Stability improvement of vector control of induction motor without speed sensor

The characteristics of a current control loop have great implications for the stability of vector control of a induction motor without speed sensors. In this paper, the authors mention the three types of current control loops and analyse exactly the stability analysis of the speed sensorless vector control system of an induction motor, taking into account the effects of each current control loop. Two types are composed of a synchronous current regulator and one of them has an additional decoupling circuit. Another is composed of a stationary current regulator. At first, the authors derive the fundamental equations and the linearized models for each control system by using the standard small signal analysis techniques. Next, the transfer functions between the rotor speed and the rotor speed command are computed. The poles and the zeros are then calculated from these transfer functions to investigate the influences of the each current control loop on the stability. Furthermore, the theoretical step responses of the rotor speed corresponding to the poles and the zeros are compared with experimental results.