slot width, it is greater than the size of hairpin windings, and the cogging torque is increased significantly with a bigger slot opening. The main advantage of the hairpin winding design is the high slot fill factors. Hence, the lower the current density, the higher torque, and efficiency are, than the normal design with the same geometry parameters. To improve the cogging torque due to the wide slot opening, the step–skew rotor slices have been arranged to minimize the torque ripple with different skewing angles.

A Multi-Objective Design Optimization for a Permanent Magnet Synchronous Machine with Hairpin Winding Intended for Transport Applications

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.

A Thermal Modeling Approach and Experimental Validation for an Oil Spray-Cooled Hairpin Winding Machine

While the electromagnetic aspects of hairpin windings are actively being investigated and discussed in recent literature, including the design rules together with the loss calculation and reduction techniques, the thermal performance and modeling aspects have received less attention to date. In hairpin windings, the conductors (pins) are comparatively larger and arranged as separate components in parallel within the slot. In contrast, conductors randomly overlap and contact each other for traditional random windings. The differences in the aforementioned winding physical characteristics result in a different methodology to develop the thermal network. This article presents a 3-D lumped parameter thermal network (LPTN) approach for an oil-spray cooled hairpin winding, which includes the slot thermal model configuration, the end-winding connections, together with different methodologies of analyzing the end-winding sprayed oil characteristics. The aforesaid thermal model captures unique features related to the winding technology and cooling mechanism, such as the nonuniform end-winding temperature caused by the uneven oil-spray cooling effects. Finally, taking an existing propulsion drive hairpin stator and a bespoke-designed test setup, the presented steady-state thermal modeling approach is experimentally validated covering various experimental tests, including different spray conditions.

Challenges in Winding Design and Thermal Calculations: Physical Model of Permanent Magnet Synchronous Machine

Interest in multilayer windings is increasing with the application of the hairpin winding technology to the manufacturing of electrical machines. Therefore, the four-layer fractional slot concentrated winding is used for the initial design of the machine in this paper. The proposed physical model of the machine uses winding with a relatively high number of turns which is inappropriate to hairpin winding. Therefore the round-wire winding is created and the three-layer winding is derived and analyzed including the effect on the slot leakage inductance. The thermal analysis is then applied to the physical model of the machine to evaluate the slot-related thermal properties of the slot and the whole machine. The measurement is compared with the finite element analysis (FEA) and the equivalent slot thermal conductivity and heat transfer coefficients of the stator and rotor are obtained.

Experimental Study on Behavior of Coolants, Particularly the Oil-Cooling Method, in Electric Vehicle Motors Using Hairpin Winding

This paper analyzes the characteristics of oil behavior in the oil-cooling of motors with hairpin winding to understand how to maximize cooling performance. The oil cooling is performed by directly spraying oil onto the motor components. The results show that as the temperature of the oil increases, the viscosity decreases, and the oil film is formed more evenly; however, oil splashing also increases. Similarly, as the flow rate increases, oil splashing also increases, but the amount of oil forming the oil film increases. However, the oil film is not affected by the rotor’s rotation. In contrast, the immersed oil is found to be closely related to the rotor’s rotation. As the rotational speed increases, the immersion oil is mixed with the air, and oil churning occurs. The mixing phenomenon increases as the temperature and flow rate of the oil increases. The higher the oil level, the greater the oil churning. As the oil is mixed with air, the heat transfer coefficient decreases, which adversely affects the thermal management of the motor. As a result, when considering the oil film and the immersion oil, the optimal oil temperature, flow rate, and oil level are at 60 °C, 0.140 kg/s, and 85 mm, respectively. The results of this paper give important information about EV motor cooling and can contribute to the development of high-performance motors.

Study of Injection Method for Maximizing Oil-Cooling Performance of Electric Vehicle Motor with Hairpin Winding

The oil injection method was studied to maximize the cooling performance of an electric vehicle motor with a hairpin winding. The cooling performance of the motor using the oil cooling method is proportional to the contact area of the oil and the coil. A numerical analysis was conducted to examine the effect of the spray nozzle type on the oil flow. The dripping nozzle forms the thickest oil film on the coil, making it the most effective for cooling of hairpin-type motors. Subsequently, an experimental study was conducted to optimize the nozzle diameter and number of nozzles. When the inlet diameter and number was 6.35 mm and 6, the oil film formation rate was 53%, yielding the most uniform oil film. Next, an experiment was performed to investigate the effects of the oil temperature and flow rate on the oil flow. The oil film formation rate was the highest (83%) when the oil temperature was 40 °C and the flow rate was 6 LPM.

