Question on transformers

I've heard it said that transformers are constant flux devices. If that is so, then at full load, when the transformer is transforming a lot of energy, does the slope of the b-h curve change? Is this picture correct for the same transformer at no load and full load?

no. they are different curves, which refer to different transformers.

So a transformer always operates at the same flux density, and with the same magnetizing force, whether loaded or not.

Would it be safe to say that additional input current beyond what it takes to magnetize the core is perfectly linked to the secondary winding current, and the output current of the transformer would then be limited by the amount of flux indicated on the b-h characteristic.

Looking for a confirmation if this statement makes sense...

If what you are saying in effect is that (excluding primary magnetising current)

NpIp=NsIs

where

Np=primary turns
Ip=primary current
Ns=secondary turns
Is=secondary current

then yes that is correct.

Don't forget - as you have suggested in another thread - that the primary and secondary load current mmf's will balance and so there is no influence on the core magnetisation. The excursions on the B-H characteristic are entirely determined by the magnetising mmf which is evident in the primary current draw under no load.

What limits the overall primary and secondary current ratings are the winding conductor sizes and the primary and secondary leakage reactances. The copper losses determine the heat dissipated (hence thermal rating) and in conjunction with the leakage reactances determine the overall voltage regulation under load.

Hope I haven't "muddied the waters" by responding to this thread as well.

When a transformer is sinusoidally excited. The core flux varies sinusoidally with time. Take a look at the links below:

http://en.wikipedia.org/wiki/Faraday's_law_of_induction


http://wiki.4hv.org/index.php/Faraday's_Law

"electromotive force around any closed loop is induced by changes in magnetic flux through the loop." If the latter was not a true statement, there would be no transformers around to make life a commodity as we have it today. A great invention...

I get it. I have no idea how I managed to misunderstand transformers all this time. If someone had told me "The excursions on the B-H characteristic are entirely determined by the magnetising mmf which is evident in the primary current draw under no load" 5 years ago I would have saved myself a lot of thought. Next step is to choose a core. Thank you tnk!