Alternating Current Test Research Articles

Time-varying model for predicting the resistivity of coral aggregate concrete

As a new type of cement-matrix composite material, coral aggregate concrete exhibits excellent properties, such as local material availability and enhanced efficiency, while being cost-effective. Coral aggregate concrete can be viable for construction on islands and reefs. Reinforced concrete structures generally exhibit low durability in marine environments, owing to long-term corrosion by chloride salts. Therefore, resistance to chloride ion attacks plays a significant role in improving the durability of concrete structures. As a non-destructive testing technique, resistivity is used for characterising the resistance of concrete to chloride ion penetration. In this study, the resistivity of coral aggregate concrete is measured using an improved alternating-current test method, and the improved method overcomes the limitations that is prone to polarization reaction and heating of the solution under exposure to voltage for an extended period. Porosity was used to analyze the reasons for the different resistivity values of coral concrete from micro-aspects. The resistivity model at 365 days was established considering the effects of temperature, humidity, admixture, and water-binder ratio on coral aggregate concrete. The hydration characteristics of cement and admixture were used to establish a time-varying model for predicting the resistivity of coral aggregate concrete.

AC voltage induced electrohydrodynamic two-phase convective boiling heat transfer in horizontal annular channels

An experimental study of alternating current (AC) induced electrohydrodynamic (EHD) flow and heat transfer augmentation for boiling inside an annular channel containing working fluid refrigerant HFC-134a has been conducted in a single-pass, counter-flow heat exchanger with a rod electrode placed concentric to a grounded tube. Experiments were conducted for an inlet quality of 0%, a heat flux of 10.2kW/m2, mass fluxes from 100kg/m2s to 500kg/m2s and applied voltages from 0kV to 8kV DC and 0kV to 24kV peak to peak AC. The AC tests were conducted for both low frequency (60Hz) and high frequency (6.6kHz) near sinusoidal waveforms. The results show that there is no apparent difference between the DC and the 6.6kHz AC test cases, whereas the 60Hz AC tests show significantly different behavior. The flow boiling heat transfer results indicate that the averaged heat transfer coefficients can be increased by as much as 3-fold for a 24kV p.p. 60Hz AC applied voltage. This also incurs a pressure drop penalty of 2.6-fold across the heat exchanger.

Electromigration of Cu interconnects under AC and DC test conditions

Electromigration (EM) of a dual damascene, test vehicle was measured using direct current (DC), alternating current (AC) followed by DC, and three rectangular-wave DC stressing conditions at 598K. In some of the experiments samples were allowed to cool to room temperature between the 598K stress cycles. We find that that only net DC time at test has a significant effect on time to fail. AC stressing for one week at 2.5MA/cm^2 prior to DC stressing to EM failure had no effect on the EM performance. A similar result is obtained with various DC and thermal cycling conditions. Thermal history of the EM test samples, and in particular cooling to room temperature, has no effect on EM lifetime. All tests, regardless of thermal history and current cycling conditions, resulted in statistically similar times to fail and distributions of those times. We conclude that the standard test methodologies, using accelerated DC stress conditions at elevated temperatures, are adequate. Since only the net DC stress time had any measurable effect on EM lifetime, the use of accelerated testing provides a good predictor for lifetime expectations under operating conditions. We also conclude that neither low current density AC stressing nor DC cycling, and associated changes to grain structure that may result, provide any measurable benefit to EM lifetime.

A study of the diffusion and trapping of hydrogen in Fe-3cr and Fe-5Cr alloys

Comparative hydrogen diffusivity measurements made in 0.1 M NaOH for Armco iron and two Fe-Cr alloys have shown a significant superiority of an alternating current (AC) technique over the direct current (DC) one. Based on the phase difference between the input and the output current recorded for 0.1 cm thick membranes at frequencies ranging from 10 to 40 mHz, the lattice diffusivities of hydrogen could be determined in all the three materials. The hydrogen diffusivity values measured at room temperature were 6.2x10−5, 5.5x10−5 and 2.8x10−5 cm 2/s for annealed Armco iron, Fe-3Cr and Fe-5Cr alloy respectively. From the latter two values the Gibbs free energy of binding H atoms to Cr atoms was calculated. Depending upon the calculation method, the AG values ranged from −9.3 to −14.4 kJ/mol. At frequencies lower than 10 mHz and in experiments carried out with 0.03cm thick membranes, inaccurate, namely too low diffusivity values were obtained. This was mainly caused by surface impediments on the entry side of membranes. In contrast with AC tests, all DC measurements were affected by spurious effects which could not be eliminated by coating the entry side of membranes with a thin layer of palladium. The effect of surface barriers on diffusivity measurements is discussed.

Corrigendum: Alternating-current tests on high-speed telegraph cables

Corrigendum: Alternating-current tests on high-speed telegraph cables

Discussion on “Alternating-current tests on high-speed telegraph cables”

Discussion on “Alternating-current tests on high-speed telegraph cables”

Alternating-current tests on high-speed telegraph cables

This paper is concerned with the alternating-current characteristics of loaded telegraph cables and their relation to transmission capacity. The general principles of transmission are reviewed, and by the use of periodic theory the performance which may be expected from a cable of given a.c. characteristics is estimated.Methods of testing are described by which complete data of the a.c. constants may be obtained during the process of manufacture. The effects of frequency, current, temperature and pressure are investigated, and the results of tests on the core for a long high-speed cable are discussed and used to estimate the performance of the cable when laid.Apparatus and alternative methods for testing the laid cable are detailed and a method of deducing the primary constants from attenuation and propagation-time tests is developed; values determined are compared with results of factory tests. Deductions regarding the value and regularity of the inductance loading are made from impedance tests.Departure and arrival curves for loaded cables are constructed from theory and compared with records obtained by test.Finally, the assumed criteria for satisfactory working, expressed in terms of attenuation, are verified by transmission tests.

Accuracy of alternating-current test instruments

The paper deals mainly with the accuracy of instruments used under maintained conditions of load. Well designed voltmeters, ammeters, and wattmeters show very small errors due to self-heating and changes in ambient temperatures. Iron-vane ammeters are described which have small errors due to direct-current hysteresis and changes in wave-forms.

Balance Methods in Alternating Current Measurement

(1) Owing to the comparatively low inherent sensitivity of alternating-current instruments, balance or opposition methods of measurement are more desirable and of wider application in alternating-current testing than in direct-current work. (2) The application of balance methods to alternating-current tests is a greater problem than in direct-current work, because of the necessity of having to oppose the measured quantity in phase position as well as in magnitude; and because of the difficulty of precisely detecting when a condition of balance is obtained. (3) A suitable source of current or voltage for opposition tests consists in the phase-shifting tranformer, to which is added a potentiometer type of rheostat. (4) A satisfactory detector is found in the D'Arsonval galvanometer or the electrodynamometer, so arranged that the indicator oscillates at an easily observable periodicity. A condition of balance exists when the oscillations have been reduced to zero. (5) In current measurement, use is made of a transformer having three windings, the measured and opposing current flowing in two of these windings, and the difference, as derived from the third, reduced to a zero value, when the ratio between the active currents becomes the turn ratio of the transformer. With this arrangement it is equally easy to measure extremely small or extremely large currents. (6) In voltage measurement a simple potentiometer arrangement of resistance is used, the ratio being set to a suitable value at the beginning of the test.