John Reband with adviser K. Haran

Previous research on weight optimization for superconducting electric machines within efficiency thresholds has focused solely on machine efficiency. Integrating these machines with aircraft for propulsion applications requires an understanding of how the machine-design parameters trade off with overall system efficiency. The propulsive efficiency of a propulsion motor and direct-drive propeller is greatly influenced by the nominal shaft speed and machine diameter when converting the supply power to useful thrust. While a large diameter may be beneficial for a light, efficient machine, the corresponding decrease in propulsive area is highly detrimental to the propeller efficiency and thus penalizes the overall efficiency.
The following method was developed to optimize the electric motor design for propulsion system weight and efficiency In order to quantify this trade-off in the context of conceptual aircraft design. Using a genetic algorithm, a space of machine designs is created for each generation. The torque and flux are modeled using finite element analysis and the losses determined analytically. With the machine modeled, the propeller performance is analyzed with blade-element theory. The fitness function determines the locus of optimal designs, maximized for the lightest machines that confer the highest system efficiency. This process is illustrated in Figure 3.

Motor weight and system efficiency optimization chart

Figure 3: Flowchart illustrating co-design optimization process

One useful output from this analysis is a relationship between motor weight and system efficiency. Quantification of the marginal increase in efficiency per-unit increase in machine mass enables aircraft designers to make informed decisions on machine size. A plot of such data is displayed in Figure 4. This work, part of the CHEETA program, is funded by NASA.

Analysis plot of motor weight vs. system efficiency

Figure 4: Plot of propulsion system efficiency vs. motor weight