An ideal transformer is an imaginary transformer that has no losses, i.e. no core losses, no copper losses and no other losses in the transformer. The efficiency of this transformer is considered to be 100%.
Ideal Transformer Model:
The ideal transformer model is developed considering that the windings of the transformer are fully inductive and the core of the transformer is loss-free. Also the transformer has zero leakage reactance (reactance is the opposition to the flow of current through the circuit element due to its inductance and capacitance).
This means, 100% of the flux passes through the core and connects to both the primary and secondary windings of the transformer. Although each winding must have some inherent resistance which causes voltage drop across it and loss of I2R. In such an ideal transformer model, the winding are assumed to be ideal (fully inductive), meaning that the winding resistance is zero.
Now if an alternating source voltage V1 is applied to the primary winding of this ideal transformer, a counter self emf E1 will be induced in the primary winding which is purely 180o in opposite phase with the supply voltage V1.
To develop a counter emf E1 on the primary winding, it draws current from the source to produce the required magnetic flux. Since the primary winding is purely inductive, the current lags the supply voltage by 90o. This current is called the magnetizing current of the transformer Iμ.
This alternating magnetic current Iμ produces an alternating magnetic flux Φ. The flux is proportional to the current that produces it so the flux will be in phase with the current. This flux also connects the secondary winding to the core of the transformer. Consequently, another emf E2 will be induced in the secondary winding, and this is the mutually induced emf as shown in the figure below.