Daniel C. Ludois

A Survey of Wireless Power Transfer and a Critical Comparison of Inductive and Capacitive Coupling for Small Gap Applications

Inductive power transfer (IPT) and capacitive power transfer (CPT) are the two most pervasive methods of wireless power transfer (WPT). IPT is the most common and is applicable to many power levels and gap distances. Conversely, CPT is only applicable for power transfer applications with inherently small gap distances due to constraints on the developed voltage. Despite limitations on gap distance, CPT has been shown to be viable in kilowatt power level applications. This paper provides a critical comparison of IPT and CPT for small gap applications, wherein the theoretical and empirical limitations of each approach are established. A survey of empirical WPT data across diverse applications in the last decade using IPT and CPT technology graphically compares the two approaches in power level, gap distance, operational frequency, and efficiency, among other aspects. The coupler volumetric power density constrained to small gap sizes is analytically established through theoretical physical limitations of IPT and CPT. Finally, guidelines for selecting IPT or CPT in small gap systems are presented.

Hierarchical Control of Bridge-of-Bridge Multilevel Power Converters

Multilevel converters are among the members of the family of power-converter topologies for realizing higher power levels and better waveform quality. In addition to the established topologies of neutral-point-clamped three-level and cascaded H-bridge converters, novel topologies that offer attractive features, such as ease of modularity and functionality, continue to be introduced. Among these, the bridge-of-bridge multilevel converters have the potential for realizing multimegawatt systems with ease. This paper is aimed at presenting a systematic approach to developing their dynamic and steady-state models, leading to a hierarchical-control approach that is intuitive to realize and versatile in application. This paper presents the dynamic and steady-state models and computer simulations that demonstrate the approach in dc-1φ/ac and 3φ/ac- 3φ/ac power-conversion applications. An experimental validation of the models using a dc-1φ/ac asymmetrical-half-bridge converter is presented.

Simplified Terminal Behavioral Model for a Modular Multilevel Converter

Modular multilevel converters (MMCs) are emerging to be an attractive approach for high-power applications. Equivalent circuit models and dynamic models for the MMC that provide a faithful representation of system behavior are quite complex given the large number of energy states and control variables. They are not particularly useful in studying the terminal behavior of the converter and for the development of an intuitive control approach to regulate power transfer. This paper reduces the complexity of the MMC analytical model to an equivalent boost-buck converter circuit while providing particularly insightful transparency of the converters physical operation from a terminal perspective. The transformed boost-buck converter model and control performance are verified using simulations and experiments using a laboratory scale prototype.

Capacitive Power Transfer for Rotor Field Current in Synchronous Machines

Permanent magnet (PM) synchronous machines are utilized in a wide variety of applications due to their many desirable characteristics, including high torque density capability and high efficiency. In the near future, however, the demand for the PM rare earth materials is projected to exceed world production. As a result, electric machines that do not rely on rare earth materials, such as wound field synchronous machines (WFSMs), are receiving renewed attention for use in traction and wind energy applications. However, WFSMs require a current delivery mechanism to the rotor such as mechanical slip rings whose components require periodic replacement and generate adverse debris within the machine enclosure. Rotary transformers may replace slip rings but also introduce rotor speed dependences and magnetic coupling difficulties. This paper proposes a capacitive noncontact power transfer technique to eliminate the need for mechanical slip rings while also avoiding the pitfalls of rotating transformer technologies. The capacitive power transfer system is compared to traditional rotor power coupling techniques and its performance is validated with experimental results.

Aerodynamic Fluid Bearings for Translational and Rotating Capacitors in Noncontact Capacitive Power Transfer Systems

Wireless power transfer (WPT) is commonly accomplished with magnetic (inductive) techniques for a wide range of applications. Electrostatic or capacitive power transfer (CPT) approaches to WPT have had limited exposure primarily due to lower achievable power density when compared to inductive WPT techniques. Recently, high-frequency (in kilohertz to megahertz) power electronics have reintroduced capacitive techniques as an option for WPT over short distances ( <; 2 mm) for applications such as slip ring replacement. To further the practicality of CPT, capacitive coupling must be maximized in an effective manner, i.e., the volumetric capacitance density of rotating/translational capacitors must be significantly increased. This paper proposes the use of aerodynamic fluid bearings to maximize capacitive coupling between stationary and moving surfaces, by minimizing their separation distance, allowing for greater surface area per unit volume. The technique allows micrometers of separation distance between moving surfaces while maintaining manufacturability and mechanical robustness. Coupling capacitance is increased up to 100 times greater than rigid plate rotating and translational CPT systems. Additional benefits include the estimation of mechanical system parameters such as speed. Operational characteristics and design highlights are presented and corroborated with experimental results for general slip ring replacement applications.

