To boost a voltage from a DC power supply by using a voltage boosting converter, generate a system voltage on a power supply line, and enhance a voltage stabilizing effect on the power supply line of a system for driving an AC motor supplied with the system voltage through an inverter.

An input voltage of the AC motor 14 is manipulated, and a fluctuation in the system voltage is suppressed so as to reduce a deviation ΔVs between a target value Vs* and a detection value Vsf of the system voltage. If a rotational speed N2 of the AC motor 14 is higher than a predetermined value or a torque instruction value T2* is higher than a predetermined value, the quantity Pm of input power manipulation required for stabilizing the system voltage is surely achieved by delaying a current vector and manipulating input power. If the rotational speed N2 of the AC motor 14 is the predetermined value or less and the torque instruction value T2* is the predetermined value or less, a fluctuation in a torque is reduced by advancing the current vector and manipulating the input power.

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