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Delo je pregled prehodnih pojavov ob sklopitvi asinhronskega stroja s transformatorjem. Analiza je bila usmerjena na napetostne in tokovne razmere na sponkah asinhronskega motorja ob njegovem direktnem zagonu preko transformatorjev različnih nazivnih moči. Cilj je bil ugotoviti vpliv nazivne moči transformatorja na tokovno-napetostne razmere ob direktnem zagonu asinhronskega motorja in potrditi ustreznost močno poenostavljene simetrične trifazne kratkostične zanke za izračun razmer v omrežju ob času direktnega zagona asinhronskega motorja. Analiza je bila izvedena v SIMULINK-u in sicer v treh korakih, pri čemer so pri vseh treh bile glavne opazovane količine napetosti in tokovi. Izbrana so bila različna razmerja moči transformatorjev v primerjavi z asinhronskim motorjem, pri čemer je bil motor vseskozi isti, moči transformatorjev pa so bile različne. V prvem koraku so bili opazovani sklopi transformatorjev različnih velikosti z nadomestno, kratkostično impedanco asinhronskega motorja. Dobljena napetost v ustaljenem stanju je bila nato uporabljena v drugem koraku za posamezna razmerja transformator – asinhronski motor. V drugem koraku so bili izračunani direktni zagoni asinhronskega motorja na idealno toge napetostne vire z amplitudami napetosti, izmerjenimi pri prvem koraku. V tretjem koraku so bili izračunani direktni zagoni izbranega asinhronskega motorja preko transformatorjev različnih nazivnih moči. Opazovan je bil tudi vpliv trenutka vklopa na potek toka. Ugotovljeno je bilo, da se pri direktnem zagonu asinhronskega motorja z večanjem nazivne moči transformatorja padec napetosti manjša, maksimalna amplituda toka pa se povečuje. Maksimalna vrednost zagonskega toka je hkrati zaradi zveznosti toka pogojena tudi s trenutkom vklopa, kar v trifaznem sistemu pomeni, da bo ena faza vedno v manj ugodnem položaju in bo tam višja špica toka kot v ostalih dveh. Zanka, sestavljena zgolj iz direktnih vej (torej iz upornosti in razsutih reaktanc primarnega in sekundarnega navitja transformatorja ter upornosti in razsutih reaktanc statorja in rotorja asinhronskega motorja) se izkaže kot dober približek za hitro in pregledno oceno padcev napetosti in zagonskih tokov. This work is an examination of transient phenomena when a 3-phase asynchronous motor is connected to a 3-phase transformer. The analysis focused on the voltage and current conditions on the terminals of the asynchronous motor during direct start via transformers of different rated powers. The aim was to determine the influence of the rated power of the transformer on the current and voltage conditions during direct start-up of the asynchronous motor and to confirm the suitability of a greatly simplified symmetrical three-phase short-circuit loop for calculating the conditions in the network at the time of asynchronous motor direct start-up. The analysis was carried out in SIMULINK in three steps, with voltages and currents being the main observed quantities. Different ratios of power between the transformers and the asynchronous motor were chosen, with the motor being the same throughout and the transformer powers being varied. In the first step, sets of transformers of various sizes were observed with an alternate, short-circuit impedance of the asynchronous motor. The obtained steady-state voltage was then used in the second step for the individual transformer- asynchronous motor power ratios. In the second step, direct starts of the asynchronous motor on ideally rigid voltage sources were calculated with the voltage amplitudes of the source being set as those in measured in the first step. In the third step, the direct starts of the selected asynchronous motor via transformers of different nominal powers were calculated. The influence of the asynchronous motor switch-on moment was also observed. The findings show that with the direct start of the asynchronous motor, the voltage drop decreases with the increase of the nominal power of the transformer, while the maximum amplitude of the start-up current increases. Due to the continuity of electric current, the maximum value of the start-up current is also conditioned by the moment of motor start-up, which in a three-phase system means that one phase will always be in a less favorable time position, resulting in a current peak on that phase. A loop consisting of only direct branches of the transformer and asynchronous motor circuit model (comprised of the resistances and leakage inductances of the primary and secondary winding of the transformer and the resistances and leakage inductances of the asynchronous motor) turns out to be a good approximation for a quick and transparent assessment of voltage drops and starting currents. |
