High-temperature Superconducting Bearing Research Articles

Loading capacity, rotation loss and torsional oscillation research on an Evershed-type hybrid superconducting bearing used for micro-thrust measurements

Characterized by their very low rotational drag, applications of high-temperature superconductor (HTS) bearings have been expanded in some high-precision instruments. We have developed a sensitive magnetic suspension stand based on an Evershed-type hybrid HTS bearing to measure μN-level thrust. The hybrid HTS bearing uses the strong attractive force of a permanent magnet (PM)-biased bearing to support the main load in the bearing, while instability in the PM-biased bearing is compensated by the magnetic stability from a HTS bearing. Compared with a single PM/HTS bearing, the hybrid HTS bearing is intended not only to support larger loads but also to suppress rotation loss and levitation drift. Loading capacity, loss and damping torque were explored for different design parameters such as bearing gaps L 1, L 2 and the field cooling height (FCH) of bulk HTSs. Spin-down testing suggested that the loss of a hybrid HTS bearing can be reduced by increasing the HTS bearing gap L 2 or reducing the FCH. The hybrid HTS bearing showed a typical oscillation behavior at extreme low frequencies, and torsional pendulum testing suggested that the spring constant k of the hybrid HTS bearing can be as low as 36.39 μN m deg–1. A micro-thrust stand based on the hybrid HTS bearing was established to test an electrospray thruster in a vacuum chamber, and the measurement result of 26.61 μN shows the ability of the micro-thrust stand to achieve μN level testing.

3D electromagnetic behaviours and discharge characteristics of superconducting flywheel energy storage system with radial‐type high‐temperature bearing

The authors have built a 2 kW/28.5 kJ superconducting flywheel energy storage system (SFESS) with a radial-type high-temperature superconducting bearing (HTSB). Its 3D dynamic electromagnetic behaviours were investigated based on the H -method, showing the non-uniform electromagnetic force due to unevenly distributed induced current may cause the superconducting magnet stator of the HTSB cracks. As the SFESS discharges, the amplitude and frequency of the discharge voltage decrease with speed, dropping by 55 and 58%, respectively, from the beginning phase (BP) to the middle phase (MP), by 41 and 37% from the MP to end phase. The decay rates of the discharge voltage and its fluctuation are both increased compared with the no-load operation. During the BP to MP, the output power is higher, the power quality is better and the harmonics are less. The amplitude and frequency of radial vibration are proportional to speed, larger than that during the no-load operation in the same case, and in the BP about 1.8 and 8.8% amplitude in the x and y directions, respectively. Its two norm is increasing by 2.6%, probably leading to the friction between the shaft and protective bearings, gradually becoming greater with speed.

Low frequency rotational loss in a high-temperature superconducting bearing and its application in micro-thrust measurement for space propulsion

High-temperature superconducting (HTS) magnetic bearings have great potential in the field of rotating machinery, such as flywheels and momentum wheels, because of their low friction and self-stabilizing capability. The study of rotational loss in HTS bearing is of great significance to achieve better efficiency. Previous research indicates that low rotational loss has a close relation with hysteresis loss in HTSs and eddy-current losses in the surrounding metal materials. A low coefficient of friction (<10−8) at low speed is attractive for high precision instruments. This paper first presents a short overview of low rotational loss in HTS bearings, and then a special design of HTS bearing is given for application in micro-thrust measurements of space propulsion. The load capacity of the bearing allows for up to 10 kg, which means that the entire thruster system can be suspended to avoid measuring the internal force. Two different measuring methods are designed to fit several operational modes of the micro-thruster, and the measured thrust level is from 1 mN to 100 mN. The bearing system shows great response to short pulse modes of the micro-thruster, and an impulse of 1 mN·s was identified accurately. The measured thrust shows great agreement with the designed values of the micro-thruster, indicating the effective application of the HTS bearing for micro-thrust measurements.

