Hi everyone, I savaged an e shaped transformer from a telephone standby power supply unit which was labelled the following:
AC 220-240V 50/60Hz
8A max
330W max

but when I measured the physical dimensions of the transformer core I found it to be :

4cm x 3.2cm = 12.8cm2 of core area

My question now is, from my understanding the power that can be delivered by an transformer is given by this formula (please correct me if wrong):

core area = √(power to be delivered)

hence: power = (core area)^2
therefore for this transformer the power that can be delivered becomes
power = (12.8)^2 = 163W. but the label on the power supply says 330W, which then leaves me in confusion that am I missing something or the given rating are wrong?

 

Maybe the core material is with a higher "mu" (u)

 

Maybe the core material is with a higher "mu" (u)

 

forgive me Ramussons but what do mean by higher "mu" I don't understand

 

forgive me Ramussons but what do mean by higher "mu" I don't understand

mu or μ stands for magnetic permeability in this context.

You have a couple of problems. What do you mean by an E core transformer? Do you mean E-I core?
Show a picture of the transformer.

The formula:
power = core area squared

is grossly incorrect.
On the left hand side the units are watts.
On the right hand side the units are cm-squared.

You cannot equate these two. Always do the dimensional analysis.

 

mu or μ stands for magnetic permeability in this context.

ohh I see....

You have a couple of problems. What do you mean by an E core transformer? Do you mean E-I core?
Show a picture of the transformer.

yes its an E/I core transformer

The formula:
power = core area squared

is grossly incorrect.
On the left hand side the units are watts.
On the right hand side the units are cm-squared.

You cannot equate these two. Always do the dimensional analysis.

I did dimensional analysis and indeed the LHS and RHS don't tally at all but I followed thid tutorial. http://www.construyasuvideorockola.com/transformador.php they also have a video on the subject which made me believe that their equation may be practically applicable.

 

I followed the old thumb rule:

VA is proportional to (Area) ^2
and 1 sq inch = 30 VA

which, in this case works out to be about 120 VA.

 

There is a series of explanations here...
http://engineerexperiences.com/category/transformer
No doubt you can find a lot of other explanations too.

The power handling of the transformer is dependent on the core material, size and frequency.
For instance, a transformer designed to run in the US will not necessarily run here in Oz as the frequency is 60Hz there and 50Hz here.
I have seen a US transformer smoke even though the voltage tap was ok because it was designed for 60Hz and a bit too close to max power. On 50Hz it did not have enough turns so the current was too high and it saturated and failed. The turns per volt are important and frequency dependant.
Core material and frequency are why switch mode power supplies can use MUCH smaller transformers than 50 or 60Hz for the same power.
And you just need the area through the core, not the volume.
There are a few calcs around that simplify it and assume laminated iron core and 60Hz.

 

Hi,

I agree fully with the "post a picture of the core" suggestion. That will tell all. Also show the dimensions on that drawing where they apply and that will help wrap this up in a hurry