Hybrid-Electric Flight 750-kW Aircraft Engine Weighs Less Than 100 Kilograms

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Fraunhofer researchers from Erlangen, Germany, have designed a lightweight electric motor for hydrogen aircraft. The drive weighs just 94 kilograms (approx. 207 lb), but delivers 750 kilowatts of power. Four separate inverters make the system particularly fail-safe.

The aviation industry is looking for technical solutions to reduce greenhouse gas emissions. The Fraunhofer Institute for Integrated Systems and Device Technology (IISB) has designed a new electric motor for this purpose. The permanent magnet synchronous machine is intended to power hybrid-electric regional aircraft in the future. It is being developed as part of the Amber project under the umbrella of the European Clean Aviation research program. The consortium is working on a two-megawatt drive system based on hydrogen fuel cells. Aircraft engine manufacturers such as Avio Aero and Ge Aerospace are also involved. The engineers are linking the electric motor in a parallel hybrid configuration with Avio Aero's Catalyst turboprop engine. The partners' aim is to reduce carbon dioxide emissions by at least 30 percent compared to aircraft from 2020.

To achieve this goal, the engine mounted directly on the gearbox has been massively downsized. Nevertheless, it delivers top performance. The most important technical data at a glance:

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• Power: 750 kilowatts nominal power (at 65 degrees Celsius oil temperature)
• Torque: 350 Newton meters
• Speed: 21,000 revolutions per minute
• Weight and dimensions: 94 kilograms, 60 centimeters long (approx. 24 in), 25 centimeters in diameter (approx. 9.8 in)
• Power density: 8 kW/kg

This extremely high power density is a decisive factor, as every gram counts in aviation.

Plug-In Coils and Extremely Thin Electrical Sheet

The engineers achieve these physical values using special manufacturing techniques for the stator. Instead of classic round wire windings, they use solid plug-in coils, so-called hairpins. These form the stator winding. An uneven number of turns per slot optimizes the current density in the hairpins and reduces alternating current losses. At the same time, the motor rotates quickly. In order to keep the resulting eddy current losses in the stator low, the institute uses 0.15 millimetre thin electrical steel of specification No15. The high power density also requires efficient thermal management: direct oil spray cooling dissipates the heat directly at the windings where it is generated. Measurements taken by the developers show that the motor achieves an efficiency of over 98 percent in a high speed range between 13,000 and 21,000 revolutions per minute.

Safety plays an overriding role in aviation. If an electronic component fails, flight operations must continue. Fraunhofer IISB therefore splits the stator mechanically and electrically. The system works with four decoupled windings that are connected to four separate inverters developed by the institute. This 4x3-phase design intercepts faults and ensures that the drive continues to operate even in the event of partial faults. The developers implement the high-voltage connection with connectors from Amphenol's HVSL series, while Ecomate Rssm connectors are used for the low-voltage connections.

Monitor Magnetic Fields Inside

In order to optimize the development, production and validation of such drives, colleagues at Fraunhofer IIS are contributing innovative sensor technologies. So-called Hallinsight sensors measure the magnetic fields three-dimensionally directly inside the rotating machine. Such data can be used to detect faults during automated production or predict wear during flight operation. From the initial concept definition to the thermal and mechanical design through to final assembly, the Erlangen-based company manufactured the engine entirely in accordance with current aviation standards. (mc)