Smaller, lighter electric motor developed for airplane

A hybrid Cessna 337 taxied down a Southern California runway and lifted into the air. The plane, a type commonly used as an air taxi between islands, had a traditional gas-powered motor in the nose and an electric engine in the back. The electric motor was equipped with an experimental silicon-carbide inverter, or motor drive, designed by the UA Power Group. 

The test flight proved that a smaller, more efficient silicon carbide-based motor drive system could replace a hybrid plane’s traditional silicon-based system. 

“We were the first university to do this for a hybrid electric aircraft. That’s a feather in our cap,” said Alan Mantooth, Distinguished Professor of electrical engineering and computer science and the lead researcher on the project. 

The results of the test flight, which took place in 2023, were just published in the journal IEEE Transactions on Power Electronics. The project was supported by a grant from the Department of Energy’s Advanced Research Projects Agency-Energy, or ARPA-E. 

THE ADVANTAGES OF SILICON CARBIDE 

Transistors are the foundation of electric circuits. They act as amplifiers or switches. The microchips in our computers and smartphones, for example, contain billions of transistors, which switch on and off to create the binary language of ones and zeros. Today, most transistors are made of silicon, which is produced by heating purified sand. 

A transistor does not instantly switch off and on. In the transition between the two states, which lasts only a fraction of a second, energy is lost. That lost energy generates heat. 

Transistors made of silicon carbide can switch 1,000 times faster than those made of silicon. The faster switching speed makes the transistor more efficient, which means all the other components, such as inductors, transformers and capacitors, can be dramatically smaller and lighter. 

“Imagine a race car with a big 350 engine that weighs hundreds of pounds. What if you had that same power, but I gave you something that would fit in your hand?” said Chris Farnell, an assistant professor of electrical engineering and computer science. Farnell was the first author on the paper about the silicon carbide-based electric plane motor drive. 

The UA Power Group is a recognized leader in the research and application of silicon carbide. 

Despite its superior performance, the higher cost of silicon carbide has hindered its wider adoption. 

“Silicon is made from dirt, and nothing is cheaper than dirt,” Mantooth said. 

The cost of producing silicon carbide, however, has been falling. And because silicon carbide transistors require smaller supporting components, the cost of the entire system is reduced. 

“If the overall system gets cheaper, then Ford cares, Toyota cares. That’s why it ends up in cars,” Mantooth said. 

Current production techniques for silicon carbide are also not yet advanced enough to economically produce the nanometer-scale devices needed for computer microchips. This fall, the UA Power Group will open the Multi-User Silicon Carbide Research and Fabrication Laboratory to advance research on silicon-carbide microchip fabrication and serve as a bridge between university researchers and semiconductor manufacturers. 

THE CHALLENGES OF AVIATION 

For the airplane project, the UA Power Group built a silicon carbide-based inverter, which converts the direct current of a battery to the alternating current needed to drive a motor. 

The reduced size of a silicon carbide-based system is particularly advantageous on a small airplane, where space is at a premium. 

“You’re able to remove stuff and give passengers more legroom,” Farnell said. 

The lighter weight of a silicon-carbide system also means the plane uses less energy to take off and cruise. 

Planes are challenging vehicles for electrical engineers. The electrical systems must have mechanical supports to withstand vibrations and the shock of landing. 

At higher altitudes, the drier air increases partial discharge, which can degrade insulation and cause electrostatic issues. 

The higher switching speed of silicon carbide also creates more electromagnetic interference, which can affect other systems on the airplane. 

The successful test flight of the Cessna 337 proved that the UA Power Group team met those challenges. 

University researchers do not often test their work outside of the lab. Even if the science does not demand a field test, the UA Power Group sees advantages in taking their work to that stage when possible. 

“The students got a second-to-none experience. They got to do some hands-on engineering in addition to their scientific work, and they went on and got great jobs,” Mantooth said. 

The other authors on the paper were Anna Corbitt, Wesley G. Schwartz and Asif Faruque, U of A graduate students at the time of the work; Yue Zhao and David Huitink, faculty in the U of A Departments of Electrical Engineering & Computer Science and Mechanical Engineering, respectively; and Nenad Miljkovic of the University of Illinois Urbana-Champaign. The industry partners were Ampaire and Wolfspeed.

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