Influence of the Pole Pitch on the Performance of Superconducting Machines for Various Applications

High temperature superconducting (HTS) machines have been proposed for drive train applications which require a high torque density or high specific power, such as wind turbines or electric aircraft. The thickness of the cryostat wall and the thermal insulation included in the magnetic air gap highly affects the torque production of a partially HTS machine. The paper aims at 1) discovering how the magnetic air gap length and pole pitch are selected by varying their ratio <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$g_{\mathrm{{mag}}}/\tau _{\mathrm{{p}}}$</tex-math></inline-formula>, and 2) revealing the effects of the ratio on various applications of partially HTS machines. The research does not depend on specific design of the cryostat and thermal insulation but models them as a part of the magnetic air gap length. The ratio <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$g_{\mathrm{{mag}}}/\tau _{\mathrm{{p}}}$</tex-math></inline-formula> was selected to cover a wide range of design possibilities. Four topologies were compared to see their suitability for different ratios of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$g_{\mathrm{{mag}}}/\tau _{\mathrm{{p}}}$</tex-math></inline-formula>. Both a distributed winding and a concentrated winding were considered. Compared to the distributed winding, the concentrated winding is more sensitive to the change of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$g_{\mathrm{{mag}}}/\tau _{\mathrm{{p}}}$</tex-math></inline-formula> because the pole pitch is relatively small. A smaller pole pitch allows for more options of topologies especially for the concentrated winding if the minimum <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$g_{\mathrm{{mag}}}$</tex-math></inline-formula> can be small enough. If the minimum <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$g_{\mathrm{{mag}}}$</tex-math></inline-formula> is too large, the distributed winding option may be better because of its less sensitivity to a rising <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$g_{\mathrm{{mag}}}/\tau _{\mathrm{{p}}}$</tex-math></inline-formula>. Regarding applications, more topology options are available for HTS machines with a small pole pitch, such as high-speed propulsion motors. In this case, iron stator and non-iron teeth topologies may also be considered in addition to the fully iron topology. The topology option space is quite limited for HTS machines with a large pole pitch, such as wind turbine generators. In this application, the fully iron topology is still most torque-productive.