Next-generation flywheels are made possible by advances in material science in rotor technology, as well as the application of magnetic bearings running in a vacuum environment. While the movement of the rotating flywheel into a vacuum eliminates parasitic drags, such as windage friction losses, mechanical bearings are not suited to operate in a vacuum or for the high speed requirements of the new designs.

Power transfer feedthroughs use copper post studs(a) or heavy gauge wire(b) and vary in design according to current and voltage needs.
However, with the advent of magnetic bearings, flywheel energy storage system (FES) engineers gained a solution for high-speed, vacuum applications that provide very low and predictable friction, the ability to run without lubrication, and high performance and efficiency in a vacuum, which also ultimately reduces the risk of catastrophic failure.

With the requirement for operation in a vacuum comes a critical design challenge facing today’s FES engineer - ensuring the vacuum integrity of the flywheel housing while meeting the needs for noise-free monitoring and high power inputs and outputs. The main cause of FES failure is a breach in the vacuum environment of the rotor, making hermetically sealed feedthroughs a critical engineering component for FES development.

Vacuum environments vary, so customized electrical feedthroughs are needed to meet unique specifications. Feedthrough design and selection is paramount due to the high vacuum environments (typically less than 1c109 Torr), large operational temperature ranges (-40F to 250F), and long life spans (20+ years) of the systems. FES engineers should pay special attention to available geometries, specifically in reference to mounting styles, such as the common o-ring and weld or copper conflat flange gasket configurations, as key issues may arise dealing with permeability and/or the ability to rework the feedthrough, seal or system, both infield and on the manufacturing floor.

Oftentimes, FES designers try to significantly reduce the system size, making it as small as possible, not taking into consideration the co-location of associated electronic and control systems and how the essential feedthroughs will be successfully situated. Thus, signal and power feedthroughs that fit into tight areas, turn corners, and still maintain vacuum require custom housing designs, often with unique geometries and specialty materials.

Control systems measure speed, temperature and voltage to make sure the system is operating at peak efficiency and peak optimal conditions. This constant monitoring via numerous Hall Effect and other sensors requires a large variety of signals moving in and out of the system, which results in high noise transmission that can compromise those critical signals. Furthermore, noisy signals can cause catastrophic failure, particularly with the enormous amount of centrifugal kinetic energy generated by the high speeds and heavy flywheel. Thus, signal feedthroughs that incorporate sealed shielded and/or twisted wires are essential in eliminating noise issues, potential failure, and to preserve signal integrity.

Power transfer feedthroughs consist of copper post studs or heavy gauge wire and will vary in design, depending on current and voltage requirements. Heavy gauge wire feedthroughs can also be used for power transfer in FES, and systems could have over 70 signal wires and will typically have 3 power leads. In all cases, small, high density feed throughs provide less risk of leakage than multiple connectors.

Understanding events such as outgassing, permeability and material compatibility is critical in developing solutions that will perform as desired over the long operation life span that most flywheel units require. In the case of encased flywheel chambers, which are submerged in a heat transfer fluid, power and signal connectors must also be leak-proof and fluid-resistant. This technique helps to contain the flywheel in case of catastrophic failure and massive energy release. Oftentimes, the vacuum environment and heat transfer fluid requires special attention in terms of material selection.

As flywheel technology continues to improve, flywheel energy storage systems (FES) are gaining in use across a wide variety of applications, from peak shaving in power utilities to energy recovery in trains and industrial equipment to rack-mounted uninterruptable power supplies. With demand rising for reliable, cost-effective, and environmentally friendly energy storage, FES is quickly coming into its own.

Compared to other energy storage solutions, FES systems have long lifetimes with minimal maintenance requirements, high energy densities (~500 kJ/kg), and large maximum power outputs. The energy efficiency (ratio of energy out per energy in) of flywheels can be as high as 90%, with power output capacities ranging from 3 kWh to 133 kWh, and an FES can typically reach full charge in as little as 15 minutes.

While the potential for FES solutions is tremendous, these projects are often at risk when the challenge of getting signals and power into and out of the vacuum environment are underestimated. Consulting hermetic feedthrough experts during the design phase can ensure that small, but necessary components are not the failure points of an otherwise successful project.

This article was written by Ed Douglas, president of Douglas Electrical Components. To learn more, please contact Mr. Douglas at 973.627.8230, or go to

Motion Control Technology Magazine

This article first appeared in the June, 2009 issue of Motion Control Technology Magazine.

Read more articles from this issue here.

Read more articles from the archives here.