# transformer core explanation required

#### crutschow

Joined Mar 14, 2008
31,110
so if i want to design a transformer that can withstand output short circuit, i should aim to have some leakage inductance
That, or sufficient winding resistance to keep it below the thermal limit of the transformer. depending upon how long it has to survive a short-circuit.

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#### sparky 1

Joined Nov 3, 2018
732
He put his finger on the point, very close.
concept: In an inductor having increase in ac current opposes change.
Analogy using your finger on the inlet of an inflated balloon. This idea has always helped me anyway.

The primary transformer winding has a field.
The secondary is shorted so it creates an equal and opposite field.
The opposing field in effect cancels the primary field.

What is that opposing field ?
Feynman describes back-emf in the subheading " The concept of self-inductance"

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#### crutschow

Joined Mar 14, 2008
31,110
The primary field is cancelled by the opposing field created by a shorted secondary.
What is the significance of that as pertains to the short circuit current of the transformer?

#### sparky 1

Joined Nov 3, 2018
732
I should have quoted the question so as not to confuse the replies thereafter and the methods of solving:

what point will primary keep on increasing current until the equilibrium is achieved. and secondly, what does the size of the core have to do here? secondary is demagnetising and primary is magnetising, so as per my understanding, core should never saturate because secondary doesn't let it magnetise at such levels.
The threadstarter was satisfied with a simple approach if later that is not sufficient there are other approaches where he can chip away at the problem of why the secondary does what it does, not that far off but a tall order this might begin with a strategy.
https://www.ieee.li/pdf/introduction_to_power_electronics/chapter_12.pdf

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