Here as students, you need to deal with 2 conditions for the correct derivation of results.
- Case 1: Transformer minus load
- Case 2: Transformer on load
Case 1 detailed: In case you are given a situation when transformer has no load, the scenario would be an open secondary side with primary side having a connection with sinusoidal alternating voltage. There is an alternating voltage present that will cause a primary winding finally resulting in an alternating flux.
In this case, both iron and copper losses are minimal and therefore there is a shift in current vector set.
Case 2 detailed: If you are given a transformer that has load in it, there is a secondary circuit that is completed via a load. In this case, there is a certain net flux that is similar to a no-load situation core. Also this core flux does not change with any amount of load added.