A six-diode bridge rectifier conducts only at the voltage peaks of each phase, so its input current is non-linear and distorts the supply with harmonics. On a large common-DC-bus paper-machine line fed by one big bridge, that can upset other equipment and breach supply limits.
For the wide paper machines we drive, the front end is a 12-pulse rectifier feeding the common DC bus. The multi-pulse transformer cancels the dominant low-order harmonics at the source, so the line stays clean without the cost and complexity of an active front end — while the same bus still supports regenerative energy return.
The choice is part of the engineering: for a winder the regenerative common-DC-bus unit is sized to the braking load; for a wide machine the 12-pulse front end is sized to keep the supply clean. Both keep the DC bus at a safe voltage and the machine running.