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Some Motors Have Design B LR Limits But Meet Design E Efficiency Levels

Oct 1, 1998 12:00 PM, By EC&M Staff

Thanks for Robert Lawrie’s article, “Watch Out For Burnouts On Hard-To-Start Loads,” in the March issue.

With continuous change and increased efficiency, many of today’s industrial motors in the 3 hp to 200 hp range run cooler, vibrate less, have increased bearing life, and last longer.

The locked rotor amps and power factor have suffered, but most motor manufacturers keep the locked amps within Design B limits. Some do this, still meeting the efficiency levels of Design E motors, which exceed EPACT levels, as shown in NEMA table 12-11.

In a table comparing 16 standard efficiency TEFC motors (CT) and premium-efficiency motors (CE), five CE motors have less locked current, but 12 are neutral (5% difference or less)—nine models have greater thermal capacity, and none have less.

Other manufacturers have similar product lines, but some can’t meet these levels. Perhaps it’s the old averaging game—one foot on ice, the other in fire, make for average comfort.

Austin H. Bonnett
V.P. Technology
U.S. Electrical Motors Div.
Emerson Electric

St. Louis, Mo.

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