Some of us may have built items many times with store-bought
transformers, and even when a project demanded some odd size, not really
a catalog item, never imagined it can be relatively straightforward to
make one's own, so we may have gotten by with the nearest stock item. Perhaps
with the following, a few of us might consider making their own, next time
we need a 38 volt, 11 ampere secondary, designed to connect to a 183 volt,
40 Hz. primary!
There are basically two metal parts to a transformer; the copper (electrical)
windings, and the iron (magnetic) laminations. Silicon-steel, actually.
Obviously, these two metallic components must be sized for the load at
hand. That is the essential point of this article.
To facilitate "shop use", I number the steps:
1. DETERMINE CORE-SIZE. A transformer's electrical (and, yes,
physical) size is determined by the Volt-Ampere (VA) requirement. No, VA
is not quite the same as Watts, but this is an electrical engineering detail
a bit off the nuts-and-bolts of the topic at hand. But, yes, one simply
multiplies the Volts times the Amperes the secondary must deliver. If more
than one secondary on the same transformer, figure the VA of each, and
add together, for a total VA rating. Then, using the VA/Core Area table
or graph, find a value in square inches. You might safely round-off to,
say, no smaller than 90% of the actual value, and higher would always be
OK, so long as installation space and funding are not considerations. If
you feel you must calculate a core-area, a polynomial, which gives you
the more conservative (larger) core-size, is provided. But for practical
use, this formula requires a computer, as the coefficients and powers make
calculation very cumbersome. Most small transformer core lamination stacks
are figured so that the core is close to square. Obviously, it is an easy
matter to stack fewer or more of the laminations of a given size to make
the core thinner or thicker than its width, and there may be electrical
or mechanical reasons for this kind of variation, now and then. But for
most small power-transformers, square is what to shoot for. You can simply
take the square root of the CA you have selected, and round this off to
the next-wider stock lamination size. Or, if it is only barely larger than
a stock size, it'd be OK to select the slightly narrower one. Example:
Say you need a core-area of 2.00 sq.in. by the graph. For a perfectly square
core, you would need a 1.414" wide center-leg. 1.375" (1-3/8") is the closest
under, and this would be just fine. 1.50" (1-1/2") is the next over, and
this is wider than really needed, considering you can "stack to area".
So, say you will use 1.375" wide center-leg laminations; 2 sq.in./1.375"
= 1.45+" You can then stack laminations to nearly exactly that thickness.
Plus or minus several will make hardly any difference, but the conservative
engineer will always go for the exact, or next larger number.
Jan
Rowland