The transformer is said to be ideal if the ohmic resistance of its winding is equal to zero. At the same time coils are pure inductive wound on a loss free core. So that iron loss is also zero and the transformer has output equal to its input and efficiency of the transformer is 100%.
In practice such a transformer is not possible to realize but can be considered for easy understanding
From the above figure shows equivalent circuit of an ideal transformer where,
V1 = primary voltage
I1 = primary current current
E1 = primary (self-induced) e.m.f
V2 = secondary voltage (terminal voltage)
I2 = secondary current current (load current)
E2 = secondary e.m.f
From figure (ii), shows the phasor diagram of an ideal transformer on no load. Since flux (Φ) is common to both the windings, it has been taken as the reference phasor. The primary e.m.f E1 & secondary e.m.f E2 lag behind the flux (Φ) by 900 i.e, E1 & E2 are in phase. But E1 is equal to V1 and 1800 out of phase with each other.
Voltage transformation ratio of the transformer:
The ratio of secondary terminal voltage (V2) to the primary applied voltage (V1) is called voltage transformation ratio of the transformer. It is denoted by K.
Turn ratio of the transformer:
The ratio of primary no. of turns (N1) to the secondary no. of turns (N2) is called turn ratio of the transformer.
Current ratio of the transformer:
The ratio of secondary current or load current (I2) to the primary current (I1) is called the current ratio of the transformer.