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。     

ITER Side Correction Coil Quench model and analysis

Previous thermohydraulic studies performed for the ITER TF, CS and PF magnet systems have brought some important information on the detection and consequences of a quench as a function of the initial conditions (deposited energy, heated length). Even if the temperature margin of the Correction Coils is high, their behavior during a quench should also be studied since a quench is likely to be triggered by potential anomalies in joints, ground fault on the instrumentation wires, etc. A model has been developed with the SuperMagnet Code (Bagnasco et al., 2010) for a Side Correction Coil (SCC2) with four pancakes cooled in parallel, each of them represented by a Thea module (with the proper Cable In Conduit Conductor characteristics). All the other coils of the PF cooling loop are hydraulically connected in parallel (top/bottom correction coils and six Poloidal Field Coils) are modeled by Flower modules with equivalent hydraulics properties. The model and the analysis results are presented for five quench initiation cases with/without fast discharge: two quenches initiated by a heat input to the innermost turn of one pancake (case 1 and case 2) and two other quenches initiated at the innermost turns of four pancakes (case 3 and case 4). In the 5th case, the quench is initiated at the middle turn of one pancake. The impact on the cooling circuit, e.g. the exceedance of the opening pressure of the quench relief valves, is detailed in case of an undetected quench (i.e. no discharge of the magnet). Particular attention is also paid to a possible secondary quench detection system based on measured thermohydraulic signals (pressure, temperature and/or helium mass flow rate). The maximum cable temperature achieved in case of a fast current discharge (primary detection by voltage) is compared to the design hot spot criterion of 150 K, which includes the contribution of helium and jacket.     

High-frequency operation of a planar-type microtransformer and itsapplication to multilayered switching regulators

The operation of a multilayered microtransformer composed of planar zig-zag coils and amorphous magnetic film is described. The transformer has a maximum efficiency of 77.5%. Its equivalent circuit is approximated by the parallel connection of the winding inductance and of the stray capacitance. Variable magnetic coupling is obtained between the primary and secondary windings by shifting the relative position of the two coils. The microtransformer is used in a magnetically controlled multilayered switching regulator. The regulator has an output of 1.4 W and an efficiency of 24%. The magnetization loss in the circuit is the same as that of the semiconductors. A two-output-type multilayered switching regulator is also proposed that has an acceptably good output characteristic at each port even though a common magnetic film is used     

Thermal shunt for quick cool-down of two-stage closed-cycle refrigerator

An efficient and easy-to-build gas circuit is described which acts as a thermal shunt between the two stages of a Gifford-McMahon cooler (GMC) during the cool-down period of an experiment. With this circuit the high cooling power of the first stage is used to cool objects with a large heat capacity attached to the second stage like a superconducting magnet or, as in our case, a dilution refrigerator (DR) with a vacuum can. The circuit is driven by a small fraction of the helium gas of the Gifford-McMahon refrigerator and precools the DR from room temperature to a temperature of 50 K. The shunt circuit reduces the cool-down time by a factor of two in our cryostat.     

Analysis and implementation of an active snubber dc/dc converter with series?parallel connection in primary and secondary sides

An active snubber dc/dc converter to achieve zero voltage switching (ZVS) on power switch is presented. In the proposed converter, the primary windings of two transformers are connected in series so that the primary currents of the two transformers are equal. The secondary sides of the isolated zeta converters are connected in the parallel to share the load current and reduce the current stresses on the secondary windings of the two transformers. A boost type of active snubber is connected in parallel with the main switch to recycle the energy stored in transformer leakage and magnetizing inductors and to limit voltage stress of the main switch. During the transition interval between the active switch and the auxiliary switch, the resonance based on the resonant inductor and the output capacitor of the power switch will allow the switch to turn on at ZVS. The principle of operation, steady-state analysis and design consideration of the proposed converter are provided. Finally, experimental results for a 360 W (12 V/30 A) prototype circuit with 150 kHz switching frequency were given to demonstrate the circuit performance and verify the feasibility of the proposed converter.     

