Induction motors are the main prime mover in industrial applications. Fault diagnosis of an induction motors fault detection is a serious issue for improving plant reliability. In order to perform proper fault diagnosis, it is essential to understand underlying principles of an induction motor operation and the effect of loading on the defect signatures. Additionally, most induction motors are driven by variable frequency drives (VFD). VFD often creates a significant interference in the overall signature on both motor and rotor side structures. Motor current signature analysis (MCSA) and vibration analysis are being promoted for diagnosing induction motor problems. Accordingly, it is important to understand and account for these factors in diagnostics.
This tutorial will provide attendees with the fundamental principles of an induction motor in a simplified manner without resorting to the underlying complex structure. We will also introduce the interfering effects the VFDs. Students will learn how to utilize their existing analysis tools to extract enough diagnostics information from the vibration and motor current signals. We will present a comparison of vibration and motor current signals for determining both electrical and mechanical faults. We will discuss results of experiments conducted with seeded fault results of broken rotor bars, air gap eccentricity, and shorted turn. Tests are performed on intentionally faulted motor with varying degrees electrical faults. Different levels of mechanical torque, unbalance and misalignment loadings are applied to the rotor side. The data is analyzed using both vibration and motor current sensors. Results indicate significant components in motor current spectrum due to electrical malfunctions except for the shorted turn. MCSA for shorted turn defect seems inconclusive. Vibration spectra results appears to be not a good indicator for the electrical faults. However, vibration data provide a better indication of mechanical defects than the motor current. The results suggest that both motor current and vibration measurements are required for more complete diagnostics of induction motors and rotors.
Key Words: Induction Motors Diagnostics, Broken Rotor Bars, Airgap Eccentricity, Shorted Turn, Vibration Analysis, Motor Current Signature Analysis, Unbalance, Misalignment