Triac Dimmer Supplying a Purely Inductive load
The key is that the current starts at zero and goes as the integral of the voltage WRT time over the interval starting at alpha, ending when this integral is again zero. 

Considering the integral as being the area under the curve, we can see that, ignoring diode drops and series resistances, for a sine wave conduction will be the area from alpha to 2Pi - alpha by symmetry.

For alpha in 0 -> Pi, current is zero outside the interval (alpha to 2Pi - alpha).
While the RMS voltage or current is easy to evaluate given this, it is not particularly interesting, far more fun to evaluate how the current varies moment to moment while the thyristor is conducting, which is of course the integral of the applied voltage during conduction. 

There will also at some angles be an unpleasantly high dV/dt which can be an issue for triacs in real systems.