The various advantages of rotating field can be stated as,
1) As everywhere a.c. is used, the generation level of a.c. voltage may
be higher as 11 KV to 33 KV. This gets induced in the armature. For
stationary armature large space can be provided to accommodate large
number of conductors and the insulations.
2) It is always better to protect high voltage winding from the
centrifugal forces caused due to the rotation. So high voltage armature
is generally kept stationary. This avoids the interaction of mechanical
and electrical stresses.
3) It is easier to collect larger currents at very high voltage from a
stationary member than from the slip ring and brush assembly. The
voltage required to be supplied to the field is very low (110 V to 220 V
d.c.) and hence can be easily supplied with the help of slip ring and
brush assembly by keeping it rotating.
4) The problem of sparking at the slip rings can be avoided by keeping
field rotating which is low voltage circuit and high voltage armature as
stationary.
5) Due to low voltage level on the field side, the insulation required
is less and hence field system has very low inertia. It is always better
to rotate low inertia system than high inertia, as efforts required to
rotate low inertia system are always less.
6) Rotating field makes the overall construction very simple. With
simple, robust mechanical construction and low inertia of rotor, it can
be driven at high speeds. So greater output can obtained from an
alternator of given size.
7) If field is rotating, to excite it be external d.c. supply two slip
rings are enough. Once each for positive and negative terminals. As
against this, in three phase rotating armature the minimum number of
slip rings required are three and can not be easily insulated due to
high voltage levels.
8) The ventilation arrangement for high voltage side can be improved if it is kept stationary.
Due to all these reasons the most of the alternators in practice
use rotating field type of arrangement. For small voltage rating
alternators rotating armature arrangement may be used.