The paper describes investigations of an electric-hydrostatic drive system, its components and its application in the clamp of a plastics injection moulding machine. The aim of the research work was to reveal the potential advantages and challenges of the new direct pump control concept as well as the suitability of hydraulic pumps for variable-speed operation. Measurements of component efficiency have proven that hydraulic components have very good efficiency compared with variable-speed electric motors. Therefore, the total efficiency of the electric-hydrostatic drive at a high load (close to corner power) cannot be expected to be noticeably better than the conventional drive system comprising an asynchronous electric motor and variable-displacement hydraulic pump, although fixed-displacement pumps can have better efficiency than variable-displacement pumps have. Improvements may be seen at part-load. The major advantages of the new motor-pump concept are reduction in power losses and noise during part-load and during idling. This was demonstrated in a test cycle. The design of hydraulic pumps applied in an electric-hydrostatic drive has to be carefully evaluated for suitability. Some piston pumps and internal gear pumps without gap compensation seem to work well under the specific load conditions of variable-speed drives, but not all units worked sufficiently during endurance testing. Owing to the high overload capability of rare earth electric motors, the dynamic performance of the electric-hydrostatic drive can be compared with that of many standard variable-displacement pumps. This was evaluated with performance tests of the clamp unit of a plastics injection moulding machine. The dry cycle time (Euromap 6 standard) achieved (1.8 s for a machine with a 1600 kN clamping force) proved the excellent controllability of the electric-hydrostatic drive and the high dynamic performance capability. The combination of a speed-controlled a.c. servomotor and fixed-displacement pump increases the number of direct pump control concepts that a hydraulic project engineer can apply to design competitive hydraulic systems for customer applications.
