Rezultaty
Tests on the full-scale prototype were carried-out in two phases:
First phase, at the Ansaldo Ricerche laboratory.
Second phase, at the independent CESI laboratory.
First phase - Full current, low voltage:
These tests were to characterise the sensor behaviour under different operating conditions (up to 3.000 Amps), it was possible to understand that there are points regarding the magnetostrictive material which will require further development at the materials science side before achieving repetitibility of results.
The electrotechnical design proved to be adequate for this sensor typology, it guarantees stable allignment of the 1 meter long stack constituted by the magnetostrictive emitter, the piezoelectric receiver and the connecting alumina bar. All measurements were recorded and analysed by other involved partners.
By this testing phase it was also possible to tune the electronic board.
The Second phase was concerned with high voltage standard tests, those currently performed on any electric equipment to be installed in substation for being operated at the usual voltage ratios, up to 400 kV.
The transformer was installed in the CESI laboratory in Milan and successfully withstood the lightning impulse test at 1.425 KV and the power frequency one, at 630 kV.
These tests are necessary to verify the integrity of the device insulation.
The transformer relies on Gas SF6 insulation.
The proposed system consists essentially of a current sensor and the associated electronic modules. The current sensor is formed by an emitter element, a transmission structure and a receiver. The emitter is based on magnetostrictive materials that generate mechanical waves under the alternate magnetic field induced by the primary current to be measured.
Thus the 50 Hz, or 60 Hz, electrical signals are converted into elastic (mechanical) waves that propagate through a dielectric coupling structure until they reach the receiver. The receiver is based on piezoelectric materials.
These materials have the property of inverse conversion of energy; thus the mechanical waves are transformed into electrical signals in the receiver. These electrical signals from the receiver are then amplified and compensated through electronic circuits.
Chen et al have studied Cobalt-ferrite Cobalt-ferrite, for potential use as a new magnetomechanical sensor material. Cobalt ferrite is a magnetostrictive material and undergoes a dimensional change when exposed to a magnetic field. According to Chen, this material shows a steep slope of magnetostriction at low applied fields, which contributes to a high sensitivity of magnetic induction to stress, hence giving high signal-to-background noise ratios in sensor applications.
The measured magnetostriction of CoFe2O4 peaks at relatively low field (300 kA/m), then decreases with increasing field, as the magnetisation of the particles or grains is rotated away from the <100> easy axis direction. This behaviour is in contrast to the behaviour of more well-known Terfenol-based magnetostrictive materials, which only peak at very high applied fields (2x106 A/m or larger).
One potential application of cobalt ferrite is the measurement of very large currents (several thousand Amperes). At present, secondary windings are employed in high voltage grids to produce proportionally reduced electrical currents suitable for measurement. This is an expensive and cumbersome practice. One possible alternative is to use a magnetostrictive material such as cobalt ferrite. Such a material would generate mechanical waves under the magnetic field induced by the primary current to be measured. Conversion of these waves into an electronic signal would allow for current measurement. Pure cobalt ferrite, having low electrical conductivity, could avoid problems associated with eddy current losses, which would be more prevalent in metal-bonded composites and more conventional magnetostrictive materials.
The preparation of cobalt ferrite by firing mixed cobalt oxide and iron oxide powders in air at 1100°C has previously been described by Chen et al. No other firing temperatures were described by those authors. In this work, tests were performed to assess the feasibility of firing the compound at a lower temperature. As the melting point of CoFe2O4 is 1570°C, it was felt that some sintering could occur at 1100°C, leading to agglomerates which might be difficult to break down. For this reason, a firing temperature of 1000°C was preferred. XRD traces of cobalt ferrite fired at 1000°C and 1100°C were identical, thus confirming the 100% conversion to cobalt ferrite on firing at 1000°C.
After the reaction, the powder compound was then re-sieved to <150µm. Using a mixer and hotplate, 3wt% PVA and 3wt% glycerine was dissolved in distilled water. The cobalt ferrite powder was blended into this solution and mixed thoroughly. The mixture was placed on evaporating dishes, dried at 95°C, ground using a mortar and pestle and re-sieved to <150µm.
