Decoupled Discrete Current Control for AC Drives at Low Sampling-to-Fundamental Frequency Ratios
Meiqi Wang, Giampaolo Buticchi, Jing Li, Chunyang Gu, David Gerada, Michele Degano, Lie Xu, Yongdong Li, He Zhang, Chris Gerada
Abstract
Implementation of proportional–integral (PI) controllers in the synchronous reference frame (SRF) is a well-established current control solution for electric drives. It is a general and effective method in digital control as long as the ratio of sampling-to-fundamental (S2F) frequency ratio, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$r_{\mathrm {S2F}}$ </tex-math></inline-formula> , remains sufficiently large. When the aforesaid condition is violated, such as operations in high-speed or high-power drives, the performance of the closed-loop system becomes incrementally poor or even unstable. This is due to the cross-coupling of the signal flow between <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$d$ </tex-math></inline-formula> - and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$q$ </tex-math></inline-formula> -axes, which is introduced by the SRF. In this article, an accurate model of current dynamics, which captures the computational delay and PWM characteristics in the discrete-time domain, is developed. This motivates the investigation of eliminating cross-coupling effects in permanent magnet synchronous motor (PMSM) drive systems. A new current control structure in the discrete-time domain is proposed targeting full compensation of cross-coupling effects of SRF while improving dynamic stiffness at low S2F ratios. The matching simulation and experimental results carried out on a 5-kW high-speed drive corroborate the theoretical analysis.