Transformer Bridge Research Articles

The potential transformer bridge with current comparator for measuring the voltage dependence of compressed-gas capacitors

In this paper, an alternative method for measuring the voltage dependent capacitance variation (VDCV) of compressed-gas capacitors is described. In the proposed method, the VDCV of the capacitor under test is measured by comparing it with a capacitor standard supplied with the secondary (low) voltage of a potential transformer. This transformer has its primary parallel connected with the capacitor under test. The paper presents and criticizes the test results obtained in which an uncertainty of at least 0.5 ppm is achieved.

A system for calibration of inductance standards based on a simple unbalanced transformer bridge

An unbalanced transformer bridge designed for inductance standard calibration is described. Basic theory of the bridge is discussed in which two components of the unbalanced voltage are measured and used to determine a ratio of compared inductance standards. Significant sources of errors are analyzed and numerical limits given. The uncertainty of comparison, in the frequency range 100 Hz-2 kHz, is assessed to be smaller than 30 ppm for 10 mH-1 H standards and smaller than 100 ppm for 1 mH-10 mH standards. The bridge is convenient for automatic operation.

Dielectric properties of transition metal oxide glasses

Abstract Dielectric properties of iron and vanadium phosphate glasses containing different glass modifiers have been studied in a broad range of frequency and temperature by the absorption current and with an ac transformer bridge. The dielectric spectra exhibit a peak that can be related to the electron transfer between iron atoms in different valency states. This peak and the dc conductivity revealed a similar activation energy. The data obtained were interpreted in the framework of a new model of relaxation in glasses recently proposed by Hunt.

Transformer bridge for the high-accuracy comparisons of standard resistors at low frequencies

A transformer bridge has been developed at IMM for comparing low value standard resistors (0.1 /spl Omega/-100 /spl Omega/) at frequencies from 0.5 to 20 Hz. Two current sources and two resistance standards are connected in series in one current circuit. The potential leads of standards are respectively connected through the differential current transformer. The null-detector consists of two identical current amplifiers with a low input resistance. Each amplifier has a Schottky noise as the dominant component. The differential current transformer produces the inverse current pulsations in the input winding of another current amplifier. At 1.2 Hz, the current noise of the null-detector is equivalent to 1 pA in 100-/spl Omega/ resistor (or 10 pA in 1-/spl Omega/ resistor), i.e., a relative sensitivity below 0.001 ppm at room temperature. The bridge has no resonant circuits and can operate using a square-wave current.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Correlation between mechanical and electrical losses in transition metal oxide glasses

The internal friction in iron phosphate glasses containing various glass modifiers has been studied in the temperature range from 120 to 700 K. The low frequency torsion pendulum technique has been employed. The dielectric loss has been investigated by two independent methods: the absorption current method and with an a.c. transformer bridge. The internal friction spectra exhibit a low temperature peak that can be related to the electron transfer between iron atoms in different valence states. It is confirmed that the dielectric loss peak and d.c. conductivity reveal a similar activation energy to the internal friction peak. The obtained data have been interpreted in the framework of a new model of relaxation in glasses recently proposed by Hunt.

A surface profile reconstruction method based on multisensor capacitive transducers

The aspects of the dimensional reconstruction of surface profiles by means of multisensor capacitive transducers are theoretically investigated. A bidimensional array-surface model is adopted. Traced profiles are related to the measured displacements between sensors and surface. Information on the displacements obtained by sensor capacitance variations during the scanning of the profile allows the reconstruction. The reconstruction accuracy relies on the dimensional stability of the array geometry. A theoretical analysis shows that three sensors are sufficient to perform the reconstruction process. Owing to these results a reconstruction system based on a three-sensor transducer and AC transformer bridges is proposed. The system performance is evaluated by computer simulations taking into account noise and deviations from ideal operating conditions. An experimental set-up based on a single sensor and a new type of autobalancing AC transformer bridge has been realized to verify the feasibility of the proposed system. The comparison between the displacement measurements of the bridge and those obtained independently by means of interferometric methods confirms the reliability of the system. Its potential performance and applications are discussed in the conclusions.

