NASA's high efficiency megawatt motor (HEMM) is being developed to achieve the performance needed by single aisle class electrified aircraft. It is a 1.4 MW electric machine designed as a generator for NASA's STARC-ABL concept vehicle. It has performance objectives of greater than 16 kW/kg electromagnetic specific power and 98% efficiency. A significant flaw in the preliminary electromagnetic design of HEMM was recently discovered. The stator teeth in the preliminary design would have caused the magnetic flux in the rotating components of HEMM to oscillate at a very high frequency (12,240 Hz). Two independent energy loss analyses are presented to show that the frequency of this oscillation would have been high enough to cause eddy current losses that significantly exceed the rotor's loss limit (51 W), despite the very small magnitude of the oscillation (＜0.01 T). To eliminate these rotor losses while continuing to meet the target performance, it was determined that the stator teeth needed to be removed and the electromagnetic geometry of the motor needed to be revised. The revised, slotless HEMM design is summarized. Also presented is a sensitivity analysis of the new design to key unknown variables such as the temperature of and number of turns in the superconducting rotor coils and the stator winding's average temperature.
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