Analysis and Improvement of Synchronous PWM-Based Closed-Loop Current Control for Machine Drive

Synchronous pulsewidth modulation (PWM) scheme is more attractive than asynchronous one in low switching to fundamental frequency ratios, for its better harmonic spectrum. But, when a customized synchronous PWM scheme is incorporated into the field-oriented control, the dynamic response speed usually needs to be limited strictly, otherwise some violent current oscillations would appear. As a negative result, the tracking speed to the command is degraded too low to be used in most applications. To address this problem, existing methods mainly introduced a phase regulation loop inside the current control loop, trying to eliminate the phase error between the sampling phases of desired and actual voltages (i.e., maintaining synchronization), which does bring benefits to the current dynamic response. However, this phase regulation loop is always designed empirically due to the lack of a mathematical model and its interaction with the current loop has not been revealed up to date. As a result, the final performance is still not satisfactory. However, the dynamic response is crucial, especially in new energy vehicle applications. To explore this problem, the phase regulation loop is mathematically modeled first in the discrete-time domain, based on which the whole system including both the current loop and the phase loop is analyzed. Consequently, the reason is discovered and the relationship between the phase loop and the current loop is clarified. Furthermore, a novel phase-error regulator is proposed allowing the phase loop to work in the deadbeat mode and the current loop. Compared with the traditional one, with the proposed phase loop the whole closed-loop system achieves a faster dynamic response and produces much smaller current oscillations. All of the design and analysis are experimentally verified on an 18-kW interior permanent magnet synchronous machines drive test rig.