February 2011 Electrical Apparatus

February 2011 Electrical Apparatus

This is a summary of the Electrical Apparatus February 2011 featured technical article,  by Richard L. Nailen, P.E.     

Motor current during acceleration is much higher than during full-speed operation. Consequences include short-time overheating within the motor, unwanted operation of overload protective devices, and a sag in supply voltage. Accurately predicting these effects is hampered by several misunderstandings and uncertainties. Although “starting current” and “percent inrush” are terms widely used in a-c motor application, neither is well defined.

For example, literature often describes motor current during starting as a typical percentage (such as 600% or 800%) meaning the ratio of that current to the full-load or nameplate value (FLC). For any given motor rating, however, the FLC varies widely between manufacturers and with motor polarity. Higher magnetizing current results in higher FLC for a slow-speed machine than for a two-pole design.

What NEMA standards define for each horsepower rating is not a percentage, or a current during acceleration, but the steady-state locked-rotor current (LRC) value independent of motor polarity. Full-load current varies with polarity. Throughout the range of standard motor speeds, the ratio LRC/FLC is therefore a fixed number divided by a variable. That, along with design variation between manufacturers, can mean a difference of 30% or more in the “percent LRC” from one product to another of the same rating. Actual LRC does not appear on motor nameplates. Rather, the nameplate gives a KVA Code Letter defining an approximate 10% range within which the LRC lies.

“Inrush” itself is undefined. Closing the motor circuit causes an initial transient current spike that typically decays within the first cycle or two and is not considered part of the LRC. It is a complex function of the motor’s relative resistance and reactance. That transient, which can peak at twice the crest value of the steady-state LRC and cause nuisance tripping of some circuit breakers, is unaccounted for in NEMA standards, nameplate data, or catalog information. The more efficient the motor, the higher that peak may be–and the higher the LRC value itself (many published values exceed NEMA Design B limits, even for motors offered in compliance with industry standards stipulating Design B).

To properly evaluate circuit voltage drop, and to choose appropriate circuit protection, a user should ask the motor manufacturer what the actual LRC should be, as well as the expected transient peak. Avoid references to a simple percentage and to such terms as “starting current.”

To order a back issue with the full article, “What do we mean by motor ‘starting’ and ‘inrush’ current?” call 312-321-9440 or visit our online webstore.

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