Control Strategies for Dual Inverter-fed Open-end Winding PM machines with Nonsinusoidal back EMFs

摘要

Permanent Magnet Synchronous Motor (PMSM) drives with a high degree of fault tolerance are required where it is crucial to guarantee continuous operation of the drive system. To have additional benefits during a fault, PMSM drives with open-ended winding can be employed. Both ends of the windings in the prototype PMSM are connected to a dual inverter arrangement of two 2-level inverters both connected to one dc source. However, considering the zero sequence voltage is clamped to zero because of the return path between the two inverters, zero sequence current circulates in the machine windings which increases losses and generates heat. Furthermore, most PMSMs generate harmonics in the back electromotive force. These harmonics if not compensated for will couple with the circulating zero sequence current, increasing the machine current further and typically beyond ratings, the total harmonic distortion of the current will increase, and the generated torque oscillates more than twice the average torque. To accurately control the current during normal operating conditions, the flux linkage harmonics are measured and incorporated with the dq0-axis current control scheme to restrict the harmonic current in the system. A revised simulation model for the PMSM drives was constructed that considers the zero sequence model and integrate the effects of nonsinusoidal back EMFs within the drive. The model can be easily extended to higher harmonic based on the obtained harmonics of the PMSM drives. Open-phase faults are then considered in this thesis. Semiconductor switches in a phase leg can be turned off and the phase winding disconnected so that the system effectively operates under two-phase vector control. A simple modified transformation under fault conditions is proposed that enables the current to be regulated in the dq-axis and retains control of the voltage. The different outcomes of utilizing a system that maintains 120 degrees between phase currents and a phase shift of 60 degrees between the remaining healthy phases are investigated. Under such conditions, possible switching states that can be used are considered. The results demonstrate that the proposed modified coordinate transformation is easy to implement and effective in solving the loss of one phase when open-phase fault occurs in the setup. The performance evaluation based on the experimental measurements are investigated during normal and faulted operating conditions.