James,
The inductive reactance of a simple choke is caused by the magnetic flux setting up a counter-e.m.f. in the windings. A transformer with its secondary open circuit behaves in a similar way to a basic choke, but as you place a load on the secondary some of the flux which was previously creating that counter-e.m.f. is now being used to induce current in the secondary circuit. So as you increase the load, the amount of counter-e.m.f. in the primary is less, and thus the reactance is reduced.
The overall impedance reflected by a transformer is the ratio of primary-to-secondary turns squared, i.e. :-
Zp / Zs = (Np / Ns) ^ 2
where Np and Ns are the number of primary and secondary turns respectively.
Now if the primary power is from a "normal" constant-voltage source, then clearly the increase in XL and Z from removing the secondary load causes a corresponding decrease in the primary current.
But where the primary is being fed from a constant-current source, the higher reactance and impedance results in the current source delivering a higher voltage on the primary to maintain the same current flow.
So in short, what the lecturer told you is correct.
[This message has been edited by pauluk (edited 03-04-2004).]
