Electrical Apparatus December 2014

Electrical Apparatus December 2014

This is a summary of the Electrical Apparatus December 2014 featured technical article,  by Richard L. Nailen, P.E. 

While fabricated steel structures furnish support for many motor drives, the larger the machine, the more likely it will rest on a concrete foundation. A century ago, when both operating speeds and machinery power densities were lower, the design of such foundations was largely empirical, based on what had been successful with steam power. In the absence of standardized tests of soil properties, the influence of soil embedment was seldom accounted for. Foundation design was aimed primarily at assuring stability against movement downward (settling), side-to-side rocking, or fore-and-aft rocking. A number of guidelines were developed governing appropriate foundation mass and dimensions; a typical rule of thumb was that foundation mass should be at least 3 to 5 times that of the supported machinery.

However, whereas concrete and steel exhibit simple and generally constant proportions between stress and deformation, soil behavior is highly variable, depending upon the mix of soil types (sand, clay, rock) and moisture content. As the science of soil mechanics evolved, along with the increasing speed and power of supported machinery, two other concerns arose. One was resonance. Foundations transmit to the surrounding soil a natural vibration frequency, which if coincident with machine operating speed can cause damaging pressure waves extending some distance away. An influential 1948 Russian book dealt with this issue in detail, except that the book’s emphasis was on foundations that were not embedded. Work has been done since then on calculations involving soil reactions against embedded foundations. Also, standardized tests have been developed by civil engineering and testing agencies to evaluate the load-bearing characteristics of specific soils.

A distinction has often been made in technical literature between design criteria for foundations supporting static or non-vibrating loads, and those for dynamic or vibratory machines such as engines or compressors. However, if resonance occurs, even well-balanced rotating machines (such as large motors) can induce damaging vibration unless the foundation has been carefully designed. Reliance on standard foundation design criteria given in building codes alone can be misleading

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