Synchronous Electrostatic Machine Torque Modulation via Complex Vector Voltage Control With a Current Source Inverter

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.