On the stability of volts-per-hertz control for induction motors

This paper deals with the stability analysis of volts-per-hertz (V/Hz) control for induction motors. The dynamics of the electrical and mechanical subsystems of the induction motor model are nonlinearly coupled by the electromagnetic torque and the back-electromotive force. Under open-loop V/Hz control, the nonlinear interaction is known to give rise to small-signal oscillations while operating at medium speeds under light loads. In this paper, it is shown that the interaction also causes a nonoscillatory unstable mode to appear at low speeds under heavy loads (despite the perfect flux level), manifesting itself as a flux collapse or surge. It is also shown that the electrical subsystem with the rotor speed input and the electromagnetic torque output has nonpassive operating regions, which indicates a risk of detrimental interactions with the mechanical subsystem. Finally, a feedback design is proposed in order to enlarge the passive and stable regions and improve the damping. The theoretical results are validated by means of simulations and experiments on a 45-kW induction motor drive.