in association with a polyphase rectifying bridge having controlled rectifiers there is employed , in addition to the normal means for controlling the firing time of the controlled rectifiers , additional circuitry for preventing the occurrence of a commutation fault . this is achieved by first producing a timing signal representative of the time intervals during which a commutation between successively fired controlled rectifiers of the bridge actually exist . this timing signal , after being properly shaped is combined with a basic reference signal representative of a predetermined value to which the rectifiers may be phased back in their firing angle without resulting in a commutation failure . stated in another way , the width of the existing commutation period serves to set a minimum to which the next commutation period must conform . when the prescribed relationships exist between the timing signal and the basic reference signal , the normal control means is overriden and the firing angle of the controlled rectifier is advanced to a point which will insure commutation .