I was asked this question the other day…
What is the difference between a hydraulic machine with a variable-drive pump motor vs. a hydraulic machine with a servo-motor-drive pump motor?
A variable-drive pump motor adjusts the motor speed to compensate for the demands of the process. These systems typically consume only 70% of the energy consumed by a traditional fixed volume pump. In this system the pump still provides more hydraulic flow than is necessary to mold the actual part and the excess flow which is diverted back to the hydraulic tank. This excess fluid results in an energy loss.
A servo-drive pump motor uses a precisely-controlled servo-motor to drive the hydraulic pump. Rather than maintaining a line pressure, diverting excessive flow, and adjusting servo valves, the flow from the pump is directly sent to the rotary or linear actuator performing the machine function. Since the pump is precisely controlled, the speeds of actions such as injection and screw rotation are controlled directly by the servo-motor. Pressure limits are easily controlled by limiting the torque of the motor. Since the no un-necessary flow is generated, the efficiency of such machines are about as high as a hydraulic machine can be. In many cases, these machines consume only 30% of the energy consumed by fixed drive pump motors.