商用交流を電源とする機器のための試験用雑音重畳回路

A new noise superimposer is proposed for noise tests of line-operated electronic apparatus. Currently, the commercial ac voltage is tainted by the noise generated in a large proportion of electronic equipment. The frequencies of the noise have a wider range than the audio frequency. Therefore, noise superimposers are needed for use in this frequency range. In the experimental circuit, the frequency response for noise signals was beyond 10 Hz∼100 kHz except near the frequency of commercial ac, and a normal low-frequency power amplifier can be used as the noise signal source. The commercial ac current hardly flows into the noise signal source, because of the principle of the transformer bridge. Appropriate selections of two impedance circuits for this bridge enable the noise signal to be sent to the load. The operating characteristics for the commercial ac power supply and the noise signal are analyzed.

Ratio transformer bridge for the measurement of dielectric constant of lossy liquids

A three-terminal capacitance bridge is developed for the measurement of the dielectric constant of lossy liquids. Using this modified ratio transformer bridge, the capacitance shunted by a resistance as low as 50 Omega is measured at 10 kHz. The capacitance error associated with the inductance of the connecting wire is compensated using the novel method of introducing an additional transformer to the existing ratio transformer bridge. Other sources of capacitance errors, such as the non-zero output impedence of the ratio transformer and the shield capacitances of the cables, are discussed.

Time domain reflectometry study of fast-ionic-conducting glasses

Dielectric data on model fast-ionic-conducting glasses are reported at various sub-Tg temperatures. A transformer bridge was used from 100 Hz to 200 kHz, while data between 85 MHz and 2 GHz were obtained by time domain reflectometry. Substantial improvements in the sensitivity and equivalent frequency range of the latter correct previous preliminary results reporting the existence and temperature dependence of apparent dielectric permittivity plateaus in the upper MHz spectral region of glasses.

Analysis of self-dual excited synchronous machine. I. Development of the general mathematical model and steady-state performance experimental verification

The use of the dual excitation system for improving the overall performance of a self-excited synchronous machines is considered, along with the replacement of the compound transformer and rectifier bridge by a potential transformer and thyristor bridge for the self-excitation system. The output DC voltage of the bridge is controlled over a wide range by an automatic feedback control circuit to vary the firing angle of the thyristors in such a way that the terminal voltage is sustained at a constant value. The mathematical models for two distinctive alternatives of the excitation system are derived. The mathematical model thus derived is suitable for transient, dynamic as well as steady-state analysis. However it should be modified to investigate the steady-state and dynamic performance. Exact steady-state operating points are achieved by solving the steady-state equations obtained from the general model. Charts describing the performance of the self-dual excited synchronous machine under steady-state operation for the two alternatives of the excitation system have been calculated at different values of the power factor, i.e., the turns ratio of the transformer and the ratio of field currents. The experimental results obtained on a 7.6 kVA induction machine converted to a d-q synchronous machine confirm the validity and accuracy of the analysis and mathematical models developed. >

Application of capacitance techniques in sensor design

By reconsidering the basic principles of capacitance, and also clarifying some of the misconceptions about the principles of guarding, which were correctly described by Kelvin more than a century ago, it is shown how guarded multi terminal capacitor geometries can be designed, analytically precalculated and correctly measured using simplified transformer bridge systems with phase-sensitive detection. An overview is given of the archetypal multi terminal capacitor configurations as well as ways to modify them for practical sensor designs. Detrimental side-effects are discussed and rules for practical designs are given. The possibilities for combining bridge circuits with simultaneous and sequential measurements of complex sensors are described. Expectations for future developments, partly based on silicon microfabrication techniques, are discussed and a comparison of the major aspects of capacitive sensors with resistive strain gauges and inductive sensors is given.

Effect of Interfacial Polarization on the Critical Behaviour of Electrical Resistivity and Dielectric Constant in Polar + Nonpolar Binary Liquid Mixtures

AbstractThe dielectric constant ϵ and the resistance R have been measured as a function of frequency in the region 1 kHz to 1 MHz. The measurements have been performed on two critical binary liquid mixtures cyclohexane + methanol and carbondisulphide + acetonitrile in the one phase region. The measurements are carried out using a double ratio transformer bridge, in a bath with millidegree temperature control. At low frequencies ϵ and R increase with T → Tc+. Above 100 kHz the resistance reaches a maximum and begins to decrease as T → Tc+. The observed behaviour is explained qualitatively on the basis of interfacial polarization mechanisms (IPM) using the ideas of the droplet model theory of binary Liquid mixtures. The analysis of the low frequency data using the functional form R or ϵ ˜ tθ shows a value of θ near 0.9. The high frequency data have been analysed with a new functional form, but the quantitative discussions of the IPM are hampered by the lack of information of its dependence upon t.

