Dynamic Properties of Back-emf Based Sensorless Drives

This paper investigates the speed sensitivity of back-emf based sensorless techniques for PM machines. The eigenvalues of the back-emf based estimation are speed dependent leading to performance degradation including reduced command tracking and disturbance rejection. At low speeds, it is shown that instability occur. The paper analyzes an operating point model of the permanent magnet synchronous machine for both sensor- and sensorless-based torque and position/velocity control. The sensorless techniques investigated include state filter-based and zero phase-lag observer-based approaches. In order to maintain disturbance rejection, the sensorless drives must operate at speeds significantly higher than the stability limit