High-temperature Superconducting Field Coils Research Articles

A Double Rotor Flux Switching Machine With HTS Field Coils for All Electric Aircraft Applications

The increasing demand for high-power density motors in electric transport industries opens a new research opportunity to develop motor topologies with less weight and high efficiency. In particular, all-electric aircraft applications require very high power density motors. This paper shows the design of a new 1 MW 20-pole/15-slot double rotor flux switching machine with high-temperature superconducting field coils and thermal management system. The proposed motor features an air-core stator. Aluminum Litz wire is used for the armature conductors, and the YBCO- high-temperature superconducting material is used for the field coils. The armature winding and field coils operate at 95K and 65K, respectively. The active part power density including rotors, armature windings and field coils obtained with the proposed design is 29.3kW/kg. The specific power density of proposed motor considering mechanical, support structure and TMS is about 18.5 kW/kg. The efficiency can be higher than 98.7% which satisfies the requirement of electric aviation.

Design and Characteristic Analysis of an Axial Flux High-Temperature Superconducting Motor for Aircraft Propulsion.

In line with global environmental regulations, the demand for eco-friendly and highly efficient aircraft propulsion systems is increasing. The combination of axial flux motors and superconductors could be a key technology used to address these needs. In this paper, an axial flux high temperature superconducting (HTS) motor for aircraft propulsion was designed and its characteristics were analyzed. A 2G HTS wire with high magnetic flux characteristic was used for the field winding of the 120 kW axial flux HTS motor, and the rotational speed and rated voltage of the motor were 2000 rpm and 220 V, respectively. The axial flux HTS motor implements a revolving armature type for solid cooling of the HTS field coil. The electromagnetic and thermal features of the motor were analyzed and designed utilizing a 3D finite element method program. The HTS coil was maintained at the target temperature by effectively designing the current lead and cooling system to minimize heat loss. These results can be effectively used in the design of propulsion systems for large commercial aircraft in the future as well as for the design of small aircraft with less than 4 seats.

Performance evaluation of a superconducting flywheel energy storage system incorporating an AC homopolar motor layout

In this paper, a novel high-temperature superconducting flywheel energy storage system (SFESS) is proposed. The SFESS adopts both a superconducting magnetic bearing and a superconducting alternating current (AC) homopolar motor. The superconducting AC homopolar motor has structural advantages in high-speed operation, however performance of the high-temperature superconducting (HTS) field coil is easily affected by the external magnetic field generated by armature windings in the process of flywheel charge and discharge, and should be paid attention. We established an electromagnetic-thermal co-calculation model of the HTS coil to analyze the coil characteristics in this application. Simulation results demonstrate that the magnetic flux density is small near the central region of the coil and is the largest at the edges of the coil. The temperature rise caused by the change of magnetic field distribution reduces the critical current density of the tapes and increases AC loss. Furthermore, we studied the grid connection control strategy of the SFESS, which adopts double closed-loop control and direct power control, and verified the feasibility of the control strategy by multi software real-time and dynamic co-simulation. Works in this paper give some guidance to further development of a 50 kW·h SFESS prototype. All the configurations of the SFESS, the method of calculating AC loss of HTS field coil, and the control strategy of the SFESS system could be extended to large commercial units.

Temperature Distribution in the Field Coil of a 500-kW HTS AC Homopolar Motor

AC homopolar motors with high-temperature superconductors (HTSs) are considered to be a promising technology for high-power applications. With superconducting components being placed exclusively in nonrotating parts, a motor can reach high rotational speeds. However, the dynamic behavior of the system creates a nonnegligible magnetic ripple field, which generates heat and makes thermal management of the cryogenic system more challenging. In this article, we present a case study for the thermal behavior of a conduction-cooled high-temperature superconducting field coil assembly in a 500-kW asynchronous homopolar motor. The analysis focuses on the investigation of the magnetic ripple field and its impact on heat generation due to ac loss in the HTS field coil and eddy current loss in copper parts present in the assembly. Using electromagnetic and thermal finite-element models, we construct a detailed description of the thermal bus design, which considers different cooling scenarios and sizing of the conductive elements. Depending on the geometry of the thermal bus, we observe heat loads from ac losses of up to 0.4 W and eddy current losses in the copper elements of up to 1 W. These heat loads translate to temperature increases as high as 10 K. Techniques to reduce the temperature rise based on thermal bus redesign are presented, along with recommendations for the most promising cooling scenarios and optimal thermal bus geometry.

