Thyristor voltage regulator
A thyristor voltage regulator was fitted so as to upgrade the ± 5% voltage tolerance previously achieved to ± 0.5%. The regulator in question is a pure voltage regulator, which uses the excitation current to keep the generator voltage constant irrespective of the load.

The generator voltage is fed via the measuring circuit transformer to the regulator and rectified by a load-side rectifier bridge. The actual value signal and regulator input voltage are derived from this DC voltage. The voltage setpoint is input via an internal reference value potentiometer or external reference value setter. After an internal setpoint/ actual value comparison, the regulator amplifier outputs a DC voltage, which is converted by a loadside pulse generator into a firing pulse with adjustable time offset for the bypass thyristor.

The bypass thyristor Vabs is situated in the excitation circuit between one phase and the anode-side output of the rectifier bridge in series with the bypass resistor Rabs (Fig. 4). The bypass circuit, parallel to excitation winding RE, is switched on in the time ranges at which valve V5 is conductive. Part of the excitation current can therefore be made to bypass the excitation winding. This takes place within every period for a fraction of that period (control angle). The excitation current is reduced in accordance with the control angle and the resistance ratio Rabs/ RE.