by Dave Olinger and Ernesto Wiedenbrug, Ph.D.
This paper discusses how voltage and current captures of motors starting at line frequency can be used to diagnose high resistance contact problems, and even detect whether these problems are up-stream or down-stream from where the measurements were taken [1-2]. These concepts are applied on startup transient captures of two 8-pole 100hp motors which are started via 80% reduced-voltage autotransformers (Fig. 1) [3]. One motor shows a high resistance contact below the point of measurement, and the other transient capture was used to diagnose a problem in the autotransformer, which has been verified to be shorts in the transformer.
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http://csaanalysis.com/PDFS/Startup%20Transients%20for%20Reduced%20Voltage%20Starts.pdf
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