August 2011 Electrical Appparatus

August 2011 Electrical Appparatus

This is a summary of the Electrical Apparatus August 2011 featured technical article,  by Richard L. Nailen, P.E.    

When purchasing any important piece of electrical apparatus, the first step is creation of an adequate specification. The next step is to choose a supplier, which calls for detailed evaluation of the general capabilities of prospective manufacturers. For critical applications, however, a further desirable step is the witnessing of certain manufacturing and test operations for the specific item ordered. That may involve requiring “hold points”–stages in the manufacturing process at which no further work is to be done pending approval of some condition.

The choice of such measures is not necessarily based on size, operating voltage, or output rating of the apparatus, but rather on how essential it is to the purchaser’s operation. In a generating station, for example, a relatively small coal car dumper drive can be as important to the plant as a boiler feed pump.

For a motor, logical hold points include the insulation of the first stator coils, fitting of journal bearings to the shaft, stacking of the rotor core, and insertion of rotor cage bars. Tests to be witnessed might include full-load temperature, locked-rotor current and torque, efficiency, and (for applications involving severe acceleration) the speed-torque curve. In-process tests, prior to machine assembly, may include a variety of dielectric tests on stator coils prior to winding.

Consider the effect on overall time and cost when calling for any witnessing. Hold points can be particularly troublesome by disrupting normal work flow. A manufacturer will always run tests beforehand, minimizing the risk of failure later when the customer is present. That adds additional test cost. Travel time and other expenses for whoever does the witnessing must also be considered, especially now that many suppliers of large apparatus are located worldwide.

Who will represent the purchaser as a witness? To judge the quality of manufacturing operations on the factory floor, the witness must have intimate knowledge of what’s needed, not just to meet the letter of a specification, but to result in a reliable product capable of long service. An engineering background can be only a basis for acquiring such knowledge and experience.

To order a back issue with the full article, “Witnessing motor manufacturing” call 312-321-9440 or visit our online webstore.

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