Simulation and calculation on rotation loss of high temperature superconducting bearings

High temperature superconducting (HTS) bearings are applied in flywheel energy storage and cryogenic liquid pump successfully due to self-stability and low-loss. Experiments and the current vector potential method (T-method) are generally used in the research of the rotation loss of superconducting bearings. Based on the B-H curve of the HTS deduced by the critical state model, the external magnetic field strength of the rotor system was obtained by using the finite element software. By means of numerical calculation, the rotation loss of HTS and the free decay curve were obtained. Furthermore, the friction coefficient of the designed HTS was calculated, which was 4.6 × 10−7. It was also found that the thickness of edge permanent magnet affected the magnetic field distribution on the surface of superconductors, and a more symmetrical magnetic field distribution can be generated by changing the thickness.

Dynamic Electromagnetic Behaviors of High-Temperature Superconductor Levitation System Based on YBCO Material from 3D Perspective

In this paper, a 3D numerical model, which was theoretically established and experimentally validated, was utilized to investigate the dynamic electromagnetic behaviors of a high-temperature superconductor (HTS) levitation system consisting of a cylindrical permanent magnet (PM) and an HTS fabricated by the YBCO material. The effects of critical current, cooling temperature, multiseeded material, and thickness of the HTS on electromagnetic behaviors were discussed from a 3D perspective. The dynamic electromagnetic behaviors indicate the induced current is mainly distributed near the top surface and edges of the HTS. As the PM approaches the HTS, the induced current density increases gradually. As the PM withdraws, it reduces initially and is then induced in the opposite direction. The levitation force increases with critical current but exhibits saturation. The lower the temperature, the larger the levitation force and the smaller the hysteresis. The induced current of the multiseeded HTS is characterized by a symmetrical distribution with the seed domain boundary or central axis. Its maximum value increases with the number of seeds but the levitation force significantly reduces. The induced current on the cross section is symmetrically distributed. As the thickness of the HTS decreases, the levitation force reduces drastically but the maximum value of the induced current increases. The reason was explained. Finally, the dynamic electromagnetic behaviors of a radial-type HTS bearing were investigated from a 3D perspective.

Technical Aspects of Critical Components for High-Temperature Superconductor Bearings-Based Flywheel Energy Storage System

Flywheel-based energy storage systems are gaining prominence in present-day energy-deficit situation. For energy storage system, the bearings and motor cum generator, for charging and discharging energy to and from the flywheel, form the vital components which have to be given due consideration. The low coefficient of friction of high-temperature superconductor bearings used in flywheel-based energy storage systems decides the energy loss factor. Yttrium Barium copper oxide (YBCO) which is one of the popular high-temperature superconductors is widely being used in high-temperature superconductor bearings. The planar type of bearings can be materialised by building the stator of the bearing with YBCO bulk high-temperature superconductor discs mounted in a circular array and rotor part can be realised by Nd–Fe–B permanent magnet ring which is to be embedded with the flywheel. The levitation force is proportional to the mean magnetization of high-temperature superconductor and the horizontal gradient of the vertical component of the magnetic field density in the superconductor. The magnetization is proportional to the product of critical current density and the grain diameter of the bulk superconductor. The high-temperature superconductor bearings outweigh the magnetic bearings because their passive nature, by virtue, does not require any external active stability balancing devices. The paper discusses of development, fabrication and load testing of prototype planar type of high-temperature superconductor bearings. For charging and discharging energy to and from the flywheel energy storage system, Halbach array-type motor cum generator is a promising candidate. The motor, with an outside rotor design, is a dipole Halbach array, establishing a uniform flux within the stator area. A uniform flux is needed for operating the motor on a superconductor bearing, where slight movement of the stator inside the air gap is inevitable. The salient construction features of the prototype Halbach array motor generator will be discussed.

Electromagnetic and Rotational Characteristics of a Superconducting Flywheel Energy Storage System Utilizing a Radial-Type High-Temperature Superconducting Bearing

A 2 kW/28.5 kJ superconducting flywheel energy storage system (SFESS) with a radial-type high-temperature superconducting (HTS) bearing was set up to study the electromagnetic and rotational characteristics. The structure of the SFESS as well as the design of its main parts was reported. A mathematical model based on the finite element method (FEM) was established to research the electromagnetic characteristics of the HTS bearing during the levitation process, which show that a part of the magnetic flux penetrates into the edge of the HTS bulks and then goes back to the opposite pole of the permanent magnet rotor (PMR). The induced current mainly distributes in the edge of the HTS bulks, indicating that larger force acts on the edge part of the HTS bulks and probably causes them to crack. The free rotations of the rotor at different steady-state speeds of 2500–5000 rpm and its radial vibration were displayed. The induced voltage of the stator winding of the motor in this process was analyzed. The rotational characteristics are related to the vibration of the rotor. Below the resonant frequency, the vibration increases significantly with the speed. Enhancing the radial stiffness to limit the vibration amplitude of the rotor is an effective approach to improve the speed.

