Q. Working of self - excited generators?

For self-excited generators, residual magnetism must be present in the ferromagnetic circuit of the machine in order to start the self-excitation process. For a dc generator, the relationship between steady-state generated emf E_a and the terminal voltage V_t is given by

where I_a is the armature current output, and R_a is the armature circuit resistance. For a dc motor, the relationship is given by

V_t = E_a + I_a R_a

where I_a is now the armature current input. Under steady-state conditions, volt-ampere characteristic curves are of interest for dc generators, and speed-torque characteristics are of interest for dc motors. Depending on the method of excitation of the field windings, a wide variety of operating characteristics can be obtained. These possibilities make the dc machine both versatile and adaptable for control.