Abstract
This article 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 backelectromotive 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 article, 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.
Highlights
T HE DEVELOPMENT of power semiconductors enabled the first pulsewidth-modulated (PWM) variable-speed induction motor drives in the 1960s [1]
We study the stability of V/Hz control by reviewing the existing results and augmenting them with new findings
It is shown that the interaction between the electrical and mechanical subsystems causes an unstable low-speed region to appear under heavy loads, in addition to the well-known unstable mid-speed region
Summary
T HE DEVELOPMENT of power semiconductors enabled the first pulsewidth-modulated (PWM) variable-speed induction motor drives in the 1960s [1]. Modern versions of V/Hz control aim to stabilize the drive by means of feedback from the stator current [14]–[18], while the RI and slip compensators are used in order to maintain the desired operating point. It is shown that the interaction between the electrical and mechanical subsystems causes an unstable low-speed region to appear under heavy loads (despite a perfect RI compensator), in addition to the well-known unstable mid-speed region. This nonoscillatory unstable mode manifests itself as a flux collapse or surge. Means of simulations and experiments on a 45-kW induction motor drive
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