Yungil Kim

Design of Damper to Protect the Field Coil of an HTS Synchronous Motor

High Temperature Superconducting (HTS) synchronous motors consist mainly of rotational (HTS coil) and stationery (copper coil) parts. The HTS coil running at a synchronous speed is not influenced by the magnetic field generated by the stator coil but is affected considerably at the initial or transient operation. Therefore, computational analysis to determine the optimal design of the damper is an important factor in protecting the field coil of a HTS synchronous motor. This paper presents the optimal design parameters of the damper for a 5 MW HTS synchronous motor using the computational analysis of the transient characteristics of the stator current, which is proportional to the alternating magnetic field of the stator coil.

Fabrication and Characterization of 4-T/203 mm RT Bore 2G HTS Magnet With No-Insulation Method

We fabricated superconducting magnet using second-generation (2G) high-temperature superconducting wire by SuNAM. Magnetic field strength at the center is 4 T, and room temperature bore diameter is 203 mm. The magnet consists of 30 double pancake coils (DPCs) with the inner diameter of 245 mm and outer diameter of 297 mm. All double pancakes were wound by no-insulation method and performance were tested separately before assemble. Tested DPCs were resistively connected by HTS tape(splice joint), and assembled coil was conduction cooled by a two-stage Gifford-McMahon cryo-cooler to the operating temperature of 8 K. The size of magnet is 452 mm in height. Current, voltage, and field strength were measured as a function of time with various ramping up and down conditions and results were compared with the simulated behavior. The coil generates 4 T when operating current ramped to 205 A by 0.03 A/s without quench. Initial cool down time was 72 h and the measure field homogeneity in 10 mm DSV was 0.015% and 0.012% in radial axis and vertical axis, respectively. The results showed that no-insulation winding method is a possible option for making compact magnet coil with sufficient structural integrity, thermal and electrical stability at the same time. The magnet showed quench at field strength of 4.49 T when ramped with 0.2 A/s to 235 A. The magnet showed same performance after recovery from quench.

Fabrication and Characterization of 3-T/102-mm RT Bore Magnet Using 2nd Generation (2G) HTS Wire With Conducting Cooling Method

A conduction-cooled high-temperature superconducting magnet using 2nd generation HTS wire, which has a room-temperature bore 102 mm in diameter, has been developed and tested up to 3 T with the operating temperature of 20 K. The magnet consists of 22 double pancake coils (DPCs) with an inner diameter of 140 mm and outer diameter of 182 mm. Twenty-two double pancake coils were tested separately at 77 K for checking the IV-curve. Selected DPCs were resistively connected by HTS tape (Splice joint), and an assembled magnet coil with the size of 182.5 mm diameter and 242 mm in height was conduction cooled by a two-stage Gifford-McMahon cryo-cooler to 20 K. Current, voltage, and field strength were measured as a function of time with various ramping up and down conditions. The resulting performance data of the assembled magnet agreed well with the expectation from FEM simulation. The aimed field homogeneity of 0.1% in 10 mm diameter sphere volume was proved when operating current was 141.6 A at 20 K with central magnetic field intensity of 2.9975 T by hall sensor. The magnetic flux density at center showed nonlinear dependence with ramping current within the range of 0.05 A/sec ~0.15 A/sec because of charging delay. However, saturated magnetic flux density showed the same value of 2.9975 T regardless of ramping rate.

Electrical Properties Analysis and Test Result of Windings for a Fully Superconducting 10 HP Homopolar Motor

This paper describes the electrical analysis and performance test result of windings of a 10 HP homopolar fully superconducting synchronous motor. The homopolar motor has high temperature superconducting (HTS) armature and field coils. The stationary field coils make the cooling system simple and easy because there is no cryo-moving part. The design of the motor and the analysis of magnetization loss generated in the superconducting armature windings are performed by finite element method. The critical current test results of the 2G superconducting wire, the pancake coil for field winding and the race track coil for armature winding are reported.

Characteristics of an HTS Pancake Coil in Persistent Current Mode Using Wind-and-Flip Winding Method

It has been well known that a coated conductor (CC), 2G superconducting tape, shows a higher critical current and better performance than the 1G, BSCCO one. However, there has still been no way to make a superconducting joint with CC, so it has been almost impossible to build a CC superconducting magnet in persistent current mode. As a viable solution for this, we have proposed a special winding technology called the Wind-and-Flip method. It shows a new possibility for the generation of a persistent current mode in a magnet with CC. First, we partially cut a wide CC tape into two pieces along the tape except at both end parts like cutting the abdomen of a fish. Then we wind two pancake coils with the cut but connected CC tape. Finally, one of the two coils is turned over, and then the magnetic fields from both coils would have the same direction with each other. In this paper, we made two high-temperature superconducting double pancake windings by the Wind-and-Flip method. One has turn-to-turn insulation layers while the other does not. They both have oxygen-free copper bobbins for the conduction cooling. First, they were magnetized by field cooling within a fine background magnet, and operated in persistent current mode at 77 K. As a result of the experiment, we measured and compared temporal stabilities for both pancake windings to investigate the possible use of these windings for magnetic resonance imaging or nuclear magnetic resonance applications. We have future plans of measuring the temporal stabilities of similar windings under lower temperatures and confirm the possibility of a high-field high-temperature superconducting magnet, which will be fabricated by stacking these pancake coils with larger cold bore size.

