The utility model relates to a medium-voltage variable frequency speed regulator applicable to single-quadrant operating load of blower, water pump, etc. and four-quadrant operating load of elevator, mill, paper machine, traction of vehicles, etc. The device is composed of a variable-frequency power units, a phase-shifting transformer and a controller. The variable-frequency speed regulator is an overlay of a plurality of low-voltage variable-frequency power units connected in series. A phase is formed from a plurality of H bridge variable-frequency power units connected in series, and three phases in Y connection are connected with a motor. The rectifying side of the variable-frequency power unit is a diode un-controllable bridge or a controllable full bridge composed of controllable-power semiconductor component. The output side is composed of each unit in mutual tandem connection. The auxiliary-edge windings of the phase-shifting transformer is divided into multiple groups. There is a phase difference between each secondary coil, and according to the voltage level and the number of stages of the variable-frequency power units, the mode of rectification of multistage phase-shifting overlay is formed from scores to hundreds of pulse series. The utility model is characterized in: wide regulating range, perfect dynamic property, efficient energy saving, high power factor, high reliability, etc.