Richard McMahon

Coaxial self-resonant systems are gaining traction as a simple yet tunable option in the design of self-resonant coils, whilst providing the typical benefits of removing the need for compensation networks and offering safer operation due to a lower voltage at the coil terminals. The authors have sourced coaxial cable, which is the simplest and cheapest cable from which a coaxial self-resonant coil can be made. It is shown in this publication that off-the-shelf coaxial cable can be used for wireless charging of electric vehicles up to 3.4 kW, and that more optimised cable/coil design may enable even higher power operation. Coil-to-coil efficiency when delivering power to a constant voltage load is around 85% but this comes with the benefit of reduced onboard space and greatly reduced coil terminal voltage. This paper is the first to thoroughly examine what is happening within the cable as it is operated in its two self-resonant modes: open mode (series-resonant) and shorted mode (parallel-resonant). The relevant aspects of coil design that one should bear in mind when constructing a kilohertz-resonant coaxial cable coil are covered and power transfer equations are derived. Coils are constructed that resonate at 260 kHz but these can be stacked in pairs to resonate at 92 kHz in a system. This frequency is achieved without the use of a ferrite core, which would further lower the operating frequency to be within the 79- 90 kHz range specified by the SAE standard for electric vehicle charging.

This paper studies integrated stator–rotor oil-injection cooling for hairpin end windings in an automotive traction motor. A particle-based three-dimensional Computational Fluid Dynamics (CFD) model is developed to examine the combined effects of stator-side and rotor-side cooling, as well as key factors influencing oil cooling rate (OCR) and heat transfer coefficient (HTC), including inlet design, rotational speed, oil temperature, and flow distribution. A thermal finite element analysis (FEA) model is also developed to predict the winding temperature distribution. A detailed experimental setup with a closed-loop fluid circuit is employed to validate simulation results. The results reveal that rotor-side injections deliver strong cooling at high speed, particularly to inner winding layers due to centrifugal oil distribution, while stator-side injection enhances outer layer coverage. When combined, the integrated cooling approach moderately enhances both HTC and OCR, while also achieving a more uniform cooling across the windings. Higher oil temperatures significantly boost both OCR and HTC, while optimized stator inlet design offers marginal gains. The findings provide actionable design guidance: implement integrated injection, direct oil toward hot spots, and tune the stator/rotor flow split to achieve an effective trade-off between cooling effectiveness and oil usage/pumping demand.

Hairpin-wound interior permanent magnet (IPM) motors are widely used in electric vehicles (EVs) for their high power density and thermal efficiency. However, pulse width modulation (PWM) control introduces high-frequency harmonics that increase AC losses, affecting motor performance. This study evaluates motor losses under three PWM strategies: Sinusoidal PWM (SPWM), Space Vector PWM (SVPWM), and Discontinuous PWM (DPWM), at various switching frequencies using a two-dimensional transient finite element model. Results show that SVPWM achieves the lowest motor losses, offering the best efficiency. DPWM, while beneficial for reducing inverter switching losses, results in significantly higher core and magnet losses due to increased harmonic content. Higher switching frequencies further reduce motor losses but lead to elevated inverter losses, presenting a trade-off between motor efficiency and inverter thermal stress. The findings provide critical guidance for selecting optimal PWM techniques and switching frequencies to balance motor and inverter performance in EV applications.

This article investigates the fluid flow and heat transfer characteristics of rotor oil injection cooling on hairpin end winding for an automotive traction motor, with a focus on comparing the performance of shaft injection and rotor lamination injection cooling systems. Computational fluid dynamics (CFD) based on a meshless moving particle simulation (MPS) method is employed to identify key factors influencing oil coverage rate (OCR) and heat transfer coefficient (HTC), including rotational speed, flow rate, flow temperature, and motor inclination. A detailed experimental validation is conducted using a closed-loop fluid system to measure average HTC. The results show that rotational speed is the primary factor in enhancing both OCR and HTC, while oil viscosity also plays a key role in cooling performance. In contrast, motor inclination generally reduces total HTC. Furthermore, a layerwise analysis of OCR and HTC reveals that inner winding layers, closer to the rotor, achieve better cooling due to stronger centrifugal forces and direct oil impingement. The study also compares oil distribution behavior between the two cooling configurations, highlighting differences in flow uniformity and cooling effectiveness. Finally, optimized cooling structures are proposed for both shaft injection and rotor lamination injection cooling, improving oil flow balance and HTC distribution.

