To increase the flux in air gap
To decrease the magnetizing current
To magnetize the poles to produce coil flux
To increase the reluctance of magnetic path
Soft iron
Cast iron
Cast steel
Stainless steel
Reduce the friction loss
Reduce the windage loss
Reduce the hysteresis loss
Reduce the eddy current loss
Shunt generator
Series generator
Differential compound generator
Cumulative compound generator
Minimizes rough commutation
Maintain constant output voltage
Neutralizes the demagnetizing effect
Decreases the excitation current of field coils