Particle size analysis revealed that after ball-milling and sieving, the base powder mix had an average particle size of 14.17mm. Attrition milling was carried out in an attempt to produce finer particles.
Pure cobalt ferrite ceramic was prepared using standard solid-state ceramic processing. Uniaxially pressed pure cobalt ferrite discs, sintered under linear ramp rate and single dwell time conditions, yielded a maximum of 90% of theoretical density. Discs made from finer particle sized powder yielded densities around 91.5%.
Rods were cold iso-statically pressed using a wet-bag isostatic press. Rubber bags were procured from Trexler Rubber Ltd (Ohio, USA). During filling the rubber bag was encased in a cardboard tube to prevent bulging of the bag. The bag was tapped during filling to aid compaction. A plunger was also used to compact the powder in the bag. A vacuum pump and bung were used to remove the air from the bag.
A rate-controlled, two step sintering profile is described to produce cobalt ferrite discs with a density of 96%. Analysis of differential shrinkage curves was found to be key in determining the optimum sintering profile. Pycnometry analysis was found to be a useful technique in terms of investigating percentages of open and closed porosity. Using the revised sintering profile, cylindrical rods of length 100mm and diameter 20mm could be isostatically pressed and sintered to a density of approximately 92%.
This value of theoretical density is considerably lower than the ~96% values recorded for the equivalent discs. One explanation could be less efficient removal of the PVA binder from the larger sample, leading to trapped pores. This theory is strengthened by the fact that a thicker 30X50 rod showed a very poor density value despite being made from fine 5.34 mm powder.
PC5A is a new piezoelectric ceramic material for the receiver with reverse signal linear range better than 0,8kV/mm and aging rate less than 0,04%/year.
These materials have the property of inverse conversion of energy; thus the mechanical waves are transformed into electrical signals in the receiver.
These electrical signals from the receiver are then amplified and compensated through electronic circuits.
The magnetostrictive properties of samples of Terfenol D were measured and compared with values given in the literature and in publications of the manufacturers. Some measurements were carried out by an independent testing laboratory, Cedrat Technologies SA in France.
The basic data was rearranged in order to study various effects, and as a result of this, some unanticipated features of the material became apparent.
Most of the detailed measurement was carried out using an alternating current with a frequency of 50 Hz. The samples of Terfenol D were in the form of solid rods measuring 20 mm in diameter by 100 mm long. The magnetic field was produced by a coil with 3268 turns and could be varied in magnitude, with a maximum amplitude of 140kA/m, to the minimum amplitude of 15 kA/m. The prestress was varied between 1MPa and 20MPa.
The samples sent for testing did not have laminations.
The first unexpected feature is that the magnetostrictive strain measured in equipment operating at 50 Hz, is much smaller than when the equipment uses a direct current. The strain measured in a field generated by a direct current can be up to three times as large as the strain developed in response to a field generated by a 50 HZ alternating current.
A second feature is that the strain sometimes becomes negative. Normally samples of Terfenol D expand in a magnetic field. However when tested at 50Hz, the strain becomes negative indicating that at some part of the cycle, the samples contract. The extent of the contraction, and the length of time during which contraction occurs, depends on the size of the current generating the magnetic field and on the prestress.
Sometimes this effect is small enough to be explained in terms of a dynamic response, occurring because of a build up of momentum as the surface of the sample moves in response to a collapsing magnetic field strength. At other times the effect is too large to be explained in this way.
It therefore seems that the experimental testing is demonstrating the existence of a transitory magnetic structure, which forms and then collapses because it is not stable, but while it exists it causes the sample to show a magnetostrictive property which is quite different from the more stable magnetic structure which is normally encountered. This transitory state seems to require more energy than the more stable state, because the extent of its formation is increased by increasing the prestress. It also seems to increase when the magnetic field strength is greater.
A third unexpected feature in the data is that the electrical current used to generate the magnetic field, is affected by the experimental arrangement as a result of self inductance. It had been anticipated that the current time relationship would be sinusoidal. However it was possible to carry out an analysis of this relationship by acquiring data points every 0.00008 seconds. This showed that there was a clear departure from a sinusoidal shape. It was also possible to estimate the timing of the change of magnetic flux by the use of an extra coil in the experimental set up. Using this it could be shown that the deformations in the sinusoidal pattern of the current occurred at the same time that the rate of change of magnetic flux reached its maximum value. This occurred very clearly when the current amplitude was small (less than 1 amp) and points to self inductance as the cause of the change in shape of the curve.
