Identification of torsional vibration features in electrical powered rotating equipment

A common torsional vibration sensing method in rotating equipment is the Time Interval Measurement System (TIMS). The method utilizes the time passage of discrete intervals on a rotating element from an incremental geometric encoder (i.e., gear, optical encoder). Ideal measurement conditions consist of a constant shaft running speed, an encoder with identical segments and no transverse motion between the sensor and shaft. In practice, these ideal conditions are rarely achieved resulting in measurement errors. Torsional vibration sensing in internal combustion engines benefits the large inherent responses which produces a high signal to noise ratio and tends to minimize the effects of some measurement issues. In electrical motor applications the torsional responses tend to be smaller and the measurement issues may become more pronounced. For health monitoring applications (i.e., shaft crack growth diagnostics) it is imperative to accurately identify and track the fault sensitive torsional features. Tracking torsional mechanical shaft line dynamics can become challenging because of the lower signal to noise ratio, high harmonic content induced from the motor speed controller and processing artifacts from the Time Interval Measurement System. This work will discuss a number of potential measurement and data processing issues in the application of TIMS for health monitoring applications. The work will focus on separating the desired shaft line health features from all apparent in the torsional response. Of particular interest will be practical items related to installation and analysis on large electrical driven equipment. Examples from laboratory and field tests will be used to describe the identification and compensatory methods that have been successfully used.