2. DETERMINE PRIMARY TURNS-COUNT. The math, next, assumes
a single primary. If you want a pair of identical primaries such as you'd
have where you want a design to be useable with either 120 VAC or
240 VAC, where you'd connect the two windings in SERIES for the higher
voltage, or PARALLEL for the lower voltage, do the math as if the higher
voltage would be used, and then simply divide the number of turns by two,
giving the turns-count for each of the two smaller primaries. Simple logic.
The General Formula for PRIMARY TURNS is N = (E · 10^8)/ (4.44·f·A·B),
where N=Number of turns; E=Primary Voltage; f=line-frequency; A=core cross-section
Area; and, B=max.flux, in lines. Assuming 60,000 lines of flux, a bit of
algebra, and we have:
N = (k·E)/ A, where k=6.256 when f=60 Hz., or, k=7.507
if f=50 Hz.
3. DETERMINE TURNS-per-VOLT. Simple: TV = N/E. E still equals Pri.
Voltage.
4. DETERMINE PRIMARY-CURRENT. Simple: I = VA/E, where I=Pri.
Current.
5. DETERMINE PRIMARY WIRE-SIZE. Copper wire must be thicker for
greater current. Wire cross-section is measured in "Circular Mils", this
value being simply the Diameter of the wire in Mils (thousandths of an
inch) squared. C.M. = D2 Copper wire tables give the diameters,
area in C.M.'s, gage-number, turns per inch and average turns per square-inch,
etc. You can find copper wire tables in electrical handbooks, the famous
ARRL manuals, etc. (For your convenience, a table of wire diameters in
mils is included in these pages.) For power-transformers of the size considered
here, a value of 750 C.M. per ampere, or 0.75 C.M./milliamp is about right.
For rather cool-running transformers, especially at full-load, 100% duty-cycle,
you might go on up to 1000 C.M./amp (1 C.M./milliamp).
Jan
Rowland