Direct torque control (DTC) of three phase induction motors allows a high dynamic performance
to be obtained with a very simple control scheme. It is based on the stator flux orientation in which
the stator flux is used as a main control variable. Therefore in DTC, determination of stator flux is
always of first and primary concern. This paper investigates the stator flux estimation problem
over the entire speed range. A hybrid stator flux estimator which combines the voltage model and
current model with a PI controller is proposed. The proposed method does not depend on the
angular speed and reduces the problems associated with pure integrators of voltage model at low
speed. An improved stator resistance adaptation scheme is used to enhance the system
performance. Thus, the drive system is capable of working from very low speed up to high speed
and exhibits good dynamic and steady-state performances. The effectiveness of the proposed
scheme is investigated, and good results have been achieved for both steady-state and dynamic
operating conditions.