US2725502A

US2725502A - Inductive apparatus

Info

Publication number: US2725502A
Application number: US288484A
Authority: US
Inventors: Jr John H Chiles; Albert J Maslin
Original assignee: Westinghouse Electric Corp
Current assignee: CBS Corp
Priority date: 1952-05-17
Filing date: 1952-05-17
Publication date: 1955-11-29
Anticipated expiration: 1972-11-29

Description

1955 J. H. CHILES, JR., EI'AL 2,725,502

INDUCTIVE APPARATUS 2 Sheets-Sheet 1 Filed May 17, 1952 Fig. l.

Volts F lg 3 INVENTORS John H. Chil Jr. and %3 wcmE 200 F lg 2 WITNESSES:

Nov. 29, 1955 J. H. CHILES, JR., ETAL INDUCTIVE APPARATUS 2 Sheets-Sheet 2 Filed May 17, 1952 Volts Fig.5

Volts 78 Fig.7

Fig.6

INVENTORS John H. Chiles qr. and

WITNESSES:

United States Pate 2,725 502 INDUCTIVE APPARATUS- John H. Chiles, Jr., and Albert I. Maslin, Sharon, Pa, assignors to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation rah-Pennsylvania Application May 17, 1952, Serial No. -288,484 4 Claims. (Cl. 317- 14) An object of this invention is to provideforminimizing any increase in the core loss of inductive apparatus that occurs when one of its windings is subjectedto: anE impulse voltage, by preventing the establishmentofra conducting path that encloses a large cross-sectional area ofthe'laminated core to thuslimit the electromagneticvoltage that can act along the path that maybe established in the core, and by grounding the core at a number of predetermined points whereby the magnitude and distribution of the electrostatic voltage induced in the-:.corei-is.chan-ged.

Another object of this invention is:to-provide:for-mini' mizing any increase in the core'loss.of'theztransformer that occurs whenone of its windingszis-subjected to an impulse voltage, by dividing the insulated :transformer core laminations into sections and separatingthe sections by material having a predetermined dielectric-strength to thereby prevent the establishment of aconducting path that encloses a large cross-sectional area ofthedam'inated' transformer core, and by groun-dingteachmf the sectionsat a predetermined pointwhereby thevmagnitude-and distribution of the electrostatic voltage induced in the core is changed.

Other objects of this invention WiIl Fbecome-apparentfrom the following description whenita keirin conjunction with the accompanying drawings',-in' Which-2 Figure 1 is a schematic illustrationof a transformer illustrating one embodiment of thisinv'ent'i'on;

Fig. 2 is a schematic illustration of a transformer core constructed in accordance-with the prior art;

Fig. 3 is a graph illustrating: voltage gradients for the transformer core of Fig. 2, when .jdifferent' factors are taken into consideration;-

Fig. 4 is a fragmentaryview of th'e'transformer core shown in Fig. l and which illustrates one -embodiment of the teachings of this invention.

Fig. 5 isa graph illustrating various-voltage gradients for the transformer core of Fig. 4wh'e'n differentfac'tors are taken into consideration; j I

Fig; 6 is a fragmentary view o'fa transformercoreillus tratin'g another embodiment of this invention;

Fig. 7;is a graph illustrating various voltage, gradients for, the transformer core .of Fig.;6 Wlienjdifferent factors are taken into consideration;

Fig. 8' is a fragmentaryview' ofa transformer core illustrating still another embodiment of this invention, and

Fig. 9'isa graph illustrating various'vol'tage gradients for inxa conventional manner.

v ance of the core 30 is considered, the

2,725,502 Patented Nov. 29, 1955 2 the transformer core of Fig. 8 when different factors are taken into'consideration.

Referring to Fig. 1 of the drawings, there is depicted a transformer 10 illustrating an embodiment constructed in accordance with the teachings of this invention. In this instance, the transformer 16) comprises a shell-type magnetic core 12;,however, it is to be understood that this in vention is equally applicable to other types of transformer cores.

' As illustrated, a primary winding 14 and a secondary winding 16 are wound in a conventional manner around the transformer winding leg 18 and are disposed in inductive relationship to the core 12, thereby subjecting the core 12 to an electrostatic and electromagnetic field when either of the transformer windings id or 16 is subjected to an impulse voltage.

Since this invention concerns the manner of constructing the transformer core 12 and grounding the same, a complete showing of a transformer case 20 and a showing of the conventional bushings disposed therein for properly insulating the primary and secondary lead-in connections is deemed unnecessary.

