Synchronous motors with controlled excitation for suddenly applied loads

IT is common practice to operate synchronous motors with full-rated field current supplied from a constant-voltage exciter. In a number of cases involving variable loads, automatic control equipment has been used to vary the excitation with the load. This control causes the field current to be reduced below the rated value during light-load periods when it is not needed and increases the excitation when the load is applied, in order to prevent the motor from pulling out of step. By this means both operating efficiency and system voltage regulation have been improved.