Star-delta switching performance during the starting of an induction motor

Abstract

Contents:During the starting of all inductiotl motor currents are very high. Through the switching of the windings from a star-to a delta-configuration the currents are diminished. But there are high current and torque transients shortly after the switching moment. These transients are dependent on the type of the star-delta-connections and on the switching moment during the run-up. This paper presents a thorough investigation of these dependencies and the resulting current and torque performance. Optimal switching moments will be discussed. The calculations are based on the presentation of the behaviour of the induction motor by space-phasors and is done by digital simulation on a computer. Great importance is put on a physical interpretation of the results. Stern-Dreieck Umschaltung wiihrend des Hochlaufs einer AsynchronmaschineObersicht: "vVXhrend des Hochlaufs eines Asynchronnlotors flieBen hohe Str6me. Diese k6nnen durch eine Stern-DreieckUmschaltung der Wicklungen verringert werden, Kurz nach dem Umschaltaugenblick entstehen jedoch hohe Strom-und Drehmomentspitzen. Diese Spitzen sin dabhgngig yon der Art der, Stern-Dreieck-Umschaltkonfiguration nnd yore Schaltaugenblick wSohrend des Hoehlaufs. Der Aufsatz beschreibt eingehend diese Abh/ingigkeiten nnd das dabei entstehende Strom-und Drehmomentverhalten. Es werden optimale Umschaltzeitpunkte diskutiert. Die ~3ereehnungen beruhen auf der Darstellung des Asynchronmotor-Verhaltens dutch Raumzeiger und wurden mit einem digitalen Simulationsprogramnl durchgefEhrt. Besonderer Wert wurde auf die physikalische Interpretation der Ergebnisse gelegt.Commonly the star-delta-switching is applied by which the currents are cut down to 2 to 3 times rated current. But after the switching again there are high current peaks. Therefore an investigation is necessary to find out whether there are optimal moments of switching and whether there can be a strategy for an optimal performance during the starting. Basic Equations for the Dynamic Behaviour of an Induction MotorIn previous papers El, 2] the presentation of the dynamic behaviour of an induction motor by spacephasors has been derived. Space-phasors give a very good picture and understanding of the physical processes inside the machine since the space-phasors of flux and current --being complex variables in a complex plane placed in the crossection of the machine --represent the rotating fieId and the current distribution by amplitude and position in the air gap of the machine. The current space-phasor for instance is defined by