what happens when T1 is longer than the timeconstant? does voltage drop or does it keep its established voltage? I think latter is true.
voltage drops. the induced voltage in the coil is in opposite direction the current pulse is coming from. after tau (voltage waveform like a sin interval [0;pi/2]) is reached it's going back to zero, because it's acting like a normal ohmic resistance.
not exactly sure but it does not really make sense that the voltage is even higher, because of the opposite direction of the induced voltage.
while T2 (low), voltage collapses, when it reaches '0v' (after length of timeconstant) the magnetic field collapses and creates a second pulse (also with the same tc).
is this comparable to the next 90 degree of a sine? or again to the first 90 degree?
It's like the third 90 degrees.
in the image notice how the voltage is instantly at a maximum (x=6), that's the voltage pulse.
(http://upload.wikimedia.org/wikipedia/de/1/1a/Auf-_und_Entladung_Spule.png)