Why 2 or 3 elements will conduct in my ac/ac converter?

[a_tek7]

ac/ac converter

Can any body tell me when and why 2 or 3 elemets will conduct together in these 2 ac/ac converter?

 

[a_tek7]

Re: ac/ac converter

and this circuit

 

[Siniša]

Re: ac/ac converter

To understand that, you should draw voltage/time graph of 3 phase system. You would see that at any time you have two phases with one polarity and one phase with oposite polarity and that is when 3 elements conduct current. Exceptions are moments when any phase passes through 0(every 60º), that is when only two elements are conducting.