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



Hi Mike, All

Lemme see what I can do:

 Original Poster: "mike" <mike-at-gmx-dot-com> 
 
 First, as I said, the info I presented was taken from "The Radio Amateurs
 Handbook", so your comments are directed at the authors, not me;-) The
edition I grabbed first was a bit old (1950) so I looked at a later version
and their recommendations are somewhat less conservative but I don't know if
that's due to a change in philosophy or a change in the core materials
available, or... The later editions give less detailed info on the core.
 
RWB: Okay, sorry about that. Redirect my "blasts" to them, please.
 
 The most common wire house wiring sizes here are #14 (15A), #12 (20A) and
 #10 (30A), but those figures are only for 3 or fewer conductors in a
 conduit. If you stuff more than that in a pipe, the ratings go down by 20%
 to limit temperature rise. Wrapped up in the center of a transformer, even
 with oil cooling, I suspect they would need to be derated even farther.

RWB: This is true, but you donīt want your house wiring to get hotter than say
35-40°C for fire safety reasons. I mean house AWG 10 is rated at 500V, but you
can put 15kV thru it w/o getting arc overs, even if two insulations touch each
other. Transformers are usually "use" rated to about 75°C. This is very
uncomfortable to the touch. Nothing you would want concealed behind walls.
 
 For a circuit 'feeding' a 10KVA pig (42A/240V) I'd use #6 or #8, depending
 on the type of insulation and the length of the run. I'd feed a 25KVA pig
 (105A/240V) with #1 or #2. As for what they use to wind a 25KVA pig, I
 don't know for sure, but I suspect that it is more like a bar than a wire.

RWB: Okay, this is the FEED wire. Your feed wire is usually larger than the
wire used to wind the transformer because you donīt want heating and resistive
losses in your feed wire.
 
 What voltage is your 7.5KVA primary running on - 240? That's 31+ amps, an
 awful lot for #14 wire. I suspect that you can get away with it because of
 the duty cycle, but how much are you wasting heating the core? And what is
 the primary voltage of the 1.65KVA MOT? 
 
 RWB: My calcs were based upon 240 volts although I might go for a different
primary winding to run it from 380V (across to phases here) for a decreased
amperage draw.BTW: I slipped on my keyboard I meant to write AWG 13. This
wouldnīt make to great a difference tho.
The primary voltage of the MOT was 240 volts.

 I did some measuring:
a.) The MOTīs primary is: AWG 19 (maybe AWG 20)
I was measuring at the connection spade. There was a little solder on the
wire.

b.) The original primary wire of my 8kVA welder (240/380 in): AWG 10/AWG13.
However, important note: This xformer was originally wound with AL wire. Using
copper you can get away with a smaller wire gauge considering the higher
resistance and lossier (do to Eddy currents) AL wire.

 Isn't the magnetizing current strictly a function of core size, core material
and turns? Why would wire size make a difference here?

RWB: Yeah, my choice of words was rotten. What I meant was the bigger the
wire, the more current it will pull (do to decreased AC resistance) without
changing output (or even no load condition).

 The CSA is the area of core that actually passes through the windings. In a
 typical 'E' core the CSA is the area of the center leg. The outer legs are
 each half of that. The required CSA depends on the core material too
 though.

RWB: We agree on the definition. I use silicon steel laminations as a
calculation basis for my CSA size. Of course I donīt know their exact
composition, so it might vary a little bit. 
 
 I'm not sure I follow you here. What would be a fifth of an 8 turn winding
 (1/25 of a 40)? I was thinkng in terms of inductance and magnetic effects,
 which should be (roughly) equal for one 8-turn winding of larger wire or
 multiple 8-turn windings of smaller wire in parallel. 
 
RWB: Well I was talking of (like Doug wanted to) paralleling the five 8 turn
windings. The magnetic effects would be similar, tru, but the resistance would
not be the same as one 40 turn winding (which I read as your comparison).
Maybe we somehow were thinking of different things here.

 That's quite alright. It's nice to know that someone is actually reading my
 posts! Whenever I join a new list I try to add whatever I can, even if it
 stirs up some controversy, before I start asking for help from others.
 
RWB: Thatīs what these lists are for.

coiler greets from Germany,
Reinhard