Figure 9-18 illustrates use of relative comparison analysis of power levels to identify certain types of defects in motor-drive systems.
Figure 9-18 – Power Level Comparison Plots for Detecting Some Types of Defects
Most problems such as cavitation, unbalance, rubbing and miss-alignment in rotating machines cause an increase in the power demand of the circuit. In a pump-motor unit, this might be detectable in the current signature but will show up more definitively in a power signature. What is entered into the on-line motor tester's parameter data fields helps in motor system diagnosis. In the most advanced software programs there are areas to enter data on bearings, gearbox meshing frequencies, belt information (e.g., number of belts), size of the pulleys used and number of fan blades, for example. The more motor data entered, the better the diagnosis presented by the analysis modules of the software. Initially it may be difficult to distinguish present power level from normal loading until a circuit problem becomes more severe, unless there is a similar motor nearby where tests can be taken to do a quick comparison. This can be invaluable if diagnosis is needed quickly. (See the case study in Chapter 15 (Volume 4) for a perfect example of this approach.) The more baselines of “good” pump-motor unit data collected for comparison the better. Figure 9-19 illustrates this point with baseline and (later) power startup wave forms superimposed on the same plot.
Figure 9-19 – Baseline and Later Power Startup Wave Forms Superimposed on the Same Plot
Tip from Motor Electrical Predictive Maintenance & Testing Vol. 3 by Jack Nicholas & Geoffery Generalovic