Design and Characteristic Analysis of a Homopolar Synchronous Machine Using a NI HTS Field Coil

This paper deals with a homopolar synchronous machine (HSM) applying high-temperature superconducting (HTS) field coils. Superconductors, especially high-temperature superconductors, have high potential as advanced materials for next-generation electrical machines due to their high critical current density and excellent mechanical strength. However, coils made with high-temperature superconductors have a high risk of being damaged in the event of a quench due to the intrinsic low normal zone propagation velocity (NZPV). Therefore, the coil protection issue has been regarded as one of the most important research fields in HTS coil applications. Currently, the most actively studied method for quench protection of the HTS coils is the no-insulation (NI) winding technique. The NI winding technique is a method of winding an HTS coil without inserting an insulating material between turns. This method can automatically bypass the current to the adjacent turn when a local quench occurs inside the HTS coil, greatly improving the operating stability of the HTS coils. Accordingly, many institutions are conducting research to develop advanced electrical machines using NI HTS coils. However, the NI HTS coil has its intrinsic charge/discharge delay problem, which makes it difficult to successfully develop electrical machines using the NI HTS coil. In this study, we investigated how this charging/discharging problem appeared when the NI HTS coil was used in an HTS homopolar synchronous machine (HSM) which is one of the electrical machines with a high possibility of applying the HTS coil in the future because it has a stationary field coil structure. For this, the characteristic resistances of HTS coils were experimentally obtained and applied to the simulation model.

Development and Testing of 2G High Temperature Superconducting (HTS) Field Coils for HTS Synchronous Machines

High temperature superconducting (HTS) synchronous machines are compact and light weight machines with higher torque to weight ratio, lower noise and better dynamic response for strategic and critical industrial applications. The development of HTS field coil is one of the main technological challenges in realizing the HTS synchronous machines, as it involves the selection of HTS tape, winding technology, handling of the HTS tape while winding on a winding machine and testing of the developed HTS coil at desired cryogenic temperatures. With advancements in HTS tape technology, recent 2G HTS tapes are offered with higher engineering current densities with respect to 1G HTS tapes even at higher magnetic fields. Hence, the replacement of 1G HTS field coils of previously designed 200 kVA HTS synchronous machine with 2G HTS field coils at the same operating cryo-temperature shows a potential to enhance its capacity to 350 kVA (more than 75% increase in machine output). In the present work the details of design, development and testing of a double pancake type 2G HTS field coil are presented for capacity enhancement of earlier 1G based 200kVA HTS synchronous machine.

Investigation on Key Parameters of NI HTS Field Coils for High Power Density Synchronous Motors

As global environment regulations are gradually strengthened, active research has been conducted on electric propulsion applications. High power density is one of the major research interests, as well as a machine's efficiency for a high-performance propulsion system. Among various electric propulsion technologies under development, a motor incorporating the no-insulation (NI) high-temperature superconducting (HTS) winding has the advantages of high current density and robustness to external disturbances, which are suitable for high specific power. However, still, there is little research on the motor with NI HTS coil. Therefore, as basic research, we investigated the potential application of the NI HTS motor, then derived required machine parameters and NI HTS coil designs in this paper. For that, first, basic parameters of the existing propulsion motors were investigated, then the target application and performance of the NI HTS motor were determined. Based on the target performance, conceptual designs of a motor with NI HTS coil were obtained by changing key design parameters including rated power, main dimensions, operating temperature, and width of air-gap. Calculated parameters were numerically compared, and expected design requirements and challenges were suggested.

Effects of stainless steel thickness and winding tension on electrical and thermal characteristics of metal insulation racetrack coils for 10-MW-class HTS wind generator

