EXCITER FOR SYNCHRONOUS MOTOR

PURPOSE:To prevent an overexcitation of a synchronous motor by providing two field current setters, setting set current values by considering the change of a current feedback resistance upon temperature rise of a closed switchboard at the time of starting and during operating, and switching a field current setter at the time of starting and during operating. CONSTITUTION:When the operation of a brushless synchronous motor SM is started, a power source of a closed switch board is heated to rise a temperature in the switchboard, the value of a current feedback resistance R is reduced, and a voltage V at both ends is reduced. When a predetermined time is elapsed after starting, a time limiting relay 41R counts up, its normal close contact 41Tb is opened, and its normal open contact 41Ta is closed. Accordingly, a current setter 70R1 is connected to a constant-excitation controller CEC through the contact 41Ta of the relay 41R during operating, and a command is so output to a thyristor rectifier THY that an exciting current becomes a predetermined value. Thus, the motor SM is prevented from being overexcited.