Document DGRkZebqxrn5JdL95bM3yjRYn

April 1956 And a trade-marl; is born . .. page 4 Fundamentals--insulation selection .. . page 17 Printed circuit technique for bushings . .. page 22 [Sew isocyanate-glass sleeving . .. page 24 First on tests . . , page 28 SC-ELEC-02740 sulation Engineering Fundamentals--I\o. 3 i i1 i L - | |) Considerations hr Insulation Selection Mjjrtf of a series of monthly articles on insulation engineering fundamentals fiv nm Lee Moses. Manager. Insulation Development Section. IT estinghouse ~itric Corporation. East Pittsburgh. Penn. Mr. Moses is u'ideir recognised fleadins authority on insulation and is the author oj more than 10(t technical . Jjictes and papers as utell as a boob on electrical insulation. He has figured b Zoortantir in many insulation developments. '_ . -.Electrical insulation is used on elec- ^ equipment because it offers high ^stance to the flow of electric current. therefore is capable of directing md guiding the current in its proper pths along the conductors. The pri- iry purpose of insulation is electrical, originally this was thought to be aonly purpose. :th the development if better insulations and larger, higher .ullage machines, engineer; have lin ked mechanical and thermal duties a insulation. When insulation require- -*ats became more severe and voltages .-ere increased, the problems were ulved by adding thicker insulation, "bese early insulations consisted main- . of untreated cotton cloth or other rganic fabrics. The first processing insisted of applying natural gums tch as shellac and boiled linseed oil . alcohol carriers. With industrial expansion and elec- ification. and the creation of many w industries using electrical devices, s requirements ior electrical insula- o pyramided by leaps and bounds, today's and tomorrow's industrial, insportation, and defense applica nts. insulation is being subjected to t increasing stresses, both physical 1 electrical, ever increasing tem peratures, and more serious contam inating environments. In the present and the future, as well as the past, the success of an insulation for a specific device depends upon the environmental and service conditions as well as upon the designer's choices of suitable mate rials and fabrication processes. The ait of selection and applying insulation ha; always been a difficult one to exercise with mathematical precision such as can be used in the solution of mechanical and electrical design prob lems. In the past, insulating materials have been selected on an empirical basis, and insulation systems have been designed by a "cut-aud-trv" approach, ^f'ith the tremendous advances that have been made in the control of insulation materials and processes, and the better understanding of their char acteristics, it can be predicted that the insulations of the future will be de signed on a more scientific basis with corresponding increases in effective ness. There is. however, no short cut to experience and understanding of the specific problems of application and use of the final product. There are many factors involved in selecting a suitable insulation svstem for every design of machine and device. The insulation engineer must cooperate closely with the electrical and mechan ical designers and there must be a common understanding of the mutual problems. This cooperation and col laboration is difficult because there have been so lew suitable yardsticks for measuring insulation properties. In the past, materials have been eval uated separately under considerably different conditions from those under which they are used. The data from such evaluations bear little relation to how the component materials will act in a composite insulation system in a machine. It must be recognized that the characteristics of insulating materials change as they are han dled and fabricated into the final product. In many cases, the dielectric levels of the materials diminish with handling. In other cases "where the sys tems are fabricated into a thoroughly integrated composite system, the prop erties may actually be greatly enhanced by processing. From this it can be seen that the selection of materials based on their individual characteris tics from handbook data may not pro duce the best design. However, the in sulation designer must start with the basic component and follow their evalu fnsuicJtoft, April 20S6 IT !| ation through the whole procedure of fabrication and use. The evaluation of insulating ma terials and systems should involve the following steps: 1. Laboratory tests on each com ponent material with the tabulation of the useful properties and character istics aimed at the solution of the final problem. . 