Stationary Coil Research Articles

Interior ballistics of a hybrid electromagnetic gun

The dynamics for the projectile inside an electromagnetic gun has been developed. The gun consists of a series of stationary cylindrical coils that are pulsed and of one inner concentric coil driven at constant current. The inner coil is the projectile. Two important design parameters are identified: one is the desired exit velocity divided by the square root of the number of stationary coils, the other is a characteristic time related to the rapidity with which the projectile attains its maximum velocity in any stationary coil.

Skull melter growth of magnetite (Fe 3O 4)

High quality equiaxed magnetite (Fe 3O 4) crystals, up to 2 cm in extension, have been grown in a skull melter in an atmosphere of CO 2 by slow lowering of the crucible out of the stationary work coil. X-ray powder diffraction and back reflection Laue photography indicate magnetite crystals, reflected polarized light microscopy of the boules reveals wüstite (Fe 1−x O) intergrowths restricted to a few microns of the grain boundaries and a thin layer (about 100 microns) of a hematite (Fe 2O 3) on the top of the boule. A comparison is made between results for skull crucibles of different sizes.

B-Clock System for the KEK Main Ring

For the monitor of magnetic field of the dipole magnets of the KEK main ring, the B-clock system was constructed. This system is useful to control the R.F. acceleration system. As the sensors of magnetic field, we use a Hall probe and a stationary search coil. The measurement is done in the power house for the main magnets, and data are transmitted to the central control room and the auxiliary equipment houses. At present the accuracy of this system is about 10-3. We plan more accurate system of 10-4 accuracy with the aid of N.M.R. magnetometer.

Flywheel energy storage—II: Magnetically suspended superflywheel

This article, the second of a two part paper, describes the general design requirements for a flywheel energy storage system. A new superflywheel energy storage system, using a spokeless, magnetically suspended, composite material pierced disk rotor is proposed. The new system is configured around a permanent magnet (“flux biased”) magnetic suspension system with active control in the radial direction and passive control in the axial direction. The storage ring is used as a moving rotor and electronic commutation of stationary armature coils is proposed. There is no mechanical contact with the rotating ring and long life and low run down losses are projected. A discussion of major components for a 10 kwh system is presented.

Electrodynamics of a magnetic levitation coil

The electrodynamics of a magnetic leviation coil is studied without the use of Maxwell's thin-plate approximation. Formulas are derived for the forces on and current fluctuation in a vertically perturbed coil and the transient response of a stationary coil in the vicinity of a permeable conducting track. All results are obtained from exact solutions of the Maxwell equations in which the displacement current is neglected. The forces are then linearized and a close coupling approximation for the reflected impedence of the coil is derived.

Instrument for ac magnetic measurements

Instruments whose operation is based on measuring the alternating (sinnsoidal) emf induced by the magnetic field in the measuring transducer are widely used in magnetic measurements. These instruments are used in particular for measuring constant and alternating field strengths of electromagnets, and for measuring alternating induction in tested specimens. In the first instance the transducer consists of a rotating search coil, and in the second instance of a stationary coil, whereas in the third case the secondary winding of the specimen is used. The best of these instruments consist of those in which the induced emf is measured by means of the compensation method, for instance, with an ac potentiometer [1, 2]. Despite their several advantages (linearity, possibility of measuring spatially nonuniform fields over a wide range of their amplitude and frequency variations) these instruments have a deficiency consisting of the fact that their induced emf amplitude is proportional not only to the amplitude of the measured magnetic field, but also to its frequency and, therefore, any instability of that frequency produces measurement errors. In measuring a constant field with a rotating coil that error is caused by the instability of the speed of rotation, and in the case of alternating fields, by the instability of the magnetizing current's frequency. For instance, in the case of the commercial mains frequency this instability can amount to :~1% and more, whereas the variation of the emf amplitude produces an error of less than 0.1%. In order to eliminate this error, frequency stabilization is used, and in measuring constant fields, compensation is applied to the voltage of the second coil, which rotates in synchronism with the first coil in the field of the additional reference magnet (see, for instance [3]). In the instrument under consideration [4] the emf is measured with a potentiometer connected in a circuit which provides proportionality between the amplitude of the compensating voltage tapped off the potentiometer and the frequency, which is proportional to the measured field amplitude. The instrument's principle of operation is shown in the schematic of Fig. 1. The instrument's output terminals 11 can be connected to any pair of the transducers' terminals 15, 12, or 7. The terminals 7 are connected to the rotating coil 8 by means of the contact rings with the brushes 9. The coil 8

Interturn Short-Circuit Detector for Turbine-Generator Rotor Windings

An effective new method of detecting interturn short circuits on round rotor windings is described. The approach used is to measure the rate of change of the air-gap flux density wave when the rotor is at operating speed and excitation is applied to the field winding. The measurements are obtained by the use of a small stationary search coil positioned near the rotor surface. The voltage outputs are recorded by Polaroid pictures from an oscilloscope screen and can be analyzed based on the information presented. The sensitivity obtained from these methods is far superior to previous methods. The data obtained at operating speed identify the coils and slots which have shorted turns, and provide a good indication of the number of turns shorted in a coil if more than one short exists.

