Two-stage Transformer Research Articles

High precision calibration of ac current-to-voltage converters

A two-stage transformer, built as a current comparator with a shielded magnetic core, is used to make a novel ac bridge for calibration of ac shunts for large currents and other ac current-to-voltage converters. A fine adjustable reference transresistance is made by current transformation and a non-reactive precision resistor connected to the secondary. By using an auxiliary transformer and resistor, all four two-stage transformer secondary terminals are brought to ground potential and consequently both magnetizing and capacitive errors are kept below one part per million.

Direct-reading absolute calibration of ac voltage ratio standards

A new system for AC voltage ratio absolute calibration has been developed at IEN. The system is based on the simplest possible application of the bootstrap method: during the step-up, the voltage differences are read directly with a vector voltmeter, purposely built to have an extremely high common-mode rejection ratio, exceeding 180 dB at 1 kHz. The system is built around a new two-stage divider, having k:10, k:11 and 1:100 ratios, a two-stage bootstrap transformer (1:10 and 1:11 ratios) with a guarded one-turn secondary stage having adjustable cross-capacitances and a two-secondaries (k:10 and k:11) transformer for easy guarding. The method is insensitive to several possible systematic errors of the voltmeter or coming from the bootstrap operation. The system has been employed during the participation of IEN in the CCEM-K7 key comparison.

A two-stage heat transformer with H 2O/LiBr for the first stage and 2,2,2-trifluoroethanol(TFE)/ N-methy1-2-pyrrolidone (NMP) for the second stage

The two-stage heat transformer (TSHT) is an advanced heat–transformer which can widen the operating range compared to that which can be achieved with a single-stage heat–transformer (SSHT). This paper analyzes three kinds of two-stage heat transformers and puts forward the two-stage heat–transformer with H 2O/LiBr for the first stage and 2,2,2-trifluoroethanol (TFE)/N-methy1-2-pyrrolidone (NMP) for the second stage. The results show that it can reach the same high delivered temperature as the two-stage heat–transformer with only TFE/NMP as working–fluid combinations and can achieve a higher system performance than the two-stage heat transformer with TFE/NMP.

A computer-controlled current-comparator-based four-terminal resistance bridge for power frequencies

A current comparator technique is applied to four-terminal resistance measurements for obtaining a highly accurate AC resistance bridge at power frequencies of 50 to 60 Hz. Active circuits are used to establish equal voltage drops between the potential terminals of the two resistances being compared. The bridge is suitable for measuring resistances from 10 /spl mu//spl Omega/ to 100 k/spl Omega/. A cascading technique using two two-stage current transformers provides extension of the ratio range to 100000000 with a maximum applied current of 10000 A. The bridge features measurement with a resolution of one part in 10/sup 7/. The total combined uncertainty (2 /spl sigma/) of the bridge, including the range extenders at power frequencies, is estimated to be less than 5 /spl mu//spl Omega///spl Omega/.

Simple absolute method for current transformer calibration

A simple mains-frequency current transformer ratio build-up scheme has been developed which enables calibration by absolute means of ratios from 5 A:5 A to 500 A:5 A with uncertainties of 0.03 parts in 10/sup 6/ and 0.05 /spl mu/rad. The build-up is based on series-parallel interchange of primary windings of two-stage transformers with magnetically shielded second stages.

Study on a new ejection–absorption heat transformer

Based on the performance analysis of the single stage, the two-stage and the double absorption heat transformer, a new ejection–absorption heat transformer is presented and analyzed in this paper. The results show that it has a simpler configuration than the double absorption heat transformer and two-stage heat transformer. The delivered useful temperature in the ejection–absorption heat transformer is higher than for a single stage heat transformer and simultaneously its system performance is raised.

A current-to-voltage converter for 5 A to 5 V and 30 Hz to 30 kHz

A current-to-voltage converter (CVC) is presented, which has three input ranges for nominal input currents of 0.2, 1, and 5 A with frequencies from 30 Hz to 30 kHz. The converter uses an amplifier-aided two-stage current transformer which operates with a very small number of ampere-turns and the primary (measurement) current path of which may be potentially elevated without its error characteristics being affected. The secondary current of 5 mA of the transformer is converted into voltage using an inverting amplifier with a feedback resistor of 1 k/spl Omega/. By fine adjustment of its frequency-dependent feedback, small errors and a small error variation are achieved over the whole frequency range. The variation of errors between 20 and 100% of nominal current in each current range is negligibly small. In the frequency range from 30 Hz to 1 kHz, relative errors of less than 10/sup -4//spl plusmn/2/spl times/10/sup -5/ are achieved. From 1 to 30 kHz, the errors increase up to 2/spl times/10/sup -4/ with /spl plusmn/5/spl times/10/sup -5/ uncertainty for the in-phase and /spl plusmn/40/spl times/10-5 for the quadrature component. The current-to-voltage converter was calibrated by means of a high-quality current shunt, which was especially developed for this purpose.

