Efforts to conserve energy have been made in a number of fields due to a shortage of energy resources, such as oil, and the need to protect the Earth’s environment. In the field of hydraulics, energy-saving pump units are very important because they consume less energy.

At present, variable displacement type hydraulic pump units (VD pump units) and inverter control type pump units (INV pump units) are widely used. The authors propose using an intermittently operated pump unit coupled with an accumulator (ACC pump unit). It is verified that the ACC pump unit is superior to the others in terms of efficiency, especially when the flow rate to the load is small or the duty ratio for the intermittent work is low.

However, the stop/restart operation of the electric motor for the pump drive is repeated frequently, and the instantaneous electric power consumed by the electric motor is more than six times the rated electricity when the pump is restarted. Therefore, we decreased the excess electricity consumption at pump restart using a method called “hydraulic assist”.

Before the electric motor is turned on, high-pressure fluid is supplied to the pump inlet from the accumulator. The pressure at the pump outlet is made to be zero by opening the valve to the tank. The pump essentially works as a hydraulic motor. Then, the electric motor is turned on. In this method, the instantaneous electric power consumption is reduced.

This method is also applied to a hydraulic pump unit driven by an internal combustion engine. The intermittent operation of the engine-driven hydraulic pump unit was accomplished using hydraulic assist to restart the pump. In this report, the energy-saving characteristics are shown.

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