Analysis of the fatigue causes on the rotor bars of squirrel cage asynchronous motors: experimental analysis and modelling of medium voltage motors

The rotor bars of squirrel cage asynchronous motors are subject to mechanical forces and electrical and thermal transient processes. During the transient operation of the machine, especially during its start up, the high values of current flowing through the cage produce small deformations in bar and end rings. These deformations, combined with the expansions and contractions caused by transient thermal processes and the presence of unavoidable manufacturing defects, lead to rotor bar fatigue. The fatigue starts with small cracks in the junction between rotor bar and endrings and ends with the complete breaking of the bars. In this paper, the causes of fatigue in squirrel cage rotors are studied. By means of several finite element models of low and medium voltage machines the forces applied on the rotor during motor start up are calculated. These forces are used as loads on mechanical models of the rotor bars. By applying the finite element method the stress distribution caused by the forces on the bars is calculated. Once this stress distribution is known fatigue analysis is also performed. The results obtained are then validated by means of destructive tests on real specimens and by the analysis of models including intentional manufacturing defects.