As illustrated, the core 12 comprises a predetermined number of. laminations 22 made from magnetic sheet material, each lamination being provided on both sides with an adherent insulating layer or film of dielectric material The laminations 22 in turn are separated into two

core sections

24 and 26 of substantially equal size by a member 28 of predetermined dielectric strength, the purpose. of which will be described hereinafter. 'As illustrated, the member 28 will be so shaped and of such a size relative to the laminations 22' as togive adequate separation and insulation as determined in" the design. In a particular application, the size of the core sections and thus the number of separated core sections required to provide a predetermined core structurewill depend on the magnitude of the electromagnetic field produced'by the current flowing through either the primary winding 14 or the secondary winding 16 due to the impulse voltage, the" size being predetermined to resist arcing between the adjacent core laminations' 22'. Each of the end lamination's 22' disposed on opposite sides of the core'lZ is grounded, the purpose of which will likewise be described hereinafter.

Referring to- Fig. 2, there is illustrated a section of a conventional laminated transformer core 30 in which one of the end laminations is grounded. The transformer core 30 comprises a plurality of laminations 32 made from magnetic sheet material which are substantially insulated from one another in a conventional manner by means of suitable dielectric material 34. One side of the core 30 is grounded by a ground connection 35.

Referring to Fig. 3, there is illustrated a graph representing: electrostatic voltage gradients for the transformer core 30 when different factors are taken into consideration. In particular, a curve 36 represents the electrostatic voltage gradient for the core 30 when only the capacitance between adjacent laminations 32 of the core 34 and the capacitance established by the capacitive coupling between the windings (not shown) and the core 39 is considered. However, if in addition to' these capacitances the resistcurve 38 represents thezvoltagegradient. When an impulse voltage is applied to eitherthe primary winding (not shown) or thesecondary winding (not shown), the transformer winding is increased in potential far above zero or ground potential.

.However, the portions of the core 30 remote from the mum at the point closest to the windings and farthest from the ground connection 35, and will decrease, as illustrated in Fig. 3, as the ground connection 35 is approached. In some cases, this electrostatic potential gradient along the core 3% is sufficiently high to cause the electrical breakdown of the dielectric material 34 between the individual laminations 32. When this takes place, the resistance of the core 30, as measured from top to bottom, is greatly reduced. In spite of this breakdown and greatly reduced resistance of the built-up core 30, however, an increase in core loss does not necessarily result. This is because the conducting path created by the breakdown may not be a closed path encircling all or part of the core crosssection. If the path, however, encircles all or part of the core cross-section, the presence of normal frequency flux in the core 31 will create a current around this path, thus resulting in an increase in core loss.

The above-discussed effect of electrostatic voltage induced in the core 39 is not the only effect that takes place when an impulse voltage is applied to either of the transformer windings (not shown). When the impulse voltage is applied, it distributes itself along the electrical length of the transformer winding (not shown), and effects a large current flow through the winding, which in turn, gives rise to a flux in the core 36 which follows the normal flux path of that core. This flux in turn will induce an electromagnetic voltage in any closed path surrounding all or part of it. Such a path may be wholly within the cross-section of the core 30 or it may correspond wholly or in part with the periphery of the crosssection of the core 30. The electromagnetic voltage induced in such a path by the flux in the core may be suflicient to break down the insulation 34 or jump the minute gaps at the periphery where there are burrs on the individual laminations 32, and thus permit the flow of current around the path, thus resulting in an increase in core loss. It is believed that both the electrostatic voltage due to the presence of the core 30 in the electrostatic field, and the electromagnetic voltage due to the electromagnetic field, cooperate in the creation of closed conducting circuits within, or surrounding the core 30, which give rise to the increase in core less experienced when either of the transformer windings (not shown) is subjected to an impulse voltage. It is further believed that the electrostatic voltage on the core 30 may effect a breakdown of the dielectric material 34 even though a closed electrical circuit is not established by such an action. Then when the electromagnetic voltage is induced in the core 30, it will complete the path or a part of the path originally established by the electrostatic voltage so as to form a completed electrical circuit within the core 30 and thus increase its core loss.