Experimental results on the electrical and thermal characteristics of GdBCO racetrack coils co-wound with stainless steel (SS) tapes of various thicknesses and winding tensions are presented in this paper. The SS thickness and winding tension are identified as the factors affecting the charging time and thermal stability of metal insulation (MI) coils. To verify these characteristics, three types of MI-SS racetrack coils were fabricated with various SS thicknesses and winding tensions: 100-μm SS thickness and 10-kgf winding tension; 100-μm SS thickness and 5-kgf winding tension; and 50-μm SS thickness and 10-kgf winding tension. Further, three MI-SS racetrack coils were characterized by sudden discharging, charging, and overcurrent tests. The sudden discharging and charging tests were performed in a steady state to investigate the decay time constant of the center magnetic field discharging the coils and the delay time of the magnetic field charging the test coils, respectively. To estimate the thermal stability of the three MI-SS coils, overcurrent tests were conducted in a transient state at 1.1 times the critical current (Ic) and 1.05 Ic. Based on the experimental results of three small-scale test coils, the total contact resistance between the layers of the test coils was calculated and applied to design a field winding using a second-generation high-temperature superconducting (HTS) tape for a 10-MW-class HTS generator used in an offshore wind turbine environment. Subsequently, an equivalent electrical circuit model was proposed to analyze the electromagnetic response and thermal stability characteristics of three HTS field coils of a 10-MW-class HTS generator with the key parameters of the HTS field coils, which were obtained using electromagnetic finite-element analysis. Finally, the charging delay time and thermal stability of the three HTS field coils were compared and analyzed in detail.

Design and Feasibility Study of a Performance Evaluation System for a Large-Scale HTS Generator Under Short-Circuit Conditions

Generally, in a high temperature superconducting (HTS) generator, a large magnetic air gap exists because of needed space for a cryostat or non-magnetic cores in the HTS generators. Consequently, the inductance becomes lower than conventional generators. As a result, the short circuit current becomes as high as more than 10 times the rated current. The design of the mechanical structure of a generator is very challenging to endure a high force and torque at the HTS field coils. Therefore, a performance evaluation system (PES) is required to test the HTS field coil and support structures of the generator. In this paper, the design and feasibility study of a PES for and HTS field coil of large-scale HTS generator are presented. The 10 MW class HTS generator and PES was designed based on the electromagnetic analysis results obtained using the finite element method. The torque and force of the HTS field coil were analyzed. As a result, the HTS field coil can be practically tested using the proposed PES whether the structure of the coil can withstand the high torque and force of the generator. The proposed PES design and short-circuit analysis results can be effectively used for the development of large-scale HTS wind power generation systems.

A Novel Fault Diagnosis Method for High-Temperature Superconducting Field Coil of Superconducting Rotating Machine

In this paper, a new method is presented for sensitive quench detection in high-temperature superconductor (HTS) rotating machinery. The normal zone propagation velocity of an HTS is about 1000 times slower than that of a low-temperature superconductor. Therefore, the propagation of normal zone resistance, which occurs when the HTS transits from the superconducting state to the normal state, is also slower. Thus, it is difficult to detect the abnormal signals by voltage measurement using voltage taps. Moreover, the monitoring signal includes noise generated by interaction between the HTS rotating machinery and the industrial environment. Therefore, when quenching occurs in the HTS rotating machinery, a thermal runaway occurs in the hot spot. Furthermore, the magnetic energy stored in the HTS coil can damage the machinery. For these reasons, a new method is proposed for sensitive quench detection that reduces the noise generated from the power supply and from the HTS rotating machinery, using both an RLC resonance circuit and fast Fourier transform analysis.

Design of an HTS module coil for a 750 kW-class superconducting wind power generator

Recently, high temperature superconducting (HTS) generators are being studied with the advantages of high efficiency, light weight and small volume compared with the conventional generators. However, the HTS generator requires a large vacuum vessel and has problems such as difficulties in maintenance of the HTS coil. These problems can be overcome through the modularization of the HTS field coil. This paper designs an HTS module coil for a 750 kW-class superconducting wind power generator. The HTS module coil was designed based on the electromagnetic analysis results obtained using a 3D finite element method. For detail structure of the module coil, the heat load analysis and the mechanical stress analysis were performed. As a result, the total heat load was minimized the temperature of the HTS coil could be kept below 30 K. The von Mises stress of the supports was less than the allowable stress of the glass-fiber reinforced plastic material. The design results of the HTS module coil can be effectively utilized to develop large scale the superconducting wind power generators.

