PURPOSE: To satisfy the scanning specification of the pump with respect to the coast down characteristics thereof by a method wherein clutches are provided for a flywheel driving shaft and an inertia transmitting shaft while both of the shafts are connected through a converting mechanism.

CONSTITUTION: Normally, when the pump 1 is being operated, the driving force of a motor 2 is transmitted to the flywheel 6, the clutch 10 is disconnected and the shaft 9a is rotated in a speed higher than that of the shaft 9b. In case an electric source interrupting accident occured, reduction of the revolving speed of the pump 1 is sooner than that of the flywheel 6 since the pump is working, therefore, the clutch 8 is disconnected, the pump 1 and the flywheel 6 rotate independently under reducing the revolving speeds thereof, a difference of the revolving numbers of the inertia transmitting shafts 9a, 9b is reduced, thereafter, when those of both shafts have become the same, the clutch 10 is connected, the pump 1 can regulate the increasement and decreasement of the revolving moment of inertia of the flywheel 6 and the reducing rate of the revolving speed thereof becomes slow.