Improved deep flux-weakening control techniques for IPMSM drives

Abstract

This thesis identified several problems that exist in deep flux-weakening control of the Interior Permanent Magnet Synchronous Machine (IPMSM) for which the characteristic current is well within the current limit circle. It then presents several important improvements of the deep flux-weakening control techniques for such an IPMSM drive. A prioritized d-axis current control technique, including maximum torque per voltage (MTPV) control, was proposed. The experimental results demonstrate that the proposed control scheme delivers wider flux-weakening speed range and improved torque performance than was possible with existing flux-weakening control methods. It is well-known that under deep flux-weakening control, there exists conflicting demand between d- and q-axis current controllers, which deteriorates torque-speed performance in the flux-weakening speed range. Recently, it was proposed that such conflict can be avoided by the single-current-regulator (SCR) based flux-weakening. However, the existing SCR method has several limitations, which were identified in this thesis and a modified SCR control scheme was proposed to eliminate the identified limitations. The modified SCR method, which includes MTPV control, delivered improved current responses, torque capability and flux-weakening speed range of the machine. This thesis also presents a method to include MTPV trajectory in a direct torque and flux control (DTFC) scheme. A closed-loop flux estimation technique, which uses an adaptive sliding mode observer (SMO), was implemented for the position sensorless control of the proposed drive. This method is known to work well at higher speed range, however, its operating characteristics in deep flux-weakening have not been demonstrated before. Using SMO based flux estimators in DTFC with MTPV trajectory control demonstrated its effectiveness in deep flux-weakening speed range. Applicability of the above mentioned deep flux-weakening control schemes in fractional-slot concentrated-winding (FSCW) IPMSM was also investigated. It was observed that deep flux-weakening control in FSCW IPMSM requires additional voltage vector compensation for both the current-vector control and DTFC schemes. An automated voltage vector compensation technique was then successfully implemented in the FSCW IPMSM along with the deep flux-weakening control schemes, achieving improved torque-speed dynamics compared to existing methods.