Question

Draw the complete phase diagram of the transformer under no load condition.

The primary input current under no load condition has to supply

Iron losses in the core, i.e. hysteresis loss and eddy current loss

Very small amount of Cu loss in primary [there being no Cu loss in secondary as it is open]

Hence the no load input current Io is not at 90degree behind V1 but lags it by an angle Φo < 90degree.

Primary input current Io has 2 component

One in phase with V1. this is known as active or working or iron loss component Iw because it mainly supplies the iron loss plus small quantity of primary Cu loss.

The other component is in quadrature with V1 and is known as magnetising component because its function is to sustain the alternating flux in the core. It is wattles.