A microprocessor-based firing circuit for thyristors working under a three-phase variable-frequency supply

A microprocessor-based firing scheme for controlling antiparallel-connected thyristors working under a three-phase variable frequency supply is presented. The firing angle is controlled by microcomputer software. The desired firing angle is given to the microprocessor and is then kept constant irrespective of supply frequency. This is achieved by measuring the frequency of one-phase (or line-to-line) voltage at every supply voltage cycle and accordingly adjusting the required time delay to get the desired firing angle for the thyristors of each phase. The required hardware is considerably reduced by using a zero-crossing detector for only the one-phase (or line-to-line) voltage of the three-phase supply. The firing instants of the thyristors connected to the other two phases are adjusted relative to the calculated instant of firing for the thyristors connected to the measured phase. The hardware and software used to implement the firing scheme are described