February 2015 Electrical Apparatus

February 2015 Electrical Apparatus

This is a summary of the Electrical Apparatus February 2015 featured technical article,  by Richard L. Nailen, P.E.    Receive this article with your subscription to Electrical Apparatus

In the stator of an a-c motor, core loss consists of two components: hysteresis, resulting from the action of magnetic flux on the molecular structure of the core material, and eddy current loss, which results from induced current circulating within and between the core laminations. Minimizing this circulation is the reason motor cores are laminated; current flow is restricted by insulting coatings on the surface of each lamination. Eddy current losses are further restricted by making laminations thinner. For practical reasons, however, the minimum thickness for industrial motors is 0.356 mm

To minimize core loss in modern high-efficiency machines, manufacturers are claiming several design and manufacturing features that are not always as effective as they appear to be. One is improvement in lamination steel metallurgy. Many years ago, laminations were generally made of silicon steel. When product cost and increased output per unit of motor weight became primary goals, less expensive steel with little or no silicon content became widely used. Today, the importance of efficiency has brought a return to either silicon steel, or advanced materials and heat treatments that offer the lower losses that were once common. So the “improvement” is there, but as a return to past performance rather than real novelty.

Also commonly claimed is some combination of lower flux density and increased core length. This can be self-defeating, however. Lower loss per unit of core volume, in a larger core, may not significantly reduce overall core loss. More important, most industrial motors are assigned to standardized frame sizes. Internal space within each frame may not permit significant increase in core length.

To reduce the eddy current component of core loss, various methods have been adopted to hold the core tightly together while minimizing the electrical short-circuiting of the laminations along the core outer diameter as occurs with welding. Some form of key or cleat is now most common for small cores, offering the possibility of reducing total core loss by 20% or more.

In any event, core loss alone does not normally offer a sizable improvement in motor efficiency. That loss is typically about 20% of total motor loss. If that 20% can be reduced by 25%, motor efficiency may increase by only 0.2$. When the U.S. standards of the National Electrical Manufacturers Association apply, this is seldom enough to justify changing the standardized efficiency value stamped on the motor’s nameplate.

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