Influence of different casting conditions and the filtering process on the electrical resistance of Al-alloy 99.7

Abstract

There are different types of electric motors; one of them is the induction motor. Such a motor consists of a winded stator and an induction rotor. The rotor squirrel cage is made with aluminium high-pressure die casting technology. Besides relatively low motor-producing costs and good repeatability, the motor efficiency and torque/speed characteristics are also important. One of the loss factors for the rotor is the loss of conductance of die cast material. This study researches how different casting conditions influence the material's electrical resistivity of Al 99.7. With melting of the alloy in an induction vacuum furnace and testing various casting conditions during the solidification (such as the vacuum, air and filtering), the temperature and electrical resistivity of the castings were measured using the four-point measuring method. The porosity proportions of the samples were examined with X-ray, as well as with optical microscopy. The presence of Fe-eutectic (a-Al + Al13Fe4) was confirmed in the investigated samples' microstructures, as was predicted by a calculation of the equilibrium phase diagram. The electrical resistivity values were additionally measured with an eddy-current measuring method. The results show a good correlation with the four-point measuring method and are in a range 29-33 nWm at 20°C. Based on the obtained results, the influence of impurities and porosity on the electrical properties of aluminium was evaluated.Obstajajo razli~ni tipi elektri~nih motorjev in eden izmed njih je tudi indukcijski motor. Indukcijski motor je sestavljen iz navitega statorja in indukcijskega rotorja. Prevodna kratkosti~na kletka indukcijskega rotorja je lahko narejena s postopkom visokotla~nega litja aluminija. Poleg relativno nizkih stro{kov izdelave sta zelo pomembni u~inkovitost in karakteristika (navor/hitrost) takega elektri~nega motorja. K skupnim izgubam motorja pri{tevamo tudi izgubo prevodni{kega materiala kratkosti~ne kletke rotorja. Cilj raziskave je ugotoviti, kako pogoji litja vplivajo na elektri~no prevodnost litega materiala Al 99.7. Taljenje in gravitacijsko litje zlitine je potekalo v indukcijski vakuumski pe~i pri razli~nih pogojih strjevanja (vakuum, zrak in filtriranje s penjenimi kerami~nimi filtri). Med strjevanjem ulitka smo spremljali temperaturo in merili elektri~no upornost vzorca s {tirito~kovno merilno metodo. Dele`poroznosti je bil analiziran z rentgenom in tudi z opti~no mikroskopijo. Prisotnost (a-Al + Al13Fe4) evtektika je bila potrjena tako, kot je predvidel predhodni termodinami~ni izra~un. Dodatno smo elektri~no upornost vzorcev merili {e z merilno metodo vrtin~nih tokov. Rezultati so primerljivi ter so v obmo~ju 29-30 nWm pri 20°C. Na podlagi pridobljenih rezultatov smo ocenili vpliv ne~isto~in poroznosti na elektri~ne lastnosti aluminija. Klju~ne besede: elektri~na upornost, ne~isto~e, poroznost, pogoji litja