Speed sensorless control of AC machines using direct flux control scheme

Due to incorrect estimation of stator flux vector around zero speeds, the performance of methods for speed sensorless control of induction motors, is very poor. We propose a simple method to accurately estimate the stator flux vector and demonstrate it´s functioning for operation at very small angular velocities. Estimation of average synchronous angular velocity in every sampling period is done by using the stator flux error vector. Using the direct flux control (DFC) method we show how to get the rotor angular velocity at and around zero value. Test results are presented for dynamic torque operation at and around zero speed. The advantage of this method is that, it uses the available information of the DFC method for achieving sensorless operation without complicating the original control scheme. Given the fact that most sensorless induction motor drive applications do not require a servo performance at zero speeds, the proposed scheme is shown to give a reasonable accuracy.