The fourth and final feature could not be observed directly in the tests. However it can be argued that if the excitation current experienced a very clearly observable self-inductance effect, the same conditions would also cause the formation of eddy currents. These would occur in the sample because it has low electrical resistance and the eddy currents would act in directions to oppose the magnetic field being generated by the electrical current in the copper coil. This would have the effect of an apparent reduction in the magnetic field experienced inside the Terfenol D samples.
Therefore, although the electric current in the copper coil surrounding the sample would suggest that the sample must be experiencing a magnetic field of a certain size, in fact because of eddy currents occurring inside the samples of Terfenol D and opposing this external magnetic field, the actual magnetic field which existed inside the samples would have a smaller magnitude. This explains why the observed magnetostrictive strain under these conditions was very much smaller than that measured at a constant direct current.
Firstly, it is important to explain why magnetic annealing is needed.
In the literature the magnetostriction of cobalt ferrite is normally given as a contraction of about 110 parts per million. However, these values do not refer to individual crystals, but to samples made up of large numbers of crystals. In these samples the crystals of cobalt ferrite have been placed together with random orientations, so that the (100) crystal axis of one crystal could be in the same direction as the (110) crystal axis of another crystal, while for most of the other crystals this direction does not correspond with any crystal axis at all.
It follows then that the quoted measurement of 110 parts per million is an average value for the contraction of a whole sample, with some crystals contracting by 590 parts per million, some crystals expanding by 90 parts per million, and most crystals contracting or expanding by some value between these two limits.
Therefore all of the crystals making up a whole sample are under high levels of stress when placed in a magnetic field, with some crystals attempting to contact by 590 parts per million while others will attempt to expand by 90 parts per million.
During magnetic annealing, the crystal axes of most of the crystals are changed so that afterwards their various crystal axes are all orientated roughly in the same direction.
The benefits of the annealing operation are as follows. It should reduce the elastic energy, which arises as a result of the polycrystalline material coming under a magnetic field, when some crystals attempt to contract and others expand. It should also produce larger magnetostrictive effects. Finally with a single crystal of cobalt ferrite it requires less energy to magnetise a sample in a particular direction. It requires more energy to magnetise when a different direction is chosen. If, therefore, in the sample, all the crystals have been aligned towards one direction, it should be possible to take advantage of this and so reduce the energy required to magnetise the whole sample.
Data in the literature suggests that a magnetic field strength of 9,000 Oersteds would be required for 72 hours at a temperature of 150°C.
The sample size was fixed by the research partners. It is a solid cylinder measuring 30 mm diameter by 50 mm long.
It was decided that the sample should be heated using a hot air gun and a jacketed plastic vessel should be used to protect the sample from any heat being generated from the copper coil.
The next question to consider was how to produce the magnetic field, and there are two possibilities. The first is to use a copper coil wrapped around the sample compartment. The alternative is to generate the magnetic field in an iron core some distance away from the sample chamber and then use the iron material to direct the magnetic field to pass through the sample in a closed magnetic loop.
Because of the heat being generated in the coil, it is necessary to remove the heat along thin copper discs which will conduct the heat from the coil to the outside of the equipment. In the outer section of the equipment there will be a large number of copper tubes of very small diameter, with air being forced up inside and outside these tubes. In this way the heat can be removed from the equipment, so that the annealing can take place for 3 days.
The jacketed tube will be fabricated in Ketron Peek 1000, which is a polyester material capable of withstanding 200?C.
A copper coil consisting of 31,440 turns can generate the magnetic field of 9,000 oersteds. However to remove the heat it is necessary to separate the coil into 60 sections using copper plates between each section. Therefore the coil must be made out of helices of copper wound in such a way that each helix consists of 131 turns. A set of 4 helices will form each section of the coil. The copper plates will be disc shaped and will be 1 mm thick.
