I have just had a major mental failure regarding transformers! Seems I don't know as much as I thought (hoped) I did!
If we take a lossless power transformer, for simplicity:
You apply mains voltage, and a magnetising current is set up in the primary, which lags the voltage. So you now have a certain amount of magnetic flux in the core. The secondary voltage is in phase with the primary voltage.
When you attach a resistive load to the secondary then the secondary load current is in phase with the secondary voltage. This causes additional current to flow in the primary, which must also be in phase with the primary voltage.
However, the secondary current does not cause additional flux because the secondary magnetomotive force (MMF) is cancelled by an opposing MMF in the primary, so the net flux remains unchanged by loading.
So how the heck does a core saturate if you apply too much load, where does the extra flux come from?!
If we take a lossless power transformer, for simplicity:
You apply mains voltage, and a magnetising current is set up in the primary, which lags the voltage. So you now have a certain amount of magnetic flux in the core. The secondary voltage is in phase with the primary voltage.
When you attach a resistive load to the secondary then the secondary load current is in phase with the secondary voltage. This causes additional current to flow in the primary, which must also be in phase with the primary voltage.
However, the secondary current does not cause additional flux because the secondary magnetomotive force (MMF) is cancelled by an opposing MMF in the primary, so the net flux remains unchanged by loading.
So how the heck does a core saturate if you apply too much load, where does the extra flux come from?!