Model for transient behavior of pulse tube cryocooler

This article describes an investigation of the transient behavior of a small (2.0 W at 85 K) pulse tube cryocooler operating at 120 Hz with an average pressure of 3.5 MPa, capable of relatively fast cool-down from ambient to about 60 K. In a series of experiments, the cold end temperature was measured as a function of time in a complete cool-down and subsequent warm-up cycle, with no heat load and different quantities of excess mass at the cold end. A transient heat transfer model was developed, that considers the effects of the cooling power extracted at the cold end and that of the heat gain at the warm end on the cool-down time. The heat gain factor was calculated from warm-up data, and found to be approximately the same for all experiments. Using the same model with cool-down data enables a determination of both the gross and net cooling power as functions of time, but more importantly – as functions of the cold end temperature. An expression was derived for the cold end temperature as a function of time for any amount of excess mass, including zero. The cool-down time of the “lean” cryocooler (with no excess mass) was found to be less than 50 s.This cool-down/warm-up method for evaluating the cooling power of a cryocooler seems simpler than steady-state experiments with a heater simulating load at the cold end. Use of the heat transfer model with data from one or two good experiments conducted in the above manner, can yield both the gross and net cooling powers of a cryocooler as functions of the cold end temperature, and allow the determination of cool-down time with any amount of excess thermal mass. While the net cooling power during cool-down differs somewhat from that under steady-state operation, the former can serve as a good measure for the latter.     

Thermo-hydraulic analysis of the gradual cool-down to 80 K of the ITER toroidal field coil

A time-dependent thermo-hydraulic simulation for an ITER toroidal field (TF) coil gradual cool-down to 80 K has been performed using a new FORTRAN code. The code is based on a 1D helium flow and 1D multi-region solid heat conduction model. The whole TF coil is simulated taking into account thermal conduction between winding pack and case, which are cooled down separately. To limit coil mechanical stresses and coolant pressure drop in the cooling channels, an improved cool-down mode has been developed based on the analysis. Typical and gradual cool-down temperature distributions of TF coil and case are presented. The results indicate that gradual cool-down to 80 K can be achieved in 3 weeks.     

Optimal cool-down time of a 4 K superconducting magnet cooled by a two-stage cryocooler

A cool-down time is one of the major factors in many cryocooler applications, especially for the design of conduction-cooled superconducting devices. Cool-down time means a time cooling a thermal mass from a room-temperature to cryogenic-temperature within a stipulated amount of time. The estimation of cool-down time seeks the elapsed time to cool the thermal object by a cryocooler during initial cool-down process. This procedure includes the dimension and properties of thermal object, heat transfer analysis for cryogenic load, thermal interface between cold mass and cryocooler, and available refrigeration capacity of cryocooler. The proposed method is applied to the specific cooling system for 3 T superconducting magnet cooled by a two-stage GM cryocooler. The result is compared with that of experiment, showing that proposed method has a good agreement with experiment. In addition, the initial cool-down time can be shortened by employing thermal link between the cold mass and first-stage of cryocooler. Through a rigorous modeling and analysis taking into account the effect of thermal link size, it is concluded that there exists an optimal cool-down time during initial cooling in conduction-cooled superconducting magnet system.     

A 3D computer code for steady-state magnetic field calculation inair

A 3-D computer code for the calculation of magnetic fields, self- and mutual-inductance coefficients, and electromagnetic forces is considered. A unique feature of this code is the presence of a number of very precise models for the calculation of the field produced by circular or noncircular coils, busbars, or any other kind of conductor of arbitrary shape. The possibility of using different coordinate systems for input/output data represents an additional user-oriented facility. The problems of singularities both for the elliptic integral calculation, used for circular coils, and for Biot-Savart's integral have been taken into account for the field calculation inside the conductors. Since this code has been conceived to solve magnetostatic problems in air-core devices for fusion research, an easy input for particular windings, such as toroidal and poloidal windings, is also provided. A means for automatic generation of the points of the current lines in busbars and in noncircular coils has been inserted in the code as well     

An accurate AC demagnetizing method using a linear inductance for high permeability cores