Thermo‐electromagnetic analysis of radial HTS magnetic bearings using a semi‐analytical method

Nowadays, after passing more than three decades from discovery of high-temperature superconductors, they are widely used in different fields including power industry. High-temperature superconducting (HTS) bearing is an electromagnetic machine that provides frictionless operation and intrinsic stability without requiring control equipment. Before entering to the fabrication stage, an accurate simulating model can satisfactorily reveal many of practical considerations. This work is based on the results of their previous study, delving into time steps of multi-physical simulations juxtaposing electromagnetic and thermal characteristics of an HTS bearing. This is attainable by using multi-physics analysis of radial HTS bearings based on a semi-analytical method called variational iteration method (VIM). This is accomplished by coupling of the Maxwell equations and thermal equilibrium. The electrical behaviour of the superconductor is described by means of a B–T-dependent modified E–J power-law relation. To verify VIM results, the multi-physics equations are also numerically solved using finite-element method. In this research, two case studies are examined; failure of the cryogenic system and incorporating new materials in the stator part of the bearing in order to improve the cooling efficiency.

Levitation Force Maximization in HTS Magnetic Bearings Formulated by a Semianalytical Approach

High-speed rotatory electromechanical devices can be practically implemented using magnetic bearings that provide a contactless operation. Besides the contactless operation, high-temperature superconducting (HTS) bearings possess an exclusive feature, i.e., self-stability. This characteristic removes requiring any control schemes and consequently an easier implementation and lower costs would be expected. This paper deals with presentation of an optimal rotor configuration in radial-type HTS bearings. A semianalytical approach called Adomian decomposition method is used to efficiently solve the governing equations of HTS bearings deduced from the H-formulation combined with the E-J power law. Control variables of the optimization process include rotor dimensions as well as magnetization direction of permanent magnets. Optimization is carried out using a powerful approach called differential evolution with maximization of levitation force around the equilibrium position as its objective function. In order to verify the proposed approach, a two-dimensional axisymmetric finite element method is also implemented.

Nonlinear vibration behaviors of high-Tc superconducting bulks in an applied permanent magnetic array field

The nonlinear vibration of high temperature superconducting (HTS) bulks in an applied permanent magnetic array (Halbach array) field, as a precondition for commercial application to HTS maglev train and HTS bearing, is systematically investigated. This article reports the actual vibration rules of HTS bulks from three aspects. First, we propose a new numerical model to simplify the calculation of levitation force. This model could provide precise simulations, especially the estimation of eigenfrequency. Second, an approximate analytic solution of the vibration of the HTS bulks is obtained by using the method of harmonic balance. Finally, to verify the results mentioned above, we measure the vertical vibration acceleration signals of an HTS maglev model, consisting of eight YBaCuO bulks, oscillating freely above a Halbach array with large displacement excitation. Higher order harmonic components, which indicate the nonlinear vibration phenomenon, are detected in the responses. All the three results are compared and agreed well with each other. This study combines the experimental and theoretical analyses and provides a deep understanding of the physical phenomenon of the nonlinear vibration and is meaningful for the vibration control of the relevant applications.

Sensitivity Analysis of Rotor Parameters in Axially Magnetized Radial HTS Magnetic Bearings Using an Analytical Method

Nowadays, high-temperature superconducting (HTS) bearings are well known in various fields requiring contactless operation, which results in decreasing loss and saving a great deal of energy. Self-stability serves as another advantage of HTS bearings, which emerges due to the flux pinning phenomenon. Consequently, the need for control equipment and excess costs disappears. One of the most important applicable characteristics to design HTS bearings is referred to as levitation force. In this paper, sensitivity analysis of rotor parameters in order to enhance levitation force is performed. For this purpose, an analytical method called the variational iteration method is applied to efficiently solve the governing nonlinear system of partial differential equations in the HTS bulks deduced from the H-formulation along with the E-J power law. A numerical two-dimensional axisymmetric finite-element method is also invoked to verify results obtained by the analytical method.

