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Old 20th November 2011, 10:31 AM   #1
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Thumbs up Iron Core Transformer Formula

Do you know the formula?
to get the best numbers of turn to this kind of transformer / trafo?
I have over simplified formula, but maybe some other formula to compare with

This my formula for EI trafo for 50 Hertz frequency

primary winding
n = 45 * V / A

secondary winding
n = 49.5 * V / A

n = number of winding turn
V = voltage input or output (volt)
A = core area (cm square)

Maybe someone help to add another formula here, please...
I want to make right trafo this time

Thanks

Last edited by JOHN BALI; 24th January 2012 at 04:30 PM.
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Old 20th November 2011, 10:37 AM   #2
AndrewT is offline AndrewT  Scotland
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the secondary shows a +10% correction for loaded output voltage.
I suspect this is a guesstimated value for regulation of the final transformer build.
Regulation can vary from <=3% to >=30%. A fixed turns ratio correction of +10% is not appropriate to all transformers.
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Old 20th November 2011, 10:44 AM   #3
AJT is offline AJT  Philippines
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are you winding on an EI core or torroid?
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Old 20th November 2011, 10:46 AM   #4
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by JOHN BALI View Post
This my formula for EI trafo for 50 Hertz frequency
Quote:
Originally Posted by Tony View Post
are you winding on an EI core or toroid?
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Old 20th November 2011, 10:51 AM   #5
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This is OVERSIMPLIFIED.

The important thing to learn is the magnetic properties of the transformer that you are trying to wind.

The first formula is

A = Square Root (W)
.......----------------...= mm2
................K

Where W is the Secondary Power + 10%

K = 0.0086 for standard E & I laminated sheet
0.0150 for T & U laminations.

Once you've got the correct iron work for the transformer.

Turns / Volt = 1 x 10e9
..................------------
...................K x B x f x A

Where K = 4.44 for a SINE wave, 4.00 for a SQUARE wave.

B = Max Flux Density - 780mT is good for reliabilty, hoever this can be pushed to 1350mT

f = Frequency 50Hz or 60Hz (Unless you are design SMPS then its kHz)

A = The Core Area as discussed above.


Once you know the Turns / Volt its a simple case of working out the Primary and Secondary Turns.

Last edited by KatieandDad; 20th November 2011 at 10:54 AM.
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Old 20th November 2011, 11:22 AM   #6
AndrewT is offline AndrewT  Scotland
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Is the first formula?
A = sqrt(W) / K
where W is the input power in Watts and K is 0.0086 for EI.
The answer in mm^2 (sqmm)

Is the second formula?
Turns/Volt = 10^9 / [ K' * B * f * A]
where K' = 4.44 for sinewave, B = 780mT for EI to stay cool and avoid the saturation knee on high mains voltage, f = mains frequency, A = core area in sqmm

Inserting values into that first equation gives the same answer as: Watts = 31 * [Core Area (in square in)]^2
using 100W and 1160sqmm = 1.8sqin

Inserting values into the second formula for 4.44, 780mT, 50Hz, 1160sqmm,
gives 5T/V
resulting in 1145T for 230Vac @ 100VA

Whereas, John's first formula result in 892T for 230Vac with 1160sqmm. This seems to confirm that Johns' first formula has assumed B=1000mT = 1T

Have I got this correct?
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Last edited by AndrewT; 20th November 2011 at 11:49 AM.
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Old 20th November 2011, 11:57 AM   #7
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1000mT is fine for commercial design.

If you've got the core material I would reduce the Magnetising Force (B).

You can ALWAYS increase the Turns / Volt, NEVER reduce them.

780mT is what the MoD employ. It also allows for Mains Transients of over 100%, not that the windings wouldn't take that for a short time anyway.
For SMPS designs the B figure needs to be MUCH lower.

Last edited by KatieandDad; 20th November 2011 at 12:03 PM.
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Old 20th November 2011, 12:00 PM   #8
AndrewT is offline AndrewT  Scotland
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But, have I interpreted your formulae correctly and drawn the correct conclusions?
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Old 20th November 2011, 12:07 PM   #9
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Looks fine to me.
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Old 20th November 2011, 12:46 PM   #10
AndrewT is offline AndrewT  Scotland
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I have only done it once so far, but would testing for the knee in the B vs V curve give confidence in having guessed at the correct identification of the core material?

I found that plotting for Vac from 10Vac to 260Vac showed that the tiny 3VA transformer was being run at far too high a voltage. 160Vac would be far cooler than running it at a nominal 240Vac, even though it is rated as 220/240Vac. Lies!! or the cheapskates thought I couldn't care about being burnt by the surface of the EI core
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