Cooling effect of oil cooling method on electric vehicle motors with hairpin winding

The cooling effect of oil cooling method on electric vehicle motors with hairpin winding is analyzed. A 160 kW motor being developed by a commercial vehicle company is applied. Firstly, the flow field of oil was analyzed. In the crown part, 96% of the oil flowed; in the welded part, 90% of the oil flowed. Secondly, an analysis of the heat transfer in the coil and stator was performed. The welded part of the coil exhibited an average temperature 4% higher than that of the crown part. Finally, the parametric study was conducted to understand the effect of the temperature and flow rate of the oil on the temperature distributions of the motor. As the flow rate increases, the maximum temperature decreases. However, there is not a big difference at more than 0.8 LPM/LPM. The results provide important information for understanding the effect of a motor using oil cooling.

Analysis of Copper Loss of Permanent Magnet Synchronous Motor With Formed Transposition Winding

In recent years, the hairpin winding flat wire motor has received more and more attention because of its high power density and high torque, but the problem of increased AC copper loss caused by the increase of its wire diameter has restricted the use of the motor. Aiming at the problem of large AC copper loss in hairpin winding flat wire motors, this paper introduces the use of formed transposition windings into permanent magnet synchronous motors. Through analysis and comparison, the transposition bar suitable for permanent magnet synchronous motor is determined, and a method of inter-turn transposition that can improve the slot full rate of the motor is proposed. Taking a 223kW permanent magnet synchronous motor as an example, the transient field of the stator bar under 360° transposition was calculated by using the field-circuit combined three-dimensional(3-D) finite element method. By calculating the data, combined with the AC copper loss calculation method of the formed transposition winding given in the article, the distribution of the DC copper loss, eddy current copper loss and circulating copper loss of the motor is obtained. Compared with hairpin windings, the use of formed transposition windings can reduce the AC copper loss of the motor to a certain extent, the feasibility of using the formed transposition winding in the permanent magnet synchronous motor is verified.

Production-oriented design of electric traction drives with hairpin winding

In recent years, the manufacturing of stators by hairpin technology has proven its ability to fulfill the requirements on quality, productivity and robustness of traction drive applications in automotive industry. However, the uncertainty and necessity of rapid product development despite fuzzy target systems still cause that processes, machines and equipment – as well as the electric design – are often in an imperfect prototype stage at the start of production ramp-up. Due to the complex interdependencies between the stator components in combination with a high sensitivity of the overall process reliability to minor adjustments of stator design features, possible production-related weaknesses in design are often recognized first in the prototype stage of the production system. In order to reduce the economic risk resulting from these volatile technological conditions, production-oriented design based on numerical simulation methods can be applied from the beginning of product development. Therefore, several techniques for numerical process modeling are presented in this paper as possibilities to consider manufacturing constraints in an early stage of product development. For this purpose, the influence of wire dimensions on the forming process of hairpin coils is investigated using the example of rotary bending as well as the twisting process of a full stator by finite element simulations. Furthermore, a numerical approach to investigate the influence of heat input during laser welding of hairpin coils on the required stripping length is introduced.

Analysis on Circulating Current Loss in the Formed Winding of Permanent Magnet Synchronous Motors

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.

Forward Converter Current Fed Equalizer for Lithium Based Batteries in Ultralight Electrical Vehicles

In this paper, the concept of a forward balancing technique fed by a buck converter for lithium-based batteries in Electrical Vehicle (EV) applications is investigated. The proposed active topology equalizes eight cells in a series in a battery pack, by using a forward converter for each battery pack and the whole battery packs, using a buck converter. The battery bank consists of four battery packs, which are in series. Therefore, the proposed system will equalize 32 cells in series. In this paper, the proposed circuit employs a single transistor used in a Zero Voltage Switch (ZVS) for the forward converter. In practice, this means a capacitor in parallel with the switch at the same time a demagnetizing of the transformer is obtained. The circuit realizes a low Electromagnetic Interference (EMI) and reduces ringing. To overcome the problem of many pins on a coil former, the transformer secondary windings are made by using hairpin winding, on a ring core. It permits, e.g., having eight secondaries and uniform output voltages. Each secondary winding is made by two hairpin turns using two zero-Ohm resistors in series. The proposed topology has less components and circuitry, and it can equalize multiple battery packs by using a single buck converter and several forward converters for each battery pack. Experimental and simulation results are performed to verify the viability of the proposed topology.