December 2008 Electrical Apparatus

December 2008 Electrical Apparatus

This is a summary of the Electrical Apparatus December 2008 featured technical article,  by Richard L. Nailen, P.E.   

As efficiency standards worldwide continue to mandate improved performance for polyphase induction motors, users must continually revise their calculations of the relative costs of repairing or replacing existing motors. Simply comparing direct cost is not adequate.

Advice for making such calculations includes a number of actions that are recommended, and a similar number to avoid. Most are well known. However, newcomers to the decision-making process may not be familiar with some, and those with more experience may overlook pitfalls in the process. Here are some of the important concerns:

Know the load. A motor often supplies less than rated or nameplate output power. Consequently, its efficiency (and that of a replacement motor) will probably differ from the published or nameplate value.

Also, the load may vary, in two ways. First, in many drives the shaft output power fluctuates from hour to hour or day to day. Second, even at constant output, motor running time may be far below the 8,750 hours per year so often cited in motor sales literature. And don’t assume motor losses will vary with load in the same way for a new motor as for the old one.

Efficiency of an existing motor must be closely estimated to determine the economic effect of replacement by a more efficient machine. No single measurement can accurately indicate what the load is. Combinations of voltage, current, speed, and power input may be necessary. Instruments must be properly calibrated if readings are to be trustworthy. Beware of unbalanced phase voltages–a condition not revealed by measurement in only one phase.

High efficiency may not mean higher power factor. Whether or not power factor will influence facility operating cost depends upon the utility rate structure. Any measures to increase premises power factor should begin with the largest, most heavily loaded motors.

Don’t expect reduced-voltage starting to save energy. It may actually increase operating cost by lengthening drive acceleration time.

Finally, do not expect the reliability of a new motor to be greater than for an older design. Expect to take just as much care as before in cleaning, lubricating, and periodically evaluating electrical and mechanical condition.

To order a back issue with the full article, “Some tips for evaluating motor operating cost” call 312-321-9440 or visit our online webstore.

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