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In a Transformer - Alternating current flows in primary winding
Causing alternating magnetic field in Iron core
Which causes alternating voltage in secondary coil.
But I am confused that what would be the polarity of the Flux caused by the secondary winding voltage.
Won't the Flux caused by the secondary voltage cancel the Flux caused by the primary voltage and thus making the net Flux zero in the coil if both the primary and secondary windings have the same number of turns?
Two differing currents flow in the primary winding of a secondary loaded transformer. The first is the magnetization current and this is purely due to the primary being regarded as an inductor. That current is 90 degrees lagging the applied primary voltage. That current sets up the magnetic field and that current induces the secondary voltage.
The other primary current occurs when the secondary is loaded. This new additional primary current produces a magnetic flux that is exactly opposite to the magnetic flux caused by the loaded secondary current. In effect, the net flux addition due to loading the secondary is zero; fluxes due to loading are cancelled leaving only the original magnetic flux due to a voltage being applied across the primary.
The excitation flux is 90 degrees out of phase VS. primary voltage. When the current flows, both primary and secondary flux due the load current is cancelled as you noted, but the magnetization flux remains unchanged (90 deg. off).
We can say that the total flux in the transformer core is the magnetization flux and it is constant. The only factors that play are primary voltage and frequency. No matter of the load current the flux remains the same.
In a Transformer - Alternating current flows in primary winding
Yes, but how much flows? See later.
Causing alternating magnetic field in Iron core
Yes, but that's not the entire current-causing-flux story, see later
Which causes alternating voltage in secondary coil.
Yes
But I am confused that what would be the polarity of the Flux caused by the secondary winding voltage.
The secondary winding voltage does not cause a flux. The only thing the secondary winding voltage might do is push a secondary current through an external load. That secondary current will generate a flux.
Won't the Flux caused by the secondary voltage cancel the Flux caused by the primary voltage and thus making the net Flux zero in the coil if both the primary and secondary windings have the same number of turns?
One way to look at it is that the flux caused by the secondary winding current cancels some of the flux caused by the primary current, leaving a residual flux that magnetises the core, and whose changes generate the primary and secondary voltage.
Another way to look at it is that the net magnetising flux in the core has to generate the primary voltage. As the primary voltage is constant, the amplitude of the magnetising flux in the core has to stay constant.
When the secondary is open circuit, the primary current equals the magnetising current and creates the core flux. When the secondary gets loaded, a secondary current flows, which would decrease the core flux, were it not for the fact that the primary current increases to cancel out the secondary current, leaving a difference between the primary and secondary fluxes that is the magnetising flux.
In a good transformer, the magnetising current required might only be a few percent of the full load currents to generate the magnetising flux. In an ideal transformer, the magnetising flux is still finite, but the magnetising current is zero, because the ideal transformer has an infinite permeability core (or approaches zero as the permeability approaches infinity)
