I have several "pulled" transformers that i need a reliable way to rate the secondary current for. How do transformer Mfg's do this?
I have heard that it is a temperature rise Vs time measurment? is this correct. where can i find info on this?
Thanks
I have heard that it is a temperature rise Vs time measurment? is this correct. where can i find info on this?
Thanks
There are so many variables the best we can do working backwards is quite rough.
Keeping this in mind:
Get some data on E&I transformers and then toroids, try Signal Transformer for their catalog and maybe Toroid Corp of Maryland.
Get a rough idea of the VA by weight.
Look up the regulation spec for that VA size, load the transformer until the voltage drops by that amount.
There is your answer.
Keeping this in mind:
Get some data on E&I transformers and then toroids, try Signal Transformer for their catalog and maybe Toroid Corp of Maryland.
Get a rough idea of the VA by weight.
Look up the regulation spec for that VA size, load the transformer until the voltage drops by that amount.
There is your answer.
So no identification marks at all ....bummer.....
Manufactures design a transformer to a particular specification/rating etc. and then test to prove that rating etc.
Without any information as mentioned above it would be extremely difficult if not impossiable to determine without.
Manufactures design a transformer to a particular specification/rating etc. and then test to prove that rating etc.
Without any information as mentioned above it would be extremely difficult if not impossiable to determine without.
You can estimate the VA rating by measuring the cross sectional area of the laminations. I put a graph of this here a year or two ago.......
Current rating............700 circular mils/amp is the usual value. Higher quality stuff may be designed as high as 1000 circular mils/amp.
You can also get handle on "quality" by measuring the magnetising current, IOW, the current through the priamry with no load. The lower the current, the more likely it will be a good transformer for audio use.
Jocko
Current rating............700 circular mils/amp is the usual value. Higher quality stuff may be designed as high as 1000 circular mils/amp.
You can also get handle on "quality" by measuring the magnetising current, IOW, the current through the priamry with no load. The lower the current, the more likely it will be a good transformer for audio use.
Jocko
Some of these have markings but as of yet have proved no value. some are quite large. and at least the primaries and secondaries are labeled.
I know one transformer is 120/240 with dual 120V secondaries. I want to run this transformer at half power. IE connect 120V to the 240V primary to produce 60-0-60 on the secondary side. But i dont know is what the current rating for the transformer is. It is quite large, about 6" overall and 2.5" thick so i am guessing 500Va or better but. there has to be a way to test these and get a reasonably accurate idea?
I just figured there had to be some such formula out there that views current vs. voltage drop vs. temp rise vs. time. or something.
Thanks
Zero Cool
I know one transformer is 120/240 with dual 120V secondaries. I want to run this transformer at half power. IE connect 120V to the 240V primary to produce 60-0-60 on the secondary side. But i dont know is what the current rating for the transformer is. It is quite large, about 6" overall and 2.5" thick so i am guessing 500Va or better but. there has to be a way to test these and get a reasonably accurate idea?
I just figured there had to be some such formula out there that views current vs. voltage drop vs. temp rise vs. time. or something.
Thanks
Zero Cool
"I know one transformer is 120/240 with dual 120V secondaries. I want to run this transformer at half power. IE connect 120V to the 240V primary to produce 60-0-60 on the secondary side. "
I've done this with good results. Leach suggests it in his 'Build a double barreled amplifier" article.
The transformers I used were a North Electric model with E&I construction and weighed in at about 11 lbs. and rated at 480VA.
With 10% regulation I was able to get 800VA out, at 100% duty cycle this would overheat.
My no load voltages were in the range of ±93V, and with a single pair of 10,000µF caps I got 600W/4R drawing 8A off the 120V line.
I used a 4A slow blow fuse to protect the transformer.
I've done this with good results. Leach suggests it in his 'Build a double barreled amplifier" article.
The transformers I used were a North Electric model with E&I construction and weighed in at about 11 lbs. and rated at 480VA.
With 10% regulation I was able to get 800VA out, at 100% duty cycle this would overheat.
My no load voltages were in the range of ±93V, and with a single pair of 10,000µF caps I got 600W/4R drawing 8A off the 120V line.
I used a 4A slow blow fuse to protect the transformer.
VA Rating
There is an article on this subject at www.schematicsforfree.mattsoft.net
I think it is in the misc section.
There is an article on this subject at www.schematicsforfree.mattsoft.net
I think it is in the misc section.
