Bulk Rotor Research Articles

New approach to study HTS bulk behavior for a fully superconducting motor design

The originality of this work is to applied Amperian Approach for modelling HTS bulk especially in electrical motor applications. This approach has proven its efficiency in electrical materials having a constant conductivity. For HTS materials, the resistivity change brutally and have a non linear comportment through any external magnetic field variations. To success the modelisation, we have to create an iterative procedure. All simulation steps are based on the power law model for computing resistivity and current densities adding the fixed point method for non linear iterations. The majority of our results concern the current densities in the HTS bulk rotor with different critical currents constatations under mgnetic field created by three phase coils of the proposed superconducting synchronous motor (SSM) prototype. Some results are consolidated by numerical way using flux 2D software.

Test equipment for a flywheel energy storage system using a magnetic bearing composed of superconducting coils and superconducting bulks

Energy storage systems are necessary for renewable energy sources such as solar power in order to stabilize their output power, which fluctuates widely depending on the weather. Since ‘flywheel energy storage systems’ (FWSSs) do not use chemical reactions, they do not deteriorate due to charge or discharge. This is an advantage of FWSSs in applications for renewable energy plants. A conventional FWSS has capacity limitation because of the mechanical bearings used to support the flywheel. Therefore, we have designed a superconducting magnetic bearing composed of a superconducting coil stator and a superconducting bulk rotor in order to solve this problem, and have experimentally manufactured a large scale FWSS with a capacity of 100 kWh and an output power of 300 kW. The superconducting magnetic bearing can levitate 4 tons and enables the flywheel to rotate smoothly. A performance confirmation test will be started soon. An overview of the superconducting FWSS is presented in this paper.

Rotation Test of a Superconducting Bulk Rotor Shielded With Superconducting Rings

In this paper, we constructed test equipment for a rotating machine that uses a superconducting bulk rotor and performed a rotation test. When a superconducting bulk rotor is used, a fluctuating magnetic field generates ac loss, it is possible to decrease its trapped magnetic flux. Performance of the rotating machine under test was influenced by the ac loss generated by a fluctuating ac magnetic field. We modeled the motor used in this paper on an axial-type synchronous motor, where the magnetic field generated by the stator coils causes the rotor to rotate. We then measured the influence of the stator coils on the superconducting bulk rotor. First, we confirmed that the temperature of the bulk rotor modules was properly controlled and could generate a trapped magnetic flux equal to that during zero-field-cooled magnetization. A technique for magnetizing two or more bulks concurrently was used. Next, we performed the rotation test at low speed. The maximum rotational speed during the test was 300 r/min. The operation time at a steady-state speed was 600 s. The maximum value of the generated magnetic field of the stator coils was 50 mT. Finally, we confirmed that a shielding ring made of superconducting wire decreased the influence of a fluctuating magnetic field.

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.

Effects of fluctuating magnetic fields on a superconducting bulk rotor shielded with superconducting rings

We study the effect of a fluctuating magnetic field, which is one of the technical problems for trapped magnetic fields in a bulk superconductor, to realize a practical bulk superconductor rotating machine. Previous research and other's research has shown that fluctuating magnetic fields reduce the strength of trapped magnetic fields in superconducting bulk modules [1, 2]. This deters development of applications of AC rotating machines because superconducting bulk modules are always exposed to a fluctuating magnetic field. Therefore, it is necessary to develop a method to control decrease of the trapped magnetic field. We propose a method to use the shielding ring of a superconducting wire to achieve this goal and the effects are confirmed experimentally [3]. We are now building test equipment for examining the performance of a shielding ring in a bulk rotating machine. This paper reports the test result for the shielding ring applied to the bulk superconducting rotor that is a part of the test equipment.

Characteristics of trapped-flux type Sm-123 bulk motor operated in liquid nitrogen

We study performance of a HTS bulk motor operated in liquid nitrogen. A disk shaped Sm-123 bulk, 120mm in diameter and 10mm in thickness, are utilized for an axial-type rotor. The locked-rotor tests and load tests are performed at 60Hz driving frequency. It is shown experimentally that the motor shows the hysteretic behavior of rotating characteristics for increment and decrement of the load, when it is pulled into the synchronous rotation. This is because the trapped magnetic fluxes in the bulk rotor at synchronism are different depending on the changing process of the load. These results are important for the clarification of the detailed mechanism of the torque generation as well as the optimal design of the HTS bulk motor.

Preliminary Test Results of Radial-Type Sintered Sm-123 Bulk Motor

We designed and fabricated a radial-type motor with a high temperature superconducting bulk rotor. The fabricated motor has 3-phase and 4-pole armature winding, and then generates rotating magnetic field at the speed of 1800 rpm under the condition of 3-phase, 60 Hz electric power. Sintered Sm-123 bulk was utilized for the solid rotor in this study. The performance of the sintered Sm-123 bulk motor was evaluated by no-load, locked-rotor and load test. Test results showed that the motor generated a little torque and rotated with large slip, even if the motor was applied 165 V/sub rms/ of the phase voltage (the stator current is 18 A/sub rms/). Because of small magneto-motive-force and spatial harmonic components in the air-gap, magnetic flux was prevented from penetrating into the HTS bulk rotor. Therefore, the screening current in the rotor flowed only at the surface of the rotor and generated torque is remained in a little value.

