Induction motor condition monitoring: Vibration analysis technique - a twice line frequency component as a diagnostic tool

Vibration analysis is one of the most successful techniques used for condition monitoring of induction motors. The efficiency of this technique for diagnosing electromagnetic problems in the motor depends on the ability to analyse vibration data. The aim of this presentation is to clarify the origin of the motor vibration at twice line frequency based on the classical theory of electromagnetic vibration, recent researches, and the author´s many years of experience in the field of vibration diagnostics on electrical machines from different industries. The vibration component at a twice line frequency is a very important indicator when evaluating an induction motor electromagnetic system condition. The correct understanding of physical phenomena that excite the vibration at this frequency is a key factor for diagnosing induction motor problems related to electromagnetic system operation. This presentation emphasizes that the most important forcing functions exciting the vibration at twice line frequency are the radial magnetic forces produced by the fundamental rotating magnetic field and a torque pulsation due to asymmetrical voltage and current in the stator winding as a result of a supply voltage imbalance in the power line and/or internal faults in the motor electrical system. Vibration excited by these sources may be significantly amplified by a resonance condition on the entire motor structure and/or on the particular motor components (for example, a stator core).