| 摘要 |
PROBLEM TO BE SOLVED: To reduce loss in energy for driving a pump by reducing the quantity of working oil to be fed as less as possible while the possible lack of working oil is being avoided when the quantity of working oil in use is changed. SOLUTION: One pump out of a plurality of pumps feeding working oil, is switched over to a pump capable of being changeable in discharge by means of inverter control so as to be controlled in such a way that the relief flow rate R for excessive working oil will be a target value R* set in advance. As for remaining pumps P2, P3..., it is considered that they can discharge a definite quantity of working oil, and when it is found to be impossible that the relief flow rate R overcomes the target value R* under a discharge flow rate control only by the variable discharge pump P1, namely, if the relief flow rate R reaches the lower limit value R min, one more pump is to be turned ON, and if the relief flow rate R reaches the upper limit value R max, one pump is to be turned OFF.
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