The Optical Sensors in FM-1

In the PPL FM-1 Machine, the plasma containment volume is shaped, in part, by a levitated superconducting ring which is stabilized and positioned by servoed currents in stationary tilt and slide coils. The instantaneous position of the ring is sensed, in the required five degrees of freedom, by external non-contacting reflex optical sensors. Design and construction of these sensors is described and results of performance tests, both off and on the FM-1, are presented.

A simple miniature furnace for routine collection of single crystal x-ray data up to 1000$deg$C on the Hilger and Watts linear diffractometer

A miniature furnace suitable for routine collection of x-ray data up to 1000°C from single crystals on the Hilger and Watts linear diffractometer, without restricting the normally allowed region of reciprocal space on the diffractometer, is described. The crystal is heated primarily by radiation from a surrounding current-heated, stationary platinum coil wound on a silica bracket. The coil is split at its middle to provide a 4 mm gap for crystal mounting and x-irradiation. The crystal, mounted on a standard goniometer head, can be rotated and centred freely, as in the room temperature case. There is no need for any radiation shields or water-cooling arrangement. Investigations up to 1500°C are possible with slight modifications of the furnace.

Flux Displacers for Pulse Generation with Superconducting Multipole Magnets

Short, steep pulses of high electrical energy are required to drive systems such as pulsed transmitters, light sources and plasma discharges. A low impedance stationary coil will generate such pulses when the magnetic flux threading the coil undergoes large and rapid changes. The rapid change can be achieved by a ``flux displacer,'' a conductor driven at high speed past the stationary coil. The applied field is provided in the present work by a superconducting magnet in order to produce a magnetic flux of the necessary magnitude and density with small operating loss, bulk and weight. The stationary coil together with the load and the flux displacer were not superconducting and were placed outside the Dewar. Two series of experiments with flux displacers driven by a rotary prime mover are discussed. The first series used relatively small, isolated specimens. These experiments served to determine the effect of displacer shape, speed and conductivity on output. The second series used relatively large displacers formed by pairs of cylindrical shell segments which are electrically and mechanically connected by end plates. These experiments give information on the influence of various system parameters on the energy, and on the duration and shape of the pulses. The results are presented in dimensionless form, so that the performance of higher-energy systems can be predicted. Ultimately, the rotary prime mover may be a lightweight explosive drive or gas turbine or, in a vehicle or aircraft, one of the propulsion engines.

An air core transformer system eliminating contact wires in the carrier frequency device of a torsion pendulum

A variation of the carrier frequency method, used for indicating the oscillations of a torsion pendulum, is described. In order to avoid additional torques caused by contact wires the transmission of the modulated carrier frequency is performed by an air core transformer consisting of two movable primary coils and two stationary secondary coils. It is a special feature of this arrangement that the coupling of primary and secondary coils does not depend on the angle of twist, provided this angle remains within a range of about 20°.

An oscillating synchronous linear machine

The paper describes the development of a new type of a.c. generator in which the moving member travels between a d.c. pole structure in a straight line with reciprocating motion. Tha induced e.m.f. appears in the moving member, which consists of a single loop of conducting material which embraces and moves along the core. The core carries stationary coils which experience an induced alternating e.m.f. by transformer action from the moving loop. These coils are not associated with the air-gap between the d.c. poles and are more easily cooled than the windings of a rotary machine. The loop, which is confined to the air-gap, can be run at high temperature, since it carries no insulation. The machine can be used as a generator to convert mechanical power supplied directly from a piston. It can also be used as a synchronous motor for such purposes as the driving of compressors without cranks; when used in this manner the machine is self-starting. Essentially a single-phase machine, the equivalent of multi-polar rotary machines can be constructed which will generate e.m.f.'s at 50c/swith mechanical oscillations at frequencies lower than 3 000 per minute.The construction of the machine is simple, since the core is made up entirely from rectangular stampings, while the windings consist of four transformer-type coils and a conducting loop.An experimental machine is described, development of which consisted largely of testing devices to minimize the internal impedance Test results are given.The paper includes a proposed design for a 33 kVA generator.