2. Tests and manufacturing experi ence to determine the effect of fabrica tion methods and processes on the in sulation system characteristics in final form. This may involve destructive tests on the actual equipment or ac celerated tests on models, such as are now being recognized by AIEE. 3. Service data should be obtained to equate actual operating life of the complete apparatus in service with the factory and laboratory tests to give the designer confidence of the adequacy of his designs. Primary Functions of Insulation The primary use for insulation in electric apparatus is to separate cir cuits and conducting parts of different voltages. There are three distinctly different methods of providing such insulation, differing in effectiveness and magnitude: 1. Separation in an insulating fluid, usually air. 2. Creepage across an insulating surface or through porous insulation. 3. Dielectric barrier action by a solid'structure or a continuous insu lating film, possessing intrinsic dielec tric strength well above that of the equivalent air spacing. All three of these methods are gen erally employed to some extent on any specific equipment. However, one or more may predominate in a particular construction depending upon design and economics. Usually it is not prac tical to completely inclose all of any circuit component in a solid dielectric sheath, even though such a system is the most effective method. The dielec tric barrier method of insulation pro vides the greatest reliability ami the most effective use of space because of the very high level of insulation strength obtainable in a given space. It is in the area of dielectric barriers where differences in materials, and techniques of fabrication, play impor tant roles in the scientific design and manufacture of insulation. Important though the basic materials are. the dielectric strength of composite insulation systems is frequently more dependent upon the technique of fab rication than upon the inherent char acteristics of the components. Identical components may be combined and processed differently and vet have di electric breakdown levels that vary bv as much as ten to one. Fibrous ma terials when treated with varnishes and resins offer excellent examples of the important effect of processing. Here the degree of consolidation of the com posite materials predominates in estab lishing the insulation breakdown level as this determines if a dielectric barrier is produced. In mica insulating products, the size and grade of the splittings as well as the manner in which they are laid and overlapped are of great importance in establishing the insulation strength level. Even here, the extent of impregnation, degree of fill, and consolidation of the composite structure, make major contributions to the insulation breakdown level. On many materials such as varnished papers, varnished cloths, and the like, where the varnish treatment is de pended upon to provide the dielectric barrier, only multiple films will create breakdown strength significantly better than the creepage spacing correspond ing to the insulation thickness. Every varnish film will have some holes, thus each additional multiple coat of varnish on a cloth or paper merely reduces the probability of there being a pinhole clear through the resultant film. The importance of these factors of process ing and fabrication, and their effect on insulation quality cannot be overem phasized. Users and manufacturers alike should fully understand that while the properties of the component insu lating materials used are important, the intrinsic potentialities of high grade insulating systems are obtained only where suitable fabrication tech niques are employed bv skilled work men with thorough quality control pro cedures. Mechanical Requirements Insulation must have satisfactory mechanical strength in the completed windings and physical supports to withstand all the forces that it mit encounter during operation. In addi tion. it must have the strength to whi- stand the abuse which occurs during fabrication, forming, processing, roe installation in the equipment. The more important of the mechanical propertia that an insulation should possess air: 1. Flexibility to withstand bendlgt and forming during appplication zai installation without loss of insulating level. 2. Tear strength particularly whet* isolated sheets of insulating material extend to provide creepage distance*. 3. 5hear strength to provide physicJ support to the winding and other sulating components, and to withstaii the magnetic and rotational stresses,'* well as vibration and shock. 4. Flexural strength particularly* the supporting members such as wedga and coil washers. 5. Tensile strength is particulaih important to insulation during windag and fabrication. 