Demodulation methods for exact induction motor rotor fault diagnostic

Dynamic faults of induction machines cause modulation. The main frequencies of modulating signals are rotation frequency at dynamic eccentricity and slip pole frequency at broken rotor bars or frame. Demodulation methods increase resolution both in magnitude region and frequency region, facilitate and precise evaluation. The paper introduces the basics of envelope analysis and two demodulation methods using Hilbert transform and space transform and compares their results together and also to the professional envelope analysis. Dimensionless relative coefficients k_r, k_sp are introduced for expressing the severity of fault and for fault comparing at various motors. Powerful measuring system enables working of all 3 envelope analyzers in real time. Measurements were made for 2 poles and 4 poles asynchronous motors with various numbers of broken rotor bars and various rotor eccentricities under various motor loads. The aim is to verify the relationship between rotor faults and values of corresponding diagnostic symptoms and to find out the accuracy of diagnostic symptoms and possible variance of results. The values of relative coefficients k_r, k_sp are compared for various measurements and the possible borders for fault severity were set.