High Power Density Electrical Machines Research Articles

Analytical Calculation and Experimental Validation of Litz Wires Axial Thermal Conductivity

Litz wires are widely used in electrical machines with reduced AC losses, compared to traditional random winding. The reduced AC losses can improve both electrical machines electromagnetic performance, such as efficiency and thermal performance. Thermal aspect is identified as a key enabler for next generation high power density electrical machines, where thermal modelling plays a critical role. Equivalent slot thermal conductivity is one of the most challenging parameters to be determined in the process of thermal model development, due to various components, such as copper, insulation, and impregnation resin. There are extensive literatures where the litz wire thermal conductivity in the radial direction are reported. However, the thermal conductivity in the axial direction is not well studied, which is critical to determine the heat transfer, such as for electrical machines with end winding spray-cooling system. In this paper, the axial thermal conductivity is investigated with analytical calculations and experimental validation. Firstly, 9 litz wire samples are selected with various types, such as rectangular and circular shapes, with varnish process and without. Two different analytical methods are then introduced and presented to calculate the litz wire thermal conductivity in axial direction, based on equivalent length theory and equivalent medium theory, respectively. Finally, experimental tests are conducted, and results are compared to those obtained from the proposed analytical methods. Guidelines are also provided to predict the litz wire axial thermal conductivity in this field.

Development of dual phase soft magnetic laminate for advanced electric machines

The development of dual phase soft magnetic laminate for advanced electric machines at General Electric is summarized, including alloy development, properties, manufacturing trials, prototype experience, potential applications, and future material development. Dual phase soft magnetic laminates provide the flexibility to tune magnetic/non-magnetic state at the desired locations, using solution nitriding treatment with protective mask/coating. This enabling material technology benefits electric machine performance by reducing flux leakage in the laminates. It also decouples the electromagnetic and mechanical design, which opens up the design space for advanced electric machines. Magnetic and mechanical properties of dual phase laminates were compared to commercial FeSi and FeCo soft magnetic laminates. Driven by the market needs for high power density and high-speed electric machines, a synchronous reluctance (SynRel) machine without PMs using dual phase soft magnetic laminates was assessed in machine electromagnetic performance as well as rotor stress analysis by finite element simulation. Successful prototype build and testing further retired manufacturing risks and advanced the technology maturity level. Future material development with higher mechanical strength, higher saturation magnetization, higher permeability, and lower loss are highly desired for ground and air transportation electrification.

Convective Heat Transfer Characteristics on End-Winding of Stator Immersed Oil-Cooled Electrical Machines for Aerospace Applications

The stator immersed oil cooling has emerged as a promising thermal management method for high power density electric machines applied in aerospace applications. However, due to the complex geometry of end-winding, the research on the convective heat transfer coefficient (CHTC) for the flooded stator is still at infancy. This article investigates the convective heat transfer characteristics on end-winding of stator immersed oil-cooled electrical machines by computational fluid dynamics (CFD) methods and experiments implemented on a motorette. The variation law of the CHTCs of end-winding with flow rate is thoroughly investigated. Subsequently, new dimensionless correlations are defined using Nusselt numbers as a function of Reynolds numbers, which are further applied into a detailed lumped-parameter thermal network (LPTN) for rapid evaluation of temperature distribution. Finally, the obtained simulation results are verified by local temperature measurements. The experimental results also show that the proposed stator immersed oil-cooled structure can withstand a current density of up to 30 A/mm2 with a corresponding continuous power density of 6.8 kW/kg.

Numerical Investigation of High-Temperature Superconducting-Coated-Conductors Subjected to Rotating Magnetic Fields

Over time, high-temperature superconductor (HTS)-coated conductors (CCs) have proven to be promising candidates for future high-efficiency and high-power density electrical machines. However, their commercialization is handicapped due to the AC dissipative loss that occurs upon exposure to external magnetic fields. In rotating electromagnetic devices, the external magnetic field is a combination of alternating and rotating magnetic fields. Most of the research is devoted to the effect of exposure of the superconductors to alternating magnetic fields only. This article presents an investigation to observe the behavior of HTSCCs under rotating magnetic fields, particularly the AC loss, using a finite-element-based homogeneous H-formulation technique. Our investigation shows that the AC loss could be considerably high when HTSCCs are exposed to rotating magnetic fields and, ultimately, could affect the cooling efficiency of future high-efficiency and high-power density electrical machines.

