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Old 7th October 2006, 08:31 AM   #1
Nixie is offline Nixie  Canada
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Default Power transformer inductances

How can one roughly estimate the primary inductance of an EI power transformer given its VA rating? I'm asking so I can simulate transformers in SPICE as coupled inductors.
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Old 7th October 2006, 09:46 AM   #2
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I measure 8-10H off of several little 12-15VA transformer primaries.
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Old 7th October 2006, 10:18 AM   #3
maudio is offline maudio  Netherlands
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There is no such relation with VA rating.

For a very rough estimation of L you need # of turns, core permeability, core cross section, air gap and length of the magnetic path. With this you can do some calculation which give you an order of magnitude. And yes, it probably will be high, in the several H range.

There is no absolute value for a transformer's inductance. It's value varies with both frequency and level at which it is measured. So modelling a transformer as coupled inductor will not give very accurate results. Might be no big deal if you just want to model a power supply but for signal transformers it's useless.

For better results consider using a more complex model that takes winding resistance, leakage, internal capacitances and core losses into account. A few terms into google should help you on the way. Still, even the more complex ones do not include the level and frequency dependency of L.
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Old 7th October 2006, 10:44 AM   #4
Nixie is offline Nixie  Canada
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Quote:
Originally posted by maudio
For a very rough estimation of L you need # of turns, core permeability, core cross section, air gap and length of the magnetic path.
Core cross section and lenght of magnetic path scale with the transformer VA. Frequency and signal level are obviously that of the mains. Obviously when mains transformers are designed, they must calculate the needed minimum inductance.

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Might be no big deal if you just want to model a power supply but for signal transformers it's useless.
Why would I model a signal transformer when I specifically wrote "power transformer"?!
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Old 7th October 2006, 11:58 AM   #5
maudio is offline maudio  Netherlands
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Quote:
Originally posted by Nixie

Core cross section and lenght of magnetic path scale with the transformer VA. Frequency and signal level are obviously that of the mains. Obviously when mains transformers are designed, they must calculate the needed minimum inductance.
Mains voltage and frequency are not the same everywhere so things are not as obvious as you suggest. Anyway, this small program can help you reverse enginering the transformers design which can help you along with your question.

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Why would I model a signal transformer when I specifically wrote "power transformer"?! [/B]
Only trying to help, you didn't tell me what you want to use it for. Many people 'abuse' power transformers for other purposes, for instance as chokes or to drive esl panels.

Also, by answering your question in a more general context the answer might be usefull for others as well. After all, this is a public forum. Sorry to offend you
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Old 7th October 2006, 12:12 PM   #6
Nixie is offline Nixie  Canada
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I can't figure out how to use that program for power transformers. For example, what do I put in the Rp field, and the primary Z?
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Old 7th October 2006, 01:01 PM   #7
maudio is offline maudio  Netherlands
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Oops... pulled the wrong link from my bookmarks, I intended magdemo. No wonder you got lost. Sorry...

The OPT program I accidently linked to before is intended to design output trannies, Rp being plate resistance from the output tubes which should be equal or < to Zprim at lowest feq of interest to prevent the output power being wasted in the primary inductance. Anyway, not very helpfull for calculating power transformers..

Magdemo may look complicated at first sight (well it did to me) but it comes with a pretty good quick design guide (in help function) which makes things rather easy. The good thing about magdemo is that it will generate a spice model for your design.

A nice article on transformers, if you're prepared to do some more reading on the subject, can be found on http://www.geofex.com/Article_Folder...es/xformer.htm

May I ask why you want to model your power transformer in spice? What do you want to achieve?
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Old 7th October 2006, 01:24 PM   #8
Nixie is offline Nixie  Canada
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I prefer to model linear power supplies by including the transformer, rather than having it as some voltage source, thus pushing the voltage source back to represent the mains.
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Old 7th October 2006, 02:29 PM   #9
maudio is offline maudio  Netherlands
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Quote:
Originally posted by Nixie
I prefer to model linear power supplies by including the transformer, rather than having it as some voltage source, thus pushing the voltage source back to represent the mains.
Ah, that makes thing a bit clearer That's not a bad idea. But you don't need Lprim for that at all. Well, that is unless you are interested in modeling the load it puts on the mains but that seems rather pointless (you don't know the mains impedance anyway).

All you need is an equivalent schematic for the transformer's secondary side. In other words: what does it look like seen from the rectifier.

To do that, take a model of a transformer and reflect everything on primary side to the secondary. At 50 or 60 Hz you can safely ignore any capacitances and leakage inductances. So all that remains is a voltage source and a series resistance (being Rsec + Rprim / N^2). Both winding resistances can be easily measured (just make sure the meter doesn't pulse the measurement current). And there you are.
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Old 7th October 2006, 02:30 PM   #10
Eva is offline Eva  Spain
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Quote:
Originally posted by Nixie
I prefer to model linear power supplies by including the transformer, rather than having it as some voltage source, thus pushing the voltage source back to represent the mains.
Then you should also have to model the mains line, since it behaves in quite a more complex way than a transformer.
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