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For years manufacturers have assigned numeric codes to their cores; these codes represent the power handling ability. This method assigns to each core a number that is the product of its window area, Wa, and core cross-section area, Ac, and is called the area product, Ap.
These numbers are used by core suppliers to summarize dimensional and electrical properties in their catalogs. They are available for laminations, C-cores, pot cores, powder cores, ferrite toroids, and toroidal tape-wound cores.
The regulation and power-handling ability of a core is related to the core geometry, Kg. Every core has its own inherent, Kg. The core geometry is relatively new, and magnetic core manufacturers do not list this coefficient.
Because of their significance, the area product, Ap, and core geometry, Kg, are treated extensively in this book. A great deal of other information is also presented for the convenience of the designer. Much of the material is in tabular form to assist the designer in making trade-offs, best-suited for his particular application in a minimum amount of time.
These relationships can now be used as new tools to simplify and standardize the process of transformer design. They make it possible to design transformers of lighter weight and smaller volume, or to optimize efficiency, without going through a cut-and-try, design procedure. While developed especially for aerospace applications, the information has wider utility, and can be used for the design of non-aerospace, as well.
Output power, P0, is of the greatest interest to the user. To the transformer designer, the apparent power, Pt, which is associated with the geometry of the transformer, is of greater importance. Assume, for the sake of simplicity, that the core of an isolation transformer has only two windings in the window area, a primary and a secondary. Also, assume that the window area, Wa, is divided up in proportion to the power-handling capability of the windings, using equal current density. The primary winding handles, Pin, and the secondary handles, P0, to the load. Since the power transformer has to be designed to accommodate the primary, Pin, and, P0, then,
The designer must be concerned with the apparent power, Pt, and power handling capability of the transformer core and windings. P, may vary by a factor, ranging from 2 to 2.828 times the input power, Pjn, depending upon the type of circuit in which the transformer is used. If the current in the rectifier transformer becomes interrupted, its effective RMS value changes. Thus, transformer size is not only determined by the load demand, but also, by application, because of the different copper losses incurred, due to the current waveform.
Reference: TRANSFORMER AND INDUCTOR DESIGN HANDBOOK
Third Edition, Revised and Expanded
COLONEL WM. T. MCLYMAN