PURPOSE: To get equivalently required inertial moment or required flow attenuation properties by having a fluid drive, a main power source, an auxiliary power source, and a controller, which detects the power stoppage of the main power source and controls the static power converter of the auxiliary power source so that it may be the required flow attenuation properties.

CONSTITUTION: A motor 3 being the power plant of a pump 2 is connected with a main power source 6 and an auxiliary power source device 9 by current paths 13. In case that the power source 6 stops, for the pump 2 and the motor 3, the revolutions drop gradually by the inertial moments that they have and the inertia of fluid, and the flow also begins to attenuate. At the same time, by detecting the power stoppage of the power source 6 and opening a switch 8, the power source 6 is cut off the motor 3 and the auxiliary power source device 9, and power is supplied from a battery 10 and a static power 11 to the motor 3 while being controlled by the controller 12, whereby the motor 3 can be driven getting required flow attenuation properties.