Proposal of humid air turbine cycle incorporated with absorption heat transformer

A detailed graphical exergy study based on the energy-utilization diagram (EUD) is applied to humid air turbine (HAT) cycle incorporated with a modified two-stage absorption heat transformer (TAHT). Employing the sensible and latent heat exchange modes in this TAHT and then introducing to the HAT cycle have clarified that this method can intensively recover the waste heat of HAT cycle and improve the system performance. Compared to a conventional HAT cycle, the overall cycle efficiency can be increased by 2 per cent points and the specific work can be increased by 7.3 per cent.

00/00592 Behavior of a two-stage absorption heat transformer combining latent and sensible heat exchange modes

00/00592 Behavior of a two-stage absorption heat transformer combining latent and sensible heat exchange modes

Behavior of a two-stage absorption heat transformer combining latent and sensible heat exchange modes

A two-stage absorption heat transformer (TAHT) and its improved version, the cycle incorpaorating latent heat and sensible heat modes, are investigated by emplying energy-utilization diagrams (EUDs). Utilizing the exhaust gas of a gas turbine as the heat source and employing the water/lithium bromide as the working fluid, it has been concluded that this is an effective way of decreasing the exergy loss by combining latent and sensible heat exchange modes. Furthermore, the temperature of the useful heat is increased. This analysis can provide a basis for synthesis and development of absorption heat transformer systems.

The two-stage metal hydride heat transformer

Metal hydrides can be utilized as working materials in different types ofprocesses in the field of energy technology. Among others, thermally driven heat transformationdevices are very promising solutions for the rational use of energy. Main applications forthermodynamic metal hydride machines are the heat transformer for generation of hightemperatures, or the heat pump/refrigerator for generation of both high and low temperatures.Such metal hydride machines can be used in a variety of applications due to their large operatingtemperature range. This paper gives an overview of the experimental setup of a prototype heattransformer for industrial use, providing useful heat (continuous output of about 7 kW at atemperature of about 190–200°C with a driving heat of 130–135°C and a cooling watertemperature of 40°C.

Performance study of a double-absorption water/calcium chloride heat transformer

In order to increase the temperature lift and efficiency of single-absorption heat transformers there are other possible arrangements. Double-absorption heat transformers have a relatively simple design and smaller size compared to two-stage heat transformers. In this work, the thermodynamic performance of the water/calcium chloride system was modelled for a double-absorption heat transformer. Results indicate that temperature lifts of up to 40°C are possible with coefficients of performance close to 0·3. © 1998 John Wiley & Sons, Ltd.

Modelling of single-stage and advanced absorption heat transformers operating with the water/carrol mixture

A thermodynamic analysis was carried out to study the performance of single-stage, two-stage and double-absorption heat transformers operating with the water/Carrol mixture, where Carrol is a mixture of lithium bromide and ethylene glycol [(CH 2OH) 2] in the ratio 1:4.5 by weight. A mathematical model to predict the theoretical performance of single-stage and advanced heat transformers operating with the water/Carrol mixture is also described. Coefficients of performance and gross temperature lifts are compared for the different heat transformers and plotted against the main temperatures of the system. A two-stage heat transformer consists of two single-stage heat transformers which can be coupled in three different ways. A double-absorption heat transformer is a single-stage heat transformer to which a dual-purpose absorber/evaporator unit has been added.

Wide bandwidth, large AC current probe for power electronics and EMI measurements

Current measurements in power electronics or in EMI domains require high-performance probes. These probes must have a very wide bandwidth, be insensitive to electromagnetic interferences, and be able to measure large direct and alternating currents (AC-DC) without distortion. This paper is limited to AC current probes. Various principles are reviewed, in particular those based on the current transformer (CT); their limitations are analyzed, and a solution based on a two-stage transformer are proposed to extend low- and high-frequency performance. It is shown that the use of new magnetic materials, such as the amorphous or the nanocrystalline, can improve large-current low-frequency operations, while preserving the smallest possible probe size. Various solutions are provided to improve high-frequency operations. Different developments are depleted; frequency and time-domain results are presented.