Battery energy storage systems (BESSs) are often used in partial-state-of-charge (PSOC) operation due to the desire for flexibility of charge and discharge. Lead–acid batteries are a good candidate to be used in battery energy storage due to their safety, recyclability, and long cycle life; however, the correct battery, cell, and regime should be chosen to ensure effective use. Manufacturers rarely publish data on PSOC performance of their batteries. During PSOC use, the charge acceptance of lead–acid batteries reduces both reversibly and, sometimes, irreversibly as the battery is cycled. Typical dynamic charge acceptance tests target the performance required in car batteries and do not adequately demonstrate the charge acceptance expected in BESS use. This paper demonstrates a representative charge acceptance degradation test which far more closely replicates the charge acceptance degradation seen in real-world PSOC BESS use using partial state of charge, coulomb control, and a charge-factor-controlled full charge. Full charges are shown to reverse the internal resistance associated with partial-state-of-charge operation. This is the case in the Leoch lead–carbon cells and 12 V battery tested. This shows that partial-state-of-charge operation degrades the charge acceptance and increases the internal resistance of a lead–acid battery, although with a charge-factor-based full-charge approach, the charge acceptance could be reset to baseline.

A correct understanding of fluid flow and heat transfer characteristics in oil injection cooling for hairpin end winding is crucial for accurate thermal analysis. This article utilizes a meshless computational fluid dynamics (CFD) approach, based on moving particle simulation (MPS), to develop a simplified numerical model. The model provides qualitative insights into fluid dynamics and heat transfer mechanism in hairpin end winding. Experimental validation is conducted to compare the numerical data and identify key factors influencing oil coverage rate (OCR) and heat transfer coefficient (HTC), including the number and placement of inlets, coolant flow rate and temperature, and rotation speed. The results indicate that increasing the number of inlets and optimizing their placement enhances cooling uniformity. Furthermore, the HTC is strongly influenced by the coolant flow rate and temperature, with an average HTC of 130 W/m²/K achieved at an inlet temperature of $40~^{\circ }$ C and a flow rate of 8 L/min. The highest HTC values are found in the lower winding layers due to the downward flow of oil. Rotation also improves the OCR and HTC by approximately 20% and 37%, respectively, at 1000 r/min, as oil droplets are sprayed onto the winding surfaces, particularly near the rotor. However, at higher rotational speeds, the cooling efficiency stabilizes, and the effect of rotation becomes less significant.

This research investigates the advantages of Kelvin-connected 4-pin discrete transistors, both MOSFETs and IGBTs, in onboard battery chargers for electric vehicles. The study compares the standard 3-pin and the extended 4-pin packages based on averaged data collected from leading manufacturers. The investigation shows significant potential power loss reduction, thermal operation mitigation, and reduced gate-drive oscillation for the 4-pin package. The benefits have been quantified by analysing the operation of actual switches in an automotive battery charger based on Boost-PFC and DC-DC LLC converters. The converters’ practical design demonstrates a procedure for integrating the Kelvin-connected package into the design methodology. The results have been verified experimentally.

Partial state of charge (PSOC) is an important use case for lead–acid batteries. Charging times in lead–acid cells and batteries can be variable, and when used in PSOC operation, the manufacturer’s recommended charge times for single-cycle use are not necessarily applicable. Knowing how long charging will take and what the variability in time required is allows for better planning of operations and algorithm creation for battery energy storage system (BESS) manufacturers. This paper details and demonstrates a procedure for identifying the charging time of cells when different charge throughputs occur prior to reaching full charge. The results showed that the charging time in PSOC operations was highly variable when a charge-factor-controlled full-charge procedure was used. Also noted were that higher voltages for the same state of charge were reached as the number of cycles following reaching full charge increased. None of the regimes tested in this paper caused any significant capacity degradation, which demonstrates that PSOC operations can be performed even on cells not specifically designed for them, provided the correct regime is chosen.