# How to determine the physical size of transformers

• posted

Hi All,

I've got a question regarding the physical size of transformers.

I have a transformer configured as an oscillator that is driving a load that is mostly capacitive but has some resistance. The max output voltage is 100V AC, and the max current is 0.7A AC, at 300Hz. So the VA value is 70 VA. The real power delivered is 25W.

I need this transformer to be as small as possible, so I'm wondering what the main factor is in determining its physical size. Is it resistive losses in the windings heating up the transformer which it needs to dissipate to remain cool enough to run, or is it core size needed to store the energy per cycle, or core losses heating it up?

I am actually thermally bolting the whole thing to a very large heatsink which is fan cooled, so this would increase the heat the tranformer could generate while still maintaining a working temperature. In that case, is it simply down to the core size needed to store the energy transferred per cycle which will determine its size?

Thanks!

Mark.

• posted

Core size and core losses. Laminate manufactures will specify losses in Watts per Pound (W/lb) If you can get away with higher magnetizing current and higher core losses then the Core area will be less. But then your winding losses will increase too, if you increase turns to compensate for less core ;( Transformer design is part art, and empirical design.

Cheers

• posted

There is value in adding an inductor across the load (or in series, as the case may be) to cancel out the substantial reactive power. The closer to

1.0 your power factor is, the easier it is to transform.

Tim

```--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms```
• posted

:> I've got a question regarding the physical size of transformers. :>

:> I have a transformer configured as an oscillator that is driving a :> load that is mostly capacitive but has some resistance. The max output :> voltage is 100V AC, and the max current is 0.7A AC, at 300Hz. So the :> VA value is 70 VA. The real power delivered is 25W. :>

:> I need this transformer to be as small as possible, so I'm wondering :> what the main factor is in determining its physical size. Is it :> resistive losses in the windings heating up the transformer which it :> needs to dissipate to remain cool enough to run, or is it core size :> needed to store the energy per cycle, or core losses heating it up? :>

:> I am actually thermally bolting the whole thing to a very large :> heatsink which is fan cooled, so this would increase the heat the :> tranformer could generate while still maintaining a working :> temperature. In that case, is it simply down to the core size needed :> to store the energy transferred per cycle which will determine its :> size? :>

:> Thanks! :>

:> Mark. :>

:>

:>

: :Core size and core losses. Laminate manufactures will specify losses in :Watts per Pound (W/lb) :If you can get away with higher magnetizing current and higher core :losses then the Core area will be less. :But then your winding losses will increase too, if you increase turns to :compensate for less core ;( :Transformer design is part art, and empirical design. : :Cheers : :

My old design engineer (now deceased) was a reasonable hand at what we technicians called the "black art" of transformer design. He always said it usually took at least 3 iterations to develop the most ideal solution for any transformer. Using all the transformer design theory and tools will rarely produce what you want at the first attempt.

• posted

markp schrieb:

Hello,

the minimum necesary weight of the core is proportional to the power transferred and inverse proportional to the frequency. If the windings with a wire thick enough for low resistive loss does not fit into the core you will nead the next larger one. If you want the transformer to be as small as possible, try to make the power as low as possible and the frequency much higher than 300 Hz. Avoid high voltages requring a lot of windings and extra isolation.

Bye

• posted

That makes sense, so the core size is proportional to the amount of energy tranferred per cycle.

So if I were to try to estimate the size of a 400Hz toroid (just core size, ignoring windings for now) for a given VA rating, I could look at a 50Hz mains version and halve all the dimensions x, y and z to give 1/8 volume?

• posted

markp schrieb:

Hello,

or simply select a core with 1/8 of the mass. It is not necessary to halve all dimensions exactly, enough mass of the core is important.

Bye

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