Multi-turn Coil Research Articles

Probe type thin film head for perpendicular magnetic recording

By experimenting with a probe type head, writing mechanism of double layer perpendicular medium, it becomes clear that the perpendicular field <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">H \gsim Hc\perp</tex> + Hd is required to write enough, where <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Hc\perp</tex> is the coercive field of the medium, Hd = kπMs ( <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k\sim2</tex> ) is the demagnetization field and Ms is the magnetization of the medium. In order to generate a strong perpendicular field, the probe type head structure was designed. Corrosion resistive Fe-Si-Ru complex film, of which saturation is 17 kG and permeability is 2000, is developed as a main pole material. A new fabrication method for the multi-turn coil within a small magnetic path is also developed to achieve this design. The output signal of the probe type head by using a Co-Cr double layer medium is about two times as large as that for Winchester type Mn-Zn ferrite head.

EXPERIMENTAL MEASUREMENTS OF THE EDDY CURRENT SIGNAL DUE TO A FLAWED, CONDUCTING HALF SPACE

Abstract An experimental investigation of the change in impedance of a practical multi-turn eddy current coil near a conducting half space has been undertaken. The real and imaginary parts of the impedance were measured as a function of both coil lift-off distance and conductivity. In addition the effect of a small flaw beneath the surface of the plane were also investigated as a function of flaw depth and flaw location. The resulting changes in impedance were compared and found to correlate well with previously derived theoretical results.

Molded hearing aid and battery charger

A molded hearing aid of the self-containing type having a rechargeable battery disposed therein, in the form of a molded plug which will fit in the ear of the user. A light emitting diode is disposed through the external surface of the plug and is internally connected in series with an iron core inductor with the series combination in parallel with the internal chargeable battery. A battery charger is provided having a cup for receiving the molded plug and having a multiturn coil wound around the periphery of the cup. The coil is connected into an oscillator circuit and tuned to cause the circuit to oscillate at about 30 kHz. The oscillator coil couples to the iron core inductor in the plastic plug thereby inducing a 30 kHz ac voltage across the coil. The light emitting diode rectifies the alternating current and is connected in a polarity so as to charge the internal battery of the hearing aid. As the charging current flows through the light emitting diode, the diode is illuminated indicating to the user that charging is taking place and indicating by its brillance when the hearing aid is located in its optimum position.

Recent developments in the use of surface electrical methods for oil and gas exploration in the Soviet Union

A great deal of interest has been expressed in Western countries during recent years on possible applications of surface electrical methods in oil and gas exploration. It has been reported that these methods are widely used in the Soviet Union, but to date few technical details have been available. In late 1979, the author visited the Soviet Union under the auspices of the Australia‐USSR Agreement on Scientific Cooperation, with the purpose of studying recent developments in electrical and electromagnetic (EM) methods. This paper presents a summary of the use and applications of those methods in oil and gas exploration, and includes a number of case histories. The methods can be broadly classified as follows: sounding for structural mapping, sounding to measure the geoelectric properties of the oil‐bearing horizon, and indirect methods which detect the presence of a geochemical plume or halo above the oil deposit. The magnetotelluric (MT) method and transient sounding in the near zone (ZSBZ) are widely used for deep sounding, especially in areas where seismic methods are ineffective. Modern ZSBZ equipment can measure over [Formula: see text] decades of time, from 0.1 msec to 44.8 sec. The transmitter consists of a large (1000 m) square loop carrying a 40 A rectangular waveform, and the receiver is a multiturn coil located at the center. Depths of exploration depend upon geologic conditions, but they are typically of the order of 3 to 4 km. Statistical data, based on logging, of the geoelectric properties of a large number of oil fields have also been collected. There is generally an increase in resistivity in both the oil‐bearing horizon and overlying layers. The resistivity of near‐surface layers appears to be partially dependent upon climatic conditions. Experimental induced‐polarization (IP) surveys have been carried out over oil and gas deposits for about ten years. Both time‐domain and frequency‐domain systems are used, and sometimes an orthogonal electric array is employed to reduce inductive coupling. Either low frequencies (less than 1 Hz) or late times (greater than several seconds) are measured. IP anomalies are thought to be caused by epigenetic pyritization located in a plume over oil fields in a geochemically active environment. The anomalies are either of the form of a wide anomaly centered over the top of the deposit, or the shape of a halo, surrounding the deposit. There is evidence that the behavior of the IP decay at very late times (several minutes) can be used to distinguish IP anomalies inside and outside the boundaries of a hydrocarbon deposit.

