Hydraulic drive system of construction machine

A hydraulic drive system of a construction machine, comprising:

• a turning motor;
• a variable displacement bi-directional pump connected to the turning motor by a pair of supply/discharge lines, such that a closed loop is formed;
• a supply pump coupled to the bi-directional pump, the supply pump supplying hydraulic oil to an actuator different from the turning motor;
• a bridging passage connecting the pair of supply/discharge lines to each other and provided with a pair of relief valves that are directed opposite to each other;
• a regulator that changes a tilting angle of the bi-directional pump;
• a turning operation valve that receives a turning operation and outputs a turning signal whose magnitude corresponds to an amount of the turning operation;
• and
• a controller that controls the regulator based on the turning signal outputted from the turning operation valve, wherein
• at a time of turning deceleration, at which the turning signal decreases, the controller calculates a motor flow rate passing through the turning motor and an instruction flow rate determined based on the turning signal,
• if the instruction flow rate is less than a reference flow rate obtained by subtracting a predetermined value from the motor flow rate, the controller controls the regulator such that the tilting angle of the bi-directional pump is adjusted to a tilting angle that realizes the reference flow rate, and
• if the instruction flow rate is not less than the reference flow rate, the controller controls the regulator such that the tilting angle of the bi-directional pump is adjusted to a tilting angle that realizes the instruction flow rate.