Capacitance measurement of magnetic specific heat

A new technique–low frequency capacitance (C) measurement–has been used to study the critical behavior of both isotropic and anisotropic antiferromagnets. We find that dC/dT is accurately proportional to the known magnetic specific heat Cm of FeF2. This indicates that both dε/dT and dl/dT scale with Cm. For cubic KNiF3, the critical exponent (Cm=At−α) has been found to be α=−0.151±.004 and A/A′=1.56±0.03. The experiments measured the capacitance of disk-shaped samples with a three-terminal capacitance technique. A ratio transformer bridge was employed, yielding a resolution in ΔC/C of about 10−8. The quality of the results is comparable to the best obtainable from conventional Cm and birefringence measurements.

Using Isotropic Magnetic Susceptibility to Delineate Glacial Tills

Measurement of isotropic magnetic susceptibility is becoming a valuable tool for delineating the strati-graphic and areal distribution of Quarternary deposits, determining their provenance, and characterizing their mineralogy. The analytical technique involves selection of a specific range in matrix size, sieving to recover this range, and measurement of magnetic susceptibility in a low-field transformer bridge. In the present study, an accurate and rapid technique is presented and applied to two tills in Minnesota to delineate one from the other and establish their differing provenance.

A Combined Transformer Bridge for Precise Comparison of Inductance with Capacitance

A combined transformer bridge, composed of two alternately balanced transformer bridges, designed for precise comparison of inductance with capacitance standards has been analyzed. A number of possible sources of error have been studied and experimentally examined in a constructed model of the combined bridge. The experimental results confirmed an ability to measure inductance in reference to capacitance with an accuracy up to 10 parts per million (ppm) in the frequency band 0.16-2 kHz.

A method of digital loss angle measurement

Describes the principle and the main measurement errors of a loss angle meter based on digital pulse time width measurement. The concept of measuring the loss angle by using a time measurement technique has many advantages. Particularly, it eliminates material capacity standards and requires a frequency standard only. The main limitations on the accuracy are due to analogue signal processing. The achieved accuracy of the instrument is comparable with that of automatic transformer bridges, but the resolution of measurement is much better, and the measuring voltage is smaller than that of automatic bridges operating in the same frequency region.

A sensitive and linear pressure transducer

The principle, construction and preliminary test results of a new capacitive pressure transducer are presented. The unit constructed may be used for the measurement of differential pressure in the medium and low ranges. The primary sensor is a U-tube manometer, comprised of two Teflon tubes filled with mercury. The two tubes are placed coaxially through the central spaces of two cylindrical cross-capacitors built on the same base. Due to the shielding effect of mercury in the U-tube, changes in the capacitance of the two cross capacitors occur whenever pressure is applied. These changes in capacitance are measured, employing an inductive voltage divider (IVD) in a transformer bridge set-up, in terms of the value of a standard capacitor. Besides its extremely high sensitivity and linearity, the scheme is considered capable of achieving a high order of accuracy and appears suitable for the calibration of other types of differential pressure gauges.

Computer-automated ratio transformer bridge

A continuously balanced impedance bridge under computer control constructed for a cryogenic thermometry application provides a few ppm resolution in resistance ratio measurements. The use of IEEE-488 computer bus-compatible commercial equipment reduces the design mainly to that of software and permits flexibility in the balancing procedure.

The reduction of loading errors of a transformer bridge using NICS

The theory and test results of a technique to reduce the loading errors of a double wound transformer bridge are presented. Two negative impedance convertors (NICS) and a second transformer with its primary winding short-circuited are employed for this purpose. Using this scheme, the errors due to loading could be decreased to very low values even with a transformer having cold rolled silicon steel core and working at audiofrequencies.

The Transformer Bridge and Magnetic Susceptibility Measurement

The Transformer Bridge and Magnetic Susceptibility Measurement