Design and Comparative Analysis of Performance Evaluation Systems for a Large-Scale HTS Generator

Generally, in a high temperature superconducting (HTS) generator, the electromagnetic force can cause problems due to high current density and magnetic field. HTS field coils must therefore be structurally designed and analyzed to ensure the stability of the generator before the HTS generator is built. As part of the stability assessment, this paper designs a performance evaluation system (PES) for large-scale HTS generators and discusses comparative analysis with a generator model. The PES for a 10 MW class HTS generator was designed based on the electromagnetic analysis results obtained using the finite-element method. The detailed structures of the swing and fixed-type PESs were investigated taking into account the HTS coil, support, and stator components. The torque and force of the HTS field coil were analyzed and the results were compared with the generator model considering the amplitude and direction of the force of the HTS field coil to confirm the effectiveness of the PESs. As a result, the output torque and tangential force of the PESs were the same as those of the HTS generator. Although both types of PES can evaluate the torque and force of an HTS generator, the fixed-type PES is more suitable for the characteristic test of HTS field coil of large-scale HTS generators.

Design and Study of a 2G HTS Synchronous Motor With Brushless HTS Flux Pump Exciters

A high temperature superconducting (HTS) synchronous motor with brushless HTS flux pump exciters was designed and is under construction. This HTS synchronous motor is composed of copper armature windings, HTS field windings, HTS flux pump exciters, vacuum chamber, rotary gas seal, etc. The double racetrack-type HTS field coils are wound with second-generation (2G) YBCO tapes and cooled directly by the cold heads. The designed operation temperature is about 30 K. An HTS flux pump was designed to be used as DC power supply to inject large currents into the HTS field coils, which eliminates the slip ring in the motor and avoids thermal load from current leads. The novel design of multiple sources enables us to control the excitation current of the HTS field winding for each pole separately and to prevent all the series field coils from stopping at the same time due to the quench of one coil. To describe the performance characteristics of the HTS motor, an electromagnetic design was carried out with 2-D finite-element method models. Finally, the total losses of HTS synchronous motor are calculated. The next step is to manufacture and install the HTS exciters, HTS field windings, etc. into the HTS synchronous motor.

A New Outer-Rotor Hybrid-Excited Flux-Switching Machine Employing the HTS Homopolar Topology

Currently, studies of flux switching machines are actively underway owing to several advantages of these machines, including their sturdy rotor structure and high output capability. This paper deals with an outer-rotor hybrid-excited flux-switching machine (FSM). The proposed machine embraces a homopolar structure and utilizes permanent magnets (PMs) for field excitation and a high-temperature superconducting (HTS) coil for flux regulation. The stator houses the HTS field coil, PMs, and armature windings. The outer rotor consists solely of an iron core. Thus, the machines are cost effective and can serve as a solution to the design and fabrication complexities of field current supplying and cooling systems. In this paper, the machine performance outcomes are analyzed using the 3D finite element method (FEM), and the validity of the proposed machine is verified.

Hardware Integration and Performance Analysis of a 10 kW HTS Wind Power Generator

The use of high temperature superconducting (HTS) generators in a large-scale wind power generation systems has drawn much attention as a contemporary research topic. In this paper, a prototype 10 kW HTS generator was designed based on the suggested design process considering electromagnetic, thermal and mechanical characteristics of the generator. We estimated the output power of the generator and operating current of the HTS field coils was decided considering the critical current and perpendicular magnetic field of the HTS wire. The thermal characteristics of the HTS generator was analysed to confirm that it was cooled to the operating temperature of 30 K. The torque tube and sticks of the HTS rotor part in the generator were designed and analysed to withstand mechanical stress considering the torque and thermal contraction. Based on these results, the HTS generator was built, and tested to prove the adequacy of the established design process of the generator. The measured cooling temperature of the fabricated generator was cooled to the target temperature of 30 K, and the magnetic field values of the HTS field coils and armature were the same as the simulation values. Finally, the output power of the generator was reached to the target output power of 10 kW. In this paper, the detailed results of the operating performance of the 10 kW HTS generator are discussed in detail.