It was important to be able to prove that it was possible to fabricate the copper wire into a flat helix with 131 turns, but with a thickness of only the diameter of the wire. This was done using a wire winder and a helix was formed and held together using a small amount of glue.
The coil is made from copper wire 0.71mm diameter, with an insulation layer of 0.033mm thick. An insulation layer was chosen which would be capable of withstanding 200°C.
Outside this section, the heat is transferred to vertical copper tubes. The tubes are 3 mm internal diameter, 6 mm external diameter and 1.5 mm thick.
Air would enter the bottom of the tubes at room temperature, or 20?C and leave at temperature between 47.9?C and 34.7?C. The total volumetric flow rate of the air would be 2.073 cubic meters per second.
The target of the project was the production of electronic modules with very high package density. Main points were the realization of solder printing and assembly of 400 components packed PCB board.
The goal of the project was the installation and learning the work with the new software "EAGLE" for PCB design. The results are design rules for full packed electronic PCB/s.
After the tests performed, it has been recommend to the task group TC57 of the INTERNATIONAL ELECTROTECHNICAL COMMISSION to change the ST (BFOC) connector (IEC 60874-10-1) by the MT-RJ connector (IEC 61754-18) as the fibre optic connector of choice in the new standards IEC-61850 parts 9.1 and 9.2.
The timing of the decision is very import for avoiding the future incompatibilities between old and new equipments due to their interfaces.
A low-cost, laser-based magnetostriction measurement system has been designed and developed. The measurement system uses a solenoid to generate a static magnetic field and a suitable non-metallic test frame to mount the laser measurement system, and the copper solenoid assembly. A specimen feeder was integrated into the test frame and was mechanically isolated from the measurement system. The magnetostriction measurement is taken using a reflected laser beam displacement sensor.
A solenoid consisting of 5555 turns was used to generate the magnetic field strength required. The choice of wire used to make the solenoid, was made taking into account the ease of winding and the current carrying capability.
A precision dual power supply constant current generator (Thurlby Thandar Instruments, UK) was used to supply a steady DC current to the solenoid coil. A laser measurement system (Model RD-50RW, Keyence UK Ltd.) was used to measure the sample displacement to an accuracy of ±1µm.
To minimize expansion problems, the solenoid former was fabricated from hard cardboard. Wooden discs (used as retaining flanges) were attached to the ends of the former. This arrangement provided significant heat insulating capability, thus limiting test sample expansion due to heat. Furthermore, it offered no resistance or distortion to the magnetic field generated by the solenoid. A temperature probe was positioned in the air space cavity to monitor temperature rises due to heating of the coil.
To test a sample, it had to be screwed into position in the centre of the solenoid. The laser head was lowered into the correct range to enable a reading. At this point, a current of 1.5A was applied and simultaneously a reading taken from the laser digital display.
The total length of wire needed to form the solenoid was 1017 metres. In order to reduce the difficulties associated with manually winding, the solenoid was made up of 7 individual coils connected in series. The resistances of the individual coils were measured using a digital multimeter (Tektronix DMM912) and showed excellent agreement with the calculated theoretical values. Five measurements were also made of the total coil resistance and yielded an average resistance of 41.93Ω.
When in operation a dramatic temperature rise occurs over a relatively short space of time. After only 300 seconds the temperature rises to 41.5?C. For this reason testing must be carried out for short durations to avoid drift of the laser beam due to thermal effects.
A DC magnetometer (AlphaLab Inc. USA) was used to measure the magnetic field inside the solenoid. The magnetic field reaches ~750 Oe (~60kA/m) in the centre of the coil. This is slightly higher than the intensity predicted theoretically. It was also observed that the magnetic field falls off towards the top and bottom of the solenoid.
In this work, two samples of Terfenol-D were sourced from commercial companies. These samples were both cylindrical rods measuring 100mm in length by 20mm diameter, which enabled them to be tested in the equipment. One sample was sourced from Etrema (USA) Ltd., the other from Tianxing Rare Earth Functional Materials (China) Ltd. (TXRE).
Both samples were tested at the maximum magnetic field level, and, as expected, exhibited positive magnetostriction, i.e. they expanded in a magnetic field. The increase varied between 36 and 47 microns.
