Control system for hybrid construction machine

A control system for a hybrid construction machine, comprising:

• an operation valve for operating a boom, the operation valve being adapted to control a boom cylinder;
• a variable displacement type of fluid pressure motor for regeneration, the fluid pressure motor being rotated by means of a return fluid discharged from the boom cylinder at the time of lowering the boom;
• a distribution mechanism adapted to adjust a flow to be distributed to the fluid pressure motor among the return fluid;
• a motor generator adapted to be rotated integrally with the fluid pressure motor;
• a variable displacement type of assist pump adapted to be rotated integrally with the motor generator;
• a distribution mechanism control unit adapted to control the distribution mechanism to maintain a lowering speed of the boom regulated in accordance with a switching amount of the operation valve;
• a tilt angle control unit adapted to control tilt angles of the fluid pressure motor and the assist pump;
• and
• a motor generator control unit adapted to maintain a rotation speed of the motor generator at a target rotation speed,
• wherein the target rotation speed at the time of boom regeneration control in which the fluid pressure motor is rotated by the return fluid is set to higher than the target rotation speed in a case where only an assist control of actuating the assist pump is carried out.