AC Loss of Superconducting Materials- Refined Loss Estimates of MgB2 Wires for Superconducting Motors and Generators

Superconducting winding are enabling for the development of the highest power density motors and generators for aircraft use. It is presently estimated that motors with standard Cu wire windings can reach a future limit of at best 20 kW/kg. On the other hand, motors with superconducting windings are estimated to be able to produce more than double this, roughly 45 kW/kg. Superconducting windings carry very large current densities (1 kA/mm ² winding J _e and more) and can generate much higher winding fields (even as high as 3-8 T in some designs), but do generate losses in the windings which must be removed at cryogenic temperatures. Here we focus on comparison of calculated and measured losses for one of the most promising superconducting materials for low loss motor and generator application, MgB ₂ . MgB ₂ is available in the form of wires (about 0.3-1 mm OD), and as triplet or 6 × 1 cables of these wires. Here we measure the hysteretic, coupling, and eddy current components of loss for a set of low-loss, multifilamentary MgB ₂ wires which have been recently developed. It is shown that present day MgB ₂ conductors are capable of 1-5 W/cm ³ in the 100-200 Hz range for field amplitudes of about 0.4 T, which makes them usable for motors and generators with sufficient attention to cooling design. Specific loss values are very dependent on frequency, field amplitude, and nature of the applied field (related to motor rotational speed, number of poles, and design) as well as conductor design. However, we compare low frequency results measured at 20 K and 0-3 T to theoretical expectation, and we then make extrapolations to frequencies up to 200 Hz (and give explicit expressions for such extrapolation). We in particular include here also the effect of maximum field amplitude variation on the average J _c , allowing us to include for the conductors a semi-bean approximation which allows for more realistic loss modelling results.