Why 3 phases motors been equipped to butt welding machines with size above 355mm

At the same power level, comparing to single phase motors, 3 phases motors provide more driving force, which is more obvious on the pipe planing tool (also called facer or trimmer).

During planing (facing or trimming) operation, certain pressure is applied to the planing tool plate, through 2 pipe ends. It's critical the output force is enough to drive the planing plate rotating. The more of pipe sizes, the more of the pipe wall thickness, the more contacting surface, the more driving force needed.

To achieve sufficient output force, 3 phases motors are more commenly used in bigger size butt welding machines, usually size starts from 355mm or 400mm.