Vibration Diagnostic Challenges: Case Studies in Electric Motor Applications

Electric motors have unique vibration characteristics mainly due to the coupling between electric and mechanical forces and the construction of their rotors. Rotors are made up of stacked parts and a copper or aluminum cage material that is casted, welded, or brazed and shrunk to fit on a shaft, which may also have welded arms. This built-up construction of different materials with different thermal reactions affects the rotor motion in a complicated manner at different speeds and thermal-loading conditions. Since motors are the primary source of rotational motion in a typical drivetrain, one will usually evaluate high vibration on the motor first before moving on to the rest of the drivetrain components. Unfortunately, other components cannot be tested alone to identify if they are the source of the problem. This often results in a loss of valuable commissioning or operating time. Vibration identification can be started by using readily available diagnostic tables, but problems can occur when the process of identification goes beyond the capability of the tables to provide a clear answer. This article discusses case studies on vibration diagnostics of electric motors where a wider scope of the drivetrain and its support must be considered to arrive at the correct diagnosis. Particular emphasis is given to the identification of the original problem and the cause of the vibration issue.