Compensation for non-synchronized position and current feedback signals in embedded digital controls of AC machines

This paper presents a technique to compensate for speed estimation errors and control performance degradations caused by non-synchronized motor position and phase current feedback signals in embedded digital controllers for vector controlled AC machines. Using a vector controlled IPM synchronous motor drive developed for hybrid electric vehicle applications as a platform, the causes and effects of such non-synchronized signals are studied. Due to the limited through-put capability of the microprocessor or DSP and the multi-task schedules often required for high performance drives, the time delay between the non-synchronized motor position and phase current signals varies over sampling cycles. Such variable time delay causes a variable phase delay in the inverse Park transformation when converting the voltage commands from the synchronous coordinate current regulator to stationary quantities. The net effects are to cause additional errors in speed estimation, steady state control errors, and possible instability of the motor control system. A novel technique to compensate dynamically for such variable time delays is developed and implemented in a TMS320LF2407 DSP based vector controller for the sample motor. Extensive tests are performed and test results demonstrated the effectiveness of the proposed solution in improving torque and speed control accuracies and stability of the drive system.