Baoyun Ge

The use of dielectric coatings in capacitive power transfer systems

Capacitive power transfer (CPT) is emerging as a competitive alternative to inductive power transfer (IPT) for non-contact applications over short distances. One distinct advantage CPT has is that the electric field is mostly contained between the coupling plates. These plates are in or near contact with each other but are galvanically isolated by an insulating coating. This paper discusses the function dielectric coating have in CPT and gives insight to their design. Ceramic coatings are presented as a viable option given their durability, high permittivity, dielectric strength and ease of application. Specifically, titanium dioxide is identified as a ceramic coating ideally suited for CPT applications. Titanium dioxide is coated on both aluminum and stainless steel substrates via relatively new sol-gel process. This process is documented step by step with the thickness and chemical composition of the coating verified by scanning electron microscope. The effective permittivity and dielectric breakdown strength of the titanium dioxide coatings are measured.

Design Concepts for a Fluid-Filled Three-Phase Axial-Peg-Style Electrostatic Rotating Machine Utilizing Variable Elastance

Rotating electric machinery is usually constructed of iron/steel laminations, copper windings, and permanent magnets. This paper investigates fluid-filled, electrostatic rotating machines for the ultimate ambition of transitioning fundamental magnetic materials to dielectrics in order to reduce production costs. The study of the axial-peg-style electrostatic rotating machine focuses on basic geometric and material knowledge and the creation of design tools. An axial-peg machine possesses interdigitated pegs (cylinders) that come into, and out of, radial alignment as the machine rotates causing variable capacitance between the stator and rotor. A prototype with peak torque of 0.7 Nm and gap field strength of 15 kV/mm was constructed. The specific torque density of the machine is 0.101 Nm/kg, comparable to fractional horsepower NEMA class induction machines. This was achieved by filling the machine with a dielectric fluid, whose relative permittivity is 7.1, rather than the ultra-high vacuum typically employed in canonical electrostatics. Experimental measurements presented include angular capacitance, peak torque, and torque-per-volt under stall conditions. Construction techniques are discussed in detail.

Evaluation of dielectric fluids for macro-scale electrostatic actuators and machinery

Recently interest towards electrostatic actuators and machinery has increased for use in MEMS, mechatronics and renewable energy applications. Effective electromechanical power conversion using electrostatics requires dielectric fluids whose permittivity and breakdown field strength can facilitate adequate electric shear stress and pressure beyond the capabilities of media such as air or vacuum. This paper discusses design points for practical electrostatic force production and evaluates available dielectric fluids in terms of electric shear stress and pressure capability. Key dielectric properties including relative permittivity and breakdown field strength are measured via custom test stand. Attractive force is measured between two electrodes mounted on load cells inside a tank filled with a dielectric liquid. Electrode surfaces may be configured to facilitate electric pressure or shear measurement. Measurements demonstrate fluids capable of reaching several PSI of electric pressure while correlating with analytical and finite element (FE) models, ultimately forming a base for electrostatic actuator/machine design.

Dielectric liquids for enhanced field force in macro scale direct drive electrostatic actuators and rotating machinery

Recently interest towards electrostatic actuators and machinery has increased for use in MEMS, mechatronics and renewable energy applications. Effective electromechanical power conversion using electrostatics requires dielectric liquids whose permittivity and breakdown field strength can facilitate adequate electric shear stress and pressure beyond the capabilities of media such as air or vacuum. This paper discusses design points for practical electrostatic force production and evaluates available off-the-shelf dielectric liquids in terms of electric pressure capability. Key dielectric properties including pressure, breakdown field strength, relative permittivity, and conductivity are measured. A custom test stand measures the attractive force between two electrodes mounted on load cells inside a tank filled with the liquid under test. Measurements demonstrate liquids capable of reaching several psi of electric pressure while correlating with analytical and finite element (FE) models, ultimately forming a basis for electrostatic actuator/machine design. The best liquid candidate, Vertrel XF, was poured into a proof of concept macro scale electrostatic machine demonstrating 0.9 N-m torque and 0.126 N-m/kg. Torque measurements verify the capability of Vertrel XF and the correlation between the electric pressure and shear stress in a rotating machine.

Three-Dimensional Printed Fluid-Filled Electrostatic Rotating Machine Designed with Conformal Mapping Methods

Recently, fluid-filled electrostatic machines demonstrated specific and volumetric torque densities that hold promise to be competitive with electromagnetic machines in niche applications, e.g., air-cooled, low-speed, and direct-drive machines. These demonstrations of variable capacitance (or elastance, which is the dual of reluctance) machines were nonoptimized from an electrostatics perspective as their geometry was heavily constrained by manufacturability issues. This paper proposes a semi-analytical design method that combines conformal mapping techniques with finite element analysis, leading to more optimal electrostatic machine geometries. Parametric sweeps of key relative dimensions establish best practices/guidelines for design. A fractional horsepower proof of concept machine was designed using the new approach and was built using stereolithographic three-dimensional printing to circumvent manufacturing constraints. The machine is mostly plastic, plated with conductor, and is, therefore, lightweight. This manufacturing approach suggests that a machine can be injected molded or cast in a single step. Measurements of the prototype demonstrate a torque density of 0.31 Nm/L and specific torque density of 0.22 Nm/kg, comparable with similar size NEMA frame fractional horsepower induction motors. Also the Nm/kV2 of the prototype machine is two orders of magnitude greater than prior nonliquid-filled work.

