A flux estimator for induction motor drives based on digital EMF integration with pre- and post- high pass filtering

Flux estimation remains a challenging part of any high performance AC induction motor drive. Generally, the easiest flux estimation method requires the integration of the back electromotive force (EMF), operation that is greatly affected by the DC offset from current sensors and by the initial conditions. This leads to a gradual appearance of a DC offset in the output estimation of the flux. Over the years many advanced control techniques have been proposed in order to obtain a reliable flux estimator, in particular at very low speeds. Generally the proposed methods involve extensive signal processing making them unattractive for practical application. In recent years the researchers have focused toward solving the flux estimation problem by using relatively simpler techniques based on some form of integrator with feedback. The present paper describes a simple digital method to obtain the induction motor flux by integrating the EMF after first passing it through an adaptive high pass filter. The output of the integrated signal is then passed through a second adaptive high pass filter in order to remove the bias caused by the initial conditions. Overall, the method is shown to greatly reduce the amount of signal processing and it can be used to obtain the flux even at very low speed using a 16-bit fixed-point processor