Rotor Flux Observer for Speed Sensorless IFOC Induction Motor at Low Speeds

In this paper, a method of rotor flux estimation is proposed to improve the characteristics performance of indirect field oriented controlled induction motor drives, at low speed and standstill. Traditionally, in voltage model rotor flux estimator, standard method is to use first order time delay element in place of pure integrator to counter the problem of dc drift and offset. Still, the performance of drive deteriorates when stator frequency is low and at standstill. Here, in addition to first order delay element, a high pass filter is included to reduce the dc offset effect. Also, reference flux space vector is added to the output of voltage model flux estimator, so that at low speed this reference value of flux is the output of the flux observer. In the proposed method, speed is estimated by comparing the torque producing stator current component computed using above flux observer, with reference stator current space vector obtained from speed PI loop. This difference is then fed to the proportional-integrator to minimize the error, generating the rotor speed. The proposed scheme is implemented using indirect field oriented control of induction motor and performance, in all operating conditions, including the critical low stator frequency range is verified using MATLAB Simulation.