The drive power transmission of production machinery may essentially be achieved via one of three technologies: mechanical, electrical or fluidic. Each technology has distinct advantages and disadvantages; however, it is not necessary to cover each of these in this article.
The losses in fluid power/hydraulic energy transmission can typically be classified as either conversion losses or transmission/control losses. Hydraulic-equipment manufacturers have continuously improved the design of their components with an eye toward increasing overall efficiencies; however, improvements in this area tend to follow the law of diminishing returns as componentry achieves ever-higher efficiency levels. Circuit technology has also changed over the years, and devices — such as variable displacement pumps with load sensing and electronic controllers — have helped achieve significant overall system efficiency gains. Below, we’ll discuss how additional efficiency gains can be achieved by combining the advantages of different drive technologies.
Historically, a typical arrangement in most hydraulic systems is a fixed-speed electric motor driving a variable-displacement hydraulic pump, which matches its output to the flow requirements of the system. In recent years, there has been interest in using variable-speed electric drives and fixed-displacement hydraulic pumps to achieve the same end. The advantage of this drive alternative is twofold: there is less energy consumption due to operating the pump and motor at a point where maximized efficiencies are achieved; and noise is reduced due to lower motor-drive speeds. A study is currently underway at a Canadian wood mill to investigate the possible savings that can de derived by using variable-speed electric servo drives and high-efficiency fixed-displacement hydraulic pumps and accumulators to replace systems currently utilizing fixed-speed asynchronous motors and pressure-compensated piston pumps. Currently, the hydraulic system consists of four 75-kW, 1,800-RPM high-efficiency motors driving pressure-compensated axial piston pumps at a pressure level of 150 bar. The machines operate 24 hours per day, five days per week.
Because the work is being performed on an existing system, we needed to benchmark its operation; therefore, data acquisition was installed to record the pressure and flow requirements during normal machine cycles. With this information, it was possible to make models of the system demands and run simulations of various machine cycles to compare operational costs using various drive technologies. When comparing the existing systems to other possible solutions, it was found that a variable-speed pump (VSP) drive offered the highest energy savings.
First, during steady-state production, the following table shows energy consumption comparisons between the original pressure-compensated pumps/fixed-speed electric motors and the new VSP drive.
(Click images to expand.)
The pressure control mode of the operation is accomplished via an algorithm inside the servo control, which adjusts motor speed in relation to the difference between commanded and actual pressure values. Therefore, when the system is not consuming any flow (pressure holding mode), the drives can be operating at nearly zero speed.
While the initial system cost is slightly higher for this new technology, the return on investment can be relatively short when considering the potential energy savings (which can be around 25 percent).
Higher gains can be achieved in systems where the pump is utilized as primary control, whereby it is providing direct control of an actuator. In cases such as these, the electric servo drive can reverse its rotation and consume power back out of the hydraulic system.
Doug Wilson is the fluid-power training manager with Bosch Rexroth Canada. For more information, visit www.boschrexroth.ca