12 puls invertor in VFD

A 3 phase full wave bridge rectifier has 6 diodes, hence the name "6 pulse".

To reduce harmonics, one trick is to use two separate 6 pulse rectifiers fed from a phase shift Delta - Wye (Star) transformer. 6 + 6 = 12 pulses. The phase shift from the transformer moves more of the harmonics into the triplen and even order ranges where they cancel each other out. 18 pulse is even better.