Energy harvesting using AC machines with high effective pole count

In this paper, we investigate ways to improve the power conversion of small, rotating, permanent magnet AC machines at low rotor speeds in energy harvesting applications. One method is to increase the pole count, which increases the generator back-emf without also increasing the I²R losses, thereby increasing both torque density and conversion efficiency. One machine topology that has a high effective pole count is a hybrid stepper motor. The impedance due to the large self inductance of these machines can be cancelled by capacitors in series with the stepper output. A circuit was designed and implemented to automatically vary the series capacitance over the entire speed range investigated. The addition of the series capacitors improved the power output by up to 700%. The results obtained for the stepper were compared to those of a DCPM brushed motor of a similar size, and results confirm that high-pole-count topologies, and specifically the hybrid stepper configuration, are an attractive choice for energy harvesting applications.