Variable frequency drive (VFD) influence on induction motor fault diagnosis


Diagnostics of an induction motors is serious issue for improving plant asset reliability.  Motor current signature analysis (MCSA) and vibration data are commonly used for diagnosing induction motor problems.  These motors are invariably driven by the variable frequency drives (VFD).  The VFD seems to distort the measured signal and can make data analysis task more difficult.  This paper will present a comparison of vibration and motor current signals for induction motor subjected to varying degrees of mechanical and electrical faults such as broken rotor bars, mechanical imbalance, and rolling element bearing faults.  This will be followed by the signal processing techniques used to remove such disturbing effect. The effects of mechanical loading such as torque, unbalance, and misalignment loadings will be discussed.  The effects of variable frequency drives (VFD) on the distortion of measured signals and how to use the information positively to enhance diagnostics capabilities will be also presented. The results suggest that both motor current and vibration measurements are required for more complete diagnostics of induction motors faults.