Multilevel motor drive for aviation takes first place in IEEE student demo competition

The combined work of a committed group of ECE ILLINOIS students was recognized at the Tenth Annual IEEE Energy Conversion Congress and Exposition (ECCE 2018) held in Portland, from September 23 – 27. Christopher Brandon Barth led a demonstration of the group’s work on “A High Frequency, Ultra Lightweight Motor Drive for Aviation Applications”. The project team includes ECE students Barth and Thomas Peter Foulkes, as well as former UIUC graduate students Pourya Assem, Oscar Azofeifa Castillo, Samantha Nicole Coday, Nathan Andrew Pallo and former post-doctoral researcher Tomas Modeer (now at SCiBreak, Stockholm, Sweden ). The group has been developing the inverter system for use in hybrid electric aircraft as part of a research project sponsored by NASA. Led by ECE ILLINOIS adjunct professor Robert Pilawa-Podgurski, the project seeks to push the boundaries of high-density inverter design and to demonstrate the advantages of a largely untapped inverter topology for this application, known as the “flying capacitor multilevel inverter” (FCML). Inverters are used in any transportation application to convert DC energy from sources such as batteries into multi-phase AC energy used to drive electric motors.

“The student demo competition provides a great opportunity to interact individually with a large number of conference attendees”, Barth explained, “this exchange can lead to many valuable contacts with both researchers and industry”. This was especially true for this demonstration. During the demo, Barth was explaining the converter operation and happened to glance down at the name tag of one of the individuals listening attentively. He immediately realized that he was speaking to the original inventor of the FCML topology Professor Thierry Meynard. “Neither myself, nor Professor Pilawa had met Professor Meynard before, so it was a bit of a surprise.” 

Although he originally developed and published the topology in 1992, Meynard expressed his respect for the work the team has done on refining and implementing the concept, and making it practically implementable for this demanding application. Over the last 3 years, the Illinois team has refined their use of state-of-the-art gallium nitride semiconductor switches and high-resolution FPGA-based control to overcome challenges which had previously prevented the architecture from being fully implemented beyond some low power prototypes. “There are a lot of challenging details to making this concept work”, Pilawa explained, “thanks to recent development in semiconductor technology, packaging, and digital control – and our own research to integrate these technologies – this concept can now be applied in high-performance motor drive.”

A significant portion of the work that has gone into optimizing this motor drive has focused on characterizing the performance of newly developed GaN FETs and ceramic capacitors. Proper converter design requires understanding how changes in the design will affect the amount of energy lost in the converter. “GaN FETs have low parasitic capacitance which gives them good performance, but they also have some second-order losses that are important for designers to understand,” explained Thomas Foulkes, “Characterizing these losses, which arise from a parameter called dynamic on-state resistance, requires careful measurements.” Sam Coday has performed similarly detailed measurements of the second-order losses in ceramic capacitors. 

Nathan Pallo has assumed the lead role in incorporating the data measured by Tom and Sam into the third and final revision of the inverter hardware. Nathan’s work experience with starts-ups WiTricity, Loci Controls, and Joby Aviation gave him a broad background in both electrical and mechanical design before returning to graduate school. The group is currently in the process of assembling the individual inverter modules into a larger multi-phase motor drive. Pourya Assem has designed the custom control hardware Barth is using to develop a hierarchical motor drive system. 

The final demonstration of a 200 kW motor drive will be conducted at Research Park in the newly dedicated 200 kW power test facility built by the POETS Engineering Research Center. (https://poets-erc.org/). The center for Power Optimization of Electro-Thermal Systems (POETS) is an NSF sponsored interdisciplinary engineering research center based at UIUC, which provides a context for researchers from Illinois, Howard University, University of Arkansas, and Stanford University to solve problems related to the cooling of power electronics. 

“We are seeking to demonstrate one of the lightest and most efficient inverters built to date” says Barth, “it is exciting to see what a smart team of dedicated researchers can accomplish.”