PSS Advances in Superconducting Motors for Aircraft

PSS just finished up a research contract for NASA in which we discovered some surprising and useful ways in which Low Temperature Superconductors (LTS) may be more suitable than High Temperature Superconductors (HTS) for making light, efficient electric motors.

In short, they’re cheaper. They’re much, much easier to design, manufacture, and use. Unlike HTS, it’s easy to make LTS electrical joints that are just as superconducting as the coils. LTS experience less heating when their internal current is changed. Crucially, you can make a “persistent switch” in which an LTS magnet is charged once and the current is trapped in the coil, persisting without the need to constantly supply current. Our LTS of choice is NbTi, the “workhorse” of the LTS family.

Interested in knowing more? Then read on!

Electric Aircraft

There are several big pushes toward electric aircraft. Air travel accounts for 2.5% of our carbon emissions. So what’s preventing us from electrifying aircraft like we did with cars? The problem is weight. An extra pound of motor or batteries costs much more in an aircraft than it does in a car.

That being said, there are dozens of research groups, companies, and agencies working on hybrid electric and fully electric aircraft. There are even serious advantages to having the freedom to place propulsion units (motors rather than jet turbines) wherever you want within the aircraft, concepts called Boundary Layer Ingestion and Distributed Electric Propulsion. The aerodynamics is complicated, but the gist is that you can get huge emissions savings even if you’re still using jet fuel and turbines, if those turbines are powering lots of little motors rather than one big jet engine.

Superconducting motors

As we said earlier, all parts of the propulsion powertrain need to be lightweight in order to make a practical electric aircraft. For decades now, superconductivity has been known as a phenomenon with the potential to decrease the weight and increase the efficiency of motors. The idea goes back to the 1960s, with several experimental LTS rotors being tested in the 1970s and 1980s before the programs ended.

But what happened in the 1980s that shifted focus away from LTS motors? The answer is the discovery of HTS. On paper, HTS looks wonderful. It is superconducting at more achievable temperatures, ~100 K versus ~8 K for LTS. It can create magnetic fields much higher than LTS. Plus, it can carry much more current than LTS, meaning the same motor can weigh significantly less if made of HTS.

Yet despite research programs going back to the 1980s and continuing today, there are still no HTS motors on the market. Why is that?

The LTS difference

Our LTS magnet. This magnet was purchased from SSI for a separate NASA contract. This magnet is operated in persistent mode, in which the magnet does not need an external power supply once it has been charged.

It turns out HTS is expensive and extremely hard to use. A magnet made of HTS would cost 20 times more than one made of LTS. HTS is weak, and when it’s under strain it can’t carry as much current. It can’t be flexed in one direction. To join two cables of HTS together into one superconducting piece, you have to grow more superconductor between them; you can’t just snap them together like extension cords.

On the other hand, LTS magnets have matured since the 1980s. Most hospitals now have an LTS magnet in the form of their MRI machine. Thousands of tons of LTS are produced yearly. LTS is cheaper, stronger, more flexible, and easier to work with. Its so-called AC losses (heating that occurs when the current is changed) are lower. Two LTS cables can be joined together to make one long LTS cable.

This latter property allows the so-called persistent mode of LTS magnets. In this mode, no external current is required to power the magnet. You charge the magnet up once, then you can disconnect it and walk away. Our LTS magnet vendor, Superconducting Systems, Inc. (SSI) of Billerica MA, has magnets that have sat persistently charged for decades.

How this affects a motor design

As part of our Phase I NASA SBIR, we designed a motor using LTS. The motor design targets small aircraft like Cessna Denali or regional airliners like Beechcraft 1900. The motor’s output power is 1 MW. The total target system weight is 100 kg. The target efficiency is 99.5%.

One of the challenges of using superconducting materials is keeping them cold. Because of the low AC losses and persistent mode of LTS, we were able to cut the heat leak down from dozens of Watts to less than 1 Watt. We were able to completely eliminate the charging subsystem and cryocooler of HTS designs. We have identified four innovative technologies that are enabled by and instrumental to the use of LTS in motors. We will be developing this technology in the coming years.

One of our innovations came from the significantly reduced heat leak into the cold rotor. Rather than use heavy, expensive cryocoolers to cool the rotor, the design suddenly came into the realm of Liquid Helium (LHe) reservoirs. Our SSI partners liken it to the difference between a refrigerator and a cooler. Use the refrigerator (cryocooler) when keeping food cold for weeks or months, but use a cooler (LHe) when making a day trip to the beach.

What’s next?

The journey of the LTS motor has just begun. Work continues at PSS. Contact us for more information or partnering opportunities.

Watch this space! Some day soon, perhaps sooner than you think, you could be flying across the country in an aircraft as renewably powered as your electric car.

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