Electrical Machines
In DC generators brushes are used for

 collecting of current without any sparkings
 reduce eddy current loss
 convert ac armature current in to dc
 collecting of voltage

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Electrical Machines
Two 3-phase AC generators are such that one has twice the linear dimensions of the other. The field windings of each are excited to give identical sinusoidal air-gap flux density waveform. Both have the same number of stator slots and identical winding patterns. The conductor/slot in a big generator is K times that of the smaller one. The value of K to get equal no load voltage at same frequency is

 4.
 1/4.
  1/2.
 8.

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