PURPOSE: To send/receive status signals through small number of signal lines with no trouble, to enable standardization of hardware or software and to perform mechanical coupling without causing degradation of characteristic of data communication system by connecting a speed control system through the data communication system with a plurality of current control systems.

CONSTITUTION: A speed control system 1 broadcasts a torque command value τand operation commands for power converters 31-33 to current control systems 11-13. The current control systems 11-13 compares the polarity of the torque command value τ with the polarity of the operation commands to the power converters 31-33. When the polarities match each other, current control for the power converters 31-33 is continues so that the operation commands match with the torque command value τ. When the speed control system 1 updates the torque command value τ, polarity information corresponding to the operational condition of the power converters 31-33 is collected through polling & response system. Control of the power converters 31-33 resumes upon matching of the polarity. By such arrangement, number of signal line can be reduced.