Rotating field, stationary armature advantages

In large alternators the field system (d.c. excited poles) is placed on the rotor and rotates, while the armature winding is placed on the stator and stays stationary. The advantages are as follows.

1. Simple insulation of the high-voltage output. The armature carries the load current at high voltage (6.6 to 33 kV). Being stationary, it can be insulated heavily and braced firmly against mechanical forces. A rotating armature would need strong support for its insulation against centrifugal force.
2. No slip rings for the output. The load current is drawn directly from the stationary terminals. A rotating armature would need large slip rings and brushes carrying heavy current at high voltage, with sparking, wear and flashover. In the rotating-field design, only two slip rings are needed, and they carry only the small d.c. field current (a few hundred amperes at 100 to 500 V).
3. Lower rotor mass and better balance. The rotor carries only the light field winding, so it is easier to balance and can run at high speed.
4. Better cooling. The stator has a large surface in contact with the frame, so heat from the high-current armature is removed easily, with ducts for air or hydrogen. The large stator also leaves room for more copper and a bigger machine rating.
5. Easy field control. The low-power d.c. field is easy to feed, and the exciter can sit on the same shaft.
6. Lower cost and higher reliability. The construction is simple, and the rating can be raised much more easily than with a rotating armature.
7. Safety. The live high-voltage parts do not rotate, so the arrangement is safer for the operator.