Characteristics of Rotating Armature Type High Temperature Superconducting Generators With Dual Field Windings for the Wind Turbine

We proposed an HTS wind power generator that has armature windings in the rotor. Field windings in the stator make a slightly higher magnetic field than the field windings in the rotor if both windings have same size of armature windings and field coils. In addition, we split the field windings into two and installed one of the split ones inside the armature, i.e., the so-called dual field windings. This structure is mechanically complex because the armature windings rotate between two stationary field windings. However, the output voltage of the 10-MW generator is 12% higher than that of the rotating field with this structural modification. We set up six different types of field winding arrangement and analyze the field magnitudes and field distributions in the armature and field windings of each one. Finally, the output voltages are calculated by a finite-element method at full load.

Persistent Current Mode of a 1-T-Class HTS Pancake Coil for NMR/MRI Applications

We had proposed a winding method, the so-called “wind-and-flip,” which enables a persistent current-mode operation of a high-temperature superconducting (HTS) pancake coil. Two pancake coils had been fabricated using this method and tested in a liquid nitrogen bath with an external magnetic field. The result showed that the time stability was not good enough to use for magnetic resonance imaging or nuclear magnetic resonance when they were magnetized by field cooling at 77 K. In this paper, the test results of a stacked HTS pancake coil for a center magnetic field of 1 T are presented. The coils were cooled to 20 K by conduction cooling using a Gifford-McMahon cryocooler. We measured the time stability of the persistent current-mode operation at 20 K, which shows a strong possibility of a high-field HTS magnet using this method.

AC Loss Analysis of Striated HTS Compact Cables for Low Loss Cable Design

Although the shape of high-temperature superconducting (HTS) tape is not good for cables, many researches are being progressed like Roebel conductors, compact cables, and HTS cable in conduit conductors. These cables can show good performances for dc application. However, the ac loss problem due to the shape of HTS tape has not been solved yet. In this paper, the ac loss characteristics of the striated HTS compact cable were analyzed. We made striations on the HTS tapes that composed the compact cable. The surface of the tape was scribed by laser. The magnetization loss was measured by the method using pick-up and cancel coils. We confirmed the effect of striation for reduction of the loss from the measured loss in straight and spiral samples.

Estimation of the Critical Current Density From the Measured Values of Perpendicular Magnetization Losses in HTS Coated Conductors

Stacking of conductors for large current capacity and striation for low loss are needed in large scale power applications of HTS superconductor. It, however, was difficult to measure superconducting properties of the HTS coated conductor non-destructively. Instead of direct measurement of the critical current of HTS coated conductor, indirect estimation from the measured magnetization loss can be used for the practical estimation of the critical current. The magnetization loss of a superconductor can be affected by the full penetrating magnetic field, and it tends to show an inflection point at the full penetrating magnetic field in the plot of magnetization loss vs. external magnetic field. The full penetrating magnetic field depends on the shape of the conductor and its critical current density, and so the effective critical current density can be estimated from the measured magnetization loss. In this paper, to prove the effectiveness of this indirect estimation of the critical current, we measured the perpendicular magnetization losses in short samples of 2G HTS coated conductors with different shapes which includes striated coated conductors were measured, and the estimated critical current densities were compared with those directly measured.

Thermal Analysis of PCS for an HTS Pancake Coil in Persistent Current Mode

It is well known that high temperature superconductors (HTS) are promising for a magnet application, but it has been almost impossible so far to apply them to the magnet in persistent current mode because of poor qualities of their superconducting joint at higher current as well as lower index values. In this paper, in order to get rid of the joints from an HTS superconducting magnet, a prototype HTS pancake coil using YBCO coated conductors by the technique of wind-and-flip pancake coil was made and will be tested later. This paper presents a design, fabrication, and thermal analysis of the prototype pancake coil. The coil will be cooled down around 20 K by conduction cooling using a GM cryocooler. The expected current drift rate for an MRI application has not been achieved, probably caused by large flux flow resistance of coated conductor at higher temperature, 77.3 K. A temperature dependency of temporal stability of the persistent current will be measured by monitoring the long-term drift of the magnetic field at different operating temperatures by Hall sensors.