Why almost all large size Synchronous machines are constructed with rotating field system type?

The following are the principal advantages of the rotating field system type
construction of Synchronous machines:
· The relatively small amount of power, about 2%, required
for field system via slip-rings and brushes.
· For the same air gap dimensions, which is normally decided
by the kVA rating, more space is available in the stator part
of the machine for providing more insulation to the system
of conductors, especially for machines rated for 11kV or
above.
· Insulation to stationary system of conductors is not
subjected to mechanical stresses due to centrifugal action.
· Stationary system of conductors can easily be braced to
prevent deformation.
· It is easy to provide cooling arrangement for a stationary
system of conductors.
· Firm stationary connection between external circuit and
system of conductors enable he machine to handle large
amount of volt-ampere as high as 500MVA.