Advances in electrostatic machine design have enhanced the torque density of macroscale electrostatic machines toward practical use. A recently developed fractional horsepower three-phase separately excited synchronous electrostatic machine (SEM) demonstrates torque densities comparable to those of air-cooled permanent-magnet-based electromagnetic machines (1.5 Nm/kg) when excited with a medium voltage (5 kV). SEMs develop torque from voltage, not from current, and therefore incur nearly zero losses at low speeds or stall. However, there is no off-the-shelf medium-voltage drive at this power level, and the appropriate control framework for these machines has yet to be established. This article presents a complex vector voltage regulator control approach as a means for modulating torque in an SEM. Ampere-second (charge) is sourced from a current source inverter (CSI) serving as the drive electronics for voltage regulation. Together, the control approach and the CSI hardware form the first high-performance electrostatic drive. Key research outcomes include the theoretical development and experimental verification of charge-oriented control via voltage regulation. Experimental results are presented for rotational and stall conditions, which are reflective of the "position and hold" applications suited to electrostatic machines. The dynamic performance of the voltage regulator is verified by measuring the controller frequency response function, dynamic stiffness, and command tracking on a separately excited SEM.
Index Terms-Complex vector control, current source inverter (CSI), electrostatic machines, torque control. NOMENCLATURE Symbols Meaning C s Stator capacitance [F]. R s Stator resistance []. C md Mutual capacitance [F]. V fd Field excitation [V]. M Back MMF or current [A]. P Pole number.
The development of electrostatic rotating machines for macroscale power conversion has been largely sidestepped, given the uncertainty of its capabilities and place in the technological hierarchy. This article reviews prior and present works in macroscale electrostatic rotating machinery and identifies the relevant machine types, their limitations, and strategies for performance improvement. The separately excited synchronous electrostatic machine presents the greatest opportunity for competitive macroscale category-two machinery, and a strategy of multiplicative gains is established. The strategy spans machine modeling, optimization, gap media (gases, liquids, and vacuum), gap maintenance, advanced manufacturing techniques, and power electronic drives/control. Ultimately, the product of innovation gains across all these areas reveals that macroscale electrostatic machinery is possible and potentially competitive with magnetic machinery for specific areas, including position and hold, low-speed direct drive, and high-voltage utility generation applications.
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