Decoupled Discrete Current Control for AC Drives at Low Sampling-to-Fundamental Frequency Ratios

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, r_{mathrm {S2F}} , 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 d - and q -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.