Why is a variable frequency drive (VFD) always powered from a phase shift transformer?
Harmonics &amp; multi-pulse rectifiers.

Industry is charged in VA (not Watts) and equally have some very harsh requirements on the harmonics they create.

A simple 6pulse rectifier will produce harmonics at 5,7,11,13...

A relatively economical method to reduce this is to goto a multipulse rectifier arrangement which can be achieved with phase shifting transformers 

Star --> Star:Delta  and two 6pulse rectifiers will produce a 12pulse setup with harmonics at 11,13,23,25....