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Old 9th March 2013, 12:36 PM   #401
JMFahey is offline JMFahey  Argentina
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Quote:
Originally Posted by AndrewT View Post
A core size of 1" by 2" cannot be a 1kVA transformer.
Assuming a flux of 1T you get ~120VA, at 1.5T ~180VA.
Maybe you could run it @ 1.8T and get at the most 225VA.

Tony,
could you check my numbers?
I design and wind (commercially) my own transformers, and besides "standard" calculations, have experimented a lot both how does real, "over the counter" iron work, and how much can it be abused without danger.
Start by saying than modern commercial iron is very good.
Don't design toroids but conventional EI types, but iron is iron, so my experiences should hold water, at least from the magnetic point of view.

My practical rule of thumb (which works in practice) is, for 50 Hz mains:
Usable V.A.= (core area^2)*1.5 , measuring in centimeters.
So in this case, and *supposing* the core is 2.5 x 5 cm ,
Usable V.A.= (2.5 x 5)^2 x 1.5= 235 V.A. *TOPS* , which perfectly agrees with AndrewT's calculation.

The advantage of toroids over my old technology EI lies in the huge "window" available and the long magnetic core circumference, both of which allow for much lower copper resistance.
In fact I have carefully measured magnetic and resistive losses, and even in my "abused" designs, resistive is still much higher than magnetic, which tells me I am not yet *that* close to the edge of the cliff.
But enough is enough.

Back to that toroid, it would be suitable for an around 150W RMS amplifier, no more.
Which is probably the real output of the "1000W" amplifier it came from
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Old 9th March 2013, 05:57 PM   #402
AndrewT is offline AndrewT  Scotland
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Thanks JMF
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regards Andrew T.
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Old 10th March 2013, 09:31 AM   #403
AJT is offline AJT  Philippines
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Quote:
Originally Posted by JMFahey View Post
I design and wind (commercially) my own transformers, and besides "standard" calculations, have experimented a lot both how does real, "over the counter" iron work, and how much can it be abused without danger.
Start by saying than modern commercial iron is very good.
Don't design toroids but conventional EI types, but iron is iron, so my experiences should hold water, at least from the magnetic point of view.

My practical rule of thumb (which works in practice) is, for 50 Hz mains:
Usable V.A.= (core area^2)*1.5 , measuring in centimeters.
So in this case, and *supposing* the core is 2.5 x 5 cm ,
Usable V.A.= (2.5 x 5)^2 x 1.5= 235 V.A. *TOPS* , which perfectly agrees with AndrewT's calculation.

The advantage of toroids over my old technology EI lies in the huge "window" available and the long magnetic core circumference, both of which allow for much lower copper resistance.
In fact I have carefully measured magnetic and resistive losses, and even in my "abused" designs, resistive is still much higher than magnetic, which tells me I am not yet *that* close to the edge of the cliff.
But enough is enough.

Back to that toroid, it would be suitable for an around 150W RMS amplifier, no more.
Which is probably the real output of the "1000W" amplifier it came from
thanks for chiming in, this thread welcomes your expertise.....
while i do built my own traffos for my projects, i am still basically a hobbyist, no intention to go commercial......

my other rule of thumb, when in doubt, treat irons as low grade if you don't know its origins, unless the laminates are 0.35mm thick and is striped....
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Old 10th March 2013, 09:41 AM   #404
AJT is offline AJT  Philippines
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Quote:
Originally Posted by donnyab View Post
building one and testing one ... if I ever get that far ha ha

still trying to get my head around all the formulas .. Pat Turner I think uses a very low B figure and that would mean his transformers are much bigger, almost like he is designing them for mild steel ?
so the 300 figure in his formula is probably too big by a factor of up to three

thanks
for my first opt build, i used the one designed by YvesM, i used it for my 6LU8 pp amp, i used 0.35mm thick RM18 laminates.....

one oldtimer fell in love with the sound of the amp that he came to the house 3 times, each time asking to hear the amp, he then went home with amp in hand...

what is said of designing OPT's, it is more an art than a science...

the Wolpert book gives design examples which you can use to make your own..math is easier to follow, imho....
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Old 10th March 2013, 02:37 PM   #405
JMFahey is offline JMFahey  Argentina
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Yes, you are right.
The iron I speak of as being "very good" is, of course, bought from a reputable Industrial supplier, and yes, *that* one can be trusted.
Now, if you buy from an unknown small supplier, he *might* have bought the cheapest he found.

I forgot: if you are recycling some old transformer of course design as if it were the cheapest iron.

I can't choose "brands", it's whatever is in the last container they opened, (all good of course or they lose important customers) but i get basically 3 types:
1) a bright greenish/yellowish striped material, same as found in Soldano transformers, some claim it's Russian silicon steel.
2) a 2 tone grey striped material , origin unknown (fir me), still very good.
3) a flat gray Brazilian type.
The only one which comes pre-cut and packaged in 20Kg cardboard boxes.
All others are imported as huge 5 Ton? rolls and are cut to size in Argentina.
Which is good because I can order special sized strips to build huge "semi-toroidal" transformers if pressed for space.

Anyway I am experimenting with SMPS to reduce weight (and some cost).
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Old 10th March 2013, 03:21 PM   #406
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JMFahey,

any photos available on the stuff you are using ?
thanks
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Old 10th March 2013, 04:10 PM   #407
JMFahey is offline JMFahey  Argentina
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You mean the EI silicon iron?
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Old 10th March 2013, 06:59 PM   #408
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hi tony, the primary winding is 220 turns, gauge 0.7 mm and the secondary is 290 turns guage 1.0 mm.
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Old 10th March 2013, 07:44 PM   #409
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JMFahey,

yep
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Old 10th March 2013, 07:50 PM   #410
JMFahey is offline JMFahey  Argentina
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Quote:
Originally Posted by donnyab View Post
JMFahey,

yep
OK, I'll grab my son's camera and shoot some laminations
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