Design Approaches for Enhancing Critical Torque Ratios in Cryogenic Induction Motors

This paper explores advanced design strategies to improve critical torque ratios (CTRs) in cryogenic squirrel-cage induction motors (IMs) used in liquefied natural gas (LNG) pumps operating at very low temperatures (VLT, <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$-163^{\circ} \mathrm{C}$</tex>). In these cryogenic conditions, the torque-speed characteristics of IMs deviate significantly from those at room temperature (<tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\text{R T}, 20^{\circ} \mathrm{C}$</tex>) due to altered material properties. Notably, the rotor resistance decreases by approximately 2.4 times at <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$-163^{\circ} \mathrm{C}$</tex>, impacting both starting and breakdown torque behaviors. This study investigates the individual effects of key design parameters, including the number of turns, coil pitch and winding layers, ring thickness, rotor skew angle, and air-gap length on CTRs. Furthermore, it evaluates a geometrically optimized design and a modified rotor conductor using lead instead of aluminum. All numerical analyses are conducted using a 2-D, nonlinear, time-stepping finite element method (FEM) under both RT and VLT conditions. These insights contribute to the development of more reliable and efficient cryogenic motors for LNG applications.