Stability Issues in V/f Controlled Medium Voltage Induction Motor Drives Considering Magnetizing Inductance Variation

In this paper, a V/f control for high-power induction motor drives is analysed. In such uses, low dynamics in terms of speed-loop bandwidth is usually required. However, in some applications, where the load consists of a long shaft with certain mechanical components and with some resonant frequencies, instability and torque oscillations can cause significant damage. For these reasons, open-loop V/f control could be a winning solution, since it avoids resonances and oscillations caused by measurement feedbacks in the control. However, instability may arise also in open-loop V/f control. A stability analysis is here carried out analytically and a new method to overcome oscillations by varying dynamically the V/f ratio is proposed. The stability analysis and the effectiveness of the proposed method are verified with simulation results on a 2.8kV-7.25MW induction motor and with experimental tests on a small-scale 380V-10 kW motor.