Graziano Sag 12 electromagnetic clutch question

I'm in the middle of a VFD retrofit for my Sag 12. The VFD will be used to power the motor but the original transformer will still provide rectified 30 volts DC for the clutches. I'm not very conversant in anything electrical and as such have "backup" in that department. My electro-guy posed a question regarding the clutches: If the clutches are activated by a coil much like the old motor contactors (which uses 24 volts AC off a different tap of the same original xformer) then what is the necessity of providing DC voltage to those clutches? Why not 30 volts AC? The rectified DC wave form looks pretty poor anyway.

Would anyone have the answer to that?

Thanks, Dave