Hybrid system for construction machine

A hybrid system for a construction machine, comprising:

• an engine;
• a main hydraulic pump which is driven by the engine;
• a plurality of actuators;
• a control valve which is connected to the main hydraulic pump via a first hydraulic fluid supply line for controlling the flow of hydraulic fluid supplied to the actuators;
• a rotary electric machine which has functions of both an electric motor and a generator;
• a rotary hydraulic device which is connected to the rotary electric machine, driven by the rotary electric machine and functions as a hydraulic pump when the rotary electric machine functions as an electric motor, and functions as a hydraulic motor and drives the rotary electric machine when the rotary electric machine functions as a generator;
• a second hydraulic fluid supply line which connects the rotary hydraulic device to the first hydraulic fluid supply line;
• an electricity storage device;
• and
• a bidirectional converter which is connected between the electricity storage device and the rotary electric machine for controlling the transfer of electric power between the electricity storage device and the rotary electric machine, wherein:
• the hybrid system comprises:
• a flow control valve device which is arranged in the first hydraulic fluid supply line and in the second hydraulic fluid supply line for controlling the ratio between the flow rate of the hydraulic fluid supplied to the control valve and the flow rate of the hydraulic fluid supplied to the rotary hydraulic device when the hydraulic fluid discharged by the main hydraulic pump is supplied to the rotary hydraulic device via the second hydraulic fluid supply line;
• a third hydraulic fluid supply line which connects the rotary hydraulic device to the first hydraulic fluid supply line downstream of the flow control valve device;
• a first switching valve which is provided in the third hydraulic fluid supply line to be capable of opening and closing;
• a drive torque detecting device which detects drive torque of the main hydraulic pump;
• and
• a control device which judges whether actuator drive torque for driving the actuators is higher than preset switching torque or not based on the drive torque of the main hydraulic pump detected by the drive torque detecting device, and
• when the actuator drive torque is equal to or lower than the switching torque, the control device switches the first switching valve to its closed position and controls the bidirectional converter so that the rotary electric machine functions as a generator, while also controlling the flow control valve device so that the flow rate of the hydraulic fluid supplied from the main hydraulic pump to the rotary hydraulic device decreases with the increase in the actuator drive torque, and
• when the actuator drive torque is higher than the switching torque, the control device switches the first switching valve to its open position, makes the rotary hydraulic device function as a hydraulic pump by controlling the bidirectional converter so that the rotary electric machine functions as an electric motor, and controls the flow control valve device so that the hydraulic fluid discharged by the main hydraulic pump is not supplied to the rotary hydraulic device.