Re: calculating inductive factor for c cores

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Malcolm Watts by way of Terry Fritz <twftesla-at-qwest-dot-net>" <m.j.watts-at-massey.ac.nz>

Hi All,
        I should have included the unit for core cross-sectional area
(Ae) in my post yesterday. The unit is *square inches*, not metric.
My apologies for omitting this.

      I'd also like to mention that I've wound and used what are
effectively C-core transformers for TC use and obtained the necessary
leakage inductance without the use of shunts by winding the primary
and secondary coils on opposite legs of the core.

A further note on the 40 - 45% window occupation for the primary (and
total secondaries): this allows for insulation and creepage distance
to occupy 10 - 20% of the winding window. The packing factor can be
improved for by the use of good materials (e.g. polyesterimide wire)
winding impregnation, exposed core insulation etc. Many rewinds and
new winds I do typically score a packing factor around 90% for
transformers with typically less than 1000V peak across their
windings. Don't forget that the space the bobbin occupies must be
factored in.

BTW - if you follow the guidelines on winding window occupation, you
can easily obtain a wire gauge as follows (the rules apply for metric
or imperial wire; be consistent with the units used):

1) calculate the area the winding will occupy (e.g. 0.4 x window area)
2) divide this area by the number of turns (gives the area per turn)
3) divide this figure by PI (this gives wire radius squared)
4) take the square root of the result (gives wire radius)
5) multiply radius by 2 to give the *total* wire diameter

NOTE: the total diameter is the wire + coating diameter. Wire tables
typically give the bare copper diameter (useful for current density
calculations). Using a micrometer, I measured the total wire diameter
for all the wire gauges I have and made a little table up in Notepad
for handy reference listing copper and total diameters side-by-side
for each.

One other note: For single phase transformers running on an MEN
system such as used in New Zealand, I regard it as good practice to
wind the primary first, connecting the end closest to the core to the
active/phase line, ensuring that the core is ALWAYS solidly and well
connected to the mains ground.

Regards,
Malcolm