Rotor bars breakage in railway traction squirrel cage induction motors and diagnosis by MCSA technique Part II : Theoretical arrangements for fault-related current sidebands

In a companion paper (Part I) the bar breakages for a 1.13 MW induction motor used in high speed railway traction drives were studied, by simulating the whole system (converter + motor) and by analyzing frequencies and amplitudes of current sidebands, for various loads (both active and inertial) and with increasing fault severity. It was recognized that faults can be characterized by (1-2s)f sideband, but results vary with load torque and drive inertia. Other sidebands appear less dependent from these disturbs, like (5+2s)f. Unfortunately, no works presented simple and general formulas about sidebands. In this paper, theoretical formulations for computation and prediction of rotor fault-related sideband frequencies and amplitudes are given, aiming to obtain relations useful for diagnostic purposes (fault severity assessment by MCSA), and by using Part I as a case-study. A decomposition by multiphase symmetrical components is performed, taking in account space harmonics. Formulas for (1-2s)f sideband amplitude are carried out, extendable to many other sidebands.