One of the most complicated control problems in variable speed AC drives is the difficulty of processing feedback signal of the outer loop in the presence of harmonics. However, conventional fixed band hysteresis controller has a variable switching frequency throughout the fundamental period. A variable-hysteresis-band controller comes as a solution to reach an optimal ripple current. An improved implementation of the constant frequency hysteresis current control of three phase voltage-source inverters is presented. A simple, self-adjusting analogue prediction of the hysteresis band is added to the phase-locked-loop control to ensure constant switching frequency, even at a high rate of output voltage change. This allows high accuracy in tracking highly distorted current waveforms and minimises the ripple in multiphase systems. In this paper, the basic principles are described, and a detailed stability analysis is done. The control performance is illustrated, both by simulated and experimental results.
Keywords: constant frequency digital hysteresis current controller, constant switching frequency, hysteresis current control, voltage source inverter, VSI