High-sensitivity DC instruments

Almost simultaneously with the invention of moving-coil galvanometers (MG) efforts began to improve their sensitivity and accuracy. Of decisive importance here were the advances made by Deprez and d'Arsonval who proposed to use a multiturn coil positioned in a cylindrical air gap of a magnetic system [i]. MG's have the moving part attached to a suspension and a scale with an illuminator that is located several meters away. These instruments have a relativeKy poor accuracy and sensitivity, especially for voltage. They are susceptible to mechanical disturbances and are therefore mounted on a main wall.

A thin film head for perpendicular magnetic recording

A thin film head of new probe type was fabricated, applicable to perpendicular recording on Co-Cr rigid disks. The head consists of a magnetic film for the main pole and multiturn spiral coils (2 layers, 10 turns) were fabricated on a ferrite substrate. The recording characteristics were measured on 14-in. sputtered Co-Cr disks having a high permeable Ni-Fe back layer. The magnetomotive force for saturation recording was 2 A-T and the output voltage was 0.65 mV at a pole thickness of 4 μm and pole width of 70 μm, at a head to medium spacing of 0.3 μm. The recording density was measured to be 17 kFRPI at a pole thickness of 0.7 μm.

Impedance of a loop surrounding a conducting cylinder

The change in complex impedance between an ideal one-turn coil surrounding and coaxial with an infinitely long circular cylinder of conductivity δ and permeability μ and a similar coil without the core has been calculated. From the exact expression, a power series in the quantity δ/b (δ=skin depth; b=radius of core) has been developed. From this result, the change in impedance of a physically realistic multiturn coil can be estimated. The theory permits a rational approach to optimization of the design of eddy-current test coils and provides a possible means of detecting changes in the radius and conductivity of the cylinder.

Fabrication of multi-turn thin film head

A new fabrication method has been developed for multi-turn thin film heads. In this method, a multi-turn coil is formed by piling two layers of flat surfaced bifilar spiral coils. A bevel-etching technique has been developed to form the flat surface of the bifilar coil. A 9-turn, 18-track thin film head has been fabricated and tested on the conventional particulate disk. The read efficiency was 88% and the MMF required for 30 dB of overwrite was 0.32AT.

Dynamic electromagnetic response of a homogeneous conducting cylinder for symmetric excitation

A stranded wire rope is idealized as a homogeneous conducting and permeable cylinder of circular cross section and of infinite length. The rope is excited by a coaxial solenoid or finite-length multi-turn coil that carries an azimuthally directed alternating current. The rope and the enclosing solenoid may have a uniform velocity relative to each other. Using a nonrelativistic analysis, the nature of this dynamic interaction is examined and numerical results are presented for parameter values that are relevant to both static and dynamic conditions in nondestructive testing of such cylindrical conductors. It is shown that for motional velocitiesv larger than about 50 m/s the dynamic interaction with the rope specimen is appreciably modified from that for the static condition (i. e., forv=0).

