POWER ASSIST DEVICE

[Problem] When load power is assisted, the number of component parts becomes greater because an AC power detector is provided in the AC power supply of an inverter. [Means for Solution] An instantaneous output power calculating section comprises a power calculating section to calculate power by using detected values of voltage and current on the output side of an inverter, and a correction calculating section to correct the calculated power. Moreover, limiter sections are provided, respectively, in charge control section and discharge control section for a chopper, and limiter values of the limiter sections are varied by the power calculated by the instantaneous output power calculating section. The charge control section and discharge control section for the chopper are configured so that a detected DC voltage of the inverter is inputted, deviation signals are obtained between the detected DC voltage and preset upper and lower limit voltages, the deviation signals are inputted to voltage control sections, and a value of charge command is calculated.