A new simple and accurate ac demagnetizing method for high permeability cores is presented. In this method, a linear inductance is connected in parallel with the winding of the core to stabilize the level ofBHminor loops at zero flux level during the whole process of ac demagnetization. The ac demagnetization error is suppressed to less than 0.5% even in 50% Ni-Fe cores. The optimal ac demagnetizing time is longer than 0.5 sec., and the optimal ac demagnetizing waveform of current source is sinusoidal. Experimentally, high demagnetizing accuracy was obtained over a frequency range of 10 Hz-2 kHz. Two cores with the same dimensions and materials are simultaneously ac demagnetized by means of a linear inductance connected in parallel with the primary windings of two cores connected series-aiding, whose secondary windings are connected parallel-opposing and short circuited; thus, the remanent flux level after ac demagnetization is nondestructively detected by destructive readout in other core. This ac demagnetizing method is useful for measuring some magnetic properties such as initial permeability and BH curves and for resetting the small-signal transformer cores and second-harmonic analog memory elements.     

Hydraulic behavior of forced-flow cooled superconducting coils for the large helical device

The large helical device (LHD) has been operated since 1998 and the 13th experimental campaign was conducted in 2009. Before final assembling, cool-down and excitation tests for the Inner Vertical (IV) field coil, which is one of the LHD poloidal field coils, were carried out in 1995. This coil, which consists of a cable-in-conduit conductor, (CICC) is cooled by the forced-flow of supercritical helium. During the tests of the IV coil, hydraulic characteristics, such as flow distribution among cooling channels and friction factors, were measured. In this paper, the consistency of the behavior of the IV coil will be presented and comparison with other fusion devices using superconducting coils will also be made at not only cryogenic temperatures but also at room temperature.     

Power Transformers Internal Insulation Design Improvements Using Electric Field Analysis Through Finite-Element Methods

Understanding the potential and electric field distribution in the insulation system of a transformer during transients is vital to its construction. Therefore, we have developed a method for electric field analysis inside a power transformer. The method consists of the following steps. (1) A lumped parameter equivalent model is constructed by dividing transformer windings into several blocks. (2) The electric circuit parameters of this model are calculated. (3) Employing the results of the transformer transient model analysis as boundary conditions, a 2-D asymmetrical electric field finite-element analysis is performed to determine electric fields through the windings. The method has been examined employing power frequency and impulse voltages. We have also demonstrated successful application of the method to a transformer and have verified the computed results by comparison with measured results.     

Soft-switching DC-DC converter with parallel-connected full-wave rectifiers

A soft-switching converter with parallel-connected full-wave rectifiers is presented. In the proposed converter, the primary windings of two transformers are connected in series. Two full-wave rectifiers with ripple current cancellation are connected in parallel at the output side to reduce the current stress of the secondary winding of the transformer. The clamp circuit, based on an auxiliary switch and a clamp capacitor, is connected in parallel with the primary side of the transformer to recycle the energy stored in the leakage inductance. The leakage inductance of transformers, the magnetising inductance and the clamp capacitance are resonant to achieve zero-voltage switching (ZVS) of the auxiliary switch. The resonance between the leakage inductance of the transformer and the output capacitance of the switch will achieve ZVS operation for the main switch in the proposed converter. The pulse-width modulation technique is adopted to regulate the output voltage. The operation principle and system analysis of the proposed converter are provided. Some experimental results for a 200 W (5V/40 A) prototype are given to demonstrate the effectiveness of the proposed converter.     

Proposal of rectifier type superconducting fault current limiter with non-inductive reactor (SFCL)

A rectifier type superconducting fault current limiter (SFCL) with non-inductive reactor has been proposed. The concept behind this SFCL is the appearance of high impedance during non-superconducting state of the coil. In a hybrid bridge circuit, two superconducting coils connected in anti-parallel: a trigger coil and a limiting coil. Both the coils are magnetically coupled with each other and have same number of turns. There is almost zero flux inside the core and therefore the total inductance is small during normal operation. At fault time when the trigger coil current reaches to a certain level, the trigger coil changes from superconducting state to normal state. This super-to-normal transition of the trigger coil changes the current ratio of the coils and therefore the flux inside the reactor is no longer zero. So, the equivalent impedance of both the coils increased thus limits the fault current. We have carried out computer simulation using EMTDC and observed the results. A preliminary experiment has already been performed using copper wired reactor with simulated super-to-normal transition resistance and magnetic switches. Both the simulation and preliminary experiment shows good results. The advantage of using hybrid bridge circuit is that the SFCL can also be used as circuit breaker. Two separate bridge circuit can be used for both trigger coil and the limiter coil. In such a case, the trigger coil can be shutdown immediately after the fault to reduce heat and thus reduce the recovery time. Again, at the end of fault when the SFCL needs to re-enter to the grid, turning off the trigger circuit in the two-bridge configuration the inrush current can be reduced. This is because the current only flows through the limiting coil. Another advantage of this type of SFCL is that no voltage sag will appear during load increasing time as long as the load current stays below the trigger current level.     

