Embedded Ring-Type Inductors Modeling With Application to Induction Heating Systems

We derive integral expressions of the equivalent impedance of ring-type coils for induction heating systems. These expressions are obtained by integrating the analytical solution of the field generated by a circular filamentary current placed between two linear and homogeneous semi-infinite media. Constant current density is assumed for the coils, since they are wound with an appropriate Litz wire. The model also gives the magnetic field in the cross section of the windings, which is necessary to calculate the proximity losses in the cables. The solution is given in terms of Bessel function integrals, showing an improvement of the convergence with respect to the filamentary model solution. We constructed several prototypes having different turns and layers and tested them with different substrates. Our measurements confirm the validity of the analytical models.      

Hysteresis losses in superconductors of round cross-section with collective interaction

The paper presents the results of an experimental study of hysteresis losses in a transverse magnetic field by samples that represent small coils made of round wire of Nb-Ti alloy. The experimental results turned out to be considerably different from the values computed by the methods described in the literature. A theoretical analysis has been conducted of the effect of the demagnetization factors of the conductor and the field, associated with magnetization of other turns of the winding, and used as the basis for obtaining sufficiently general closed-form types of dependence which make it possible to compute the losses with a high degree of accuracy     

A theoretical study of the impact of magnetic field of Hall current on the parameters of a high-current vacuum-arc discharge

It is demonstrated that the magnetic field of Hall current in a short high-current vacuum-arc discharge significantly distorts the external axial magnetic field and affects the distribution of current density in the discharge gap. This effect decreases with increasing external magnetic field, with decreasing arc current, and with decreasing ratio of the length of discharge gap to its transverse dimension. A 2D magnetohydrodynamic mathematical model is used to calculate the discharge parameters for different values of induction of external magnetic field. The calculation results are compared with similar results obtained using a 1.5D model in which the impact made by magnetic field of Hall current is ignored. It is inferred that the simpler 1.5D model may be employed in calculations of parameters of vacuum arc in a wide range of variation of arc current and of induction of external magnetic field.     

Study of the direct current breakage process in an external nonuniform magnetic field

We study experimentally the influence of an external nonuniform magnetic field with transverse and longitudinal components on the electric characteristics, plasma configuration, and cathode spot arrangement of vaccum-arc discharge. It is revealed that for a cylindrical cathode, cathode spots are nonuniformly distributed on the cathode surface, the spot configuration has no axial symmetry, and the arrangement of spots changes in time depending on the induction value of the external pulsed magnetic field. With an increase in induction, spots (on average) are arranged closer to the cathode, i.e., displaced to the region of weaker transverse field. For two cathode geometries, the probabilities of direct current breakage depending on the induction of the external nonuniform magnetic field are experimentally determined. To determine the conditions of current breakage, a three-dimensional mathematical model of ion motion is suggested and the current breakage criterion is formulated. The trajectories of fast cathode ions in an electrode system with a ring cathode have been calculated using the model. Calculations were performed in a nonuniform magnetic field, the radial and axial components of which have been measured experimentally. It is shown that conditions of current breakage determined with this criterion agree with the results of experiment.     

Test Coils Made of Tl-1223 Tapes

Silver sheathed Tl-1223 tapes were prepared by a powder-in-tube process. The critical current density of short samples was 18 kA/cm² at 77 K. Longer tapes up to 1.2 m, prepared by sequential pressing, had a critical current density of 12 kA/cm². From these tapes we have wound two coils. A solenoid coil with 5 windings was made of 8 tapes with a total length of 4.5 m. At 77 K the critical current of the coil was 23 A in the self generated magnetic field (18 Gauss at the centre of the coil). Using an iron yoke the critical current remained at 22 A while the generated magnetic field increased to 120 Gauss. A pancake coil with 15 windings, made of 5 tapes with a total length of 5 m, generated a magnetic field of 149 Gauss at the critical current of 12 A. From measurements of the critical current density of our tapes in applied magnetic fields, we conclude that coils made of Tl-1223 tapes can be used to generate higher magnetic fields at 77 K.     

Measurement of time constants for superconducting cables with Hall probes

The time constants of coupling losses have been evaluated for the composite-type aluminum stabilized superconducting cables used for the pool-cooled helical coils of the Large Helical Device (LHD). A new method using Hall probes was incorporated to measure the local field change generated by the screening currents in the conductors. During an exponential discharge of the external magnetic field, the current decay was found to be well fitted according to a simple magnetic diffusion equation with a geometrical shape factor. Measurements in the magnetic fields both perpendicular and parallel to the wide surface of the flat cable were performed. The shielding effect of the pure aluminum stabilizer was confirmed by removing it from the cable, which then revealed a longer time constant.     

