The use of variable-speed electric drives to power hydraulic pumps has grown with the development of high-speed controls, quick-response motors, and improved software that give these motor-pump combinations the power and responsiveness to match systems controlled by electrohydraulic valves or a variable-displacement pump, while also reducing energy consumption.
Traditionally, constant-speed ac induction motors have been used to drive the pumps in stationary hydraulic systems, with hydraulic power controlled by flow- and pressure-regulating valves or a variable-displacement pump. However, the constantly-running motor can consume as much as 50% of full-load current, even when the system is not under load. In addition, pumps and motors in conventional hydraulic power units (HPUs) often are oversized to meet maximum duty-cycle demands. By contrast, a variable-frequency drive or servodrive can manage an electric motor’s operating torque and speed more efficiently. Instead of running continuously at full speed, the motor rotates only fast enough to meet system demand at any given time.
A hydraulic system with a variable-speed drive combines the robust, power-dense benefits of hydraulics with the intelligence and ease of integration into automation systems provided by electric drives. Energy efficiency is increased because pump speed is easily adjusted to flow demands, and the motor isn’t running constantly at high speed, regardless of load. Noise is also reduced, especially during operations where pump flow demand is below maximum. Varying the pump speed varies the flow and therefore matches the hydraulic power delivered to the exact amount needed at any point in a machine’s duty cycle, whether during operation, in standby, or while maintaining pressure. In addition, because these systems generally require less oil and consequently a smaller reservoir, the machine footprint can be reduced.
Although variable-speed drives commonly are paired with fixed-displacement pumps, additional benefits can come from teaming a variable-speed drive with a variable-displacement pump in many applications. “By combining a variable-speed electric drive and a variable-displacement hydrostatic drive, we can drastically reduce the motor size, which lowers both installation costs and electrical parasitic losses,” says Rashid Aidun, application engineer—VAS team, Parker Hannifin.
“Combining a high-response variable-displacement pump with a VFD-controlled induction motor results in greater efficiency and a much higher system response than a variable-speed unit alone,” adds Paul Stavrou, manager of system applications, Bosch Rexroth. “Using a variable-displacement piston pump makes it possible to adjust pressure, pump displacement, and motor speed (torque and speed) to optimal working points, thereby minimizing electric, hydraulic, and mechanical losses.”
In many cases, variable-speed pump drives are used to power hydraulic motors in applications such as mobile or material handling equipment. However, they provide equal benefits when used to power actuators. Electrohydraulic actuators (EHAs) combine the best of electromechanical and electrohydraulic technology to convert power from electric to hydraulic to mechanical. EHAs are attractive in applications such as machine tools, since the actuators only consume power when the systems demand it. They also eliminate the need for thermal management, as well as oil and filter changes.
The benefits of variable-speed pump drives are paying off in applications that include machining and metal forming equipment, plastics and rubber machinery, die casting machines, pulp and paper machinery, hydraulic test stands, material handling equipment, and many other industrial applications.
What follows is a summary of major variable-speed drives and systems currently available:
Bosch Rexroth’s Sytronix FcP drive systems include an induction-rated motor, a hydraulic pump and an electronic variable-frequency drive as an assembly.
Bosch Rexroth, Charlotte, N.C., officials say their Sytronix variable-speed pump drives help reduce energy consumption and operational noise. They combine two of the company’s well-established technologies to deliver the reliability of powerful hydraulics and the energy-efficiency and dynamics of compact electrical drive technology. The hydraulic control intelligence is integrated directly into the electrical drive to adapt the motor torque to the machine’s pressure and flow-rate requirements. If the system does not require full power, the control automatically reduces the torque of the pump drive, and thus decreases energy consumption and noise level. Practical application has shown that energy savings of up to 80% can be realized with Sytronix systems.
Bosch Rexroth’s Sytronix FcP drive systems are preferred for constant-pressure power units in the low power range. They include an induction-rated motor, a hydraulic pump and an EFC electronic variable-frequency drive as an assembly. The FcP runs at a lower power range consistent with PGF internal gear pumps. In higher pressure and power ranges, the FcP may use internal gear pumps from the company’s PGH family or its A10 or A4axial piston pumps. The torque and load on the electric motor are reduced by using variable pumps, especially in high power ranges, so that smaller drive sizes can be used.
