Electrical techniques: current, flux and power monitoring
This chapter shows that electrical techniques are powerful tools for the condition monitoring of electrical machines, particularly axial leakage flux, current and power, offering the potential to provide a general condition-monitoring signal for the machine. The availability of high-quality, digitally sampled mechanical vibration and electrical terminal data from electrical machines opens the possibility for more comprehensive monitoring of the machine and prune mover or driven machine combinations. However, these signals generally require broad bandwidth (>50 kHz) and a high data rate for adequate analysis. Therefore the principal difficulty of applying these techniques is the complexity of the necessary spectral analysis and interpretation of their content. This situation is made more difficult if variable speed drives are involved because tune domain signals may no longer be stationary and will also be polluted by harmonics from the power electronic drive. Comprehensive monitoring of an electrical machine can be achieved by measuring shaft flux, current, power and electrical discharge activity. These are broad bandwidth (generally >50 kHz) signals requiring complex analysis. Shaft flux, current and power signals are capable of detecting faults in both the electrical and mechanical parts of a drive train. Shaft voltage or current is an ineffective condition monitoring technique for electrical machines. Shaft flux monitoring is non-mvasive and uses a single sensor but it is complex to analyse and untested in the field. Current monitoring is also non-mvasive, but uses existing sensors and has established itself as motor current spectral analysis, a reliable and widely accepted technique for machine monitoring. Power monitoring is also non-mvasive, uses existing sensors but requires less bandwidth (<10 kHz) and less complex spectral interpretation to detect faults but is not yet widely accepted, so it deserves investigation for future development.
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