Effect of Change in Field Excitation

Author: E.E. Kimberly

If alternators (a) and (b) in Fig. 20-8 are supplying a load jointly and the field excitation of machine (a) is reduced, the division of power load will not be affected. However, a circulating current will be caused to flow around the load circuit formed by the two alternators. This circulating current will be leading with respect to the voltage of alternator (a) and will thus supply the excitation deficiency by armature reaction. Furthermore, the circulating current will be lagging with respect to the voltage of alternator (b) and so will detract from its excitation. The line voltage will decrease somewhat because of the increased impedance drops in the two alternators, but the division of power load will remain constant.