Rotor cage faults detection in induction motors by motor current demodulation analysis

Dynamic rotor faults of induction motors, mainly broken or cracked rotor bars and dynamic rotor eccentricity, cause stator currents modulation. The detailed motor current analysis revealed meaningful phase modulation and its relation to amplitude modulation at increasing motor load and inertia load. It is shown, that by increasing motor load mainly phase modulation increases and by increasing inertia load a phase shift between both modulations currents changes. This influences the MCSA spectral sidebands amplitudes and therefore it is not possible to determine the real fault size. The paper introduces a new method of motor current amplitude demodulation analysis - MCDA and shows that spectral amplitudes almost do not change with increasing load and inertia load. Moreover MCDA directly analyzes fault signals without great motor stator current and increases resolution both in amplitude and frequency domains, facilitates and precises evaluation. Two demodulation methods using Hilbert transform and space transform are introduced.