Hyun-Mook Kim

High Temperature Superconducting Motor Cooled by On-Board Cryocooler

HTS (high temperature superconducting) motor that utilizes superconductor as the field winding has advantage of compactness, light weight and high efficiency, but it has an inherent disadvantage that the rotor must be cooled down at cryogenic temperature. Conventionally, HTS rotor was cooled by circulation of cryogenic fluid such as helium or neon, and the cooling system was installed outside of the motor to prevent it from being rotated. In this paper, however, the prototype of the HTS motor with an on-board cryocooler, is fabricated and tested. The experimental system is composed of the stator with conventional copper winding, the rotor with superconductor, and the rotating cryocooler as a novel concept of cooling system. The rotor is fabricated as the race-track coil with 2G, YBCO tape and situated in the 110 mm diameter cryostat. It is designed to minimize heat invasion from room temperature environment and be compatible with the coaxial structure of the cryocooler. The cryocooler is in physically and thermally contact with the HTS rotor while being rotated together. Two kinds of refrigerator, a Stirling refrigerator and an inline-type PTR (Pulse Tube Refrigerator) are used individually as the on-board cryocooler. The whole system is carefully integrated and fabricated for rotating stability. Several temperature sensors and voltage taps are installed to monitor the conditions of the HTS rotor and the cryocooler while their signals are acquired by the bluetooth data acquisition system during the operation. The HTS motor is successfully operated with 120 rpm of rotating speed. In the rotating test, 70 A current is supplied to the superconducting rotor. Further developmental design issues for on-board cryocooling system are addressed and discussed in this paper.

Detection of Laser Excited Lamb‐Wave Using Air‐Coupled Transducer and Wave Mode Identification Using Wavelet Transform

In this study, the laser source with arrayed linear slits generated Lamb‐wave with wavelengths, and an air‐coupled transducer received them. Modes of leaky Lamb‐wave at a plate had various oblique angles in air. By changing oblique angle of an air‐coupled transducer, a selected mode was received. Also, the received signal was obtained by wavelet transform for the analysis in domain of time‐frequency. We compared the theoretical dispersion curve and the spectroscopy by wavelet transform.

Non-Contact Tube Inspection Technique Using Laser Generation of Guided Wave and Its Reception by Air-Coupled Transducer

Ultrasonic guided wave has been widely used for the tube inspection. The conventional method is to use piezoelectric transducers that should be contacted to the target surface. In recent years, however, the non-contact method is strongly required in the automation of inspection process for the manufacturers as well as in dangerous environmental in-service inspection. In this paper, we have proposed a non-contact inspection method generating the ultrasonic guided wave by laser and receiving it by an air-coupled transducer. This method can generate and receive the guided wave of a specific mode with selectivity, which makes the interpretation of received signal clearer and resultantly improves the accuracy of inspection. Also the detected signal was analyzed by using the wavelet transform and it was shown that the wavelet analysis is useful for the mode identification. The proposed method was applied to the tube of 1mm thickness and 20mm outer diameter with three different types of artificial notch defects; through-wall, inside and outside of tube. The size of defects were 2∼8mm long in the circumferential direction, with 100μm width. It was proven that all kinds of defects were detectable. Finally we have developed a practical automatic inspection system, in which the inspection result is displayed in a 2-D image.Copyright

Nondestructive Evaluation of Degraded 2.25Cr-1Mo Steel and Estimation of Nonlinear Acoustic Effect Using Bispectral Analysis

The nonlinear acoustic effect has been considered as an effective tool for the evaluation of material degradation. In this paper, the applicability of the nonlinear acoustic effect using bispectrum analysis to the evaluation of degraded 2.25Cr-1Mo steel is investigated. First, artificial aging was performed to simulate the microstructural degradation in 2.25Cr-1Mo steel arising from long time exposure at 538°C. Second, ultrasonic nonlinear parameter was quantitatively measured by bispectrum and power spectrum. The nonlinear acoustic parameter using bispectrum analysis was found to be clearly sensitive to material degradation.Copyright

Laser Generation of Focused Ultrasonic Wave

An arc‐shaped line array slit is used for the laser generation of focused laser‐ultrasonic wave. The spatially expanded Nd:YAG pulse laser is illuminated through the arc‐shaped line array slit on the surface of a sample to generate the ultrasonic wave of the same pattern as the slit. Then the generated ultrasonic wave is focused at the focal point of which distance from the slit position is dependent on the curvature of slit arc. The relationship between the characteristics of the generated ultrasonic wave including the focusing performance and several design parameters such like as slit width and slit interval are investigated. By using the focused wave we can upgrade the inspection ability for the small size defect with the improvement of spatial resolution. Also this method can be combined with the scanning mechanism to get image just like we can get by the scanning acoustic microscope(SAM).

Reliability Assessment of Ultrasonic Nondestructive Inspection Data Using Monte Carlo Simulation

Ultrasonic NDE is one of important technologies in the life‐time maintenance of nuclear power plant. Ultrasonic inspection system is consisted of the operator, equipment and procedure. The reliability of ultrasonic inspection system is affected by its ability. The performance demonstration round robin was conducted to quantify the capability of ultrasonic inspection for in‐service. The small number of teams who employed procedures that met or exceeded ASME Sec. XI Code requirements detected the piping of nuclear power plant with various cracks to evaluate the capability of detection and sizing. In this paper, the statistical reliability assessment of ultrasonic nondestructive inspection data using Monte Carlo simulation is presented. The results of the probability of detection (POD) analysis using Monte Carlo simulation are compared to these of logistic probability model. In these results, Monte Carlo simulation was found to be very useful to the reliability assessment for the small NDE hit/miss data sets.

Evaluation of Thickness Reduction in a Thin Plate Using a Non‐Contact Guided Wave Technique

Ultrasonic guided waves are widely being studied and successfully applied to various non‐destructive tests with the advantage of a long range inspection. Recently, non‐contact methods are also adopted and combined with the guided wave techniques. In this paper, an advanced technique for the nondestructive detection of thinning defects simulating hidden corrosion in thin plates using non‐contact guided waves is presented. The proposed approach uses EMAT(Electro‐Magnetic Acoustic Transducer) for the non‐contact generation and detection of guided plate waves in aluminum plates. Interesting features of the dispersive behavior in selected wave modes are used for the detection of plate thinning. The experimental results show that the mode cutoff measurements provide a qualitative measurement of thinning defects and change in the mode group velocity can be used as quantitative parameter of thinning depth measurement.