When an alternating voltage is applied to one of transformer windings, generally by definition the primary, a current will flow which sets up an alternating m.m.f. and hence an alternating flux in the core. This alternating flux in linking both windings induces an e.m.f. in each of them. In the primary winding this is the 'back-e.m.f' and, if the transformer were perfect, it would oppose the primary applied voltage to the extent that no current would flow. In reality, the current which flows is the transformer magnetizing current. In the secondary winding the induced e.m.f. is the secondary open-circuit voltage. If a load is connected to the secondary winding which permits the flow of secondary current, then this current creates a demagnetizing m.m.f. thus destroying the balance between primary applied voltage and back-e.m.f. To restore the balance an increased primary current must be drawn from the supply to provide an exactly equivalent m.m.f. so that equilibrium is once more established when this additional primary current creates ampere-turns balance with those of the secondary.
Since there is no difference between the voltage induced in a single turn whether it is part of either the primary or the secondary winding, then the total voltage induced in each of the windings by the common flux must be proportional to the number of turns.
We should note that the exciting current may not be restricted to the primary winding when load current flows but may be shared by both windings and other closed electrical circuit (even transformer metallic case) which influence in linkage magnetic fluxes. Because the nature of circulating magnetizing currents and related magnetic flux are unique in physical view and we can't consider existing of each without another, similar a body and its shade. Even in single coil we can suppose a lot of magnetizing current circuit in parallel which total current flow in it and all of them influence in produced magnetizing flux. Indeed This fact can lead to legitimate circuit models with shunt branches on either side of transformers related to importance of its technical applications. These are all mathematically equivalent since impedances can be transferred across the ideal transformer present in the circuit. We usually demonstrated this for series impedance for importance of it in transformer behavior study.
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