Vibration diagnostic challenges 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 fit on a shaft which may also have welded arms. This built-up construction of different materials with different thermal reactions affects 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 drive train, high vibration will usually be evaluated on the motor first before moving onto the rest of the drive train components. Unfortunately other components cannot be tested alone to identify if they are the source of the problem. This often results in losing valuable commissioning or operating time. Vibration identifications can be started by using readily available diagnostic tables. Problems can occur when the process of identification goes beyond the capability of the tables to provide a clear answer. This paper discusses case studies on vibration diagnostics of electric motors, where a wider scope of the drive train and its support must be considered in order to arrive at the correct diagnosis. Particular emphasis is given to the identification of the original problem and cause of the vibration issue.