Rotating field vs stationary armature advantages

In large alternators, the armature winding is on the stator and the field system rotates, instead of the opposite arrangement. The advantages are:

1. Easy collection of output: the high-power armature output is taken directly from stationary terminals, with no slip rings and brushes in the main power circuit. Only the DC field supply, which is at low voltage (about 125 to 600 V) and is a small fraction of the machine rating, passes through two slip rings.
2. Easier insulation for high voltage: a stationary armature can be insulated well for 11 kV to 33 kV, because it is not subject to centrifugal forces. In a rotating armature, such voltages would cause sparking and insulation breakdown at the slip rings.
3. Better cooling: the large armature winding and core, where most of the heat is generated, are stationary and can be cooled by ducts with air or hydrogen, or by water in the stator.
4. Light rotor: the field winding is simpler and lighter, so the rotor has lower inertia and can be built stronger against centrifugal stress.
5. Smaller slip-ring losses and cost: the two small rings carry only the DC field current, so there is less wear, sparking and maintenance.
6. Better mechanical balance and simpler construction of the rotor, and a heavier, rigid frame for the stator.