Cooling, thermal design, and stability of a superconducting motor
A Cessna-type aircraft is to be powered by a motor that utilizes high temperature superconducting (HTS) components in the inductor. This motor is a novel design that uses both BSCCO (bismuth strontium calcium copper oxide) tape wound in pancake shapes and single domain, bulk YBCO (yttrium barium copper oxide) plates to create powerful magnetic fields capable of meeting requirements for an aircraft. The HTS inductor design generates a magnetic field outward to a rotating, non-superconducting armature. The BSSCO pancakes generate a magnetic field to trap magnetic flux in the YBCO plates via the Field Cooling method (FC). The use of FC creates very powerful magnetic fields but requires a multi-step cooling schedule for the inductor design. To properly trap flux using FC, the BSCCO pancakes must first be cooled to the operating temperature and generate an applied magnetic field while the bulk YBCO plates remain above the critical temperature. The YBCO plates are then cooled near the operating temperature, thus trapping flux in the plates. The current in the pancakes is then reversed and the magnetic fields are generated, then the YBCO plates are further cooled to the steady state temperature. This process of cooling the BSCCO pancakes and then cooling the YBCO plates is called the 3-stage cooling.
The cooling of the motor is by conduction due to the mobile application of aero-propulsion. The conduction-cooled inductor is constructed along with a cooling apparatus that includes an aluminum central cylinder attached to a cryocooler, G10 rings, and heaters that aid in the 3-stage cooling process. Simulations were performed that model the heat loads, cooling schedule from room temperature to operational temperature, and the 3-stage cooling to aid in the design. These modeling results show the temperature gradients in the inductor and HTS components and are verified experimentally. A full-scale, mockup inductor has been constructed and is cooled with a cryocooler in a cryostat. The cooling inductor final design is shown with modeling results and the proof-of-principle or a motor utilizing HTS materials in the inductor has been provided. A prototype of the motor should be built and tested based on these electromagnetic and cooling designs.
The use of heaters near the YBCO plates is required in the 3-stage cooling design. The YBCO has trapped-flux that is dependent on the operating temperature and the stability of the trapped-flux is critical to the motor design. Work has been done that experimentally tests the stability of trapped flux in YBCO plates. A heat impulse is inputted into a YBCO sample that is fully penetrated in current via FC. The experiments were performed in a sample chamber that has temperature and applied magnetic field controllability. The change in the magnetic field and temperature of the sample is measured and analyzed before and after the heat pulse using Hall probes. The experimental data suggests that there is no thermal runaway loss in the trapped-magnetic flux for a small heat input and an operating temperature for which the sample has maximum stability.
A cooling apparatus was designed to cool the inductor of a HTS motor. The electro-magnetic design utilizes field cooling to trap flux and this was accomplished with 3-stage cooling process. The cooling design was validated using simulations and experimental data. The cooling apparatus showed the feasibility of the inductor to trap flux in the plates. The stability of the trapped flux was also studied. Experimental data shows that there is no thermal runaway when heat is inputted into a sample and an operating temperature exists that suggests a maximum stability. The physics of the stability experiment was uncovered using an analytical model and a FEA model. Also shown was the effect of the cooling environment on the sample during the heat impulse. The stability models showed that the data are the results of the cooling environment and the competing effects of current density and specific heat, both functions of temperature. (Abstract shortened by UMI.)