Referring to Fig. 4, the transformer core laminations 22 of Fig. 1 are separated into the two

core sections

24 and 26 by the dielectric member 28, and each end lamination of the laminations 22 disposed on opposite ends of the core 12 is grounded. The laminations 22 in turn are insulated from one another by means of dielectric material 42 formed by the adherent insulating layers disposed on both sides of each of the laminations 22. By subdividing the core 12 by means of the dielectric member 28, the occurrence of a conducting path enclosing a large cross-sectional area of the core 12 is prevented. Thus the electromagnetic voltage which can act along any path that may be established within the

core sections

24 and 26 is limited. In particular, if the transformer core is divided into N sections, the voltage induced electromagnetically in any closed circuit involving the core structure will be reduced by the factor when compared with the voltage induced electromagnetically in a core not so subdivided. In practice, it has been found that when the dielectric member 28 has a thickness of between /1 and of an inch, depending upon the rating of the transformer, the thickness of the member 28 is suflicient to prevent an electrical breakdown thereacross. However, it has been found by investigation that the thickness of the dielectric member 23 may be reduced to an insulating value substantially equal to the insulating value of the sum of the dielectric material 4-2 provided in either of the

core sections

24 or 26. For instance, the laminations 22 disposed on opposite sides of the dielectric material 28 may be bent towards one another at their edges during the course of manufacturing the core 12, thus limiting the effectiveness of the dielectric material 28. Therefore, in practice, it has been found desirable to increase the thickness of the dielectric material 28 to offset such an effect produced by the bending of the edges of the laminations 22 disposed on opposite sides of the material 28. However, the thickness of the dielectric material 28 can be properly determined by one skilled in the transformer art taking into consideration all the problems involved.

In practice, it has been found that the dielectric material may be formed from pressboard, varnished cloth, resin bonded laminate, or any recognized insulating material having relatively high compressive strength.

In order to change the magnitude and distribution of the electrostatic voltage induced in the core 32 when either the primary winding 14 or the secondary winding 16 is subjected to a surge voltage, each of the end laminations 22 disposed on opposite sides of the core 12 is grounded by the

ground connections

44 and 46, respectively. Thus, by grounding the core 12 as illustrated in Fig. 4, the magnitude of the electrostatic voltage appearing across either the

core section

24 or 26 is reduced to substantially /2 the magnitude of that voltage appearing across the core 30 illustrated in Fig. 2 for the same given set of conditions. This can be more clearly seen by reference to curve 48 of Fig. 5 which represents a distribution of the electrostatic voltage across the core 12 when taking into consideration only the capacitance established between adjacent laminations 22 and the capacitance established by the capacitive coupling between the

windings

14 and 16 and the core 12. On the other hand, curve 5 represents the distribution of voltage on the core 12 when in addition to these capacitances, the resistance of the core 12 is taken into consideration. It is believed that the actual voltage gradient for the core 12 lies somewhere between the

curves

48 and 50 and it is illustrated by the curve 52. From the curve 52, it can be seen that there is a considerable voltage drop across the dielectric material 28. Due to this voltage drop, the dilference in voltage between adjacent laminations 22 in the core section 26 is reduced, thus affording further protection against the formation of conducting paths within the core section 26, which effect an increase in the core loss.

Referring to Fig. 6, there is illustrated another embodiment of this invention, in which for the purpose of simplifying the drawings, only a magnetic core 54 is illustrated. However, it is to be understood that the core 54 has disposed in inductive relationship therewith windingssimilar to the primary winding 14 and secondary winding 16 illustrated in Fig. l and the core 54 is likewise disposed in a transformer case (not shown). The magnetic core 54, made from magnetic sheet material, comprises a predetermined number of laminations 55, and it is divided into

core sections

56, 58 and 60 by means of sheets of

material

62 and 64 of predetermined dielectric strength. As illustrated, the core section 58 is approximately twice the size of either the core section 56 or the core section 60. The sheets of

material

62 and 64 are made from the same material discussed with reference to the dielectric material 28 illustrated in Fig. 4. In practice, the thickness of the sheets of

material

62 and 64 is also the same as the thickness of the dielectric material 28 illustrated in Fig. 4, and the thickness is determined by the same factors discussed with reference to the dielectric material 28.

The sheets of

dielectric material

62 and 64 are disposed in the core 54 in order to prevent the establishment of a conducting path. that encloses a large cross-sectional area of 'the"core"54', and thus prevent the establishment of a large induced electromagnetic voltage across the laminations 55, which have'disposed therebetween a suitable dielectric material 66.v In particular, the magnitude of the electromagnetic voltage induced in the

core sections

56 and 60, comparedto the magnitude of the electromagneticvoltage induced in the core 38 of 'Fig. 2 for the same given set. of conditions is only one-fourth as great, while the magnitude of the electromagnetic voltage induced in the core section-58 as compared to the magnitude of the electromagnetic voltage induced in the core30' of Fig. 2 for the same given set of conditions is only one-half as great.

