Electrical Apparatus magazine, May 2003 Verifying Motor Efficiency: No Easy Task

Most standard general purpose three-phase induction motors sold in the United States (regardless of where they're manufactured) must now exhibit full-load efficiencies as tabulated in the Energy Policy Act (EPACT) of 1992. Defined as "nominal," such an efficiency represents an average for a large number of motors of identical design.

The same efficiencies are required by standards of the National Electrical Manufacturers Association (NEMA), and are to be marked on the motor nameplate. For each individual motor, the NEMA standards also tabulate a required "minimum" efficiency, representing a motor loss 20% greater than that associated with the nominal efficiency.

A user concerned about the efficiency of a single motor must understand that because the nominal efficiency is an average value, it cannot be verified by testing any single unit. The U. S. Department of Energy (DOE) has therefore established an efficiency enforcement procedure. If a user has reason to believe (usually from a laboratory test) that a motor does not meet the law's requirement, a complaint to the DOE initiates a sampling process imposed upon the manufacturer. Several motors are tested and the results analyzed according to a complex statistical procedure; a decision is then made as to whether or not the product complies with the law.

Some agencies have evaluated motor performance by a test program classifying motor designs into three categories. A fully acceptable "green" group exhibits nominal afficiencies meeting both NEMA and EPACT levels, and minimum efficiencies within the NEMA requirement. An unacceptable "red" group fails both of those criteria. In between, a "yellow" group meets the NEMA minimum efficiency, but testing indicates that an average (nominal) efficiency may be too low. Only the DOE sampling procedure can evaluate performance in that category.

In any event, the only useful tests are those performed under laboratory conditions in facilities certified by the Department of Energy. Field tests cannot be expected to match those conditions.

Industry news...

Featured Technical Article
May 2003

On this page is a summary of the Electrical Apparatus May 2003 featured technical article, by Richard L. Nailen, P.E. , "Verifying Motor Efficiency: No Easy Task" Die Überprüfung der Leistungsfähigkeit von Motoren ist keine leichte Aufgabe ... La 'vérification' du rendement d'un moteur: une tâche difficile ... La Verificactión del Rendimiento de un motor: No es Fácil

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