Comparative Analysis of Overmodulation and Six-step Strategies for PMSM Drives

Sammanfattning

Permanent magnet synchronous motors (PMSMs) are widely used in electric vehicle drives because of their high efficiency and high power density. In high speed operation, however, the available inverter voltage becomes a major limitation on the achievable torque and power. This thesis investigates voltage extension strategies for PMSM drives, with the main focus on modulation methods operating beyond the linear Space Vector PWM(SVPWM) range. Five modulation methods are compared across the transition from linear modulation to overmodulation and six-step operation: SVPWM, Uncontrolled overmodulation, Bolognani’s overmodulation, δ-based overmodulation, and Dynamic overmodulation. Since these methods are mainly applied in the voltage-limited high speed region, two field weakening approaches, lookup-table-based method with and without closed-loop voltage-feedback controller, are evaluated to support stable operation beyond the base speed. A simulation framework based on a nonlinear PMSM model is developed for the comparison. The investigated methods are assessed in terms of voltage utilization, torque and power capability, dynamic response, and current harmonic distortion under both steady-state and transient operating conditions. The results show that the δ-based and Bolognani’s overmodulation achieve the highest voltage utilization and increase the maximum torque and power capability by up to 8.8% and 8.4% compared with conventional SVPWM operation. The dynamic overmodulation provides the fastest transient response, reducing the current settling time to 0.3 ms. The closed-loop voltage feedback field weakening controller maintains stable operation across the SVPWM, overmodulation, and six-step regions. The results also show that increased voltage utilization is accompanied by higher current harmonic distortion, indicating a trade-off between high speed performance improvement and current quality. These findings provide a basis for selecting suitable modulation and field weakening strategies for future PMSM drive platforms according to different performance priorities.