Application of Direct Current generators and their parallel operation

   Practically all d.c. generator installed on vessels for supplying auxiliary power are of the self-excited type .

   Shunt-wound generators are usually used as the exciters of separately excited generators and have the added feature of charging accumulative batteries . The latter is associated with the fact that with a reverse current they aren’t demagnetized in view of the unchangeable current direction in the field winding .

   Series-wound generators have the voltage which varies abruptly with the load and , therefore , this type of d.c. generators is not used on board ships.

   Compound-wound generators are not subject to frequent overloads and short-circuits , for the series field winding demagnetizes them . they are employed in welders as well as in some electric drives on the voltage-control system .

    Separately excited generators find application where a wide-range voltage regulation is required , thet is in the electric drives of steering gears , windlasses , winches etc. as well as in the electric propulsion plants as main generators and exciters .

    When putting d.c. generators in parallel operation , it is necessary that two condition be met : 1) their polarity should be the same as the one in the mains to which they are connected ;  2) their e.m.f. is required to be equal to the mains voltage . The device responsible for fulfilling the indicated conditions is a voltmeter of a magnetic electric system . In order to transmit the power from one generator operating in parallel to another , the excitation currents of the generator to which the power is transferred , should be increased but those of the generator from which it is collected , should be reduced . The generators are required to be loaded as uniformly and proportionally to their rated capacities as possible . If this condition is not fulfilled , the efficiency will be lower .