6. Bond strength between wire* particularly on mush-wound co3* which may be subjected to vibratw# and shock, and to rotating field subjected to centrifugal forces, 7. Abrasion resistance is import** to insulating surfaces and indivicW parts to withstand handling dni*( fabrication of coils and assembly iff* equipment. Sometimes this is tant later because operating ha may produce coil movement. Electrical Characteristics 1. Dielectric breakdown streng the most important electrical projJ of most insulations. This is particu true where the insulation is used^ dielectric barrier: and mav not important on physical support; creepage surfaces. Serious conswfi tion must be given the possibility^ reduction in the level of this pro may occur during forming and U lation. " 2. Insulation resistivity mt 5 high so that the insulation is eff* and so that minimum heating dielectric occurs during uperatios 3. Power factor is a variabl1 quirement depending upon the en<j of the material or apparatus, dielectric loss is always a de property although its importanc 18 April 1956 ; with the application. Power factor the end uses, and protection offered to of similar pi operlie.-: and >4i organi. ot a reliable indication of dielectric the insulation bv the equipment en varnishes i enamel' a? applied to con ngth. closure. ductors. Dielectric constant may or may 4. Ozone degradation is usually the Class B--Class B insulation consist- P; significant depending upon result of corona which is ordinarily of of mica, asbestos, fiber glass and simi fiber a distorted voltage distribution little importance on machines operat lar inorganic materials in built-up nportant. An unbalanced design ing below six thousand volts. form with organic binding substances. result from using different ma- 5, Flammability is being given more A small proportion of Class A ma jfals having widely different aieiec- and more emphasis as an insulation terials may be used for structural pur constants. particularly at higher requirement. Often, fires will follow poses onlv.** Fiber glass or asbestos rating voltages as the dielectric electrical failures which may result in magnet wire insulations are included will be concentrated on the ma- more consequential damage than re in this temperature class. These may having the lower dielectric con- sulted from the original failure. Thus, include supplementary organic ma resistance to fire and self-extinguishing terials. such as polyvinylacetal or poly Corona resistance is always a properties are highly desirable. amide films. tirable property, but it is much more jjlportant for operation at six thousand and above, where corona activity greater. i. Arc resistance may be important ljo some parts of a device if it may be ijected to arcing but often is of little consequence. Track resistance is desirable :iiriiere surfaces may be subject to high y'^jcctric stresses on occasions, but where arc should ever occur. Moisture resistance is a highly desirable property in most electrical in solation. Most materials have good electrical properties when dry, but these properties deteriorate rapidly when the surfaces are wet or moisture pi absorbed into the solid insulation. The differences in effect of moisture | n a material may be a determining I factor in the practical selection of the | best material, process, or design. f ; Chemical Characteristics Temperature Classification of Insulation It has long been recognized that the operating temperature of insulation is a major factor in determining its life expectancy. The rating of electric ma chinery is dependent to a large extent on its temperature rise, and therefore on the hottest-spot temperature of the insulation. Because of this limitation, temperature is important in establish ing machine design characteristics. The temperature classification of insulating materials was first proposed by Lamme and Steinmetz in an AIEE paper in 1913. Shortly thereafter, AIEE Stand ard No. 1 was written classifying in sulating materials according to their temperature stability. This original classification has been used with rela tively little change since it was first established over forty years ago. The major changes have been adding of new insulating materials, such as fiber Class H--Class H insulation consists of i 1 i mica, asbestos, fiber glass and similar inorganic materials in built-up form with b nding substances com posed of silicone compounds, or ma terials with equivalent properties; (2i silicone compounds in rubberv or resinous forms, or materials with equiv alent properties, A minute proportion of organic materials may be used onlv where essential for structural purposes during manufacture.** Class C--Class C insulation consists entirely of mica, porcelain, glass, quartz and similar inorganic materials. * An insulation is considered to be "impregnated" when a suitable sub stance replaces the air between its fibers, even if this substance does not completely fill the spaces between the insulated conductors. The impregnat ing substances in order to be con sidered suitable, must have good insu lating properties: must entirely cover i Thermal stability of insulation glass, and new insulation classes, such the fibers and render them adherent to | tan be considered a chemical character- as H, as they were developed. This each other and to the conductor; must Jctic. It is one of the most imnortant classification of insulating materials not produce interstices