The variable-speed hydraulic pump, with its potential for energy savings, power density and temporary noise reduction, is currently the dominant pressure supply system in the media. It provides a clear example of how hydraulics can successfully participate in the evolution of electric drives.
Promotions for the variable-speed pump like to suggest that standalone individual drives are the future for hydraulics. Electrohydraulic actuators have their uses, of course, but it is important to remember that electric motors, and their converters, are what make electromechanical drives so costly. Each drive needs a fixed motor, which makes the drive large and expensive. If this principle is applied to hydraulics, it restricts the choice of appropriate pumps to those variants that are suitable for four-quadrant operation. This limits options and increases costs as well.
The multiple use of the electric motor in the central supply, together with the large range of suitable two-quadrant pumps (pumps in which the P- and T-attachments on the pump remain in place unchanged, regardless of the direction of flow; that is, the high pressure is always on P, and T is always connected to the tank), provide good economic arguments in favor of hydraulics as opposed to electromechanical systems. 
Because the central supply is connected to the drive with pipes and hoses – this is why hydraulic drives are often so compact – the power can be fed around any corner.
When marrying hydraulic pumps with electric motors, one has to take into account the behavior of each system, as determined by its construction type, with respect to its hydraulic, electrical and mechanical characteristics. A variable hydraulic flow can be created with an adjustable-flow pump, a fixed-displacement pump on a variable-speed electric motor, or a combination of both: a variable-flow pump on a variable-speed electric motor (Figure 1).
Figure 1
Studies so far suggest that the last of these is the most energy-efficient combination, but also the most expensive. With smaller flow rates in particular, this expense cannot be justified, and it is preferable to adopt more economical solutions involving fixed-displacement pumps. The long-established option of a variable-flow pump on a constant-speed asynchronous motor is almost unanimously considered to be the benchmark solution.
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Dr.-Ing. Gerd Scheffel is industrial applications engineering manager for Parker in Kaarst, Germany. He holds a doctor’s degree in hydraulics at Aachen University. He started his career as hydraulics engineering manager at Towler Hydraulics which was acquired by Parker in 1984. He served as general manager from 1989 to 2012 of the Hydraulic Controls Division and managing director of Parker Germany from 1991 to 2014.
Dr.-Ing. Roland Bublitz is industrial applications engineering manager for Parker in Kaarst, Germany. He holds a diploma and a PhD in mechanical engineering both from the RWTH Aachen University in Germany. He began his career at Parker in 2003 and worked previously as research assistant at the Institute for Fluid Power Drives and Controls at RWTH Aachen University.
This article first appeared in MDA Technologies Internal Edition, October 2015.
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