June 2013 Electrical Apparatus

June 2013 Electrical Apparatus

This is a summary of the Electrical Apparatus June 2013 featured technical article,  by Richard L. Nailen, P.E.    Receive this article with your subscription to Electrical Apparatus

When purchasing an alternating-current motor, specifiers often request a copy of a routine test report. Expectations differ as to what constitutes such a test, or what June be learned from the report. The National Electrical Manufacturers Association defines it in MG 1 standards, somewhat differently for ratings above or below 500 hp (373 kW). Included for any size polyphase machine, in addition to winding hipot, is measurement of winding resistance; no-load current, frequency, and speed at “normal” voltage; and locked-rotor current.

The purpose of routine testing is to quickly check insulation condition and certain operating characteristics against what’s expected from design or previous testing. This reveals such typical manufacturing mistakes as a wrong winding connection. Unusual motor noise or bad bearings are also apparent.

For the user, however, the value of a test report is questionable. Much attention has recently been drawn to phase unbalance, in current, resistance, or impedance, as indicators of specific defects. Statistically reliable evidence of such cause-and-effect relationships is scarce. Unbalanced current is not a good benchmark, because magnetic dissymmetry in stator or rotor cores can cause magnetizing current unbalance that largely disappears when motor load is applied.

Tested locked-rotor current cannot be readily correlated with on-site experience. It’s a steady-state value that will be considerably exceeded during the first cycle after the motor is energized, decaying rapidly near full speed.

Finally, routine test results cannot be used to check full-load, power factor, revolutions per minute, or temperature rise. In general, then, the usual routine test report is of little value in subsequent maintenance planning.

To order a back issue with the full article, “Routine motor tests: What do they tell us?” call 312-321-9440 or visit our online webstore.

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