A) from what i have learnt that the use of 50Hz or 60Hz as rated frequencies didn't came from nothing ,we have learnt that the resistive load doesn't effect by frequency this is not completely true but the effect will be minminzed for 50 and 60 Hz
b) In generators we would like to induce voltage from the rotor to the stator .this can be satisfied if the rotor has rotating magnatic field so we have two condition that are to be satisfied :
1) magnatic field
2)the rotation of this magnatic field
The magnatic field is supplied by a dc supply in the mean of brushed and comutators in small generators and (brushless excitors"internal supply " ) for large generators ..so this supply will produce current in the rotor that will produce a non-rotating magnatic field ....
so using the prime mover will make this magnetic field rotating and so we will have induced voltage on the stator winding