Two-stage metal hydride heat transformer laboratory model

Metal hydrides can be utilized advantageously as working materials in a number of devices which can contribute significantly to an environmentally clean technology. Among others, there are heat transformers which allow us to use low quality thermal energy (e.g. waste heat) to generate high quality thermal energy or cold. This paper gives a detailed description of a heat transformer development for industrial application.

Self-calibratable voltage transformer with part-per-million accuracy

The design and construction of a 1000 V/100 V, two-stage voltage transformer with burden compensation, which has errors of no more than a few parts per million over its working range of voltage levels and burdens, is described. By dividing the primary winding into ten equal sections and by enclosing the primary and secondary windings in electrostatic screens, it is shown that the transformer can be self-calibrated by a build-up technique, without reference to standard transformers or other potential dividers.

Energy recovery and plume reduction from an industrial spray drying unit using an absorption heat transformer

Spray drying is a widely used unit operation in the chemical industry. It is highly energy intensive, requiring hot, dry airstreams, at temperatures up to 550°C, to dry a wide range of products. Exhaust air from a dryer is usually vented to the atmosphere with little or no heat recovery. At best only the sensible heat associated with the air stream is recovered, although the majority of the stream's energy is in the form of latent heat associated with the evaporated water. Exhaust airstreams usually contain moisture to cause visible plumes upon leaving the dryer stack. A two-stage absorption heat transformer (A.H.T.) has been designed and contructed to investigate the potential for dehumidifying and reheating a simulated dryer exhaust stream to make it suitable for recycling to the dryer inlet. The amount of air vented to atmosphere and also the amount of wasted heat would be reduced by incorporating an A.H.T. into the drying operation. Performance data for the A.H.T. indicates that an airstream can be reheated to a temperature of 160°C, using a lithium bromide solution of 68% w/w, with a circulation ratio (LiBr: steam flow) of 14.8. Temperature lifts between 50 and 70°C are possible in the reheat column when using a low circulation ratio and a high LiBr concentration. Experiments show that a humid air stream can be dehumidified to a level suitable for recycling by direct contact with a concentrated lithium bromide stream.

Programmable precision IVD for the audio frequency range

A programmable 22-bit inductive voltage divider (IVD) for the frequency range from 100 Hz to 10 kHz is described. The design consists of a two-stage transformer for the higher six bits, a one-stage transformer for the next four bits and a multiplying DA converter for the lower 12 bits. The in-phase and quadrature errors of the two-stage transformer which constitute the largest part of the overall error were smaller than 2*10/sup -6/ and 3*10/sup -6/ at 10 kHz and smaller than 2*10/sup -7/ and 5*10/sup -7/ at 120 Hz.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Coupling a two-stage absorption heat transformer with finite heat sources and sinks

The coupling among a two-stage heat transformer (TSHT) and finite heat sources and sinks is considered. A novel configuration is proposed, in which the evaporator of the second stage is fed both by the absorber of the first stage and by the external source. A lumped-parameter mathematical model is used to compute the performance of the proposed TSHT. It has been found that the range of applicability is widened, when compared with the usual configuration. Moreover, the use of a two-stage apparatus allows one to noticeably increase the performance compared with that obtainable with a simpler single-stage transformer. Thus, it is possible to increase the output heat and/or the user temperature, to reduce the discharge temperature, or to use lower quality energy sources. The results also allow one to draw some more general conclusions on the applicability of absorption heat transformers of different configurations, when different heat sources/sinks are considered.

Selection of alloys and their influence on the operational characteristics of a two-stage metal hydride heat transformer

A heat transformer can upgrade heat to a higher temperature. A two-stage heat transformer has a greater temperature upgrading potential than a single-stage heat transformer, e.g. heat can be upgraded from a level of about 130–140°C to temperatures of about 200°C. A practical method to select suitable hydrides to be used in a two-stage heat transformer is presented. The example discussed shows that the selected alloys result in a reasonable operation of the two-stage heat transformer. Three different evaluation criteria viz. coefficient of performance, alloy output and temperature output, are introduced to compare the operational characteristics of heat transformers with different alloys; the influence of some metal hydride properties on the operational characteristics is also discussed.