Abstract: The original 12-phase rectifier control system for the motor in φ76mm mandrel mill, consisting of two 3-phase fully-controlled tandem bridges with an alternative angle 30°, is analysed and the causes which lead to abnormal operation of the system are pointed oat. Based on the analysis and experiments, the original design has been innovated by two sets of tandem bridges in which the pulse with lead angle 30° can be compensated by the pulse with delay angle 30°. The innovaled system operates stably and reliably and can be used to regulate speed in a wide range.