The company’s Sytronix DRn drive systems are appropriate for constant-pressure power unit applications in the medium- and high-power ranges. The DRn system consists of a drive-pump-module with an asynchronous motor and a frequency converter as the control unit. DRn pressure control systems feature programming that monitors load demands and automatically adjusts speed at constant pressure.
Green Hydraulic Power’s turnkey system incorporates a Siemens Sinamics servo pump, which is based on a variable-speed drive powering an electric motor coupled to an internal-gear pump.
MJC Engineering, Huntington Beach, Calif., has created a turnkey solution that includes system integration, engineering, software technology, and programming under the Green Hydraulic Power (GHP) label. The system incorporates a Siemens Sinamics variable-speed drive powering an internal-gear hydraulic pump. MJC claims up to 70% energy savings over conventional hydraulic systems, as well as noise reduction by 20 dBA, less heat generation, lower cooling requirements, less oil volume, and a smaller footprint.
MJC, a custom machine tool builder, had been looking for ways to improve the energy efficiency of its machines. The company turned to Siemens, its longtime automation supplier, to help design a new hydraulic power unit. “We saw that the servo pump could control pressure and flow and precisely convert electric energy into hydraulic power,” says Carl Lorentzen, president of MJC. “In some cases, the use of control valves is completely or partially eliminated.” Lorentzen subsequently established Green Hydraulic Power, Inc. as a separate enterprise that now offers the unit for OEM integration by other machine builders. The GHP line is available in two styles: one with a VFD asynchronous motor and internal gear pump, and the other with a servo inverter, synchronous servo motor, and internal gear pump.
Parker Drive-Controlled Pump (DCP) and GVM-series synchronous motor are teamed with AC30 variable-speed drive to provide exceptional levels of control, from simple open-loop pumps and fans to closed-loop process line applications.
Parker Hannifin Corp., Cleveland, offers its Drive Controlled Pump (DCP) technology through its Hydraulic Pump and Power Systems Div., Columbus, Ohio. The DCP provides a synergistic approach that integrates hydraulic power units, electronic drives, electric motors, and hydraulic pumps to meet each local load demand within a hydraulic system. The product line integrates an ac drive unit, which includes variable frequency drives and electronics and either an induction or servo motor, matched to either a vane pump or axial piston pump or a combination of the two technologies.
Parker also offers fully customized DCP Solutions that may combine a variety of electric motor technologies, from induction to servo motors, partnered with a diversified selection of the company’s hydraulic pump technologies, including vane, helical gear and axial piston pumps. Parker’s custom field-tested control algorithms manage the electric motor’s operating torque and speed, producing the precise, variable pressure and flow required at any given point in the machine or duty cycle.
To provide the combined benefits of variable speed and variable displacement in a simpler package, Parker offers a version of its PVplus axial-piston pump, a dual-displacement pump that can be switched between two continuously adjustable settings. When combined with a variable-speed drive, it provides advantages that include lower acquisition costs for the motor and frequency converter, high operating pressure, high productivity, and high energy-efficiency.
Kyntronics developed this electrohydraulic actuator, a self-contained modular assembly that contains a complete hydraulic system. Power is transmitted by a variable-speed motor and control that drive a hydraulic gear pump, while built-in transducers provide closed-loop force and position control.
Kyntronics, Eastlake, Ohio, has developed a self-contained modular actuator that not only drives a hydraulic pump with a variable-speed motor, but also incorporates volume compensation to account for piston rod volume. By employing a technology known as electrohydraulic actuation (EHA), it combines most of the advantages of hydraulics with those of electromechanical actuators, but without many of the disadvantages.
An EHA typically integrates a hydraulic cylinder with a controller, motor, pump, reservoir, and control manifold into a self-contained solution. The Kyntronics solution combines a drive, servomotor, manifold, and cylinder within a self-contained package. A key element of the Kyntronics EHA is a rod-volume compensating mechanism, which compensates for the differential volume between the cylinder’s rod- and cap-end volumes as the piston rod extends and contracts.
The modular system starts with a bidirectional ac or dc servomotor driving a gear pump. This eliminates the need for a proportional directional valve because the motor controls flow rate and direction. One beneficial feature is the way the system accounts for the differential volume between a cylinder’s cap and rod ends. To compensate for this difference, the hydraulic cylinder includes an outer low-pressure cylinder, which creates a chamber between the two cylinders. This chamber contains compressible foam that serves the same purpose as an accumulator in a conventional system, but the foam does not require an initial precharge or any maintenance.
