Definitely a great question. It deserves a great answer - wish I was able to give one
I'll keep it short and sweet..
The transformer will "idle" with either no secondary load connected or the feeders to the secondary not connected.
At idle, the primary will draw only the current needed to magnetize the core, plus some current from losses in the windings, the core and external from the circuitry. You feel the power developed from these losses as heat at the transformer.
Small losses in the secondary coils will also be applied.
The reason that current in the primary is so low [almost zero] is that the magnetic flux in the core is stable [no flow], which results in an almost equal Inductive Reactance and Counter EMF imposed to the primary current, which results in nearly no flow of input current [or input KVA].
When a load is connected to the secondary, the stability of the core is changed, due to the change of XL in the secondary [the load allows current to flow out of the coil].
This results in a change in the core's flux [now is flowing], which ends up changing the XL on the primary windings.
Current increases until a steady state level is accomplished, then it remains at that level [until some value on either primary or secondary - or both -changes].
At startup, there is a high current draw on the primary - even if the secondary is not connected. It stabilizes over a period of time [usually by no more than 2 seconds].
This is similar to the inrush of an AC Induction motor, which by the way, is just a transformer with a rotating secondary winding!!
John-Tx has the idea!! Good job!
Hope this is understandable
I'll get to your other questions ASAP, so don't think I have ignored them.
Will submit that theory message before this weekend - or die
....
Scott SET