CONTROLLER FOR THYRISTOR CONVERTER

PURPOSE:To prevent the commutation failure of a converter with the end limited value as the necessary minimum limit value by limiting a phase control signal by the sum of a superposed command value and a margin angle command value and then converting it to a control value by reverse cosine calculation. CONSTITUTION:A thyristor converter 1 for driving a DC motor 3 is controlled by a phase control signal Ralpha in response to a deviation between a current command and a current detection signal through a phase angle limiter 8, a reverse cosine calculator 9 and a pulse generator 10. At this time, a limiter 8 is composed so that the output of a superposing angle calculator 11 and a commutation margin angle set signal Rgamma are added to output an end limited value R and applied to a proportional calculator 13 to produce a phase angle command Ralpha'. Accordingly, since the end limit is limited before converting to the control angle by the calculator 9, and the commutation failure can be prevented by reducing the end limited limit to the minimum value irrespective of the nonlinear property of the load current and the superposing commutation angle.