Firing angle control on thyristor

does anyone know if there is a way to make the thyristor’s angle change during the simulation ? I need the angle of the thyristor to change from 30 degrees to 60 degrees after 3 wave cycles and after 3 waves chenge again to 90 degrees in the same simulation. but I don’t know how to do this. I tried to use the alpha controller but i dont think it work. if anyone know how to do this plz share it with me. Sorry for my english its not my first language.