Development and testing of a 1 G based high temperature superconducting (HTS) double pancake coil for HTS synchronous machines

High Temperature Superconducting (HTS) synchronous machines are highly efficient, compact, lighter, low noise, high output power to weight ratio and better dynamic response machines compared to conventional copper conductor based machines. HTS synchronous machines are used worldwide for various strategic and critical industrial applications, because of the above mentioned advantages. Successful development of HTS synchronous machine requires HTS, electrical, mechanical, cryogenic and vacuum technologies. The major components of a HTS rotor and copper winding stator topology synchronous machine are air gap copper winding stator, HTS rotor, excitation system, cryogenic cooling and vacuum systems. HTS rotor consists of HTS tape wound pole coils, which generate DC magnetic field when DC current passes through them. There will be no joule heat in the HTS pole coils due to superconductivity. Each pole coil consists of few numbers of field coils (in the form of double pancake coils) which are stacked to form a pole. One of the main technical challenges for manufacturing HTS rotor is development of HTS field coil. The development of a HTS field coil involves the selection of HTS tape, winding technique, handling of the HTS tapes while winding and testing of the HTS coil at cryogenic temperatures of 77 K and 35 K. In the present work, a HTS racetrack double pancake coil using 1 G HTS tapes was fabricated and tested. The details of design, development and cryogenic testing of 1 G HTS tapes wound double pancake coil are presented in this paper.

Electromagnetic Design of Superconducting Synchronous Motors for Electric Aircraft Propulsion

This paper evaluates the electromagnetic characteristics as to two types of superconducting motors: one is the superconducting motors adopting high-temperature superconducting field coils and copper armature windings; and the other is fully-superconducting motors, which have high-temperature superconducting field coils and MgB 2 superconducting armature windings. Analytical formulas and FEM are employed for the designs of 1.5-, 3.0-, and 5.0-MW motors. We consider how to reduce ac loss in MgB 2 winding structure as well. The analysis of our results, which was conducted mainly over output density and efficiency, suggests that the adoption of superconducting motors is more realistic in realizing electrical aircraft propulsion system in terms of higher output; the analysis results show that the fully-superconducting motors have a potential to reach over 16 kw/kg.

Characteristic Analysis of a 1-kW-Class HTS Motor Considering Armature Current Information

We present the results of a two-dimensional electromagnetic finite-element analysis of a 1-kW-class high-temperature superconducting (HTS) wound-field synchronous motor that considers armature-current information, i.e., current phase angle β and current I a , for maximum torque per ampere and flux-weakening control. The torque performance was analyzed using the d- and q-axis inductances, which were estimated when β and I a were varied. Then, the characteristics of the magnetic field distribution and operating current margin of the HTS field coils were investigated based on various β and I a values. Finally, suitable β and I a were estimated as references for a future study.

Characteristic Analysis of an HTS Flux-Switching Synchronous Generator With NI-Type HTS Field Coils

This paper presents the characteristics analysis of the flux-switching synchronous generator (FSG) applying no-insulation (NI)-type high-temperature superconducting (HTS) field coils. The HTS FSG can be cost effective, since the field coils and armature windings are both installed in the stator, while the rotor is only made of iron core with tooth. Along with low cost, since HTS field coils in the FSGs are located in the stationary part of the generator, FSG can be a solution for the design complexities of the excitation and cooling systems for HTS field coils. In addition, several studies on HTS coils without turn-to-turn insulation (NI) have been reported for HTS field coils for applying in electrical rotating machines. The NI-type winding technique enhances the stability of the HTS field coil since the overcurrent is automatically bypassed through the contacts between winding turns. Nevertheless, there is still an issue about whether the NI-type HTS field coil can be applied to the electrical rotating machines. In this study, we fabricated and tested an NI-type HTS field coil. In addition, a 600-kW HTS FSG was designed using a fundamental design process and the characteristics analysis of the generator was performed based on the test results of the fabricated NI-type field coil. The results show that the HTS FSG having NI-type HTS field coils is able to operate normally under normal operating conditions. However, the field current charging problem issue has been revealed through this study. Therefore, further research will need to improve the problem.

Load Test of 3-MW HTS Motor for Ship Propulsion

This paper describes the test facility and load test program for a 3-MW high-temperature superconducting (HTS) motor for ship propulsion. The test facility was used to conduct a constant-load test and a variable-load test. The program consisted of the constant-load test, the variable-load test, and the inspection after tests. During the constant-load test, the motor efficiency was measured at various rotor speeds and torques, and the 100-h cumulative endurance test at 3 MW was completed successful. The variable-load test is composed of three different modes that simulated an emergency maneuver of full astern to stop the ship and avoid a collision, and they have been verified. After the series of tests, the HTS motor was taken apart and an inspection confirmed that there was no problem with the HTS field coils or other main parts. This program demonstrated the reliability of the 3-MW HTS motor for ship propulsion.