Myunghwan Ku

Properties of High Temperature Superconducting Magnet With Optimized Air Gap Between Pancake Windings

In a magnet with pancake windings, inserting a gap between the pancake windings can increase the central magnetic field. This paper shows the optimized length of each gap is able to maximize the central magnetic field. Inserting a gap also improves field uniformity because the pancake windings are spread across a wider area. A high temperature superconducting (HTS) magnet consisting of BSCCO-2223 pancake windings was chosen for the calculation model to show the effectiveness of the proposed method. The E-J relation was used to determine the critical current, and an evolution strategy was adopted for the optimization of gap length. The results of calculations show that the critical current and the central magnetic field increased by 86.0% and 33.7%, respectively, for a magnet consisting of ten pancake windings.

The Optimal Air Gap Between the Pancake Windings in a HTS Magnet Consisting of Insert and Outsert Magnet

In a high temperature superconducting magnet consisting of pancake windings, the perpendicular magnetic field considerably reduces the value of the critical current of the outer pancake windings due to anisotropy. An air gap was inserted between each pancake winding in this paper to reduce the decrement of the critical current in each pancake winding. In a low temperature superconducting magnet, the central magnetic field decreases when there is an air gap between the pancake windings. On the other hand, the central magnetic field of a HTS magnet increases when an air gap is provided. The properties of the HTS insert/outsert magnet having an air gap between the pancake windings are examined in this paper. YBCO wire and BSCCO wire were used in the insert and the outsert magnets, respectively. An E - J relation and the evolution strategy were adopted to calculate the optimum critical currents of both magnets. The calculation results showed that there was an optimum air gap which maximized the central magnetic field. The optimum air gap was dependent on the specifications of the HTS magnet.

Characteristics of Open-Loop Current Sensor with Temperature Compensation Circuit

Open-type current sensors have been commonly used for DC motor controller, AC variable controller and Uninterruptible Power Supply. Recently they have begun to be used more widely, as the growth of renewable energy and smart-grid in power system. Considering most of the open-type current sensors are imported, developing the core technology needed to produce open-type current sensors is required. This paper describes the development and test results of open-type current sensors. Design of C type magnetic core, selection and test of a Hall sensor, design of current source circuit and signal conditioning circuit are described. 100A class DIP(Dual In-line Package) type and SMD(Surface Mount Devide) type open-type current sensors was made and tested. Test results show that the developed open-type current sensor satisfies the accuracy requirement of 2% and linearity requirement of 2% at 100 A of DC and AC current of 60Hz. Temperature compensation was carried out by using a temperature compensation circuit with NTC(Negative Temperature Coefficient) thermistor and the effect of the temperature compensation are described.

Properties of an inductance type sensor for position control of a long stroke actuator

In large actuators for water gate control of hydraulic dams, precise position measurement of the actuator cylinder is very important. An inductance type sensor can be used to measure the position of the actuator cylinder. This paper presents the effects of the slot shape on the properties of the inductance type sensor consisting of a permanent magnet and pick-up coils. The inductance type sensor is able to measure the position of actuator travelling over 10 m with 0.5 mm resolution. Result of the calculation is able to provide the data for choosing the appropriate shape of the slots to produce the proper magnitude of flux linkage and to minimize the mechanical stress imposed on the cylinder.

Design and fabrication of a 300A class general-purpose current sensor

Current sensors are used widely in the fields of current control, monitoring, and measuring. They have become more popular with the increasing demand for smart grids in a power network, generation of renewable energy, electric cars, and hybrid cars. Although open loop Hall effect current sensors have merits, such as low cost, small size, and weight, they have low accuracy. This paper describes the design and fabrication of a 300A open loop current sensor that has high accuracy and temperature performance. The core of the current sensor was calculated numerically and the signal conditioning circuits were designed using circuit analysis software. The characteristics of the manufactured open loop current sensor of 300 A class was measured at currents up to 300 A. According to the test of the current sensor, the accuracy error and linearity error were 0.75% and 0.19%, respectively. When the temperature compensation was carried out with the relevant circuit, the temperature coefficients were less than 0.012%/°C at temperatures between -25°C and 85°C.

Efficient Calculation Method of the Critical Current of Nested High-Field Magnets

A load line method has been used to calculate the critical current of superconducting magnets. However, in high-temperature superconducting tapes that have a rectangular cross section, as the critical currents vary depending on the direction of an external magnetic field, the load line method cannot be used to precisely calculate the critical currents of the HTS magnets because it cannot fully consider the magnetic anisotropy of the HTS tape. In nested HTS magnets that have multiple layers, calculating the critical current considering the magnetic anisotropy is very time consuming. This paper presents an efficient calculation method to calculate the critical current of nested HTS magnets. The secant method and the fixed-point iteration method are used to reduce the calculation time. The critical currents of two nested HTS magnets are calculated: a double magnet consisting of a YBCO inner coil and a BSCCO outer coil and a triple magnet consisting of a YBCO inner coil, a YBCO middle coil, and a BSCCO outer coil. The calculation results demonstrate that it is possible to very effectively calculate the critical current of nested HTS magnets by using a combination of the secant method and the fixed-point iteration method.

Characteristics of an HTS Magnet Wound With 12-mm Zr-Doped HTS Tape

This paper examined the properties of an high-temperature superconducting (HTS) magnet, such as the Ic-B relation and critical current, with pancake windings of a 12-mm-wide Zr-doped HTS tape. Considering the width of the Zr-doped HTS tape was 12 mm, which was three times wider than that of a conventional 4 mm tape, the current distribution at the cross-section of the 12-mm tape was assumed to be non-uniform. The HTS magnet consisting of four pancake windings of the 12-mm tape was chosen as the model for calculations and measurements. The inner diameter and the number of turns of the pancake winding were 30 mm and 88 turns, respectively. When the critical current of the HTS magnet was calculated assuming the same critical current for all pancake windings, the critical current of the whole magnet was limited by the middle pancake windings. The limiting of the magnet current by the middle pancake windings was due to properties of the Zr-doped HTS tape. After allocating the pancake windings with higher critical current to the middle part of the HTS magnet, the critical current was limited by the top and bottom pancake windings.

Test Results of Central Magnetic Field and Field Uniformity of a High Temperature Superconducting Magnet With Gap

Due to the magnetic anisotropy of a High Temperature Superconducting (HTS) tape, inserting a gap between pancake windings can increase the central magnetic field and improve the field uniformity of HTS magnets consisting of pancake windings. Although inserting gaps with constant length can increase the central magnetic field, optimizing the length of each gap to maximize the central magnetic field is preferred. This paper shows the test results of HTS magnets with and without optimized gaps. Both magnets consisted of 12 BSCCO-2223 pancake windings. The test results were compared with the calculation results, and confirmed that the central magnetic field of the HTS magnet with optimized gap was increased compared with the HTS magnet without gap. The magnetic field uniformity of the HTS magnet was also improved.

Performance Evaluation of an YBCO Insert and a BSCCO Outsert Magnet Manufactured With Double Pancake Windings

When there is an air gap between the pancake windings in an HTS superconducting magnet, the central magnetic field is increased. To prove the effectiveness of the air gap for increasing the central magnetic field, an HTS magnet with an outsert and an insert magnet was manufactured and tested. The test results proved that the air gap between the pancake windings of the outsert magnet increased the central magnetic field by 11.0%. The critical current of the outsert magnet with the air gap increased by 73.9%. The uniformity of the magnetic field near the center of the magnet also increased. The measured values were compared with the calculated values.