Why Rotating Field is used over Rotating Armature in AC Generators

Why Rotating Field is used over Rotating Armature in AC Generators?

In AC generators used for industrial purpose, field windings are taken as rotors and armature windings are taken as stators, why is it so?

In small rating generators general convention is followed that is field windings are used as stator and armature is used as rotors similar to that in DC generators. But in industrial generators, rotating field is preferred over rotating armature because of high current generated in high rating industrial generators. There are many more explanations present to justify the above mentioned reason. Some of them are briefed below:

Problems in collecting current:
  1. Feasibility in Handling: Since high rating generators produce large amount of current corresponding to voltages ranging from 11kV to 33 kV, so large number of conductors are required to obtain the current and insulations are required to handle such a large current. This is possible only if the part dealing with high current is kept stationary.
  2. Ease to collect current from stationary parts: It is comparatively easier to collect current from stationary parts of the generator rather than rotating parts formed with the help of slip rings and brushes.
  3. Ease in Excitation of Field circuit: When rotating field is used, just 110 to 220 V DC supply is used for the excitation of field circuit which can be easily done using the help of slip rings and brushes rather than using these things for rotating armature where the voltage generated ranges from 11kV to 33 kV.
  4. Sparking in Slip rings: If armature is used as rotor, the high currents produced may cause sparking in the slip rings and may damage the generator setup, so slip rings are preferred only for the low voltage circuits.

Rotation Problems:
  1. Speed to rotation: Since field circuit is smaller than armature circuit both in weight and size, so the inertia of field circuit is lower compared to its counterpart. So it facilitates in high speed rotation and produces output with much enhanced efficiency.
  2. Interaction of Forces: If armature is rotated, then centrifugal forces arise and since high currents are produced, so this setup is avoided so as to keep the generator free from the interaction of mechanical and electrical forces which may otherwise damage the machine.

Ventilation and Insulation
  1. Insulation: When armature is used as rotor for 3-Phase circuits, it becomes difficult to provide insulations to slip rings for 3-phase circuits.
  2. Ventilation: When the machine part dealing with high voltage is kept stationary, then there are higher chances of getting it exposed to environment for ventilation purpose which otherwise gets heated up due to high current flowing through it.

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