Hardware control of an active magnetic shield

The presented hardware controller controls the current in an active shield consisting of a number of compensation coils at well-chosen positions. In combination with a converter and the active shield, the controller reduces the fundamental component of the magnetic stray field of an induction heater by generating a magnetic counter field in a defined target area. The controller uses two input signals (from two magnetic field sensors) and generates amplitude and phase information for the converter. Based on this information, the latter produces the compensation current wave that is sent to the compensation coils to minimise the magnetic field. The frequency range of the controller is 1-100 kHz. Its dynamic behaviour is explained in theory and validated by experiments     

Boundary integral equations analysis of induction devices with rotational symmetry

The application of boundary integral equations (BIE's) for the analysis of linear induction devices with rotational symmetry is considered. One-dimensional Fredholm integral equations are derived for the tangential field components at the boundary of a conducting medium with constant scalar conductivity and permeability excited by a time-harmonic azimuthal current source. The important special case of a short right circular conducting cylinder (magnetic or nonmagnetic) coaxial with one or more short coils is treated in detail. The explicit form of the kernels and the numerical solution technique are presented. Numerical results are presented for typical induction heating applications where the load length as well as the coil length are finite. Results are also presented for the magnetostatic problem of finding the demagnetization factors for short magnetic rods. In each case the results are compared with published results and the accuracy and efficiency of the proposed approach are demonstrated.     

Generation of quadrupole magnetic field for trapping atoms in Cs fountain being developed at NPL India

We describe the development of a constant current source for magneto-optical trap (MOT) coils. The system is designed to supply maximum 5 A current in continuous or pulsed mode to the anti-Helmholtz (AH) coils which produce quadrupole magnetic field inside the chamber for trapping of Cs atoms. To obtain a stable field gradient the coils are operated at low current to minimize current fluctuations. With AH coils having 100 turns and 5 Ω resistances, we achieved rise and fall times of current as 5 ms and 7 ms, respectively. With a field gradient of 6 G/cm at the center of the MOT, 7 number of atoms trapped is about 10. To prevent burning of coils due to failure of current source, a thermal protection circuit has been incorporated. Measurements made for switching times of current profile in coils is also reported.     

一种圆鞍形磁屏蔽线圈的设计与性能研究

以毕奥-萨伐尔定律为基础,推导了一种圆鞍形线圈在空间任意点处的磁场分布表达式,优化了一种线圈结构,证明了圆鞍形线圈可以产生一定区域的稳恒磁场,并定量地给出了线圈内部磁场的空间分布情况。所设计线圈产生的磁场中,1%均匀度的磁场均匀区占轴上区域的28%,0.1%均匀度的磁场均匀区占轴上区域的10%;在1 mA的电流下,单圈线圈最大磁场补偿值为15 nT,相对偏差量不超过0.14 nT,验证了所推导表达式的正确性,为多维磁场补偿系统设计与研究提供了理论依据和参考。     

A model for calculating a.c. losses in multistage superconducting cables

Superconducting magnets in tokamaks for fusion experiments are subjected to fast variations in magnetic field. As the high current conductors used in these magnets are made of multistage cables, these variations induce interstrand coupling currents that create losses. These losses are usually characterized by the so-called time constant of the conductor. A model is given to calculate this time constant. Working formulas are also proposed to calculate the current induced in the different cabling stages. This model takes into account the strand characteristics and the detailed cabling pattern. Using it, a method is also given to deduce the time constant from resistive measurements. The influence of the resistive barrier (chrome plating, CuNi shell, outer bronze matrix) is pointed out. Finally, this model is applied to a conductor that is foreseen for the toroidal coils of the International Thermonuclear Experimental Reactor (ITER).     

System for coil simulation measurements of losses and instabilities in superconducting wires

A new system for measuring ac losses in superconductors under coil simulation conditions is described. It is aimed to investigate instabilities in local points of a coil. Currents up to 150 A and magnetic fields up to 0.4 T can be applied simultaneously and can be varied independently. The magnetic and electric fields on both sides of the sample are measured by means of pick-up coils and potential leads, respectively. By this method the instantaneous values of the power introduced through each of both sample surfaces can be measured separately. The validity of the method and the performance of the system are demonstrated by measuring the losses of a copper-clad single-core Nb-Ti-Zr wire for several values of the ratio between the current and the magnetic field, which are inphase.     

