Author Topic: Losses in transformers  (Read 3214 times)

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Offline spanacc

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Losses in transformers
« on: March 21, 2012, 01:13:26 pm »
Hi guys
I am new on this forum and i want to ask a question: How does one calculate losses in transformers ?
If i have a transformer with which i want to step down the mains voltage from 220V into something like 48V, how big are the losses in it ?
Is it a function of current drawn ?

spanacc
 

Offline spanacc

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Re: Losses in transformers
« Reply #1 on: March 21, 2012, 02:35:47 pm »
Is this a complete answer ? I have found this using good old google
--------------------------------------------------------------------------------------
Various energy losses occur in transformers:
Copper losses, the resistance to the current flow in the windings which heats the conductors.
Iron losses, which are of two kinds:
    eddy current losses which flow in the laminations of the core caused by the magnetization and re-magnetisation of the core, which also causes heatingof the core. Eddy current losses can be greatly reduced by not making the out of a solid piece of iron.
    That is why transformer cores are generally made of lots of separate thin "laminations" which are insulated from one another by being bonded together using an epoxy resin adhesive.
    hysteresis losses which are again caused by the magnetizing and de-magnetising of the core.
    These are reduced by making the laminations of silicon steels, which have lower hysteresis losses than plain iron.
Together all these losses lead to a total efficiency of about 97 to 98%, which will alter depending upon the load current that the transformer is supplying.
 

Offline T4P

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Re: Losses in transformers
« Reply #2 on: March 21, 2012, 04:27:15 pm »
97% to 98% is not a good margin , it's actually way lower then that .
Leakage inductance is also another thing , the less the better , that's the reason why toroidal cores and R-cores exist , they have FAR less leakage inductance making them more powerful per sq-cm .
But they are usually too large to consider them small  :)
Looking at a standard RS toroidal 160VA transformer , the magnetizing current is 120mA at 230V
230*0.12 = 27.6W magnetizing current :o
Iron loss ( core losses ) : 7W
Copper loss ( winding losses ) : 18.8W
So that's about 53W lost . I'm not too sure if my 27.6W magnetizing current is correct or not
So that's about 66.7% efficiency . And to be fair , that's some cheapy RS brand transformer , nuvotem's transformer has a hell lot less losses and is alot smaller but 2 times more expensive , 6W less iron losses , 0.8W less copper losses and 25W less magnetizing wattage
The reason why switching mode transformers are so small it's because the higher the frequency , the less time current needs to stay in the core , therefore only needing less space , provided hysteresis losses is kept down .
« Last Edit: March 21, 2012, 04:33:32 pm by Dave.S »
 

Offline spanacc

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Re: Losses in transformers
« Reply #3 on: March 22, 2012, 09:42:46 pm »
Hello Dave,
I thought magnetizing current is too small to take into consideration when having some load on the output. I found this when searching the web :
"Exciting current is the amount of amperage a transformer draws under a no load condition"
Anyway, even without that the efficiency is about 80% which is still less than what i have found online in my second post.

Thank you for your answer
 

Offline T4P

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Re: Losses in transformers
« Reply #4 on: March 22, 2012, 09:50:37 pm »
97 to 98% is pure bullshit .
If they were talking about SMPS , yes , but even most SMPS struggle to do 96% .
 

Offline IanB

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Re: Losses in transformers
« Reply #5 on: March 22, 2012, 09:57:28 pm »
Together all these losses lead to a total efficiency of about 97 to 98%, which will alter depending upon the load current that the transformer is supplying.

The efficiency of a transformer is to some extent a design parameter. You can get a higher efficiency if you build a bigger and more expensive transformer.

An efficiency of 97-98% is entirely typical of large utility transformers used in power distribution (even 99% is possible in some cases).

For the kind of small transformers found in consumer devices the efficiency will be somewhat lower. It just isn't sensible or economical to strive for such high efficiencies in tiny transformers.
« Last Edit: March 22, 2012, 09:59:08 pm by IanB »
I'm not an EE--what am I doing here?
 

Offline spanacc

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Re: Losses in transformers
« Reply #6 on: March 23, 2012, 12:07:40 am »
So a good quality transformer has to have at least thick wires to minimize copper losses ?

 

Offline IanB

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Re: Losses in transformers
« Reply #7 on: March 23, 2012, 12:25:44 am »
Yes, it has to have thicker wires, but the thicker wires take up more space, so the core windows have to be bigger to make room for the fatter wires, and now the core itself has to be bigger to preserve the required flux density, and everything cascades like dominoes. In a really oversimplified view of things, more efficient = bigger.
I'm not an EE--what am I doing here?
 

Offline T4P

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Re: Losses in transformers
« Reply #8 on: March 23, 2012, 06:21:03 am »
Yes, it has to have thicker wires, but the thicker wires take up more space, so the core windows have to be bigger to make room for the fatter wires, and now the core itself has to be bigger to preserve the required flux density, and everything cascades like dominoes. In a really oversimplified view of things, more efficient = bigger.
Everything goes bigger in the pursuit of efficiency , in return , lots of power .
 


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