Dryout Heat Fluxes for Inductively Heated Particulate Beds

Experimental observations of the dryout heat fluxes for inductively heated particulate beds have been made. The data were obtained when steel and lead particles in the size distribution 295–787 microns were placed in a 4.7-cm dia pyrex glass jar and inductively heated by passing radio frequency current through a 13.3-cm dia multiturn work coil encircling the jar. Distilled water, methanol and acetone were used as coolants in the experiments, while the bed height was varied from 1.9 to 8.9 cm. Different mechanisms for the dryout in deep and shallow beds have been identified. Dryout in shallow beds is believed to occur when the vapor velocity in the gas jets exceeds a certain critical velocity at which choking of the vapor, leading to obstruction in the flow of the liquid towards the bed occurs. However, deep beds dry out when gravitational force can no longer maintain a downward coolant flow rate necessary to dissipate the heat generated in the bed. Finally, the heat flux data of the present investigation and that from two previous investigations made at Argonne Laboratory and at UCLA have been correlated with semitheoretical correlations based on the proposed hydrodynamic models.

The relationship between the electric field in a semi-infinite conductive region and the power input to a circular coil on or above the surface

The electric field induced in a semi-infinite isotropic homogeneous region by a time-varying current in a multi-turn circular coil positioned on or above the surface of the region is related to the power supplied to the coil. In this derivation, the electric field (normalised in terms of the conductivity of the region, the distance of the field point from the surface and the applied power) is expressed as a function of the ratios of the linear dimensions which characterise the geometry of the system to the distance of the field point from the surface and of a quantity which involves frequency, the Q factor of the coil, conductivity of the region, and the distance of the field point from the surface.

A field-theoretical approach to magnetic induction heating of thin circular plates

A field-theoretical approach is used to analyze the subject of magnetic induction heating of thin circular plates by planar coils. Closed-form solutions for the electric and magnetic fields are found to the basic field problem of a single circular loop carrying current at a frequency ω in the presence of a plate characterized by a permeability μ and conductivity σ. By using these fields, expressions are derived for the complex Poynting vector at the surface of the plate, and for the induced EMF in the coil. The theory is extended to include multiturn coils and a field-dependent permeability, and a specific multiturn coil and plate combination is chosen as an example. The complex amplitude of the magnetic field and the Poynting vector are calculated along the surface of the plate using iterative methods to assure self-consistency with the field dependent permeability of the plate. By using Fourier transform techniques, the transient coil current and coil voltage waveforms are calculated under the experimental conditions used to take data on the sample coil and plate. The absorbed power is calculated from these waveforms and is found to be within 10 percent of the measured power absorption for all levels of operation from 50 to 2000 W. The calculated coil current waveform is compared with the measured waveform and is found to be in very good agreement in both shape and period.

A sensitive displacement transducer using an extremely reentrant 84 MHz cavity oscillator

A single-turn toroidal optimization of the inductor in the LC circuit of a capacitive displacement transducer is described. It has a higher Q than the usual multiturn coil and allows the construction of a rigid LC resonator which does not use any insulators. When this resonator is used in conjunction with a tunnel diode oscillator, the sensitivity of the capacitor to position changes is conservatively estimated as 7×10−12 cm.

MAGNETITE ASSAYS FROM MAGNETIC SUSCEPTIBILITY MEASUREMENTS IN TACONITE PRODUCTION BLAST HOLES, NORTHERN MINNESOTA

A study was made at U.S. Steel Corp.’s Minntac open‐pit taconite mine in northern Minnesota to assess the applicability of relating magnetic susceptibility measurements made in production blast holes to the magnetite content of the strata penetrated. The measurements were made by continuously recording the changes in self‐inductance of variously constructed air‐core, multiturn coils as they were raised in the holes. These coils were designed so that the magnetic flux which coupled with the wallrock was parallel to the horizontally bedded Biwabik iron‐formation. An analysis of the response for these coils showed that the recorded inductance changes (apparent magnetic susceptibilities) are related linearly to the average weight percent Fe as magnetite within a practical degree of accuracy. In one test hole, 36 assayed intervals, each 18 inches long, fitted a linear line of regression to the corresponding apparent magnetic susceptibilities within a standard error of ±1.2 weight percent Fe. When averaged over three ranges of Fe content, the values fitted a straight line to within ±0.5 weight percent Fe. As susceptibility depends on the mode of occurrence of the magnetite, extrapolation of these results to other areas will require further testing. This method of assay should provide control for the efficient processing of taconite ores. Since the method is rapid and inexpensive, measurements could be made in scores of these accessible holes to delineate cut‐off boundaries between ore and waste. A potential use of these continuous measurements would be found in establishing lithologic contacts from the detailed responses in selected horizons of the iron‐formation.

