Three Segment Vector Pulsewidth Modulation With Improved Harmonic Performance and Linear Overmodulation Gain for Low-Carrier Ratios

High-power converters and/or high-speed motor drives often operate at low-carrier ratios, particularly at higher fundamental frequencies (and consequential higher modulation indices), because of device switching limitations. For such applications, advanced modulation strategies are required to minimize current quality issues, together with complex corrections to manage a (usually) nonlinear progression into the overmodulation and six-step regions so that converter capacity and/or motor operating speeds can be maximized. To minimize harmonics, these strategies are typically synchronous with the target fundamental frequency, which complicates their use in closed loop systems where a constant calculation frequency is preferable for optimum controller performance and stability. This article addresses this issue, by combining concepts of synchronous optimal pulsewidth modulation (SOPWM) and magnetic flux synthesis using space vector pulsewidth modulation (SVPWM), to create a fixed calculation frequency three-segment vector pulsewidth modulation strategy (TSVPWM). Similar to SOPWM, TSVPWM generates a “corner hexagonal” magnetic flux linkage that achieves a comparable harmonic performance to SOPWM at high modulation indices. TSVPWM maintains a linear gain across the entire modulation range with a smooth transition into six-step operation, and can be easily integrated with high-performance motor control strategies because of its constant calculation frequency. Its performance has been validated using a high-speed motor at 380 V and 12 000 r/min.