Plasma Propulsion by a Rapidly Varying Magnetic Field

An investigation has been made of the motion of a plasma under the action of a rapidly varying magnetic field from a stationary coil. The analysis has been done for two cases: (1) the plasma is assumed completely diamagnetic so that the ionization history of the plasma may be ignored and (2) the nondiamagnetic case where the ionization process is considered in detail. For the diamagnetic case the analysis yields a coupled set of nonlinear ordinary differential equations which have been put in similarity form and solved for a wide range of the similarity parameters. An illustrative example is given to show how to use the results to obtain detailed information for a given physical system. The calculations indicate that, for field frequencies of about one megacycle, velocities of the order 107 cm/sec may be attained, but the efficiency of conversion of stored energy into kinetic energy is rather low (about 5–10%) and cannot be varied much by possible changes in the physical and geometrical parameters of the system. Because of the complexity of the analysis, a systematic series of solutions has not been made for the nondiamagnetic case. Instead, a typical example has been worked out to indicate the major differences to be expected. The results show that the nondiamagnetic plasma yields an efficiency slightly less than that obtained for the corresponding diamagnetic plasma. The ionization process inside the plasma levels off very rapidly so that the electron density increases by only a few percent during the motion of the plasma, but the amount of ionization has very little effect upon the efficiency of the system. The indications are that the efficiency of a plasma driven by a magnetic field from a stationary coil is probably too low for practical applications.

Analysis of partly symmetrical machines by means of unitary transformation

The instantaneous impedance matrices of machines having electric and magnetic circuits on but one side of the air gap symmetrically arranged are shown to be Hermitian and unitary. Unitary matrices containing variable base vectors are used to transform the operational equations relating voltages and currents in six circuits, viz., three stationary armature coils and three rotating field coils on salient pole. The transformed equation is compact, convenient and amenable to transient solutions. It is correlated to previous results obtained by Park's transformation in a simple manner. An analysis is then made on asynchronous operation under balanced terminal voltages with the main field short circuited. The results include the well-known Georges phenomenon of induction motor with single phase rotor.

The Magnetic Cross Valve

This is the discussion of a basically new electromagnetic device in which two stationary coils with no mutual inductance to each other in the ordinary definition of the term, transfer energy from one coil at one frequency to the other coil at a second frequency through the medium of a vector or field of flux which follows a lissajous locus or mode of motion. It is compared to rotary electromagnetic devices in which the more simple lissajous mode of motion or circle is used and to the transformer which has but one degree of freedom. Saturation of the core legs in sequence to become active, and alternating at a frequency different than the power source.

A New Method for Stripping Venomous Snakes

The usual method of evacuating the venom glands of poisonous snakes—viz., manual manipulation, is in a great many cases accompanied by traumatic injury of sufficient severity to interfere with subsequent functioning of these structures. This results, shortly, in a decrease in venom production and a product of inferior quality. Experience has shown that by employing electrical stimuli in place of manual manipulation, the venom can be obtained without injury to the glands and without affecting materially its synthesis or quality. The procedure as carried out in this laboratory is simple and safe and the necessary apparatus is inexpensive. The apparatus consists of an induction coil which can be regulated to deliver from five to ten volts. The type used in this laboratory is one of very simple construction with a stationary secondary coil and a fixed soft-iron wire core.

An automatic magnetic field stabilizer of high sensitivity

The present paper describes a method whereby the field of an electromagnet can be kept constant automatically to at least one part in fifty thousand ( i. e. , to 0⋅2 gauss in 10,000 gauss) for long periods. The method has the great advantage of not merely ensuring that the current through the magnet (or, say, the voltage across the magnet windings) is kept constant, but that the field itself , as measured by a fluxmeter, is kept within one-fifth of a gauss (or less) of some desired value. Thus, it is not necessary to know whether a fluctuation of the field is caused by unsteadiness of the exciting battery voltage, variations of the magnet coil resistance with temperature, etc. ; in all cases, the stabilizer automatically readjusts the exciting current until the field is brought back to its original value. Further, as explained at the end of the paper, the stabilizer can be made to bring about an increase or decrease in the field of a definite number of gauss, and to continue to maintain the field at this new value. Also with very slight modifications, which are so obvious as to require no further explanation, the stabilizer could be made to maintain a current, or a potential difference, constant just as easily as a magnetic field. The method has been applied with success to the annular magnetic field used in the Cavendish Laboratory. The magnet is used for the analysis of α-particle groups. Under the influence of suitable fields of the order of 10,000 gauss, α-particles of various velocities are made to travel in a semicircle of 40 cm. radius, and are focussed on to the slit of a counting chamber. Groups of various velocities are focussed in turn by varying the field, their velocities being determined from the values of the fields. It is extremely important, therefore, to know the field strength with accuracy and to maintain it constant, at each of these values, during the experiment. In some experiments already described this was done manually. An observer watched the spot of light reflected from the mirror of a fluxmeter, the coil of which was connected to a stationary search coil situated in the magnetic field. A change of one-fifth of a gauss in the magnetic field produced a deflection of the fluxmeter spot of about 1 mm., and by suitable adjustment of the field current by means of a mercury rheostat it could be arranged that the spot was kept within half a millimetre of its zero portion. This, however, required one observer to spend his whole time in stabilizing the field during an experiment which might last several hours.

Measurement of Alternating Current of Low Value

Alternating-current ammeters of the dynamometer type when used for measuring small currents are usually connected so that the moving coil is in series with the stationary or field coils.