So I work in Power Distribution and we have many 12 pulse rectifier installed on our sites. I understand the concept of three-phase full wave rectifiers and that the output voltage in DC is (roughly) 1.35 x Vrms.

On 12 pulse rectifiers though, the ones we have, the secondary windings on the transformer are delta and star. Since this gives us 6 phases (2 lots of 3 phases out by 30 degrees), why is the output 2.7 x Vrms?

What I am getting at is, why are we now summing the rectified output of these two windings when the full-wave bridges on both windings are in series?

At first I thought it is because it's essentially like having two power sources in series but these are out of phase by 30 degrees. Not just that, but for the current to flow from the load and back to the neutral, it requires 60 degrees worth of diode conduction.

Thank you. I've probably made this completely unclear and I do apologise but I can't seem to explain it better for the time being. I'm sure as answers and questions roll in I'll be able to explain it better.

Thanks for all your help guys.

 

... the rectified output of these two windings when the full-wave bridges on both windings are in series?

I think this answers your own question. They're independent voltages, so placing them in series adds them.

 

Thanks for your help