Motors and generators with power densities in the 30-40 kW/kg are of great interest for electric aircraft propulsion, including both fixed wing and VTOL systems. Superconducting are enabling for such motors. However, at higher frequencies we must begin to consider the influence of skin depth effects as well as harmonics. For superconductors, there are three different regimes of skin depth effects, each with their own critical frequencies and loss characteristics. These are important because not only the main operational frequencies are important for loss generation, but also the higher frequencies from the inverters which drive the motors. Here we explore the AC loss characteristics of MgB2 superconducting wires as a function of frequency up to inverter relevant frequencies (e.g. 10 kHz). We makes estimates for three critical frequencies, fc1, fc2, and fc3, as well as the losses in these regimes. We compare these estimates to measurements made on a 1 m OD MgB2 ring segment coil. The excitations are in the rage of 50 Hz to 5 kHz, and the amperage injected is up to 3 A; the goal here is to explore harmonic losses. Losses are measured by balancing heat generation and cryocooler power in a conduction cooled cryostat. The losses are seen to be proportional to the square of the frequency in this regime. The effects of high frequency harmonics are then explored, focusing on functional dependence. Comparisons are then made between these losses and those to be expected from Al-Litz wire operating in the same regimes and a range of temperatures. Limiting cases for loss comparisons to ambient and cryogenic conductors are explored.

Keywords: MgB2, AC Loss, High Frequency, harmonics