in a thyristor chopper circuit used for the control of the rotational speed of an electric motor and provided with a protective system for preventing problems caused by a commutation failure in the chopper circuit , the protective system is provided with means for detecting the occurrence of commutation failure and means for certifying the operative condition or validity of the commutation failure detecting means . the operative condition certifying means is provided with means for supplying the commutation failure detecting means with a checking signal corresponding to a signal which is applied to the commutation failure detecting means upon the occurrence of the commutation failure so that whether the protective system is in a normally operative or valid condition can be certified at any time desired .