Authored by:
Neal Gigliotti
Manager, Plastics Group
Jan Komsta
Manager, New Technologies & Simulation
Bosch Rexroth Corp.
Bethlehem, Pa.
Edited by Kenneth J. Korane
ken.korane@penton.com
Key points:

• Traditional hydraulics run at a constant speed and generate flow without regard to the actual power needed by a machine.
• Variablespeed electric drives rotate pumps at their mostefficient speed to supply only the hydraulic power a process requires.
• Engineers should consider using variablespeed pump drives whenever a machine cycle has fluctuating flows.
Resources:

Bosch Rexroth

Energy consumption is often the major contributor to overall costs of owning and operating industrial plants and machinery. On average, energy consumption accounts for between 20 and 30% of total equipment operating costs. And in energy-intensive hydraulic applications, the share of energy costs are often far greater. In such cases, variable-speed pump technology quickly pays for itself.

Fixed versus variable
Traditional hydraulics run continuously at a constant speed to generate the power required to operate a machine. In these circuits, an electric motor typically runs at its rated speed, say 1,800 rpm, and drives a fixed-speed pump. The pump, in turn, rotates continuously and generates flow without regard to the actual power needed by the machine at any given moment. The varying hydraulic flow a machine demands is regulated via internal adjustment mechanisms in the pump, or by diverting excess flow back to the tank.

Innovations over the years have improved the efficiency of fixed-speed pump circuits, but applying variable-speed techniques raises potential energy savings to a new level. Such systems use variable-speed electric drives to rotate pumps at their most-efficient speed — often less than 200 rpm — while supplying only the hydraulic power a process requires at any given moment. Depending on the application, this can produce energy savings between 30 and 80%, compared to fixed-speed operation, without sacrificing performance or limiting operating pressure.

In addition, lower energy consumption indirectly reduces the emission of CO2, helping companies significantly reduce their carbon footprint.

Operating the pump and motor at less than full speed also means they generate less noise. In some cases, noise emissions have been reduced by as much as 20 dB(A). This eliminates potential hazards to workers and the need for personnel-protection measures. And the cost and hassle of secondary noise containment or damping measures is cut accordingly. This adds to the potential savings of variable-speed drives, helping pay off the investment in relatively short order.

Due to lower energy consumption, another benefit is that the hydraulic fluid does not heat up excessively. In many cases, it is possible to eliminate heat exchangers or fans to cool the power unit. This, in turn, permits a more-compact design, which can reduce a system’s complexity and up-front costs. Commissioning becomes easier and quicker, and it simplifies troubleshooting and maintenance.

Finally, variable-speed pump drives can be integrated into existing equipment without major redesigns, thus letting them increase the energy efficiency in machines already installed on the plant floor.

High-performance drives
A number of fluid-power and motor manufacturers offer variable-speed pump drives, and the available combinations of pumps and motors seems to be growing by the day. For example, Rexroth’s Sytronix product line has more than 140 modular, variable-speed pump/drive combinations, covering a broad spectrum of applications for demand-based control of hydraulic systems. The variablespeed pump drives currently fall into three general groups:

The frequency-controlled Sytronix FcP 5000 pump drive is the basic version. It consists of an asynchronous standard motor and a frequency inverter driving an internal or external gear pump. This combination offers a good price/performance ratio, especially for high-power applications. It’s primarily used in constant-pressure systems. It maintains system pressure by matching flow with demand. For example, the unit’s PGH-3x internal gear pump has been designed specifically for variable-speed operation, providing high efficiency over a wide speed range. It is can run at more than 3,000 rpm, but in partial-load operation it reduces speed to less than 200 rpm without affecting operating pressure. This also substantially reduces noise from the hydraulic power unit.

The FcP 5000 is well suited for clamping and tensioning operations, such as required in machine tools. Depending on cycle characteristics, energy consumption falls by 30 to 80%, average flow is reduced over the entire machine cycle, and the oil stays cooler.

The second line, the DFEn 5000, consists of an axialpiston, variable-displacement pump driven by an asynchronous standard motor and a frequency inverter. It’s for high-performance, high-power applications.

Digital onboard electronics calculate the best operating speeds for the pump and motor. Destroking the variabledisplacement pump lowers the flow rate and motor load, with optimum speed for maintaining pressure generally between 300 and 400 rpm. During production breaks, the software can stop the pump without affecting the operating pressure. With machines that operate on a cyclical basis, a teach-in process can be used to bring the system back up to speed just before a needed increase in flow rate.