Performance Comparison of Superconducting Machines With Induction Motors

The demand for high power density electrical machines grows in aircraft and ships for their propulsion systems. Superconducting machines can be an attractive alternative for these applications. In this context, the present work presents a study comparing a superconducting motor with a squirrel-cage induction motor operating at room and cryogenic temperatures. The superconducting electric machine uses 2G tapes stacked on the rotor. The study is based on experiments with bench prototypes of conventional induction motors and superconducting motors having equal dimensions. The rotor is composed of three superconducting tape rows, which are stacked in a spiral shape around a soft ferromagnetic core. The operating principle of this machine considers the interaction of the rotating field produced by the stator with the magnetic field trapped in the superconducting tapes stacked in the rotor. A commercial 0.5-hp squirrel-cage induction motor was adapted for the tests in different circumstances for operation in an extensive temperature range. The induction machine was tested at liquid nitrogen temperature and room temperature. The results of all prototypes were compared. The authors observed that, up to 80% of the rated load, the superconducting machine works in a synchronous regime, and for higher torque’s levels, it is asynchronous. Besides, it increased the torque 1.24 times and 2.43 times for the induction motor operating at room temperature and 77 K, respectively. The results showed that this superconducting machine has the potential for high power density applications.

Toward more electric powertrains in aircraft: Technical challenges and advancements

The main purpose of this article is to provide an instructive review of the technological challenges hindering the road toward more electric powertrains in aircraft. Hybrid, all-electric, and turboelectric powertrain architectures are discussed as possible fuel consumption and weight reduction solutions. Among these architectures, the short-term implementation of hybrid and all-electric architectures is limited, particularly for large-capacity aircraft due to the low energy/power density levels achievable by state-of-the-art electrical energy storage systems. Conversely, turboelectric architectures with advanced distributed propulsion and boundary layer ingestion are set to lead the efforts toward more electric powertrains. At the center of this transition, power converters and high-power density electric machines, i.e., electric motors and generators, and their corresponding thermal management systems are analyzed as the key devices enabling the more electric powertrain. Moreover, to further increase the fuel efficiency and power density of the aircraft, the benefits and challenges of implementing higher voltage powertrains are described. Lastly, based on the findings collected in this article, the projected roadmap toward more electric aircraft powertrains is presented. Herein, the individual targets for each technology, i.e., batteries, electric machines, and power converters, and how they translate to future aircraft prototypes are illustrated.

Minimization of AC Copper Loss in Permanent Magnet Machines by Transposed Coil Connection

With the increasing demand for high power density and high-speed electrical machines, ac copper loss issue becomes more severe and poses a significant challenge for machine design. This article proposes a suppression technique by transposing the coil connection in the same branch to balance the inductances of parallel strands, thus reducing the circulating current ac loss. Compared to the commonly used transposition method implemented within one coil (e.g., Litz wire), the proposed method can be beneficial to reduce the transposition points and have no influence on the slot filling factor. The proposed idea is investigated by the field-circuit coupled finite element analysis (FEA), taking the bundle-level circulating current into account. It shows that the copper loss can be reduced by 17% compared to the nontransposed one at 400 Hz for a studied 12-slot 10-pole permanent magnet machine. The influence of winding layout and slot/pole combination on the effectiveness of the proposed transposition method is also investigated. Finally, experiments are implemented on a designed test-rig to validate the FEA method and the effectiveness of the new transposition.

Cooling of Windings in Electric Machines via 3-D Printed Heat Exchanger

This article presents a novel use of three-dimensional printed direct winding heat exchangers (3-D-DWHX) to improve the thermal management of the stator in high power density electric machines. The 3-D-DWHX is in direct contact with the stator windings, enabling high continuous current densities. The use of dielectric polymer for the heat exchanger allows direct contact with the windings without an additional insulation material. By occupying the otherwise unused space between double layer concentrated windings, there is no significant impact on the electromagnetic design, and a smaller machine can be realized. The result is an increased continuous power rating and power density of electric machines. The polycarbonate-aluminum flake 3-D-DWHX was validated in a nonencapsulated motorette test, achieving a continuous current density of 20.4 ARMS/mm2 for a hotspot temperature of 148 °C. Finite element analysis shows that the same design, when encapsulated, can achieve 33.3 ARMS/mm2 current density with a hotspot temperature of 195 °C, corresponding to a continuous specific power of 11.8 kW/kg.

