Advances on the control of multiphase ac drives under converter overmodulation

Resumo

Multiphase motors with an n-phase configuration have gained considerable attention in recent years due to their superior performance over traditional three-phase systems, offering enhanced fault tolerance, increased power density, reduced torque ripple, and lower per-phase currents. These advantages are largely attributed to the increased degrees of freedom available in multiphase systems (n > 3) making them well-suited for advanced ac variable-speed drive applications, such as fault-tolerant or high-power electric vehicles. A critical objective in such applications is maximizing the dc-link voltage utilization, increasing the achievable modulation index M and thus extending the motor speed range. This dissertation investigates overmodulation (OVM) techniques for multiphase ac drives, which allow the modulation index to exceed linear pulsewidth modulation (PWM) limits by injecting low-order current harmonics. Such harmonics, however, can increase the current total harmonic distortion (THD), leading to additional loss. In three-phase systems, achieving the desired M in OVM requires injecting torque-producing harmonics. In contrast, multiphase motors can exploit additional non-torque current-producing subspaces (termed subspaces) to inject harmonics while avoiding torque ripple. Existing high-performance OVM methods, particularly for five-phase machines, have not been generalized to higher n-phase systems or thoroughly assessed for their current- THD minimization potential. This work addresses key gaps in the state of the art about OVM in multiphase drives. A generalized carrier-based (CB) PWM strategy is proposed for symmetrical multiphase motors with odd n 5, achieving minimum voltage distortion (MVD) with computational simplicity. Additionally, a comparative evaluation of existing OVM methods for five-phase drives in both OVM1 and OVM2 ranges is conducted to assess current THD, with a novel minimum current distortion (MCD) technique introduced as a benchmark. Moreover, a MCD strategy applicable to any n, winding arrangement, and neutral-point configuration is developed. A study that compares previous methods for many kinds of multiphase machines with the new generalized MCD strategies as benchmarking is also performed. In comparison with previous OVM methods, this generalized MCD consistently ensures minimal harmonic loss in any considered scenario. In fact, this is the first time that OVM with reduced THD is addressed for many widespread multiphase systems, e.g., n = 6; 12 asymmetrical motors with a single neutral point. Finally, a comprehensive comparative study considering voltage and current THD, switching losses and harmonic stator copper loss is put forward to establish the most suitable OVM technique for symmetrical n = 6 motors. In this comparative study, the feasibility of using three-phase discontinuous PWM methods is also explored and a simpler MVD method is presented for this kind of machine. To sum up, this dissertation provides essential advancements for implementing efficient, high-performance OVM in multiphase AC drive applications. Contributions of this thesis have been published in four JCR-indexed journal papers in the first quartile (Q1) and presented at one international conference.