' In order to change the magnitude and distribution of theel-ectrostatic voltage induced in the core 54, the

core sections

56, 58 and. 60 are provided with.

ground connections

70, 72 and 74, respectively. As illustrated the ground connections 70' and 74 are connected to the outside endflaminations .55 of the core sections 56 and 68 respectively, which end laminations are disposed on opposite ends of the core 54. The ground connection 72 is connected nearthe mid-pointof the core section 58, but this isnot essentialto the functioning of the invention. However, it has been found that good results are obtained whenthe ground connection- 72 is connected to substantially the mid-point of the core section 58.

Inparticular, referring to Fig. 7, there is illustrated a plurality. of: electrostatic voltage gradient curves for the core 541 when different factors are taken into consideration. If only the capacitance between adjacent laminations 55 and the capacitance established by the capacitive couplingbetween the core 5.4and its inductively related windings (not shown )'is taken into consideration, an electrostatic voltage gradient-curve 76 1s obtained for the core 54. "On the other hand, if in addition to these capacitancesthe resistance of the core 54 is taken into consideration, a voltage gradient curve '78 is obtained for the core. 54; Thus by grounding the core 54 at the various points shown in the drawing, the maximum magnitude of the'voltageappearing on the core 54'is only one-fourth as great as the maximum magnitude of the electrostatic voltage appearing on the core 30- illustrated in Fig. 2.

Itisbelieved that the actual voltage curve for the core 54 lies somewhere between the

curves

76 and 78 and it is illustrated by a curve 88. The voltage drop across the sheets of dielectric material '62, and 64, as can be seen fromsthe electrostatic voltage gradient curve 88, reduces the voltage difference between the adjacent laminations 55 ofthe core, section 60 and between the. adjacent-lamination'slSS offthe upper portion ofv the core section 58. Such an action reduces the electrostatic voltage stress on these particular; portions of "the core 54.

Referring to Fig. 8, there is illustrated another embodiment of this invention. In order to simplify the drawings only a magnetic core 84 is illustrated. However, in practice. windings (not shown) similar to the windingst14 and 16 of Fig. 1 are disposed in inductive relationship with the core 84. Likewise the core 84 is disposed in a transformer case (not shown). In this particular embodiment the magnetic core 84 having a predetermined number of laminations 85 is divided into three

core sections

86, 88 and 90 of substantially equal size by means of sheets of

material

92 and 94 of predetermined dielectric strength, which are similar to the sheets of

material

62 and 64 illustrated in Fig. 6. Dielectric material 87 is disposed between each of the adjacent laminations 85 in a conventional manner. In accordance with this invention, the sheets of

dielectric material

92 and 94 are provided in order to prevent the establishment of a conducting path that encloses a large cross-sectional area of the core 84 and thus limit the magnitude of the electromagnetic voltage which can act along any path which could be formed within any of the three

core sections

86, 88 or 90. When the core 84 is subdivided into N core sections of substantielly qual s e, th o tage induc e e t omagnetilly in ny c o d ci cu t nvolving he core 841 w ll be reduced by the factor when compared with the voltage induced in a core not so subdivided, as illustrated in'Fig. 2.

The voltage induced electrostatically across the core 84 is inversely proportional to the capacity between a ground point and a lamination 85 most remote from the.

In particular, referring to Fig. 9, the maximum electrostatic voltage appearing across any of the

core sections

86, 88 or 90 is only one-sixth as great as compared to the voltage to ground of the core section 30 illustrated in Fig. 2.

Referring to Fig. 9, a curve 102 represents the electrostatic voltage distribution on the core 84 when taking into consideration only the capacitance between adjacent laminations and the capacitance established by the capacitive-coupling between the core 84 and the windings (not shown) disposed in inductive relationship therewith. However, if in addition to these capacitances the resistance ofthe core 84 is taken into consideration, a voltage gradient curve 104 is obtained. Again it is believed that the actual voltage distribution curve for the core 84 lies somewhere between the

curves

102 and 104 and this actual distribution curve is illustrated by a curve 106. Owing to the relatively large voltage drop across the sheets'of dielectric material, 92 and 94, the electrostatic voltage diiference between certain adjacent laminations 85 is reduced. In particular, this reduction occurs in the upper portions ofthe

core sections

86, 88 and 90, respectivelyl In' accordance with this invention when a plurality of core sections are produced by the insertion of dielectric material such asthe dielectric member 28, the probability of all the core sections breaking down and having high core loss is very much less than the probability for a single.

core not so subdivided. Thus, even though there may be a breakdown the use of multiple core sections separated in accordance with this invention is effective in lowering as much. Therefore, the ratio of strength to voltage is increased by the expedient of dividing the core into core sections in accordance with this invention.

