Electrical Apparatus magazine, January 2014

Electrical Apparatus magazine, January 2014

This is a summary of the Electrical Apparatus January 2014 featured technical article,  by Richard L. Nailen, P.E. 

International standards now mandate full-load efficiency of most standard induction motors. In the U.S., federal regulations and standards issued by the National Electrical Manufacturers Association govern both the required efficiency and the way it must be marked on motor nameplates.

In managing industrial and commercial energy usage, motor efficiency must be considered in two situations. When a motor fails, economics may favor replacement rather than repair. An element in this choice is the efficiency of the replacement motor. Also, life cycle costing can dictate replacement prior to failure of a low-efficiency motor with one costing less to operate.

A motor user may therefore wish to determine the actual efficiency of the existing motor as it is installed. Since the early 1980’s, many different methods have been developed for this purpose. Only three quantities are involved in such a determination: electrical power input, mechanical power output, and motor internal losses. An accurate measurement of any two of those three yields efficiency.

How accurate the measurements can be is debatable. Most published methods involve either calculated, estimated, or assumed values of some quantity not readily measured on a job site. To make such measurements requires intrusive steps such as disconnecting the motor electrically or uncoupling it mechanically. Quantities commonly either estimated or calculated according to some arbitrary formula are shaft torque, certain motor losses, motor equivalent circuit constants, and motor temperature. All this is complicated by the tolerances (set by standards or otherwise) on nameplate current and speed, which are involved in some methods of estimating efficiency in the field. A further source of uncertainty in the result is in the instrumentation itself, which is necessarily less accurate than that used by the motor manufacturer.

Even if the often-stated goal is determination of efficiency within three percentage points, that is too broad a measure to support the choice between motor designs that offer tolerances of one-half a point or less.

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