Hybrid Asynchronous/Synchronous Pulse Width Modulation Technique for Permanent Magnet Synchronous Motor Drives in Traction Applications

Abstract

Asynchronous pulse width modulation (PWM) and synchronous optimal PWM (SOPWM) combined hybrid PWM technique is a well-established solution in railway traction applications. This technique achieves low current total harmonic distortion (THD) over a wide speed range and extends the motor operation range while maintaining the switching frequency within the desired range. Therefore, the hybrid asynchronous/synchronous PWM is a particularly well-suited modulation strategy for adjustable speed drives where a wide speed range is necessary, such as electrified transportation, industry, and high-speed machines. Despite its advantages, the widespread adoption of the hybrid PWM technique in other traction applications has been hindered primarily due to the challenges associated with SOPWM in closed-loop control. This thesis aims to resolve the main challenges related to SOPWM to enable the applicability of hybrid PWM in permanent magnet synchronous motor (PMSM) drives and other motor drive applications. To create an optimal hybrid PWM scheme, this thesis conducts a comprehensive review of PWM techniques for three-phase, two-level voltage source inverters (2L-VSIs), providing an in-depth comparative analysis of the commonly used PWM techniques based on experimental investigations. The selected PWM techniques are assessed based on key performance metrics, including output power quality, DC-link voltage utilization, dynamic response, and complexity. Furthermore, this review study examines the existing challenges and the impact of digital controllers, wide-bandgap semiconductors, and model predictive control on PWM techniques to forecast future trends in the field. Based on the comprehensive comparative study, the SOPWM exhibits significantly low current THD at lower switching-to-fundamental frequency ratios, proving its suitability for high-speed regions, as anticipated. Therefore, this thesis employs asynchronous PWM at low speed and the SOPWM at high speed, similar to the conventional hybrid PWM scheme. A notable drawback of the SOPWM technique lies in its requirement for substantial memory space on the microcontroller for storing high-resolution pre-optimized optimal switching angles (OSAs). This drawback is exacerbated especially in motor drive applications, where multiple sets of OSAs with different numbers of commutations are necessary to maintain the switching frequency within the desired range across a wide speed range. A novel method for reducing the look-up table size of OSAs in SOPWM is introduced to tackle this issue. The proposed method generates new switching angles with a minimum number of points, which can ensure that the error between the new switching angles and the original OSAs remains within the desired threshold. Experimental and simulation results are provided to demonstrate the effectiveness of the proposed method. Furthermore, this thesis investigates the influence of OSA error on voltage THD, current THD, and voltage magnitude, revealing the impact of OSA error on SOPWM performance. This study investigates conventional SOPWM with current THD minimization for PMSM drives. The main issues related to the discontinuities of the OSA are addressed by introducing hysteresis bands around the discontinuities by extending the local switching angles by means of a new post-optimization method that optimizes the OSA in forward and reverse modulation index directions depending on the beginning and end of the discontinuity. Moreover, hardware limitations are considered during the optimization and optimal pulse pattern (OPP) generation stages. As a result, an enhanced SOPWM that is more suitable for motor drive applications has been developed. The effectiveness of the proposed method is validated through experimental and simulation results. The presence of deadtime is inevitable in VSIs, and it introduces significant errors in the magnitude and phase of the generated voltage waveform, resulting in increased THD in both voltage and current and degrading the overall performance of SOPWM. This thesis proposes a novel deadtime compensation method for SOPWM to mitigate these issues by effectively correcting the deadtime-induced errors in the OPP. This correction is achieved through real-time manipulation of the reference voltage angle used for OPP generation, ensuring accurate switching and minimum THD. Simulation and experimental results are provided to demonstrate the effectiveness of the proposed method. Furthermore, this thesis also presents the impact of deadtime on traditional carrier-based PWM and SOPWM, highlighting comparative performance metrics and emphasizing the importance of the proposed deadtime compensation method for SOPWM in enhancing power quality. Finally, this thesis introduces an enhanced hybrid PWM technique suitable for integration with the well-established field-oriented control (FOC) strategy. The proposed technique relies on an innovative robust SOPWM and simple smooth transition schemes to improve the robustness of hybrid PWM against noise in the control loop introduced by the measured currents and rotor position. Furthermore, a simplified extended back-emf Luenberger observer and phase-locked loop on the reference voltage angle are implemented in the FOC to further enhance the hybrid PWM performance. Experimental results are provided to demonstrate the effectiveness of the proposed hybrid PWM technique and the simplified observer in the closed-loop control of a PMSM drive.