Patent ID: 7399165
Filing Date: 2008-07-15
Classification: F04B,F04C

Abstract:
1. A pump unit comprising: a first fixed-capacity pump having large capacity; a second fixed-capacity pump having small capacity that is smaller that of the first fixed-capacity pump; a first discharge line connected to the first fixed-capacity type pump; a second discharge line connected to the second fixed-capacity type pump; a variable-speed motor arranged for driving the first and second fixed-capacity type pumps; a switching valve arranged to selectively connect or disconnect the first discharge line and the second discharge line with each other; a pressure sensor arranged to detect a pressure of the second discharge line; and a control device arranged to receive a signal from the pressure sensor and a signal representing a rotational speed of the variable-speed motor, the control device being configured to control the switching valve and the variable-speed motor so that operation is selectively performed in a first mode in which the first discharge line and the second line are disconnected with each other to make the first fixed-capacity type pump unloaded such that a constant-horsepower operation is performed, and a second mode in which the first discharge line and the second discharge line are connected with each other such that a constant-horsepower operation is performed, wherein the switching of the switching valve from connecting state to disconnecting state is effected based on whether one of the rotational speed of the variable-speed motor and the pressure detected by the pressure sensor has reached a first predetermined set value, while the switching of the switching valve from disconnecting state to connecting state is effected based on whether the other of the rotational speed of the variable-speed motor and the pressure detected by the pressure sensor has reached a second predetermined set value.