AC Coil Research Articles

Experimental study on quench current of superconducting coils for 50-60 Hz use

The quench current of a superconducting coil wound with a multifilamentary superconducting wire for 50-60 Hz use was measured for various frequencies by using a variable-voltage variable-frequency power supply composed of a cycloconverter or an inverter circuit to investigate characteristics of the superconducting AC coil and solve the current degradation problem. The test coil was wound by using wire containing no copper. Because of the instability of the wire, the effect of wire motion was observed, and quench current of the coil varied significantly. Generally, superconducting wires for AC use are unstable because the copper content of the wire is lower than for wires for DC or pulse use. From experimental results, it has become clear that a fixing method for wires to a bobbin keeping a good cooling condition is the key to solving the current degradation problem.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Development in the Theory and Analysis of Eddy Current Sensing of Velocity in Liquid Metals

We have performed a theoretical study of the magnetic fields produced by a single turn ac coil encircling a moving conductive rod or tube of flowing liquid metal. A solution exact to first order in the velocity has been obtained as a closed-form integral. Radial variation of the velocity profile is included. The solution allows a calculation of the voltages induced in two secondary coils wired in opposition for application as a velocimeter. With secondary coils wired in series, the calculation provides the classical impedance vs. frequency curves which allow determination of sample conductivity and diameter. The solution has been used to optimize design for a sensor to be used for monitoring speed and tube bore diameter in a liquid metal atomizer system. An additional solution has been obtained for arbitrary magnitude of velocity, but for constant velocity profile.

Possibility of ac applications of high-T c superconductors in a flux-flow state

For the ac applications of high-Tc superconductors, the possibility of a ‘‘flux-flow conductor’’ is proposed. The ac loss of a high-Tc superconductor in the flux-flow state is theoretically examined. Two applications, a power transmission cable and a winding of an ac coil, are considered. The ac loss of a ‘‘flux-flow superconducting cable’’ may become smaller than that of a ‘‘critical state cable’’ in ordinary conditions, because the upper critical fields of high-Tc oxides are large. On the other hand, the ac loss of a ‘‘flux-flow ac superconducting wire’’ becomes advantageous only in a limited condition, namely, high frequency, low current density, and low magnetic field.

AC screening measurement for the characterization of ceramic superconductors

Abstract The effect of 2.5 MeV electron irradiation on ceramic sintered YBa2Cu3O7 was investigated by in situ measurements at 20 K of the screening efficiency of a sample between two small AC coils. The complex transmittivity, T, was measured as a function of the superposed static magnetic field HDC for various irradiation doses. From the analysis of T(HDC) curves following a model presented in the preceding paper, we deduce a decrease of the characteristic magnetic filed of Josephson junctions after irradiation and a very slight increase of intergrain critical current JcJ at very low irradiation doses followed by a decrease. An increase of the width of hysteresis loops in T(HDC) curves was also observed after irradiation.

Nonintrusive methods for monitoring the operational readiness of solenoid-operated values

Solenoid-operated valves (SOVs) are being studied at Oak Ridge National Laboratory as part of the USNRC Nuclear Plant Aging Research (NPAR) Program. The primary of the study is to identify, evaluate, and recommend methods for inspection, surveillance, monitoring and maintenance of SOVs that can ensure their operational readiness — that is, their ability to perform required safety functions under all anticipated operating conditions. An earlier (Phase 1) study described SOV failure modes and causes and identified measurable parameters that might be used to monitor the various degradations that lead to functional failure. The present (Phase II) study focuses on devising and then demonstrating the effectiveness of techniques and/or equipment with which to measure the previously identified performance paramaters (or others) and thus detect and ascertain the trend of the progress of any degradation that may be occuring. Several nonintrusive techniques are presently under investigation. These include monitoring coil mean temperature during operation by means of DC coil resistance, monitoring valve position by means of coil AC impedance, verifying free SOV plunger movement by measuring current and volatage at their critical bistable (pickup/dropout) values, and detecting the presence of shorted turns or insulation breakdown within the solenoid coil using interrupted-current test methods. A technique slated for later investigation is monitoring coil hum or chatteringm, using acoustic sensors or electrical noise indications. Experimental results are presented demonstrating the technical feasibility and practicality of the diagnostic techniques being assessed in this continuing study.

Stability of epoxy-impregnated AC superconducting winding

AC superconducting composite wires for 50-60-Hz application are susceptible to disturbances due to wire motions, because matrices of the wires are highly resistive CuNi. Instability of the wires is one of the most serious problems in developing AC wires of large current capacity. An effective technique to reduce the wire motions is to impregnate the windings. It is demonstrated that epoxy impregnation is an effective technique to stabilize AC superconducting windings and that the AC losses can be cooled sufficiently by heat conduction through the epoxy. The characteristics of epoxy-impregnated 100-kVA-class AC coils are experimentally and theoretically compared to those of non-impregnated coils. >

Controlled characteristics of a superconducting AC coil by pulse width modulated current

The authors propose the application of superconducting AC coils to superconducting magnet energy storage. The stored energy of the superconducting coil is to be supplied to the AC power line (60 Hz) via a power converter, and the pulsation of supplied power controlled in the frequency range between DC and several tens of hertz. A variable-voltage variable-frequency drive system was developed to study the possibility and characteristics of this sort of SMES system. The developed system is composed of a small superconducting AC coil and MOSFET pulse-width modulation converters. The combination of superconducting AC coil and PWM control can be utilized for SMES stabilization of a power system.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A Rapid Cycling Synchrotron Magnet with Separate AC and DC Circuits

In present rapid cycling synchrotron magnets ac and dc currents flow in the same coil to give the desired field. The circuit reactance is made zero at dc and the operating frequency by running the magnet in series with an external parallel resonant LC current. We propose to return the ac flux in a gap next to the synchrotron. The dc coil encloses the ac magnetic circuit and thus links no ac flux. A shorted turn between the dc coil and ac flux enhances the separation of the two circuits. Several interesting developments are possible. The dc coil could be a stable superconductor to save power. The ac flux return gap could be identical with the synchrotron gap and contain a second synchrotron. This would double the output of the system. If the return flux gap were used for a booster, the ac coil power could be greatly reduced or radiation hardening of the ac coil could be simplified.

Equipment and materials

A range of 5 basic types of industrial, all purpose, relays offer a choice of ac or dc coils ‐6A capacity. Operating at 2·4VA for ac and 1·2W dc, the D range conforms with accepted coil power requirements. High impact flame retardant moulding materials, with good insulation properties, are used throughout. Plug‐in types, with either solder or screw‐terminal sockets and panel mounted types suitable for either termination or Faston connectors, are both supplied with a clear polycarbonate clip‐on dust cover. A narrow profile of both the 2 pole and 3 pole plug‐in versions enables close stacking and consequently space saving with additional uniformity of appearance when mounted on an equipment panel.

University of Virginia Superconducting Wind-Tunnel Balance

The design of an electromagnetic balance using superconducting coils is reported. Both dc and ac coils are used to support aerodynamic models in a supersonic (Mach 3) wind tunnel and to simultaneously measure the forces acting on them along 3 orthogonal axes. Major design characteristics include: adoption of symmetrical coil arrangement to provide maximum space for the wind tunnel; 3 gradient-coil pairs capable of being driven between 0 and 350 A at a frequency of 30 Hz by specially designed power supplies; a vertical wind tunnel with a 6-in. test section located in the axial room-temperature access of a 250-liter liquid-helium Dewar. Results on ac losses for prototype gradient coils wound of three different superconducting materials are reported.