Key
drivers for fixed displacement or dual displacement pump systems are energy
efficiency and the need for new hydraulic motion control concepts in markets
where energy usage plays a vital role. These pump systems use tight integration
with a drive and an electric motor to achieve intelligent operation and reduced
energy consumption.
"The energy efficiency of the system is
higher, especially when the system is working under partial load," says Dave
Geiger, hydraulic systems engineering manager for the Moog Industrial Group.
"Under full load conditions, the performance compared to the variable
displacement system is nearly identical. With a medium load, the efficiency of
the new system is 20 to 30 percent higher compared to the classic system. If
you are running without load, or in a standby mode, energy consumption is even
up to 90 percent less."
The classic system for variable
displacement pumps consists of a pump and an induction motor, which is directly
connected to the power grid. In this typical system, the motor runs at a
constant speed, normally at 1,500 to 1,800 rpm, and uses an internal mechanism
to change the output flow of the pump.
The new system consists of integrated
building block products from Moog including a fixed displacement Radial Piston
Pump (RKP-II), the Maximum Dynamic Brushless Servo Motor and the Modular
Multi-Axis Programmable Motion Control Servodrive (MSD). Users can change the
speed of the motor and pump, which enables control of the fluid flow.
Two contributing factors lead to a
higher overall efficiency of the whole system. The heart of the innovation is
centered on the drive because of its control algorithms but also on the use of high-efficiency,
low-leakage technology. The MSD drive provides pressure and flow functionality
and, depending on pressure and flow demand values, the drive decides what
torque and speed settings are required.
From the pump and motor side, pump and motor
characteristics are measured at the factory and saved in the drive, so the
system has critical information about the attached motor and pump. This creates
a more intelligent system, which is integrated on the drive side and has the
ability to communicate via fieldbus to external systems.
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Further improving measure is a dual displacement
pump design, which provides the ability to change between two displacements. In
this case, the controller takes into account the actual displacement. This is
similar to a gear shift on a bicycle, and is especially important if the application
has a load holding phase. In that situation, the application normally needs low
flow, but also high pressure. The dual displacement pump changes to the low
displacement, reducing the required motor torque.
Depending on the operating conditions, the
pump intelligently switches from one displacement to the other. This switchover
is controlled by the drive and the algorithm.
With a dual displacement pump design, it's
possible to switch to the smaller displacement, which requires a reduced torque.
And as a result, the motor size required for the system can be reduced. This
leads to energy savings during the application's holding phase due to more
efficient operation of the motor.
Besides energy savings, the
technology offers additional advantages including a more compact design and much
easier system integration. The design achieves more compactness primarily because
the synchronous motor is much smaller than the corresponding induction motor.
With older systems, any fieldbus
used in a system would be interfaced through the pump control because it is
linked to the automation system. But now the fieldbus is through the drive
electronics, which is identical to an electromechanical axis.
The new system is easier to integrate,
because it looks like an electromechanical system. Since their interface is the
drive electronics (MSD drive), the interface is absolutely identical to an
electromechanical axis and the interface is more or less the same whether they
use the electrohydraulic or an electromechanical system. Integration is much
more straightforward and the system achieves a smaller footprint.
This variable speed pump system is targeted
for applications using variable displacement pumps today. These pumps change
flow using an internal mechanism but now the new technology can replace that
approach for use in applications such as die casting, injection molding and
wrapping or bending machines. Whenever the focus of the application is on
energy savings, compact design or easier integration, this recently adopted technology
provides an effective solution.
When comparing the cost of Moog's system with
a system that uses a variable displacement pump and a constant-speed induction
motor, the initial cost will be higher. But calculations and experience show
that the total cost of ownership will be lower. And, typically after two years,
energy savings has paid back the higher initial investment.
With mixed technology systems, whether you use
electromechanical or electrohydraulic systems, what matters in the end is the
impact and benefit of the technology on the application. With the Moog variable
speed pump system, the human machine interface is the same regardless of the
technology and topics like energy savings and safety that are driving the use
of the technology. The system has the ability to combine the best of both
electrohydraulic and electromechanical technology, to create the best solution
for a specific project.
The technology was actually
developed several years ago, but now adoption is dramatically increasing with
Chinese and German builders of plastics and injection molding machines looking
carefully at total cost of ownership as design criteria.
"The keys are upfront costs and what it costs
to operate and maintain the machine," says Geiger. "The cost of ownership for
electric systems was higher than expected because the initial purchase price is
higher. But upkeep is also more expensive because on an injection molding
machine, for example, the electromechanical devices are built into the
framework of the machinery. It's not like an actuator that can be easily bolted
in and later removed because it is integral to the machine."
Geiger says that every few years,
when an electric machine is rebuilt, it has to be totally disassembled and
re-assembled. The variable speed pump technology is attractive because the
energy consumption is not far away from electric and, while the performance is
a little lower, in most applications the higher level of performance from a
repeatability view point is not needed.
The value of the variable speed
pumps is the 20 to 30 percent energy savings versus variable displacement
technology, plus the ability to changeover machines without moving to a
completely electric solution. With a view of long-term cost of ownership, the
cost of building the machine is very inexpensive and similar to a hydraulic
machine. Plus, the process of rebuilding in the future is much lower because
you can disassemble the actuators, put in new seal packages, replace some hose
fittings and replace the pump cartridge.
Geiger says the key to higher
adoption is the overall value of the variable speed pump. When there is a
requirement for high performance and, in particular, high repeatability machines,
OEMs move to an electric solution. And for high-speed machines, users typically
move to servo valve hydraulics. Those are the areas where the variable speed
pump technology is not a fit, but it does fit in 90 percent of applications.
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