within itself as J ^properties of an insulation or an in by AIEE, ASA, NEMA. and others a consequence of evaporation of the . aolation system. defines and classifies insulations as solvent or through any other cause; - 3*^ Resistance to the common sol- follows: must not flow during the operation ol 7jents and oils may be of great impor- Class 0--Class 0 insulation consists the machine at full working load or at ' " fance depending upon the end use. of cotton, silk, paper, and similar or the temperature limit specified: and solvent cleaning is damaging ganic materials when neither impreg must not unduly deteriorate under pro losome materials. Most insulation com- nated* or immersed in a liquid dielec longed exposure to operating tempera j ponents and varnishes include solvents tric. ture. - which should not be damaging to those Class A--Class A insulation consists **The electrical and mechanical materials adjacent thereto. Manv elec of: (1) cotton, silk, paper, and similar properties of the insulated winding trical machines and devices are ex posed to oil vapors which should not :^be deleterious to the insulation char acteristics. organic materials when either impreg nated* or immersed in a liquid dielec tric; (21 molded and laminated ma terials with cellulose filler, phenolic must not be impaired by the applica tion of the hottest-spot temperature per mitted for the specific insulation class. The word "impaired" is here used in S. Resistance to weak acids and resins and other resins of similar prop the sense of causing any change which -alkalis is a desirable property although . erties; (31 films and sheets of cellulose could disqualify the insulating ma -tJhe need will be determined Iargelv by acetate and other cellulose derivatives terial for continuously performing its lnsulation. April 1956 1 9 intended junction.; whether creepage spacing, mechanical support, or dielec tric barrier action. Table I--Typical Temperature Allowances for Specific Electric Machinery It should be noted that this classifica Permissible rise, deg, C* tion of insulations originally applied primarily to materials. The extension of this classification to insulation sys tems and apparatus has been an evolu tionary Drocess. The present letter sys tem of temperature classification is based almost entirely on whether the essential components are organic or inorganic: and whether or not the Insulation class Hottest-spot temperature, deg. C By thermometer** 0 90 . 35 A 105 50 B 130 70 C Not established H 180 no By resistance or by , embedded detector** - ' 45 * 60 * SO : 0 120 f composite insulation is impregnated with resin, and if so, what resin. In the last five years, a new concept has been developed jointly bv designers and users of electric equipment. This is the concept of "functional evalua tion" of insulation systems which is based on classifying insulations by tests rather than by their chemical * Based upon a 40 C ambient. Based upon a hot-spot allowance icith the thermometer method oj 153 C jor Class 0 and Class A insulation; 200 C jor Class B: 30 C jor Class H and a hot-spot allowance with the resistance method oj 5 = C jor Class 0 and Class A insulation; 100 C jor Class B; 20 C jor Class HOther types oj equipment have different hottest-spot temperature allowances and permissible observable rises. compositions. The advent of many new kinds of svnthetic resins in the past materials and simple combinations usually is not related to its mechanical 15 vears has made it impractical to thereof. There will be distinctly differ strength. The electrical strength gen continue to classify insulating materials ent test procedures for the complete erally increases in the early stages of according to their chemical composi insulation systems to permit testing thermal aging, even while mechanical tion. Many of the synthetic organics them according to the specific condi strength is decreasing. Often, the di have vastlv better thermal endurance tions of use on equipment. An excellent electric strength of an insulation will and electrical properties than their example of the latter type of test pro not fall below its initial level until its counterparts which occur in nature. It cedure is AIEE Standard No, 1-C. mechanical strength has been reduced has been found that insulation systems The relationship between the life of to near the failure point. Unless dis made up of some of these synthetics an insulated winding and the operating turbed mechanically, the dielectric cannot be classified as to their temper temperature is very complex. The sub barriers and spacing separators may ature stability except by testing them ject required a great deal of study and provide adequate insulation for long in specific combinations under the con test work before engineers could draw periods. When physical disintegration ditions of use. while they are perform any rational conclusions as to tempera of the insulation occurs, electrical ing their intended