With noncyclical machines, sensing demand lets the controller increase speed to the required minimum shortly before additional flow is required, for instance in case of a batch change. That way, the full output of the pump is available if and when it is needed.

Finally, for high-end applications, the Sytronix SvP 7000 servopump drives offer the greatest responsiveness and precision. They take advantage of the high dynamic capabilities of permanently excited synchronous servomotors to boost efficiency and substantially reduce energy consumption, for example in presses and plastics-processing machines.

The drives use an internal gear or axial-piston pump. A pressure transducer senses hydraulic pressure, which is controlled by software in the frequency converter. Here, pressure control has a limiting effect on pump speed. Closed-loop comparison of required and actual values for pressure and flow can be sent to the higher-level machine controller via analog signals (0 to 10 V) or fieldbus. As in the other versions, the pumps are designed for variable speed operations, combining low leakage and noise with high overall efficiency.

The SvP 7000 with an internal gear pump can be used in open hydraulic circuits to control pressure and flow, and for simple positioning. And highly dynamic position control is possible with an axial-piston pump in a closed-loop circuit. Precisely metered flow allows for direct control of cylinder movements, letting the unit position an axis without the need for control valves. Thus, it can considerably simplify hydraulic systems.

And several servo-variable pump drives can be combined into one cascading system. Ethernet-based, realtime communication between several controls using Sercos III lets users synchronize an almost unlimited number of SvP 7000s to satisfy most any application demand.

Typical applications
Variable-speed pump drives aren’t warranted for every hydraulic system. But engineers should consider them whenever a machine cycle has fluctuating flows. “Intelligent” servodrives sense demand and regulate speed and, thus, power consumption of the electric motors. The more time a machine spends running under partial-load conditions, the greater the potential energy savings. Applications where variable-speed pump drive offer substantial energy savings include: • Machine tools • Plastics-processing machines • Die casting • Presses • Woodworking and papermaking.

For instance, variable-speed pump drives in plasticsprocessing machines reduce pressure-holding losses during pauses and partial-load operation. High-performance units, like the SvP 7000, accelerate twice as fast as conventional variable-speed pumps, shortening cycle times for higher productivity. Digital speed control ensures high repeatability, and the setup compensates for pressure-related leakage losses. With sophisticated closed-loop pressure control, quick pressure changes are possible while minimizing under or overshoots. Software can even prevent cavitation when changing from high to low pressures.

In, hot-chamber die-casting machines, closed-loop control corrects for deviations in pressure and speed, and compensates for leakage past the pump. It can also simplify the hydraulic circuit. In one redesign, an SvP 7000 eliminated the need for separate pumps for high and low pressure circuits, replaced some high-response control valves, and supplied an accumulator-charging circuit for the shot cylinder.

For press automation, variable-speed controls efficiently generate the required flow. In addition, the drives improve efficiency by eliminating throttling losses across control lands. Required oil flow is precisely metered, and the electric motor switched off when the press does not need flow or pressure. When retrofitting existing presses, they clearly improve a machine’s capacity while significantly reducing energy costs and noise.

In continuous papermaking, variable-speed pump drives can significantly improve energy efficiency. For example, during pressure-holding functions for rolling up paper, intelligently reducing speed directly saves up to 60% on electricity compared with power units with nonregulated fixed displacement motors. At the same time, variable-speed pump drives can run faster, permitting smaller-size units with lower cooling demands. Thus, they cut operating costs and space requirements.

In all these applications, variable-speed pump drives maintain the reliability and power density of hydraulics, and combine it with the flexibility of electric drives. The extended diagnostics options of these closed-loop drives also opens up additional options for preventing expensive machine failures, thanks to condition monitoring.

And keep in mind the possibility for upgrading existing plant machinery by replacing fixed-displacement units with variable-speed pump drives. This lets users quickly reduce energy consumption without having to invest in new equipment. Simulation programs are available that evaluate a machine’s hydraulic circuits and determine possible savings for an application, before investing in hardware. And because existing hydraulic circuits often remain unchanged, the switch can be made with surprisingly little effort. It also opens up possibilities for replacing classic, constant-speed hydraulic power units with variable-speed pump drives.

© 2012 Penton Media, Inc.