H-type dipole model for future plan at KEK

For the bending magnet of the electron ring of the so-called TRISTAN project at KEK, the H-type dipole, full scale model with 4 m in length, has been investigated at first to examine the feasibility to the electron storage ring from some points such as magnet performance, accessibility for the field measurement and vacuum chamber installation, manufacturing problem and magnet cost. Here, the magnet performance is treated. To measure the magnetic field inside the air gap, the specially designed flip coils were used. Fields of 11 points at the same longitudinal positions could be measured at once by rotating probe containing 11 flip coils on its axis.     

匝间短路故障对永磁同步电机失磁影响的分析与研究

目的 研究不同相带第1匝发生短路故障对永磁电机失磁的影响。方法 利用有限元法, 在对电机的设计参数及绕组结构进行介绍的基础上, 运用ansoft软件建立了永磁电机模型, 并计算了不同故障类型对永磁电机的失磁影响; 提取了不同故障下, 永磁电机短路线圈的电流变化波形和磁密矢量分布图; 从绕组分布的2个不同方面,讨论了不同相带的第1匝发生短路故障时, 去磁磁场的大小、 失磁区域的大小和失磁区域分布之间的关系。结果 失磁区域的大小和分布与短路线圈的空间位置有关。结论 该研究结果可以作为判断短路线圈与永磁体失磁位置的依据, 也为永磁同步电机的设计提供了技术参考。     

Analysis of a novel laminated coil using eddy currents for AC highmagnetic field

The analysis and performance of an eddy current type laminated coil for a high AC magnetic fields are described. It is usually difficult to obtain a high AC magnetic field using an air-gap coil because of eddy currents. The present coil circumvents this limitation by making use of the magnetic shielding effect of eddy currents. Two different realizations of the coil are proposed to provide design flexibility. The field distributions are analyzed by the two-dimensional finite-element method. The coils can also be applied to an induction electromagnetic pump     

Simulation of the Magnetic Induction Vector of a Magnetic Core to be Used in FFC NMR Relaxometry

This paper is a contribution for the assessment and comparison of magnet properties based on magnetic field characteristics particularly concerning the magnetic induction uniformity in the air gaps. For this aim, a solver was developed and implemented to determine the magnetic field of a magnetic core to be used in Fast Field Cycling (FFC) Nuclear Magnetic Resonance (NMR) relaxometry. The electromagnetic field computation is based on a 2D finite-element method (FEM) using both the scalar and the vector potential formulation. Results for the magnetic field lines and the magnetic induction vector in the air gap are presented. The target magnetic induction is 0.2 T, which is a typical requirement of the FFC NMR technique, which can be achieved with a magnetic core based on permanent magnets or coils. In addition, this application requires high magnetic induction uniformity. To achieve this goal, a solution including superconducting pieces is analyzed. Results are compared with a different FEM program.     

Analysis of magnetic field patterns of single Josephson tunnel junctions with large idle regions

We have experimentally investigated the effect of a combined lateral and longitudinal idle region on the magnetic field dependence of the critical current of Nb/Al−AlO_x/Nb Josephson junctions. All the samples were high quality devices with a good uniformity and reproducibility of the electrical parameters. An excellent agreement has been found with calculations of the magnetic field patterns made using a simple model of junction with idle region. A comparison with the magnetic field response of the same devices after a proper trimming of the idle region has also been done.     

多阶段活塞式磁流变阻尼器的电磁线圈研究

分析磁流变阻尼器采用多阶段活塞式结构的优点,系统地研究磁流变阻尼器的电磁线圈特性。基于有限元软件构建磁流变阻尼器的仿真模型,分析得出在并联的电磁线圈中通入方向相反的电流,可以最大程度地提高磁路利用率。调整磁路中有效长度的分布和改变活塞齿槽断面为梯形结构,可以改善磁通的分布,确保整个阻尼通道内部的磁感应强度分布的均匀性。     

Effective magnetization and forces due to eddy currents

A simple method for evaluating the effective magnetization due to eddy currents excited by AC magnetic fields in finite conductive objects is presented. The values of magnetization enable the estimation of the forces exerted on the object by the effect of eddy currents. The method relies on assuming a similarity between eddy current magnetization and the magnetization due to diamagnetic effects, which is easier to evaluate. Its validity is checked by comparing results from the eddy current forces with data obtained from conventional but much more complicated methods or from experimental data. The approach may be useful for evaluating the eddy current losses in finite conductive objects excited by AC magnetic fields or the influence of small conductive objects on AC excitation coils. The method combines techniques related to the conventional evaluation of eddy currents in certain 1-D geometries with techniques that approximate behavior of the AC magnetic field in finite objects in a way similar to that followed when the magnetic forces acting on a diamagnetic object are calculated