Thanks, I found 3 articles there but all deal with how to identify primary and secondary etc etc. only one mentioned anything about how to determin current rating and all it said was if you can touch it and count to 3 its within ratings.....
Many thanks for the pointer...But i am still looking for something more solid.
Zero Cool
Many thanks for the pointer...But i am still looking for something more solid.
Zero Cool
"Many thanks for the pointer...But i am still looking for something more solid."
What part of my post didn't you understand?
Core losses in a toroid can be ignored, especially when running 120V into a 240V primary.
The loss is now all I^2R, the same amount of current produces the same amount of heat.
Since the voltages are half, the VA will be half.
Weigh it.
Estimate VA based on weight.
This is close enough.
You could double check this by % of regulation if you have the equipment.
It doesn't sound like you do.
Remember to de-rate by 50% for 120V on 240V primary.
What part of my post didn't you understand?
Core losses in a toroid can be ignored, especially when running 120V into a 240V primary.
The loss is now all I^2R, the same amount of current produces the same amount of heat.
Since the voltages are half, the VA will be half.
Weigh it.
Estimate VA based on weight.
This is close enough.
You could double check this by % of regulation if you have the equipment.
It doesn't sound like you do.
Remember to de-rate by 50% for 120V on 240V primary.
VA
There are also formulas you can use based on the cross sectional area of the core. I have only seen these for "EI" lamination transformers. These, however, only tell you the total VA of the entore transformer and not of individual winding because the formulas are meant to work the other way around. WHere you determone the VA you want then size the core accordingly. The Transformer design program on the schematicsforfree web site uses there formulas. I wrote the program in the 80s and I have since forgoten the formulas. I got them from a Sams book called "Practical Transformer Design". I think the source code is in the program as I wrote it in GW Basic.
There are also formulas you can use based on the cross sectional area of the core. I have only seen these for "EI" lamination transformers. These, however, only tell you the total VA of the entore transformer and not of individual winding because the formulas are meant to work the other way around. WHere you determone the VA you want then size the core accordingly. The Transformer design program on the schematicsforfree web site uses there formulas. I wrote the program in the 80s and I have since forgoten the formulas. I got them from a Sams book called "Practical Transformer Design". I think the source code is in the program as I wrote it in GW Basic.
Ummm No....
But i was hoping to find a more definitive answer and so far all i have gotten is how to guess. if thats all there is, then thats all there is.
I was hoping someone would say something like:
apply a variable load to the transformer and when you reach X% of drop then your at max VA.
Or,
Apply a load and Monitor Tempurature rise over time and if the core reaches Xdegress in X Minutes then your at max VA.
OR something similar that is closer then a guess.
No problem, if there is no such formula.
But i was hoping to find a more definitive answer and so far all i have gotten is how to guess. if thats all there is, then thats all there is.
I was hoping someone would say something like:
apply a variable load to the transformer and when you reach X% of drop then your at max VA.
Or,
Apply a load and Monitor Tempurature rise over time and if the core reaches Xdegress in X Minutes then your at max VA.
OR something similar that is closer then a guess.
No problem, if there is no such formula.
That isn't how transformers are designed. Or how to evaluate them. No one that I know would suggest such methods, as they are almost meaningless.
The "guesses" that we supplied are more than adequate to do what needs to be done.
Jocko
The "guesses" that we supplied are more than adequate to do what needs to be done.
Jocko
efficiency
Transformer efficiency is about 98%...
Just a clue...
However, i am not sure how it can be used
Gajanan Phadte
Transformer efficiency is about 98%...
Just a clue...
However, i am not sure how it can be used
Gajanan Phadte
" That isn't how transformers are designed. Or how to evaluate them. No one that I know would suggest such methods, as they are almost meaningless."
I've got five minutes, tell me all you know about transformers.
My first paid position in electronics was in large transformer and motor repair, the subject is a lot more complicated that you think, and yet quite simple at the same time.
"I was hoping someone would say something like:
apply a variable load to the transformer and when you reach X% of drop then your at max VA."
What in the world do you think:
"Look up the regulation spec for that VA size, load the transformer until the voltage drops by that amount."
MEANS?
I've got five minutes, tell me all you know about transformers.
My first paid position in electronics was in large transformer and motor repair, the subject is a lot more complicated that you think, and yet quite simple at the same time.
"I was hoping someone would say something like:
apply a variable load to the transformer and when you reach X% of drop then your at max VA."
What in the world do you think:
"Look up the regulation spec for that VA size, load the transformer until the voltage drops by that amount."
MEANS?
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