Noninterrupting Method for Measuring Pulsed Charged Particle Beam Currents

A noninterrupting method for measuring a charged particle beam current consisting of a series of periodic pulses is presented. The pulsed beam is allowed to pass through a high permeability toroid on which is wound a multiturn coil. If the distance traveled by a beam particle during one pulsing period is large compared with the toroid dimensions, then the voltage induced in the coil consists of a series of impulses, the time integral of each impulse being proportional to the beam current. Since this voltage is equal to that induced in the coil by a one-turn loop wound on the toroid and carrying a current identical to the beam current, it is possible to vary this current, and employ the multiturn coil as a null indicator to indicate equality of the beam and loop currents. If now a capacitor adjusted to resonate with the multiturn coil is placed across the coil, the sensitivity of the null circuit is increased by a factor equal to the Q of the resonant circuit. Using this resonant magnetometer and a 1000-cps chopping rate, currents of 3×10−7 A peak have been measured with an accuracy of ±10−8 A.

Flux Concentrator for High-Intensity Pulsed Magnetic Fields

Use of multiturn coil structure in the production of several hundred kilogauss pulsed fields is discussed with specific emphasis on coil life. The magnetic force that shortens the coil life is reduced by introducing a flux concentration device. Into the multiturn coil is inserted a rigid, slotted metal slug that generates induction current in such a way as to shift the magnetic flux from the multiturn to the central vacuum region and serves as a flux concentrator. Electromagnetically and mechanically this scheme is a convenient synthesis of the multiturn and single turn coils. The advantage of this scheme is most apparent for tape-wound coil structures, and a quantitative analysis of the flux pattern relevant to this coil system is given. An analog computer is employed in solving the time dependence of the field. The results are documented by experimental tests carried out with a 3000-μf, 4-kv capacitor bank.

Multiturn coils for use in vacuum

Coils for use inside sealed-off vacuum devices have usually been limited to a small number of spaced turns. Where larger numbers of turns were required, the use of some form of oxidized wire was sometimes resorted to.

A Small Coincident-Current Magnetic Memory

This paper describes a small coincident-current memory used for buffer storage. Such a memory as part of the self-checking card-to-magnetic-tape converter, an auxiliary of the Univac System, is now in production. Typical advantages of a small coincident-current memory in computer input-output equipment, as well as some of the problems encountered in its application, are described. This memory affords the card-to-tape converter a great degree of flexibility, making it possible to read cards sidewise and to check and edit information with a minimum of hardware and complexity. Memory cells consist of metallic-tape cores wound with multi-turn coils. The low currents required permit operation of the memory directly from the card-sensing brushes on writing and from a diode function-table on reading. The functional aspects of the memory and its associated electrical circuitry are described. Information concerning the physical nature of the memory, specifications of the cores, and some of the tests performed in their inspection is also presented.

Magnetic Heating of Transmission Line Clamps

Magnetic heating of transmission line clamps is reduced by the use of nonmagnetic parts. An economical method is the use of a non-ferrous body. Of the three methods used to measure clamp power loss, the multiturn coil circuit is recommended. The results of tests made with the multiturn coil show the marked difference between the power loss of the all-ferrous clamp and that of the nonferrous-body clamp. If clamp and conductor losses are known, the temperature rise of a conductor inside a clamp can be estimated.

Transient Voltages in Rotating Machines

The unequal distribution of voltage in rotating machine multiturn coils caused by switching transients, and means for alleviating the stresses occasioned by these transients are discussed in this paper. A brief description also is given of field measurements of switching transients and of tests to determine the effectiveness of capacitors as a protection against switching transients. It is shown that capacitors connected between each phase and ground near the machine terminals gives effective protection.