Transformer equations under load

[Glenn G]

My question is about the theory when a current is drawn from secondary coils ...

So ac in the primary produces a changing field that links itself producing a back emf in primary (in ideal transformer back emf equals Vp from the supply but in a real transformer the is a small net current in primary)

The changing field also links secondary producing induced voltage and as current flows there is also a back emf in secondary, can think of mag field generated by secondary feeds back to primary

??Does this field fed back from secondary cancel out part of the back emf in primary such that in primary therefore there is a bigger difference between Vp and emf so larger current in primary?

 

[jim hardy]

??Does this field fed back from secondary cancel out part of the back emf in primary such that in primary therefore there is a bigger difference between Vp and emf so larger current in primary?

I think you already know the answer, just you don't trust your reasoning...

We use words to paint a word picture of an idea in our mind.
Those words are received by a listener and he paints in his mind a picture of an idea.

Are the two ideas the same ? There are four translations in between the two ideas.

That's why it is always better to resort to math and pictures. This is after all a physics forum.

Counter emf is proportional to flux, e = -ndΦ/dt
and at constant applied voltage flux level is constant
that relation holds for both windings
flux Φ is proportional to total of all magnetomotive forces present, ∑mmf (which is amp-turns), divided by a constant for the core - its reluctance..

so Φ = $\frac {∑amp-turns } {Reluctance}$so assuming the picture in my mind of your question is same as the picture in your mind
yes, secondary amp turns cancel primary amp turns
and more primary amps flow
to restore flux
hence counter-emf
back to equlibrium
where counter-emf = applied voltage.

see http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/transf.html

Always test your intuition against your math. When they agree you can begin to trust your intuition.
Then it's safe to build on the picture that's in your mind.

old jim

 

[anorlunda]

We have many previous threads on this topic.

https://www.physicsforums.com/threads/question-about-magnetic-circuits-in-a-transformer-core.955253/
https://www.physicsforums.com/posts/5889911/
https://www.physicsforums.com/threa...-in-a-transformer-conceptual-question.664589/
https://www.physicsforums.com/threa...tion-in-current-emf-and-magnetic-flux.592713/

 

[jim hardy]

Always test your intuition against your math. When they agree you can begin to trust your intuition.
Then it's safe to build on the picture that's in your mind.

"""Algebra, which is adapted to its purpose in every species of expression, in the most simple, most exact, and best manner possible, is at the same time a language and an analytical method...""