Design and manufacture of a toroidal-type SMES for combination with real-time digital simulator (RTDS)

The authors designed and manufactured a toroidal-type superconducting magnetic energy storage (SMES) system. The toroidal-type SMES was designed using a 3D CAD program. The toroidal-type magnet consists of 30 double pancake coils (DPCs). The single pancake coils (SPCs), which constitute the double pancake coils, are arranged at an angle of 6° from each other, based on the central axis of the toroidal-type magnet. The cooling method used for the toroidal-type SMES is the conduction cooling type. When the cooling method for the toroidal-type SMES was designed, the two-stage Gifford–McMahon (GM) refrigerator was considered. The Bi-2223 HTS wire, which was made by soldering brass on both sides of the superconductor, is used for the magnet winding. Finally, the authors connected the toroidal-type SMES to a real-time digital simulator (RSCAD/RTDS) to simulate voltage sag compensation in a power utility.     

A new procedure for performance prediction of air conditioning coils

A new numerical model has been developed for the simulation of pate-fin-tube type of cooling and dehumidifying coils widely used in the air conditioning industry. The model discretizes the coil into nodes along the coolant path and carries out repetitive marches from the coolant inlet to its outlet while simultaneously updating the values of the air stream properties for obtaining quick convergence. Unlike previous models, this model is capable of analyzing complicated circuiting arrangements and can take into account the variation in heat transfer coefficient along the coil. Since the heat transfer and pressure drop through each circuit are calculated separately, the model can be helpful in pointing out any maldistribution of coolant in various circuits of the coil.     

A circuit model for nonlinear simulation of radio-frequency filters using bulk acoustic wave resonators

This paper describes a circuit model for the analysis of nonlinearity in the filters based on radiofrequency (RF) bulk acoustic wave (BAW) resonators. The nonlinear output is expressed by a current source connected parallel to the linear resonator. Amplitude of the nonlinear current source is programmed proportional to the product of linear currents flowing in the resonator. Thus, the nonlinear analysis is performed by the common linear analysis, even for complex device structures. The analysis is applied to a ladder-type RF BAW filter, and frequency dependence of the nonlinear output is discussed. Furthermore, this analysis is verified through comparison with experiments.     

Cool-down of foam insulated cryogenic transfer lines

An experimental programme is presented for estimating cool-down time for foam insulated cryogenic transfer lines. The results show that there is good agreement between theoretical and experimental cool-down curves. The integral method is shown to be an accurate way to determine the position of cooling front as the cool-down proceeds. It was also found that the mass flow rate does not significantly affect the cool-down of short transfer lines.     

Inductance Calculations for Circular Coils of Rectangular Cross Section and Parallel Axes Using Bessel and Struve Functions

A simple method for calculating the mutual and self inductances of circular coils of rectangular cross section and parallel axes is presented. The method applies to non-coaxial as well as coaxial coils, and self inductance can be calculated by considering two identical coils which coincide in space. It is assumed that current density is homogeneous in the coil windings. The inductances are given in terms of one-dimensional integrals involving Bessel and Struve functions, and an exact solution is given for one of these integrals. The remaining terms can be evaluated numerically to great accuracy using computer packages such as Mathematica. The method is compared with other exact methods for the coaxial case, and with a filamentary method for the non-coaxial case. Excellent agreement was found in all cases, and the exact method presented here agrees with another exact coaxial method to great numerical accuracy.     

The wolfson magnetic levitation project

The Wolfson magnetic levitation project is concerned with the construction of a 500 m long test track and vehicle, levitated electrodynamically by means of superconducting magnets. The general design features and present status of the project are described. The particular aspects which are discussed include the guideway design, the cryogenic system and superconducting coils, and both the linear synchronous and commutator motor propulsion systems having powered track windings.