IEEE 98-2002 pdf download IEEE Standard for the Preparation of Test Procedures for the Thermal Evaluation of Solid Electrical Insulating Materials
4. General considerations
The practice to assess the thermal endurance of insulating materials is to age the materials at higher temper.atures than the intended service temperature and to conduct diagnostic tests at ambient temperature tdetermine retention of properties.
41 Results of thermal endurance tests
The results of thermal endurance tests can be described by the Arrhenius relationship betwcen the logarithniof time to a certain degree of property change and the reciprocal of the absolute temperature, K. Thisrelationship is often used for extrapolating the times to the same degree of property change at lower temper.atures. Performing thermal endurance tests at three or more temperatures serves a double purpose: first, theconfidence limits of the extrapolation are improved and, second,a check is provided of the basic assumptiorthat a linear relationship exists betwcen the logarith of timc and the reciprocal absolute temperaturc.Thislinearity may not exist if there is more than one dominant deterioration mechanism in the range of agingtemperatures An extrapolation of the results may not be justified if this check is negative
The aging curve may be extrapolated (see 7.3) to a leneth of time (usually between 20 000 h and 40 000 h)which is specified in the thermal cvaluation procedure for a particular insulating matcrial. The Tl for thatmaterial is the number that is equal to the value of the temperature obtained from the life-temperature curvcat the arbitrarily specified time
The agine curve gives the most complete information about the results of an accelerated thermal aging testSuch curves can also be used to compare aterials expeditiously.
4.2 End-point criteria
The Tl of the material is determined by end-point criteria based on the material’s ability to withstand stressessuch as clectric ficld, mechanical tension, or flexure, ctc or is based on changcs in a property of thcmaterial. All the properties of a material subjected to heat aging do not deteriorate at the same rate andconsequently, TIs should be obtained for all properties related to possible modes of failure of the material inservice. A material may therefore be assigned more than one TI derived from the measurement of differeniproperties
4.3 Test procedures and conditions
Different test procedures have been and are to be developed for diferent kinds of insulating materials suctas impregnating varnishes, sheet insulation, etc. Experience has shown that some insulating materials incombination infuence one another duringthe thermal aging process. This interaction may increase ordecrease the relative thermal life compared with the life of either material tested alone. For example, thcthermal life of enameled magnet wire may be either increased or decreased, depending upon the insulatingvarnish used to impregnate windings containing the enameled wire. Metals in contact with insulating materials may also affect the thermal aging of the materials. Since insulating materials are often utilized incombination,test procedures may provide for evaluating combinations of materials.
4.4 Variability
It is recognized that insulating materials will be used under widely different conditions. Factors such asyoltage stress, irradiation, mechanical shock and vibration, environmental conditioning, and chemical con.tamination shall. therefore, be carefully considered. The test procedure may be modifed to provide methodsfor evaluating the effects of these factors as the material deteriorates thermally. Because materials are used in
IEEE 98-2002 pdf download
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