A simple speed sensorless control for variable frequency induction motor drives

This paper presents a method for speed control of an AC drive with variable frequency PWM inverter without a speed sensor. The proposed control algorithm is based on speed compensation as a function of the load torque. The load torque estimate is obtained with a single current sensor in the DC link of the inverter. Most of the problems that are the consequence of cross-coupling and numerous nonlinearities in the system, are solved with the use of pre-formed look-up table, that gives the stator frequency as a function of the reference speed and the load. The control system is implemented using digital techniques, and the steady state error is less than half of the stator frequency resolution. Theoretical analyses and experimental verification supports the validity of the proposed control concept, and its usefulness to practical application.