Although this invention has been described with reference to two or three core sections established by the dielectric material disposed between adjacent core sections, it

is to be understood that this invention can be practiced by dividing a transformer core into more than three core sections by means of dielectric material, such as the material 28 of Fig. 1. It is also to be understood that the size of each core section and its relative size as compared to the other core sections of the transformer core can be determined by one skilled in the transformer art.

Since certain changes may be made in the above-described apparatus, and difierent embodiments of the invention may be made without departing from the spirit and scope thereof, it is intended that all matter contained in the foregoing description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

We claim as our invention:

1. In a transformer, the combination comprising, a winding, a magnetic core having a predetermined number of laminations substantially insulated from one another by dielectric material and disposed in inductive relationship to the winding, thereby subjecting the core to an electrostatic and electromagnetic field when the transformer is subje ed to an impulse voltage, a member having a preset ined dielectric strength disposed in the core separating it into two core sections, to prevent the occurrence of a conducting path that encloses a large cross-sectional area of the core and thus limit the electro magnetic voltage which can act along any path that may be established within the two core sections, and thereby minimize arcing between adjacent core laminations, and means for grounding each of the end laminations disposed on opposite ends of the core, whereby the magnitude and distribution of the electrostatic voltage induced in the core is changed, to minimize the increase in the core loss of the transformer which may occur when the winding is subiected to the impulse voltage.

2. In a transformer, the combination comprising, a winding, a magnetic core having a predetermined number of laminations substantially insulated from one another by dielectric material and disposed in inductive relationship to the winding, subjecting the core to an electrostatic and electromagnetic field when the transformer is subjected to an impulse voltage, members having predetermined dielectric strengths disposed in the core separating it into three core sections, to prevent the occurrence of a conducting path that encloses a large crss-sectional area of the core and thus limit the electromagnetic voltage which can act along any path that may be established within any of the three core sections, and thereby minimize arcing between adjacent core laminations, and means for grounding each of the end laminations on opposite ends of the core and for grounding the midpoint of the intermediate of the three core sections, whereby the magnitude and distribution of the electrostatic voltage induced in the core is changed, to minimize the increase in the core loss of the transformer when the winding is subjected to the impulse voltage.

3. in a transformer, the combination comprising, a winding, a magnetic core having a predetermined number of laminations substantially insulated from one another by dielectric material and-disposed in inductive relationship to the winding, subjecting the core to an electrostatic and electromagnetic field when the transformer is subjected to an impulse voltage, members havir" predetermined dielectric strengths disposed in the core separating it into three core sections to prevent the occurrence of a conducting path that encloses a large crosssectional area of the core and thus limit the electromagnetic voltage which can act along any path that may be established within any of the three core sections, and thereby minimize arcing between adjacent core laminations, the intermediate core section being of a larger size than the two end core sections, and means for grounding each of the end laminations disposed on opposite ends of the core and for grounding the mid-point of the intermediate of the three core sections, whereby the magnitude and distribution of the electrostatic voltage induced in the core is changed, to minimize the increase in the core loss of the transformer when the winding is subjected to the impulse voltage.

4. In inductive apparatus, in combination, a winding, a magnetic core having a predetermined number of laminations substantially insulated from one another by dielectric material and disposed in inductive relationship to the winding, subjecting the magnetic core to an electrostatic and electromagnetic field when the inductive apparatus is subjected to an impulse voltage, means having a predetermined dielectric strength disposed in the magnetic core separating it into a plurality of core sections to prevent the occurrence of a conducting path that encloses a large cross-sectional area of the magnetic core and thus limit the electromagnetic voltage which can act along any path which may be established within the plurality of core sections, the size and thus the number of the core sections depending on the magnitude of the electromagnetic field to be met and the size of each core section being predetermined to resist arcing between adjacent core laminations, and means for grounding each of the end laminations disposed on opposite ends of the magnetic core, whereby the magnitude and distribution of the electrostatic voltage induced in the magnetic core is changed, to minimize the increase in the core loss of the inductive apparatus which occurs when the winding is sub jected to the impulse voltage.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Japanese publicationNo. 18,505 to C. Okawa; lished November 25, 1939.

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