July 2010 Electrical Apparatus

July 2010 Electrical Apparatus

This is a summary of the Electrical Apparatus July 2010* featured technical article,  by Richard L. Nailen, P.E.    

A widely accepted principle of squirrel-cage motor design and manufacturer is that long service life requires rotor bars to be tight in their slots. Creating suitable tightness initially–and maintaining it through many years of thermal cycling (especially during acceleration)–is a difficult problem not completely solved by any of the many methods used by motor manufacturers.

Assembly involves several contradictory requirements. Bars must fit loosely enough to permit fitting them into slots without damage to bars or core laminations. They must fit tightly enough to eliminate vibration leading to eventual fatigue failure. Yet they must remain loose enough for removal if replacement is ever needed.

Complicating these requirements is the use of many different bar and slot shapes. Control of windage and resistive losses, accelerating torque, and noise has resulted in radical departures from the simple rectangular bar shape secured in place by slight interference between bar and slot at the corners, scraped away as the bar is driven axially into position.

Whatever the bar’s shape, the manufacture of the bar itself must allow some dimensional variation. The slot can be precisely formed in each lamination. However, stacking many such laminations into a core, using a mandrel or other assembly tooling, inevitably results in slight variation in slot position from one lamination to the next. The consequent slot stagger effectively reduces the space for the bar, to an extent difficult to either predict or measure.

One approach to tightness is to build suitable locating surfaces into the bar itself. Another is to eliminate slot stagger by broaching the slots prior to bar insertion. This often calls for special tooling, and the operation is costly. A third method, not suited to all bar shapes, is to create a loose fit for easy insertion, then indent the top of each bar to upset the metal and create a tight fit.

Once in place, bars unavoidably shift their position axially with each temperature change in the rotor, particularly during acceleration. Because copper and aluminum conductors exhibit much higher rates of thermal expansion and contraction than steel, the bars will therefore move lengthwise relative to the slots.

Whereas bars that break because of thermal, vibratory, and centrifugal stress can be fairly easily found in a number of ways, bars that are merely loose in their slots are much more difficult to detect. And looseness, unfortunately, is always a matter of degree.

The Department of Energy can’t be accused of not doing its homework. Its documentation for this Rule consists of more than 1,300 pages, and much work was also done by NEMA and others. But the task has proven to be far more complex than the setting of appropriate standards for larger polyphase motors, and no, we’re not quite there yet.

*This article was published in two parts, in the July 2010 and October 2010 issues. To order back issues with both parts of the full article, “Can rotor bar tightness be assured?” call 312-321-9440 or visit our online webstore.

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