Investigation of HTS Tape Stacks for Levitation Applications

Currently, high-temperature superconductor (HTS) bearings are most commonly designed with bulk REBa 2 Cu 3 O 7-x (REBCO), but these HTS bulks have several drawbacks, such as brittleness and the complexity of sample preparation. An alternative for levitation systems is to use stacks of REBCO coated conductors (HTS tapes). The main advantages of HTS tapes are the simplicity of manufacturing elements in the required shape and high strength stability. For the development of HTS bearings, it is important to characterize both the levitation force along the axis connecting the magnet and the superconductor, and the stability of the system in response to lateral displacement. In this paper, we present new results of measurements of the levitation force between a permanent magnet and a stack of HTS tapes containing from 10 to 100 tapes of 12 mm × 12 mm. Measurements were conducted in zero-field cooling mode. We observed hysteresis of the levitation force in the case of lateral displacement. In addition, we have investigated the decay in the levitation force due to lateral displacement with each cycle of movement. It is shown that the decay rate tends to be constant on increasing the number of tapes in the stack.

Operation and Improvement of Liquid Nitrogen Pumps with Radial High- Temperature Superconductor Bearings

This paper reviews the advantages of replacing the mechanical bearings of low-temperature pumps by radial high- temperature superconductor (HTS) bearings. Radial HTS bearings have the advantage of being non-abrasive, so that the working life is increased significantly. In this article, two types of liquid nitrogen pump with radial HTS bearings are proposed. To reduce heat leakage, one pump uses a permanent magnet (PM) coupling and the other uses a long hollow pipe coupling. Successful stable operation of these two pumps means that radial HTS bearings have the potential to be applied in liquid nitrogen pumps. Test results show that the flow rate is influenced mainly by rotational speed but not by the coupling component. Further designs of the two types of pump for practical applications are described, and their characteristics are analyzed: the pump with a PM coupling has lower heat leakage, whereas the pump with a long hollow pipe coupling can solve the force creep problem of the HTS bearing. The design of the pump with a long hollow pipe coupling is based on the pump that is already in practical use, and therefore has greater feasibility for practical applications. Finally, improvements of the liquid nitrogen pump by improving the structure of the pump and the performance of the radial HTS bearing are discussed.

Analysis of Levitation Characteristics of a Radial-Type High-Temperature Superconducting Bearing Based on Numerical Simulation

A two-dimensional (2D) axisymmetric finite-element model was proposed to simulate the levitation characteristics of a radial-type high-temperature superconducting (HTS) bearing. The E-J power law together with an H-formulation was used to consider the superconducting state. The numerical model was successfully implemented using a FORTRAN program. A comparison of the experimental results with the simulation was given to validate the model. It shows that they have good agreement. The electromagnetic behavior inside the HTS bulks was also analyzed. The model presented in this paper can be used to optimize the design of a radial-type HTS bearing.

Measurement and Calculation Method of the Radial Stiffness of Radial High-Temperature Superconducting Bearings

Stiffness of radial high-temperature superconducting (HTS) bearings is widely researched by experiments (measurement method). These researches include radial stiffness and axial stiffness at different temperatures and rotational speeds, cross stiffness, and so on. The measurement method can obtain the precise value of stiffness, while it takes so much time and cost. Simulation calculation method is suitable to model radial HTS bearings which have different structures, and it is easier to obtain stiffness than the measurement method. The problem is that the calculation precision is sensitive to the parameters in the simulation calculation. The measurement and calculation method of the radial stiffness of radial HTS bearings has the strong points of saving time and cost, and high precision. In this paper, this method is improved. Radial stiffness between HTS bulk and permanent magnet (PM) rotor is analyzed basing on a new mechanical model, and equation for the radial stiffness of radial HTS bearings is deduced. The error of calculation is reduced to 2.86 % according to the measured values. Application of this method for calculating the radial stiffness of radial HTS bearings with different number of HTS stator layers is introduced.

