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Old 23rd April 2016, 08:11 PM   #1
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Default Power Transformer Data

I was wondering if there is a way to calculate some more information from the standard VA rating found in power transformer datasheets? This is for an EI core transformer.

For example if I had a line isolation transformer that was rated for 60VA can I find the winding resistance and inductance? We know that the intended frequency of operation is 60Hz. We know that the transformer will not saturate at currents 500mA and under. We know the turns ratio is 1:1.

I was looking through some old transformer design papers and was curious if someone could help me and see if I am on the right track. It says;

Calculate inductive reactance, XL: XL = V / I so 240 = 120 / .5

Calculate required inductance, L: L = XL / 2 * pi * f so .636 = 240 / 2 * pi * 60

Is this close? Can winding resistance be found too?

Thanks,

-bird
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Old 23rd April 2016, 08:55 PM   #2
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Inductance can be calculated, resistance can be measured.
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Old 23rd April 2016, 09:32 PM   #3
JMFahey is offline JMFahey  Argentina
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Quote:
Originally Posted by famousmockingbird View Post
I was wondering if there is a way to calculate some more information from the standard VA rating found in power transformer datasheets? This is for an EI core transformer.
Not exactly (just not to say plain "no)
Design is always a compromise, so according to designer preferences transformers can be designed in different ways, yet all be acceptable .
He might minimize iron used to save weight/size in which case he will work closer to saturation, or minimize copper, in which case he'll have higher resistive losses , or design "no strings attached" , either for HiFi or personal use, all will use different core sizes, different copper turns = different inductance.
Besides, gap influences inductance a lot, and you have an uncontrolled gap even in a regular interleaved iron EI transformer, because cut ends are rough and do not perfectly stack up.
So much so that to improve that, expensive "C" cores are wound, cut, and mating surfaces *polished* to minimize gap.
That's why best is just to measure the d*mn things, as suggested by JonSnell

Quote:
Calculate inductive reactance, XL: XL = V / I so 240 = 120 / .5
Your formula is fine, but with wrong data, that's not inductive reactance, this is not a choke but a transformer, so that is the load impedance reflected to the primary.
The actual inductance can be calculated by measuring primary current with secondary unloaded.
In fact it's a standard test suggested in transformer design manuals.

Good design practice is to choose primary inductance high enough so at 50/60Hz transformer pasess relatively small üseless" idle current, compared to useful current to be delivered to the load.


Quote:
Calculate required inductance, L: L = XL / 2 * pi * f so .636 = 240 / 2 * pi * 60

Is this close? Can winding resistance be found too?
What you had calculated first was not XL
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Last edited by JMFahey; 23rd April 2016 at 09:37 PM.
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Old 23rd April 2016, 10:27 PM   #4
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Thanks! Makes perfect sense.

Quote:
Originally Posted by JMFahey View Post
Your formula is fine, but with wrong data, that's not inductive reactance, this is not a choke but a transformer, so that is the load impedance reflected to the primary. The actual inductance can be calculated by measuring primary current with secondary unloaded.
In fact it's a standard test suggested in transformer design manuals.

So can I just leave the secondaries floating (unloaded) and use my digital meter in series with primary to read AC current? Or is it best to not place a digital meter in series and instead use a current transducer over one of the primaries to read current? Anyway after I take this current reading I assume I apply it to; V = L * ( dI / dt )

Okay I believe this is a similar answer I was reading.

How can I calculate the inductance of the primary of a transformer given a specific load on the secondary? - Electrical Engineering Stack Exchange
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Old 23rd April 2016, 10:39 PM   #5
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Actually I have done something with signal transformers which I believe I can do here.

I guessed at what the primary inductance was, so with this transformer lets say 10mH. With the audio transformer I figured it was around 25H but this is different. The XL of 10mH @ 1kHz is 62.8. This just gives you an idea of what size trimpot to use. Then just input a sine wave of say 1v @ 1kHz into the primary with the secondary unloaded, have a 1k trimpot in series with primary and adjust it until you read -3db on a scope across the resistor? Basically you just look for -3db on the scope which then the XL should equal the value of resistance? Calculate back for the inductance.
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Old 24th April 2016, 02:36 PM   #6
DF96 is offline DF96  England
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Quote:
Originally Posted by famousmockingbird
We know that the transformer will not saturate at currents 500mA and under.
Load does not cause core saturation; it is lack of load which can do this for a cheap transformer. 60VA at 120V means that it should not overheat at currents of 500mA and under; it might saturate with no load or if you try to draw only a few mA from the secondary.

Quote:
For example if I had a line isolation transformer that was rated for 60VA can I find the winding resistance and inductance?
No, not by calculation. You could perhaps estimate the resistance by assuming, say, 10% voltage droop on full load.
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Old 24th April 2016, 09:25 PM   #7
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Hi, yes I found the thread I remember reading from a long time ago;

Hammond Transformer Winding Resistance -- 369GX

I will just measure things it's the only way to go. For a quick sim in PSUDII I can see using the aforementioned method for a ballpark figure.
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