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The development of magnetic bearing technology has advanced the science of high-speed rotating machinery. Offering oil-free and high-speed, virtually zero-friction operation up to 100,000 rpm with the ability to function in high-vacuum environments, magnetic bearings are finding increased use in applications ranging from high-efficiency HVAC compressors to semiconductor, biotech and green technologies including windmills, turbines and flywheel energy storage. With the advent of magnetic bearings and magnetic-mechanical hybrids, engineers gained bearing solutions with very low and predictable friction, the ability to run without lubrication, and the capability of high performance in a vacuum – the ideal bearings for high-speed vacuum applications. However, these same engineers also gained the challenge of supplying power, control and monitoring cables into and out of a sealed vacuum environment, leading to a need for reliable hermetic seals.
Engineering and specifying hermetic seals for magnetic bearing applications are relatively new design challenges. While early applications and experiments in NASA laboratories could make use of homegrown ceramic or metal hermetic seals, developing and testing a customized seal for commercial applications and quantities presents different types of challenges. One such challenge lies in the quantity of leads required to get through a small space, because the larger the hole in the chamber, the greater the risk of leakage. Smaller, high-density feedthroughs are better than multiple hermetic connectors due to the same risk of leakage. Then there are the issues with mating a hermetic connector in an environment that is submerged, which is often the case with flywheel applications, since flywheel chambers are sometimes encased in heat transfer fluid. Further, there is the risk associated with the use of welding to hermetically seal the feedthrough to the chamber wall. Often, process control issues with welding create unanticipated temperature fluctuations that could damage both delicate electronics as well as the electrical feedthroughs.
Epoxies Make the Difference
Fortunately the past decade has seen tremendous advances in material science and the application of these new materials to high-vacuum hermetic seal and feedthrough technology. Modern material science has developed epoxy compounds able to withstand temperature and environmental conditions previously only met by glass, metal and ceramic seals.
With epoxy seals, standard or custom connectors are sealed in a fully-potted, high-performance epoxy compound. Epoxy-based hermetic seals offer unlimited design options with performance equal to or better than older technologies, at a lower price and with faster turn-around time.
Epoxy-based hermetic seals also enable greater configuration flexibility while maintaining high performance. With designers often trying to significantly reduce system size, co-location of associated electronic and control systems can present challenges in how feedthroughs will be successfully situated. Thus, control and power feedthroughs that fit into tight areas and turn corners while maintaining vacuum require custom housing designs, often with unique geometries and specialty materials. As epoxy seals can be configured in any form factor, accommodating highly customized applications is no problem.
Getting Power Into a Sealed Environment
While today’s magnetic bearings enable new technologies and efficiencies, they are also vulnerable to new types of potential problems which require an evolved understanding of how to prevent them. These advanced applications can be put at risk when the challenge of getting signals and power into and out of the vacuum environment is underestimated. Often the vacuum environment and heat transfer fluid require that special attention be paid to material selection. For control systems on high-speed rotating machinery, speed, temperature and vibration all need constant monitoring via numerous thermistors and other sensors, often incorporating shielded and/or twisted wires to maintain signal integrity. For power transfer, copper post studs or heavy-gauge wire feedthroughs must be accommodated, depending on current requirements. In all cases, small and high density feedthroughs provide less risk of leakage than multiple connectors. In the case of chambers that are submerged in a heat transfer fluid, feedthroughs must also be leakproof and resistant to whatever fluid is in use.
Understanding parameters such as out-gassing, permeability and material compatibility is critical in developing solutions that will perform as desired over the 20+ years of operation that most of these units require. Consulting hermetic feedthrough experts during the design phase can ensure that these small but necessary components do not become the failure points for an otherwise successful project. Over the years, it has been our experience that missteps often occur when engineers:
- Underestimate the importance of the feedthrough in their final design or product;
- Overestimate their ability to build something in-house;
- Overestimate the ease of finding the right feedthrough solution;
- Underestimate the lead time that will be involved when they find a vendor;
- End up with delays, expenses and embarrassment that a little preplanning could have eliminated.
It’s true that there are some situations in which the application is straightforward, and an off-the-shelf feedthrough may suffice. But there are frequently design, material or envelope considerations that require a customized solution. For example, if an existing bulkhead already has NPT penetrations, a matching feedthrough should be sourced to hopefully avoid drilling and rethreading the penetration.
Often, a design may require more wires than can fit into an off-the-shelf feedthrough. Rather than adding another penetration or enlarging the existing penetration (if either option is even feasible), a better solution is to design a feedthrough to precisely match the conductor requirements within the existing envelope.
Design engineers know that magnetic bearing failure due to loss of vacuum can be catastrophic. Purchasing departments know that commercializing a product requires a reliable, cost-effective supplier of every component – including the hermetic seals. Traditional metal and ceramic seals are costly, not easily customized, and have long lead times for prototyping and delivery. By using new technology in epoxy-based hermetic feedthroughs, failure due to vacuum loss is decreased, cost of hermetic feedthroughs is decreased and reliability is increased to the benefit of engineers and purchasing departments.