Effect of laminated core on rotor mode shape of large high speed induction motor

Electric motors is the backbone of many industrial process, however it consumes of large portion of energy. Increasing energy costs create incentives for movement toward higher efficiency systems. Large high speed induction motors which are operating on variable frequency drives with flexible rotor dynamic designs are often required to have an accurate prediction of its modal characteristics (natural frequencies, mode shapes) and its critical speed. The construction of rotors for large induction motors consists of laminated core made of Ferro-magnetic steel, which alters the overall rotor system stiffness and hence influences its modal characteristics. The aim of this paper is to investigate the effects of laminated core on the rotor mode shapes. To this end, experimental modal analysis was carried out for shaft and for rotor core mounted on shaft. The extracted modes were studied and the associated Finite Element modal analysis was carried out. It is shown that the modal characteristics of the shaft are significantly different from the rotor core as it leads to stiffening of the system and also influences the location of nodes. Preliminary analysis for identifying the core stiffness from its modal characteristics is presented. Based upon the identified parameters rotor-dynamic analysis is performed for critical speed calculation. The effect of core stiffness in critical speeds along with the observations from field testing is highlighted and the plans for future research originating from this work are summarized.