Eaton’s PVM variable piston pump, left, and PowerXL DG1 series variable-frequency drive create a system that can match the precise load requirements of any point in a duty cycle.
Eaton Corp., Eden Prairie, Minn., highlights the energy savings of its variable-speed-drive pump solutions for equipment in the machine building, manufacturing, oil and gas, and machine industries. A complete solution can include one or several of Eaton’s PVM or Hydrokraft variable-displacement piston pumps, or VMQ fixed-displacement vane pumps. When combined with the smart control of Eaton’s variable-speed PowerXL DG1 general purpose drive or SPX 9000 high performance drive, the system can match the precise load requirements of the duty cycle.
A major system benefit is energy savings of up to 70%. Instead of operating at a constant speed, the variable-speed drive pumps can be controlled precisely to match the load requirements of the current duty cycle via intelligent control, which slows or stops the machine when the duty cycle allows. Quieter operation is another advantage, since slowing or shutting the machine down when it does not need to be running reduces noise significantly. This helps protect operators’ hearing and meets more stringent noise regulations. Because more efficient machines produce less heat, manufacturers can downsize or eliminate oil coolers with variable-speed pump drives.
They also reduce the amount of hydraulic oil needed, which allows for a smaller tank and helps extend the life of seals and oil. Pumps also can be downsized to cut costs. Because the system can run at higher RPMs, pump displacement can be reduced without reducing flow. All these benefits help shrink the machine’s overall footprint and reduce costs.
Daikin’s EcoRich hydraulic power unit combines an IPM brushless dc motor and inverter drive system with a tandem pump to automatically control the pump based on load pressure during operation, standby, and pressure maintenance.
All World Machinery Supply, Roscoe, Ill., a member of the Daiken Group, reports Daikin’s Hybrid Hydraulic Power Units seamlessly integrate oil hydraulic control and inverter motor technologies and can reduce power consumption significantly. Its patented inverter drive is teamed with an IPM motor (one type of dc brushless motor), manufactured by Daikin Industries, that uses a rare-earth permanent magnet deeply positioned in the rotor to generate magnet torque and reluctance torque. This electromagnetic structure attains high torque and efficiency, with low heat generation. The energy-saving hybrid-control HPUs are designed specifically for industrial applications such as metal cutting, metal forming, injection molding, and many other uses. A keypad lets the user adjust pressure and flow settings manually. The company says the units can reduce factors that include energy consumption, heat, oil requirements, and noise, resulting in cost savings that can provide a 12- to 18-month complete return on investment.
Moog’s Electrohydrostatic Pump Unit combines the elements of a conventional hydraulic power unit into a lighter, more compact, modular assembly that can be mounted directly to a cylinder via a manifold. Its servomotor driven pump also makes the EPU substantially more energy efficient than conventional HPUs, especially during dwell periods.
Moog Inc., East Aurora, N.Y., has a speed-controlled pump system that allows machine builders to lower energy consumption by 30% or more compared to conventional approaches. The Electrohydrostatic Pump Unit (EPU) is intended to combined benefits of both hydraulic and electric actuation. The Moog EPU is at the heart of an electrohydrostatic actuation system and helps make the decentralized drive system possible. Doing so eliminates the need for an associated piping to reduce the overall machine footprint. The compact design of the EPU lets users mount it directly onto a cylinder, thereby reducing space requirements on each motion axis even more.
Because they have fewer fluid connections, the self-contained units greatly reduce the potential for leaks. The clean, simple design also improves reliability, maintenance, and total cost of ownership compared to conventional units. The electrohydrostatic pump units can be used for a wide range of applications, such as metal forming, injection molding, die-casting, gas and steam turbines, and wind turbine pitch control.
“Our EPU is intended for companies that want a standard, out-of-the-box product that can be easily integrated into their hydrostatic transmission or new machine designs,” says Dr. Achim Helbig, innovation manager at Moog. “It has a mounting pattern that allows direct mounting onto a cylinder via a manifold. This reduces plumbing time and costs, installation time, and maintenance work can be considerably simplified.”