Single Active Switch Power Electronics for Kilowatt Scale Capacitive Power Transfer

The development of capacitive power transfer (CPT) as a competitive wireless/contactless power transfer solution over short distances is proving viable in both consumer and industrial electronic products/systems. The CPT is usually applied in low-power applications, due to small coupling capacitance. Recent research has increased the coupling capacitance from the pF to the nF scale, enabling extension of CPT to kilowatt power level applications. This paper addresses the need of efficient power electronics suitable for CPT at higher power levels, while remaining cost effective. Therefore, to reduce the cost and losses single-switch-single-diode topologies are investigated. Four single active switch CPT topologies based on the canonical Ćuk, SEPIC, Zeta, and Buck-boost converters are proposed and investigated. Performance tradeoffs within the context of a CPT system are presented and corroborated with experimental results. A prototype single active switch converter demonstrates 1-kW power transfer at a frequency of 200 kHz with >90% efficiency.

Wireless electric vehicle charging via capacitive power transfer through a conformal bumper

Wireless power transfer (WPT) is emerging as a practical means for electric vehicle (EV) charging. Of the three main approaches to WPT, resonant inductive, inductive, and capacitive coupling, capacitive power transfer (CPT) is proposed herein to charge an EV at a kilowatt scale power level. CPT implementation replaces copper coils and ferrous core focusing/shield materials of inductive approaches with foil surfaces making CPT cost effective and structurally simple to implement, while maintaining efficient power transfer capability. This paper addresses each facet of kilowatt scale CPT system development, namely achieving high coupling capacitance between the vehicle and charging station and the associated drive power electronics. High capacitive coupling is achieved through a conformal (flexible and compressive) foam transmitter bumper that molds and contours itself to the vehicle to minimize air gap during charging. An experimental docking station to charge a Corbin Sparrow EV 156V battery pack was built and measured throughput power is demonstrated at >1kW with a coupling capacitance of 10nF operating at 540kHz.

Capacitive Power Transfer Through a Conformal Bumper for Electric Vehicle Charging

Wireless power transfer (WPT) is emerging as a practical means for electric vehicle (EV) charging. Of the most common approaches to WPT, inductive coupling, and capacitive coupling, capacitive power transfer (CPT) is proposed to charge an EV at a kilowatt scale power level. CPT implementation replaces copper coils and permeable focusing/shielding materials of inductive approaches with foil surfaces, making CPT a cost effective and structurally simple system to implement while maintaining efficient power transfer capability. This paper addresses the primary technical hurdles to kilowatt scale CPT system development, namely, safe field confinement by achieving high coupling capacitance between the vehicle and the charging station. High capacitive coupling is achieved through a conformal (flexible and compressive) transmitter bumper that molds and contours itself to the vehicle. This minimizes the air gap and confines the field during charging. Here, a conformal surface demonstrates 3-5 times more coupling capacitance than its rigid counterpart of equal area. The associated power electronics are also discussed in detail, utilizing a Class E2 amplifier/rectifier. An experimental docking station was built to charge the 156 V battery pack of a Corbin Sparrow EV and measured throughput power is demonstrated at 1 kW at ~90% efficiency via a coupling capacitance of 10 nF operating at 530 kHz.

Capacitive power transfer for slip ring replacement in wound field synchronous machines

Permanent magnet synchronous machines are utilized in a wide variety of applications due to their many desirable characteristics, including high torque density capability and high efficiency. In the near future, however, the demand for the permanent magnet rare earth materials is projected to exceed world production. As a result, electric machines which do not rely on rare earth materials, such as wound field synchronous machines (WFSMs), are receiving renewed attention for use in traction and wind energy applications. However, WFSMs require a current delivery mechanism to the rotor such as mechanical slip rings whose components require periodic replacement and generate adverse debris within the machine enclosure. Rotary transformers may replace slip rings but also introduce rotor speed dependencies and magnetic coupling difficulties. This paper proposes a capacitive non-contact power transfer technique to eliminate the need for mechanical slip rings while also avoiding the pitfalls of rotating transformer technologies. The capacitive power transfer system is compared to traditional rotor power coupling techniques and its performance is validated with experimental results.

Simplified dynamics and control of Modular Multilevel Converter based on a terminal behavioral model

Modular Multilevel Converters (MMCs) are emerging to be an attractive approach for high power applications. Equivalent circuit models and dynamic models for the MMC that provide a faithful representation of system behavior are quite complex given the large number of energy states and control variables. They are not particularly useful in studying the terminal behavior of the converter and for the development of an intuitive control approach to regulate power transfer. This paper reduces the complexity of the MMC analytical model to an equivalent boost-buck converter circuit while providing particularly insightful transparency of the converters physical operation from a terminal perspective. A control approach which manages throughput power as well as the MMCs internal capacitive states based on terminal quantities is developed eliminating individual feedback of module capacitor voltages to a centralized controller. An extension of the control to mitigate the circulating currents within the MMC caused by bridge voltage harmonics is also developed. The transformed boost-buck converter model and control performance are verified using simulations and experiments using a laboratory scale prototype.