Graphene: future of high power density electrical machines for electric vehicle applications

The key challenges faced by many of the industries in the present scenario is to develop a high power density electrical machine. The advancements in material technology help the industries to enhance the power density as well as the efficiency of an electrical machine to a greater extent. Conductors play a major role in improving the machine's performance. In this paper, the role of conducting materials in high power density electrical machines are discussed. Graphene is an advanced nanomaterial that exhibits excellent conductivities with low mass density. Using graphene coated windings, the weight and volume of the machine can be reduced significantly. In order to understand the prominent features of the nanomaterial based conductors, a case study has been carried out in different electric vehicles such as Toyota Prius (2017), Nissan LEAF and BMW i3. Moreover, a major constraint in high power density machine is the temperature rise, which is discussed at the end.

Graphene: future of high power density electrical machines for electric vehicle applications

Graphene: future of high power density electrical machines for electric vehicle applications

Extended End-Winding Cooling Insert for High Power Density Electric Machines With Concentrated Windings

Concentrated windings are favourable in electric motors with a high torque density because of the achievable high winding filling factor and their short end-windings. Nevertheless, the performance of the cooling system has the biggest impact on the reachable torque density limits. This article introduces a thermally conductive insert piece that encloses the stator end-windings with extensions in the stator slots. This results in an extended contact surface between the stator windings and the machine housing, which is advantageous for the conductive heat transfer. The introduced indirect cooling concept is compatible with traditional cooling techniques such as forced air and water jacket cooling. The concept is illustrated with 3D thermal finite element simulations, including the anisotropic thermal material properties, as well as ad hoc measurements on a stator pole unit mock-up of a 6-4 switched reluctance machine. Where the stator with reference cooling system was only able to handle a continuous current density of 19.0 A/mm $^2$ , 26.5 A/mm $^2$ could be obtained in the winding of the stator with the extended end-winding cooling inserts.

High Power Density Electrical Machines for Electric Vehicles—Comprehensive Review Based on Material Technology

The growth of electric and hybrid electric vehicles has drawn more attention toward the development of high power density electrical machines. The materials used in the fabrication of electrical machines play a crucial role in enhancing their power density. In this paper, the significance of advanced magnetic materials for lamination, nano-material based conducting materials, and high energy product permanent magnets (PMs) is discussed in detail. Finally, a case study has been presented with a 5 kW PM synchronous machine in order to understand the advantages of the discussed materials.

Reduction of Winding AC Losses by Accurate Conductor Placement in High Frequency Electrical Machines

In the design of electrical machines, the increase in power density has become one of the main research themes. With most of the power loss in high power density electrical machines often being generated in the stator winding assembly, it is necessary to reduce these losses. The effects of strands and bundle positions in the slot on the ac losses in the winding are often overlooked. Taking as a case-study an existing high-frequency machine, this article analyzes and provides an in-depth insight into such effects. Exploiting the rapid advancements in precision three-dimensional printing, it is found and experimentally verified that by controlling the position of the conductors within the top of the slot the ac losses can be markedly reduced.

Stator Current Vector Determination Under Consideration of Local Iron Loss Distribution for Partial Load Operation of PMSM

Iron losses have a large share in the overall losses of high power density electrical machines operating as variable speed drives. Especially in the partial load area, the ratio of iron losses is dominant with respect to the copper losses. Therefore, commonly used control strategies, such as maximum torque per ampere or maximum torque per voltage, aiming at minimizing the current for given restrictions as the maximum voltage do not select the best direct and quadrature currents to maximize the efficiency or minimize the overall losses at each operating point. This paper elaborates a stator current vector determination strategy considering the iron loss distribution at all operating points, comparing different iron loss models in a machine with distributed windings. Depending on the operating point and the operational conditions of the machine, the overall losses can be reduced up to 7%.

Piezoelectric Fan Cooling: A Novel High Reliability Electric Machine Thermal Management Solution

Electric machine thermal management is critical for the correct operation of high power density electrical machines. This is, however, challenging to achieve in safety-critical applications where the reliability of the cooling system needs to be significantly higher than conventional solutions. Piezoelectric fans are presented here as a novel fault-tolerant forced cooling convective system for electric machines. Particle image velocimetry techniques in conjunction with infrared thermal measurements were implemented to map and quantify the flow fields generated by such a cooling arrangement as well as to determine the effective cooling enhancement. This paper also outlines the main design variables for such a system and highlights the main considerations to be accounted for to optimize the cooling potential. For the specific machine presented in this paper, the optimal fin/fan geometry resulted in mean flows in excess of 2.41 m/s and turbulence values in excess of 2.38 m/s, which resulted in an average convective heat transfer coefficient enhancement of 364.8% on the fin base and a further enhancement of 53.6% on each of the fin side walls. This, in turn, led to a 61.8% reduction in the electric machine heat sink cooling mass.