When inverted, the Etrema sample exhibited higher magnetostriction. This can be explained in terms of residual magnetism in the sample, which was created during the sample manufacturing process. When placed in the solenoid, the new magnetic field opposed the residual magnetism in the sample. However, when the sample was inverted the residual magnetism was re-enforced.
No data for Etrema s sample (without pre-stressing) was available for comparison with the result generated in the equipment. However, Etrema quote a general magnetostriction range of 800-1200ppm for a field strength of 2000 Oe. At a magnetic field strength of 750 Oe, one would expect results similar to those obtained in this work, ie. 375ppm.
Ways of improving the testing conditions are outlined below.
--The reading from the laser equipment is a digital display, and sometimes this is not constant but varies ± 1mm.
--A water cooled solenoid could avoid the heating of the sample during testing.
--A DC current is switched on by turning two knobs from zero to a predetermined position. This manual procedure must be executed as quickly as possible to avoid heat being generated in the coil, and introduces operator variability. A more sophisticated device to produce an exact current automatically is required.
--The sample support mechanism could be improved to prevent undesirable sample movement, thus yielding more reliable results.
The magnetostrictive properties of a sample of cobalt ferrite were measured and compared with values given in the literature. Some measurements were carried out by an independent testing laboratory, Cedrat Technologies SA in France.
The basic data was rearranged in order to study various effects, and as a result of this, some unanticipated features of the material became apparent.
Most of the detailed measurement was carried out using an alternating current with a frequency of 50 Hz. The sample of cobalt ferrite was in the form of a solid rod measuring 20 mm in diameter by 100 mm long. The magnetic field was produced by a coil with 3268 turns and could be varied in magnitude, with a maximum amplitude of 140kA/m, and a minimum amplitude of 15 kA/m. The prestress was varied between 0MPa and 10MPa.
The sample sent for testing did not have laminations.
The following conclusions can be drawn from these measurements, where the magnetostrictive properties of cobalt ferrite are compared with those of Terfenol D for use as an emitter in a project. The aim of the project was to develop a sensor to measure large electric currents
1. Signal range and linearity.
Both Terfenol D and cobalt ferrite have much larger signal ranges than the originally specified 30 kA/m. For Terfenol D the magneto-strictive response is always non-linear, but for cobalt ferrite, depending on the magnitude of the pre-stress, the magneto-strictive response can be made almost linear for a large part of the signal range.
2. Zero Crossing.
Zero crossing occurs for both materials. It occurs very strongly when Terfenol D is used as the magneto-strictive material, and it is not possible to minimise its effects by choosing the best conditions of testing. It is a very much smaller effect when using cobalt ferrite. It can be reduced further when large magnetic field strengths are present, and when large pre-stresses are applied.
3. Magneto-strictive Coefficient.
In almost all the conditions for testing Terfenol D, the magneto-strictive coefficient exceeded the specified minimum of 1.3*10-9 m/A. In some cases the coefficient was nearly 10 times the required value. However in most of the conditions for testing the cobalt ferrite, the coefficient was less than the specified minimum. In fact if parameters are chosen to minimise other undesirable effects, the coefficient becomes less than one third of the specified minimum.
4. Hysteresis
Both materials show hysteresis, and it is always smaller when the magnetic field strength range is greater. However the hysteresis feature is always smaller when cobalt ferrite is being tested in comparison with Terfenol D.
5. Avoidance of Eddy Currents
No direct measurements were made to confirm the existence of eddy currents. However from indirect evidence it is assumed that eddy currents develop in the Terfenol D material with detrimental effects. It is also assumed that these can be avoided when cobalt ferrite is used as the emitter material, because of its low electrical conductivity.
6. Summary.
In 4 out of the 5 criteria used to specify desirable characteristics for the emitter material, cobalt ferrite is shown to be superior to Terfenol D. It is understood that careful design methods can be used to compensate for the limited magneto-strictive coefficient of cobalt ferrite. Therefore this material is recommended as the most suitable for the emitter.
Wyszukiwanie danych OpenAIRE...
Podczas wyszukiwania danych OpenAIRE wystąpił błąd
Brak wyników