Electrical Apparatus April 2015

Coming in April!

This is a synopsis of the Electrical Apparatus April 2015 featured technical article,  by Richard L. Nailen, P.E.    Receive this article with your subscription to Electrical Apparatus. To advertise in this issue call 312-626-2316.

When an electric motor has failed and is no longer operable, the choice between repair and replacement is straightforward both technically and economically. Choosing replacement prior to failure—either because of some perceived advantage of a newer machine or because scheduling the replacement in advance of need offers some operating advantage—is more complex.

Predicting the time when failure will occur is not possible based on any measured motor condition. Periodic testing can indicate increasing deterioration of the insulation, for example, but no one can say how much life remains. Furthermore, the failure rate for any newly installed replacement apparatus is always higher than it will be after the “infant mortality” period.

Some authorities advocate condition monitoring of motors in service as a means of estimating a time to failure so that replacement can be made before that time arrives. However, such a procedure can properly serve only to determine when some maintenance activity can be appropriately scheduled, such as relubrication, or a check of shaft alignment. Observation of a trend in some condition cannot reliably predict when to expect breakdown.

Some have argued that the newer, higher-efficiency motors tend o be less reliable than older units. This has been shown to be untrue—but it is equally unrealistic to expect the newer motor to be less reliable. Inadequate maintenance is the leading cause of motor failure and is likely to affect a replacement motor just as adversely as an existing one.

When any motor is replaced after it has failed, a cost is incurred to remove it and install the replacement. This cost is mandatory. If the replacement is made prior to failure, however, the cost is a direct addition to the price of replacement and must be taken into account.

In any event, the user must determine if an exact replacement is available. The manufacturer may no longer exist. Will a product from another source meet all the dimensional and performance characteristics of the existing motor?

For example, replacement with a higher-efficiency design may appear economically attractive, but will its lower losses be accompanied by a lower accelerating torque (though still within industry standards)?

Keep in mind that most motors operate at 2/3 to 3/4 of rated load, whereas efficiency guarantees apply only at full load. Although the locked-rotor current for a standard motor must meet a standard limit, the short-term inrush may differ enough to require some change in the circuit protection.

Management may elect to keep a duplicate motor on hand for quick replacement in an emergency. Unless proper long-term storage is provided, along with periodic evaluation of motor condition, such a spare may prove unusable when the need arises.

To advertise in Electrical Apparatus, call  312-626-2316.  To subscribe, visit http://ea-renew.com or telephone 312-321-9440.  To order a single copy of the issue containing this article after it has been published, go to http://barksbooks.com and order the April 2015 issue. 

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