An ideal transformer is one which has no ohmic resistance and no magnetic leakage flux i.e. all the flux produced in the core links with primary as well as secondary winding. In other words, we are postulating the following:
Bahaviour and Phasor diagram
The primary winding is connected to an alternating voltage source V1, while the secondary winding is left open. A current Im flows through the primary winding. Since the primary coil is pure inductive, the current Im lags behind the applied voltage V1 by 900. This current sets up alternating flux in the core and magnetises it. Hence it is called magnetising current. Flux is in phase with Im. The alternating flux links with both primary and secondary windings. When it links with primary, it produces self induced e.m.f. E1 in opposite direction to that of applied voltage V1 according to Lenz’s law. When this flux Φ links with secondary winding, it produces mutually induced e.m.f. E2 in opposite direction to that of applied voltage.
In an ideal transformer there is no power loss, so that, output must be equal to input.
E2I2cos Φ = E1I1cos Φ
E2I2 = E1I1
The induced emf in primary and secondary winding is directly proportional to number of primary and secondary winding turns.
E1 α N1
E2 α N2
The ratio of primary to secondary induced emfs is equal to the ratio of primary to secondary turns is called transformation ratio. It is denoted by letter K.