Analysis of a frequency converter equipped with diode rectifier and small DC link capacitor
Author
Ofeigsson, Thordur
Term
10. term
Publication year
2008
Abstract
Mange lavpris-frekvensomformere bruger en simpel diodensretter og en lille DC-link-kondensator. Det holder prisen nede, men giver store rippel og spændingsspidser i DC-linket, fordi den lille kondensator ikke kan udglatte ensretterens harmoniske. Når en sådan omformer driver en kortslutningsrotor-asynkronmotor (en almindelig industrimotor), kan hurtige hastighedsændringer skabe DC-link-overspændinger, som udløser beskyttelse og forringer den dynamiske ydeevne. Projektet undersøger, hvor stabilt dette omformer–motorsystem er, og hvordan forskellige designvalg påvirker stabiliteten. Vi opstiller en matematisk model af systemet og lineariserer den for at få overføringsfunktioner, der viser, hvordan input påvirker DC-link-spændingen. Med klassisk reguleringsteori analyserer vi stabilitetsmargener og kontrollerer dem med simuleringer i Matlab/Simulink. For at passe til fokus på lav pris designer vi en sensorløs V/f-styring (forholdet mellem spænding og frekvens) og verificerer den i simulation. Vi implementerer open-loop V/f-styring og space-vector-modulation på en digital signalprocessor (DSP) for at observere systemet i praksis. Eksperimentelle resultater blev indsamlet og dokumenteret.
Many low-cost variable-speed drives use a simple diode rectifier and a small DC-link capacitor. This keeps costs down but allows large ripple and spikes in the DC-link voltage, because the small capacitor cannot sufficiently smooth the rectifier’s harmonics. When such a converter drives a squirrel-cage induction motor (a common industrial motor), rapid speed changes can create DC-link overvoltages that trigger protection and degrade the drive’s dynamic performance. This project examines how stable this converter–motor system is and how different design choices affect stability. We build a mathematical model of the system and linearize it to obtain transfer functions that show how inputs influence the DC-link voltage. Using standard control theory, we analyze the stability margins and then check them with Matlab/Simulink simulations. To match the low-cost focus, we design a sensorless V/f (voltage-to-frequency) control strategy and verify it in simulation. We implement open-loop V/f control and space vector modulation on a digital signal processor (DSP) to observe the system in practice. Experimental results were collected and documented.
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