Electricity: measuring and testing – A material property using electrostatic phenomenon – Corona induced
Reexamination Certificate
1999-02-08
2001-01-30
Metjahic, Safet (Department: 2858)
Electricity: measuring and testing
A material property using electrostatic phenomenon
Corona induced
C324S514000, C324S515000
Reexamination Certificate
active
06181139
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for testing of the insulating ability and of an insulation on an electric conductor, such as an enameled wire or a cable, and of the type.
2. Description of the Prior Art
U.S. Pat. No. 3,413,541 discloses an apparatus, where the difference of voltage for the testing is established between the electric conductor and a mechanical arrangement with rolls; and from U.S. Pat. No. 2,488,578 it is furthermore known to use hanging chains to form connection to the conductor, which insulation has to be tested. Furthermore it is known to transfer the testing voltage to the conductor by means of metal balls or the like, through which the conductor is moved.
These methods have in common that an insulation defect on the conductor, for instance in the form of a crack or a hole, will be registered as a voltage flashover. The voltage has to be chosen so that it does not exceed the puncture voltage of the insulating material, but is larger than the puncture voltage in atmospheric air at the same distance as the thickness of the insulating material.
It is furthermore known to use a contact wheel, where the wire to be insulation tested extends in an angular notch along the periphery of the wheel. That is that puncture may only occur where the sides of the notch touch the wire. An arrangement, where the wheel is substituted by a conducting media, such as salt water or mercury, which is known from the description in Danish patent application No. 2281/88, does not have this defect; but causes pollution of the wire to an unacceptable degree, which requires a subsequent cleaning process. The liquid may be substituted by a metal powder or by metal balls. This will not cause pollution of the surface of the wire; but the surface of the wire may be damaged by grinding.
WO 90/05311 furthermore discloses a test method making use of a superimposed high A.C. voltage on the testing voltage, whereby a puncture may be obtained where the insulation layer is defective. This requires that the superimposed voltage is adjusted to a voltage between the puncture strength for air, or in this case a gas which ionizes easily, Argon, and for the measured insulating material. However, in this case ionizing only occurs if a defect in the insulation occurs.
DD-A-209 912 and DD-A-209 913 discloses a test method, where the conducting media also may be replaced by a sufficient quantity of electric charge carriers, ions, which over a suitable axial distance is arranged to surround the wire. Such an electric conducting cloud may be a plasma cloud, or ionized gaz. A plasma cloud generated by a lighted electric arc is capable of carrying an electric current, by ionizing of for instance a noble gas between the electrodes. An electric arc is aggressive, both towards the electrode materials, the insulating materials and in this connection the item tested, due to the very high temperature to maintain the arc, and thereby the ionizing. This results in unfavourable properties in he form of sporadic damage of the test item and the insulating material and poor durability of the electrodes. However the test method disclosed in DD-A-209 912 is simplified by obtaining a varying division of the supplied voltage and with it a potential displacement between the light arc and the test voltage by means of an ohmic voltage divider.
U.S. Pat. No. 4,151,461 discloses, that the ionizing also may be obtained by continuously discharging of sparks; but to secure continuous ionizing it requires high power, 150-800 watts at 100-600 Hz, and it therefore causes that the electrodes to be quickly worn down.
U.S. Pat. No. 3,263,165 furthermore discloses, that the ionizing of atmospheric air also may be provided by means of a radioactive isotope, which may establish a current through the atmospheric air. Due to environmental reasons is not expedient, and requires intensive maintenance.
EP-A-0 264 482 discloses the use of laserbeams to generate the charge carriers, and EP-A-0 367 379 furthermore discloses, that electron emitting materials, such as chemical compounds of barium or calcium may emit electrons—but only by heating to high temperatures.
The aforementioned six methods have the important advantage, that the insulating material and the wire are not touched mechanical, which is why there is neither wear, tearing or physical damage to the surface of the insulation. Hardening of the metal core of the item, which normally may happen by mechanical bending around rolls or the like, does not occur. Such hardening is very unfavorable such as, for example, copper wire for spools in electric machines etc.
SUMMARY OF THE INVENTION
The invention is a method for testing the insulating ability of the insulation on an electric conductor by means of a careful, touch-free manner by means of simple provisions to establish a sufficient quantity of electric charge carriers in the form of an ion cloud or corona to test quickly and effectively, whether the electric conductor is completely covered by a sufficient layer of insulating material, and thereby is capable of insulating for an electric voltage.
The method according to the invention is distinctive in that the ion cloud is established by a high frequency high voltage generator with low power, and that defects in the insulation are determined by means of a D.C. voltage established in an electric serial connection with the ion cloud, so that insulation defects in the testing item will cause an electric current through the electric charge carrier in the form of ions, and at the electric current is registered by a current measuring circuit. In a careful, touch-free manner and by means of simple provisions it hereby becomes possible currently—on-line—to establish a sufficient quantity of electric charge carriers in the form of an ion cloud or corona to test quickly and effectively, whether the electric conductor is completely covered by a sufficient layer of insulating material, and thereby is capable of insulating for electric voltage. It should be emphasized that—in order to avoid damaging heat protection of the test item—there is made use of high-frequency and high voltage with low power to establish a permanent corona between the testing electrodes.
The invention furthermore relates to an apparatus for use with the method according to the invention, which apparatus comprises a system of testing electrodes providing a central passage for a tested item, an electric conductor, so that the tested item may be moved continuously through the passage, a high-frequency high voltage generator with low power, being adapted to ionize the atmospheric air in the passage and form a permanent ion cloud or corona, a measuring voltage generator and a current measuring circuit, and wherein the measuring voltage generator forms a current path through the ion cloud or the corona to the surface of the tested item, if insulation defects may occur, as the current measuring circuit also is connected with the electric conductor of the tested item, and wherein the testing electrode system is provided as two sets of electrodes being alternatively placed in a circle around the passage for the tested item in such a manner, that the electrodes mutually are equally distributed along the circle that is by way of example mutually displaced 45°.
The apparatus according to the invention is furthermore with one set of the electrodes insulated from the other set of electrodes in such a manner that a permanent ion cloud or corona is formed because of the capacitive current generated by the voltage from the high voltage generator.
In an advantageous manner the insulating material between the set of electrodes is of such type (shape and material), that the ionizing does not cause puncture, sparks, but continuous discharges in the form of a permanent ion cloud or corona.
In order to optimize the effect of the preferably permanent ion cloud or corona the electrodes are provided so that only between the respective circuits a common current path is formed through the ionize
Joergensen Peter
Olesen Joergen
Antonelli Terry Stout & Kraus LLP
Dansk System Elektronik A/S
Deb Anjan K
Metjahic Safet
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