Synchronization of an axial-type Bi-2223 bulk motor operated in liquid nitrogen

We fabricated and tested an axial-type Bi-2223 bulk motor that was operated in liquidnitrogen. The rotating characteristics were tested as a function of temperature, which wascontrolled by evacuating the metal cryostat with a rotary vacuum pump. The tests wereperformed for different air-gap lengths. It was found for the first time that thesynchronization was realized when the operating temperature as well as the armaturecurrent was set to the appropriate range. Furthermore, the no-load test results showed thatthe rotating speed had no hysteretic characteristics through the increasing anddecreasing process of the armature current even after the motor was pulled-in,and this result indicated that there were no trapped fluxoids in the bulk rotor.

Characteristic analysis of hysteresis-type Bi-2223 bulk motor with the use of equivalent circuit

Characteristics of a hysteresis-type Bi-2223 bulk motor were studied based on the equivalent circuit. The Bi-2223 bulk motor consists of a disk shaped Bi-2223 bulk rotor and a 3-phase, 4-pole stator. The locked-rotor test results were discussed in this study. Stator impedance was directly obtained through the measurement, and then the rotor resistance was estimated from the equivalent circuit with some assumptions. In order to show the validity of the equivalent circuit, the experimental results were compared with the calculated results using the circuit. It was shown that the calculated results of the torque curves agree semi-quantitatively with the experimental results. Based on the equivalent circuit, temperature dependence of the rotor resistance is also to be presented and discussed.

Investigation of magnetic characteristics in HTS bulk materials for motor applications

Magnetic characteristics of a Bi-2223 bulk rotor that was used for the HTS motor was investigated at 77.3 K. Spatial distribution of the magnetic flux density in the air-gap of axial-type HTS motor was measured in a rotating magnetic field at 60 Hz frequency. Numerical simulation based on 3D finite element method was also performed. It was shown that the pinning property of the bulk rotor influences the harmonic components of the air-gap flux density as well as motor characteristics.

Characteristics of axial-type HTS motor under different temperature conditions

We tested an axial-type Bi-2223 bulk motor under different magnetic field and temperature conditions. Distributed armature windings with a pole number of four were used in order to compensate for the harmonic components in the air-gap magnetic flux density. The temperature was varied by evacuating the metal cryostat with a rotary vacuum pump. The fabricated motor demonstrated hysteresis characteristics when the temperature of the system was decreased and the magnetomotive force was adjusted to appropriate values. Then, synchronous speed at no-load condition was fulfilled. On the other hand, slip of rotational speed as well as inductive torque was induced when temperature and magnetic conditions were not satisfied. These results can be explained based on the pinning properties and AC loss inside the bulk rotor.

Performance of axial-type motor with Bi-2223 HTS bulk rotor

We fabricated and tested an axial-type motor with a Bi-2223 bulk to investigate its applicability. The motor was immersed in liquid nitrogen with zero-field cooling method. Load and locked rotor tests were carried out in a rotating magnetic field at 60 Hz frequency. Load tests showed that the fabricated motor does not reach synchronous speed, and torque declines with increasing rotational speed. These properties are similar to those of induction motor with high rotor resistance. On the other hand, locked rotor test indicates that the torque vs. magnetic field characteristics is qualitatively described by the area of hysteresis loop which is measured on a bulk surface with a hall sensor. That is, the fabricated motor shows both characteristics of conventional induction and hysteresis motors.

Ｂｉ‐２２２３バルク回転子を適用したアキシャル型高温超電導モータの電磁的基礎特性

We have tested an axial-type motor that was fabricated using a disk-shaped Bi-2223 bulk rotor. Two-pole and three-phase copper coils were used for concentrated armature windings. Lock and load tests were performed as a function of temperature at a frequency of 60Hz. In order to examine the test results, the spatial harmonics of the air-gap magnetic flux density were measured at room temperature and 77.3K, and then analyzed by Fast Fourier Transformation. Hysteresis curves of the Bi-2223 bulk rotor were also measured in a rotating magnetic field at 77.3K. It was shown that the air-gap magnetic flux density includes large components of 5th and 7th harmonics due to the salient and concentrated armature windings, and these harmonics may be one of the main reasons for asynchronous torque as well as large slip operation. On the other hand, these harmonics are suppressed when the armature current is increased due to the heating with AC loss. Furthermore, it can be deduced from a comparison between the stational torque and the area of hysteresis loops that torque generation varies from hysteresis-dominant to induction-dominant mechanisms at the point the gradient of torque vs. armature current curve changes. These results are reasonably consistent with the temperature dependence of motor characteristics.

Fundamental experiments of axial-type BSCCO-bulk superconducting motor model

An axial type HTS hysteresis motor has the same structure as a conventional axial flux type permanent magnet synchronous motor. The permanent magnet rotor of a conventional permanent magnet synchronous motor is replaced by a rotor made from HTS materials with the objective of increasing power densities and reducing losses. The behavior of a BSCCO bulk rotor has been tested in rotating magnetic fields produced by two poles and three phase motor coils fed by a PWM inverter at 77 K. To characterize the torque output capability of the HTS hysteresis motor, speed versus torque and current versus torque are tested. Experimental results for the HTS hysteresis motor are presented.

Study on electric motors with bulk superconductors in the rotor

We have studied electric motors using YBCO bulk superconductors in the rotor. The superconductor can be used as a screening material, a magnetic hysteresis material, a flux source, or a secondary conductor, and form several types of motors. This paper describes experimental results of torque characteristics obtained with a small electric motor using a superconducting bulk rotor, which was operated in the synchronous and slip modes. Dependence of the torque on the stator three-phase current (gap flux density) and the slip frequency will be analyzed.