functions. Here ture effects and make predictions as to failure is usually prompt. AIEE Stand again, it is important to recognize the the probable life of insulation under ard No. I clearly states that how long different functions that insulating ma specific operating temperatures. It must an insulation will last electrically de terials and systems must perform in be recognized that insulation does not pends not. ony on the class of material service. The functions may be physical, fail by immediately breaking down used, but also on the magnitude of the chemical, or electrical and thev mav when it reaches any critical tempera physical forces tending to disrupt it_. provide low voltage spacing or high ture. Failure occurs by a gradual me Thus, the thermal endurance of an in voltage dielectric barrier action. chanical deterioration with time at sulation is dependent on how much This new concept of the temperature temperature. The question, "How hot time the insulation is in use and ho* classification of insulating materials may an insulation be permitted to much load is carried bv the equipment and insulation systems according to operate?" can be answered onlv with during the actual period of use. Thfi functional evaluation by tests was the knowledge of how long must it factors of variable load and intermit-' recognized in the 1954 revision of last. It is the time-temperature rela tent use are particularly imuortant in,, AI EE Standard No. 1. Future revisions tionship which determines the rate at determining insulation life when equip of AIEE Standard Mo. 1 are contem which the mechanical strength of most ment is overloaded and normal operat plated wherein methods for functional insulation decreases. As organic ma ing temperatures are exceeded. ^ evaluation of insulation, and tempera terials age thermally, they become Until recently, the engineer's con-" ture classification of insulating ma weaker and brittle, and are more sus cepts of insulation aging time beett_ terials and insulation svstems. will be ceptible to disintegration under the in based on a simple mathematical rel*f. provided in official AIEE Testing Pro fluence of vibration, shock, differential tionship wherein the insulation lif*. cedures. Separate test procedures will expansion, and other mechanical forces. was assumed to be reduced bv half fr. provide for the evaluation of insulating The dielectric strength of an insulation each approximately 7 tn ;jJ C 20 Insulation, lonf i in temperature, $hiie this was ful approximation, recent more prehensive tests have indicated that lation aging is an inverse function e absolute temperature and ra ces treatment as a chemical rate omenon- Special graph paper is jlable with an inverse absolute ternture scale on the abscissa and a .rithmic scale for life on the ordiWhere the thermal life of insulaJ, fits this concept, as expressed in following formula, the life data 'ts a straight line on this special iph paper: here y ~ A e61 y = Insulation life in hours ior days! x = F.eciprocal of the abso lute temperature ( 273 + C ) e = 2.718 t A and b = Empirical constants J oc.OoO \i SO,QOQ \ X X'\ \ \| \[ ! i X \ !\ I 6,600 I\ i! XN A\ ibX ,,______ 7 Jii $60 i\ l/) D O^ too u so ... . A typical example of the application of this concept and mathematical treat ment to the life of an insulation system is shown in Figure 1. This curve recog nises the variability of insulation life and instead of precise!' defining life as a single line, shows that it may have a wide range during which probability oi failure varies. Tests and operating experience have indicated that for most equipment. 957V of the failures will lie within such a range as illustrated with a few random failures above the maximum and below the minimum limits of this range. It is generallv believed that thermal aging of insulation makes it vulnerable to the many other damaging and con taminating influences. On the early stages of thermal degradation, the varnish treatment and internal resin bonds become brittle, weak, and porous. One criterion of increasing vulnerability and impending trouble is ^K 1 X \ | 'K \ \ \ \ \ \ believed tv be the los> of the initial level of moisture resistance. Other de teriorating influences may then be the cause of actuai failure, and the absence of roasted-out insulation mat obscure the fact that thermal aging was in fa?l tiie underly ing cause. Continued next issue. NBS Electron Tube Information Service The National Bureau of Standardhas established a tube information service for accumulating and dissemin ating technical data on both domestic and foreign radio tubes. At the present time, nearly 10.000 cards, filed by tube type number, are appropriately ref erenced to manufacturers' source ma terial. In addition, about 10 percent of these cards, selected mainlv from the high-use miniature and subminiature types, have been coded on punched cards for mechanical soninc. 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