Re: Self-built power transformer (some design parameters)

To: tesla-at-pupman-dot-com
Subject: Re: Self-built power transformer (some design parameters)
From: "mike" <mike-at-gmx-dot-com> (by way of Terry Fritz <terryf-at-verinet-dot-com>)
Date: Sun, 04 Oct 1998 10:51:28 -0600
Approved: terryf-at-verinet-dot-com


Hi:

 Here's some info, mostly taken from "The Radio Amateurs Handbook", that
may be helpful in designing a homebrew transformer. Note that this assumes
the use of a laminated silicon steel core capable of supporting a flux
density of at least 50,000 lines/sq. in.

 Suggested parameters for a 1KW transformer:

  Primary wire size:  #10 AWG
       Primary turns:  132
Core cross section: 2.5" x 2.5"

An average value for calculating the number of primary turns needed is:

   turns = 7.5 x (primary volts/core cross section in sq. in)

>From this we can calculate that an 8-turn primary would work if your core
cross section is roughly equal to the primary voltage (120 / 240 sq. in.)!
Quite a large chunk of iron. (Don't forget that five 8-turn windings in
parallel is not equivalent to one 40-turn winding of larger wire.)

As for the wire size, the recommended size is 1500 circular mils per amp
for continuous duty or 1000 cm/amp for intermittent use. #24 has an area of
404 cm, so it would only be good for 270 mA continuous or 404 mA
intermittent and 5 in parallel would only be good for about 2 amps in
intermittent service. For 30 Amps you need something like #6 (or the
equivalent) for intermittent duty. (One thing to remember here is that most
wire tables list the ampacity of wire under free-air conditions, not
confined in the core of a transformer!)  Another way to look at is that
five #24s in parallel is equivalent to one #17, and you woudn't wire your
30 amp feeder circuit with #17 wire. While it might carry the current it
would get quite warm, even in free air;-)

Hope this helps,

- MIke


> Slight misinterpretation, it appears. I was talking about 8 primary
turns.
> I would put
> the windings on top of each other and parallel them, thus giving me an
> inductor with the
> same value but capable of handling more current. According to my
> information, 32 AWG can
> take 1 or 2 amps, and 24 can take 6, so with 5 layers paralleled, it can
> take 30. It'll
> be in oil anyway, so I have some margin for error.