Analyses and Tests of HTS Bearing For Flywheel Energy System

Flywheel energy system (FES) is a non-pollution energy storage system which uses high-speed rotation flywheel to store energy. As high temperature superconducting (HTS) bearings have very low friction and self-stability characteristic, they are applied to FES increasingly. In this paper, for the optimal structure of permanent magnet rotor, 3 topologies were presented and their magnetic field characteristics were analyzed. For effective cooling of YBCO bulks, a thermal model of the superconductor magnet stator using alumina shims was set up and its temperature distribution was analyzed. The result shows that the structure of the stator in which alumina shims were put between the neighboring YBCO bulks is better than that without alumina shims, and the temperature of YBCO bulks in the structure is lower. Then a radial-type HTS bearing prototype with outer diameter of 200 mm, inner diameter of 40 mm and height of 120 mm was built. The axial force, the axial stiffness and the levitation characteristics of the device were measured and analyzed in field cooling and zero-field cooling respectively.

Results and Analysis of an Accident in 35-kWh SFES

A 35-kWh superconductor flywheel energy storage (SFES) system using hybrid bearing sets, which is composed of a high-temperature superconductor bearing and an active magnet damper, has been developed in the Korea Electric Power Corporation Research Institute (KEPRI). To keep the flywheel system safe, backup bearings were adapted, and a ball bearing was selected through simulations. The 35-kWh SFES was tested up to 6500 r/min successfully. During rotation tests, there were few accidents. An accident occurred while decreasing speed, at around 4000 r/min. Although the backup bearing was damaged to keep the flywheel system safe, the shaft was also damaged. Contact due to the elongation of the flywheel shaft was considered as the reason for the accident. Some results of the accident are described and analyzed.

The improved damping of superconductor bearings for 35 kWh superconductor flywheel energy storage system

A 35kWh Superconductor Flywheel Energy Storage system (SFES) using hybrid bearing sets, which is composed of a high temperature superconductor (HTS) bearing and an active magnet damper (AMD), has been developed at KEPCO Research Institute (KEPRI). Damping is a source of energy loss but necessary for the stability of the flywheel system. We found that the damping of HTS bearings can be improved by thermal insulating bolts, which play a role of passive type external damper. To investigate the source of the increased damping, damping coefficients were measured with HTS bearings using insulating bolts made of three kinds of polymer materials. The damping coefficient was raised over 3000Ns/m in the case of PEEK bolts. The value was almost a quarter of the AMD. In this study, thermoelastic and Coulomb friction damping mechanisms are discussed. The main damping mechanism was the thermoelastic damping of the bolts themselves. And interfacial gap between the insulating bolt and metal chamber, which increased during the cooling process, was considered to be the cause of the anisotropic damping coefficients. Finally, the effects of the HTS bearings on the first critical speed are shown.

A Measurement Testing Setup of the Characteristic Properties for High Temperature Superconducting bearing Systems

Abstract A measurement testing setup of the characteristic properties for high temperature superconducting bearing (HTSB) systems is introduced in this paper. The equipment setup can measure the radial and axial forces, temperature, oscillation and rotary speed of the HTSB simultaneously, so the relation between these properties can be investigated. For example, the decrease of the radial and axial forces, and the increase of the temperature can be measured and analyzed during different rotary speeds of the permanent magnetic rotor. The measurement ranges for the radial and axial force will reach 5 kN and 10 kN. The temperature of the superconductors in HTSB can then be controlled by adjusting the air press inside the dewar which is an easier method compared with a cryocooler.

Damping of Sub-Synchronous Whirl in Rotors With High-Temperature Superconducting Bearings

For large flywheel rotors supported by high-temperature superconducting (HTS) bearings, the amplitude of sub-synchronous whirl can become so large that it limits the maximum speed to a value less than that dictated by its mechanical strength. Internal damping, in the form of hysteretic loss in the rotor material, creates a force that adds energy to the forward whirl. We examine the effect of internal damping as the cause of the sub-synchronous whirl and show that theoretical predictions of the onset criteria agree with experimental data. We next show how it is possible to damp the whirl from a theoretical context and discuss a practical approach to accomplish this that has had success in our laboratory experiments.