A 1-phase 48-pole axial peg style electrostatic rotating machine utilizing variable elastance

Starting from the duality of magnetic reluctance and electric elastance, this paper discusses design features of an axial peg style electrostatic rotating machine. Axial peg style refers to the geometry of the machine. Interdigitated pegs (cylinders) come into, and out of, radial alignment as the machine rotates causing variable capacitance between the stator and rotor. A prototype with peak torque of 0.9 N-m and gap field strength of 14 kV/mm was constructed. The specific torque density of the machine is 0.126 N-m/kg, comparable to fractional horsepower NEMA class induction machines. This was achieved by filling the machine with a dielectric fluid whose relative permittivity is 7.1, rather than ultra-high vacuum typically employed in canonical macro scale electrostatic machine designs. Selected experimental measurements presented include angular capacitance, stall torque, average torque and losses under stall conditions.

A 3D printed fluid filled variable elastance electrostatic machine optimized with conformal mapping

Recently, fluid filled electrostatic machines have demonstrated specific and volumetric torque density that hold promise to be competitive with electromagnetic machines in niche applications. These demonstrations of variable elastance (dual of reluctance) machines were non-optimized from an electrostatics perspective as their geometry was heavily constrained due to manufacturability. Higher performance electromechanical power conversion for electrostatics requires optimal geometric design. This paper proposes a semi-analytical method incorporating conformal mapping techniques with finite element (FE) analysis to optimize a variable elastance electrostatic machine in a low speed direct drive applications. As a proof of concept, an optimized geometry was built using additive manufacturing, specifically stereolithographic 3D printing, to circumvent geometry constraints. By building the machine from plastic plated with conductor, it is lightweight with improved torque density. This manufacturing approach suggests that a machine can be injected molded or cast in a single step. Experimental results support both the design and manufacturing approaches and the resulting machine is benchmarked against previous work.

Design concepts for a 3-phase axial peg style electrostatic rotating machine utilizing variable elastance

Rotating electric machinery is usually constructed of iron/steel laminations, copper windings, and permanent magnets. This paper investigates fluid-filled, electrostatic rotating machines for the ultimate ambition of transitioning fundamental magnetic materials to dielectrics in order to reduce production costs. The study of the axial-peg-style electrostatic rotating machine focuses on basic geometric and material knowledge and the creation of design tools. An axial peg machine possesses interdigitated pegs (cylinders) that come into, and out of, radial alignment as the machine rotates causing variable capacitance between the stator and rotor. A prototype with peak torque of 0.7N-m and gap field strength of 15kV/mm was constructed. The specific torque density of the machine is 0.101N-m/kg, comparable to fractional horsepower NEMA class induction machines. This was achieved by filling the machine with a dielectric fluid, whose relative permittivity is 7.1, rather than the ultra-high vacuum typically employed in canonical electrostatics. Experimental measurements presented include angular capacitance, peak torque and torque-per-volt under stall conditions. Construction techniques are discussed in detail.

A dq-axis framework for electrostatic synchronous machines and charge oriented control

Recently, innovations such as advanced dielectric liquids and additive manufacturing have enabled electrostatic machines at the fractional horsepower scale that have competitive performance with their conventional electromagnetic counterparts. The emergence highlights a need for circuit modeling that guides machine design and drive controls, rooted in the canon of well-established electromagnetic machinery practices. This paper combines prior approaches and expands upon them to form a unified electrostatic machine dq-axis framework capable of providing insight on the capacitances of interest for torque production as well as control. Maximum torque per volt and charge oriented control are presented with a torque equation whose terms are parsed into field, elastance, and induction torque mechanisms. Complex vector voltage regulators form the heart of drive controls. The model is confirmed with finite element analysis (FEA).

A switched elastance electrostatic machine constructed from sustainable elements for rotational actuators

A rotating electric machine built entirely of sustainable lightweight materials is presented for rotary actuator applications. The machine uses electrostatic forces instead of magnetic, enabling different materials. Here, aluminum, plastic, and printed circuit boards are used as they are low cost and easily recycled at end of life. The electrostatic machine developed is of the axial flux variety, utilizing the switched elastance mode of operation, i.e. a dual of a switched reluctance machine. Cascaded disks whose conducting vanes/poles (surfaces) align and misalign during rotation are immersed in a dielectric liquid of sufficient energy density to enable higher torque output. The liquid drag torque limits efficiency at higher speeds as a motor, but is ideal for low speed rotary actuation. A prototype electrostatic machine was built in a NEMA 42 frame form factor for general comparison. While operating at 9 kV, the machine provides 1.9 N-m average torque constituting torque densities of 0.5 N-m/kg (specific) and 1.2 N-m/L (volumetric). The machines low conduction loss and large internal surface area eliminates virtually all cooling needs and stall torque can be held indefinitely.