Designing an Electromagnetic Shield to Protect Rotor Components in Superconducting Synchronous Machines

AC loss presents a major design consideration for superconducting rotating machines. Superconducting dc field coils on the rotor are exposed to high-order-harmonic ripple fields from the ac stator, which can create an excessive heat load on the cryogenic system. To mitigate these effects, an electromagnetic (EM) shield surrounding the outermost surface of the rotor is usually employed to intercept stator harmonics before they can interact with the superconducting coils and other components inside the cold environment. Due to the important role of the EM shield, it is necessary to correctly characterize its behavior. The EM shield is characterized by a self-inductance and its mutual-inductance with the stator coils. These parameters can be calculated easily using 2-D analytical formulations or 2-D finite-element analysis (FEA). However, these methods tend to underestimate the inductances compared to 3-D FEA simulations which are more accurate but also more time-consuming. In this paper, we determine correction factors necessary to bring shield parameters calculated by 2-D analytical methods closer to the values obtained by 3-D FEA simulation. The corrections are independent of motor radius, and are applicable to both small and large machines. Their use enables rapid machine optimization using 2-D methods.