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Testing aircraft subsystems is, by necessity, an exacting process. With crew and passenger safety resting on the outcome, no one would argue about the need for these tests to be reliable.
That’s also why historically there have been few complaints about the amount of time—and energy—these tests typically consume. However, that doesn’t mean aircraft manufacturers wouldn’t welcome a more efficient method of proving their products’ flight readiness.
In fact, the U.S. Dept. of Defense (DoD) already is taking delivery of a new generation of helicopter test equipment that will drastically reduce the time, dollars, and energy expended in testing aircraft powertrains. The DoD will be replacing 20 aging dedicated test stands with five new flexible test machines.
These new test systems will allow the DoD to institute new streamlined testing processes that sidestep the labor-intensive setup and tear-down that accompanies the use of current-generation test stands. Eliminating these steps will enable aircraft manufacturers to conduct multiple testing operations in a single shift, as opposed to the single test most can conduct today.
The potential energy savings are even more impressive. Using just one of these new systems to test transmissions at a U.S. Army helicopter repair center is expected to yield $500,000 in energy cost savings in a year.
Reducing lost energy
These savings are possible because these new systems regenerate energy during the testing process. That’s a sharp contrast from the test systems currently employed by most aerospace manufacturers, which lose 100% of their energy in the form of wasted heat.
This new class of electrical regeneration test systems can capture most of that energy to be put back into the power grid or used to run the machine itself. In most cases, electrical regeneration systems are less expensive to build, have lower operating costs, require less maintenance, and have greater reliability. When compared with traditional testing systems, these systems will pay for themselves over time.
The basic premise of electrical regeneration is to replace traditional energy-wasting braking methods with a method that produces recoverable energy, which is then fed back into the machine or the local power grid. The commercial automotive industry has been an early adopter of this technology, with large numbers of electrical regeneration dynamometers being used to test engines and powertrain components.
The previously mentioned DoD repair center currently is using test stands built decades ago to fulfill a single purpose, requiring excessive energy consumption and heavy cost expenditures. Thus, the DoD’s willingness to replace more than 20 dedicated powertrain test stands with five flexible systems that can test multiple transmissions and gearboxes.
The current helicopter transmission and gearbox test systems use traditional braking technology that results in wasted energy by converting power to lost heat.
For the DoD helicopter project, electrical regeneration systems use a common dc bus architecture, which allows for only one ac to dc conversion in the motoring direction. The regenerative braking power goes straight to another inverter, which is motoring via the common dc bus link. This method eliminates two conversion points where energy would be lost and increases overall efficiency.
In addition, the common bus solution paired with the active front end (AFE) has the ability for power factor correction, further increasing the overall savings of a common bus system. All AFE drives allow for unity power factor and low total harmonic distortion (THD) that meets IEEE 519 harmonic standards, which allows the drive systems to improve the present power factor displacement in the customer’s facility.
The energy savings on this project have exceeded expectations. The new systems are six times more energy efficient than the single-purpose helicopter test stands. The new system designed for testing the main transmission—when running at full capacity—will cost approximately $400 less per hour to operate than current systems.
The potential for energy savings is even greater with turbo shaft engine test centers, which can lose as much as 5.5 million kWh of electricity during a typical year, enough energy to power 1,500 residential homes.
Current turbo shaft engine test systems use water braking technology that results in wasted energy by converting power to lost heat.
In addition to wasting energy, there are other disadvantages to using liquid brake test systems. The high rate of energy conversion taking place inside the brake erodes the internal components, requiring them to have to be rebuilt or replaced more often. With these frequent maintenance activities impacting the system’s structure and alignment, test data accuracy and repeatability are jeopardized. Also, the erosion process is sensitive to the quality of the incoming cooling water, which must be continually filtered and chemically treated, adding cost to the testing process.
By selecting an electrical regeneration engine test system, these disadvantages are avoided, and most of the test system’s energy can be fed back to the power grid in the form of electricity. This type of engine test cell technology is analogous to wind turbines, using essentially the same conversion equipment, switching gear, and energy management systems.
Showing global potential
Globally, electrical regeneration systems are being used in many other ways for energy conservation, including:
At least one test system supplier already is designing and building a wide array of mechanical and electrical energy regenerating test systems for the aerospace, automotive, and heavy vehicle industries and is committed to incorporating energy-saving technologies into all its designs. This technology promises to revolutionize the world of engine and powertrain testing.
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