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Old 15th December 2010, 03:23 PM   #11
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I have purchased transformers that were very much in saturation at normal line and also maybe with 20% to go. It's Totally up in the air. If you didn't design it you can't guess, although it's easy to tell once you plug them in. No load temperature rise and noise is a dead giveaway. I have some Talema 200VA toroids that must draw a watt or less at idle, and I think I paid 10 bucks a piece for them. Then I recently bought some Dayton plate amps and had to throw out the supplied transformers because they got hot just sitting there. I think they laminated them out of tin cans and put on about 3/4 of the required primary.
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Old 15th December 2010, 06:14 PM   #12
DF96 is offline DF96  England
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Quote:
Saturation in a transformer is independent of the load current.
There was a long discussion about this somewhere. The conclusion was that load current reduces core flux by creating ohmic drop in the primary. Not a huge effect, but if a mains transformer is not saturating off-load then it will not saturate at any load.
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Old 16th December 2010, 06:08 AM   #13
AJT is offline AJT  Philippines
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Quote:
To clarify one point.

Measuring the Primary current with the secondary open circuit. Is that the same as measuring the primary current with the secondary removed?
yes.....no load current consisting of magnetizing(exciting current) and hysterissis/eddy current losses
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Old 16th December 2010, 06:13 AM   #14
AJT is offline AJT  Philippines
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Quote:
I wind power supply transformers with a maximum of 1,2 T at 230 Vac. Mostly not over 1 T.
i wind my traffos at 0.7T, cores available in my country are not so good compared to M6 irons from the states...
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Old 16th December 2010, 07:43 AM   #15
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Designing for 130/260V rms WHEN HOT is probably enough
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Old 16th December 2010, 10:09 AM   #16
AndrewT is offline AndrewT  Scotland
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That's the second time "hot" has been mentioned.
What is it about "hot" that makes the test condition more severe?
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Old 16th December 2010, 11:19 AM   #17
AJT is offline AJT  Philippines
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there is such a thing as a 10 second rule.......if you hold the traffo in your hand and you are able to stand the heat for 10 seconds, then the traffo is not that hot.....
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Old 16th December 2010, 11:47 AM   #18
AndrewT is offline AndrewT  Scotland
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Hi,
all my own design gear that is mains driven run with the transformer feeling almost cold.

I have touched some plug in the wall type transformers that get too hot to touch !
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Old 16th December 2010, 11:54 AM   #19
AJT is offline AJT  Philippines
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these are the traffos that i have made recently:
Click the image to open in full size.
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Old 20th December 2010, 01:11 AM   #20
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Quote:
Originally Posted by AndrewT View Post
That's the second time "hot" has been mentioned.
What is it about "hot" that makes the test condition more severe?
Winding resistance goes up with rising temperature, causing output drop and a potential runaway problem.

I don't think iron saturation varies too much with temperature, unlike ferrite which performs badly when it gets warmer eg Powdered-Iron and Ferrite Materials for Inductor Cores Page of
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