US3569883A - Electrical winding - Google Patents

Electrical winding Download PDF

Info

Publication number
US3569883A
US3569883A US857952A US3569883DA US3569883A US 3569883 A US3569883 A US 3569883A US 857952 A US857952 A US 857952A US 3569883D A US3569883D A US 3569883DA US 3569883 A US3569883 A US 3569883A
Authority
US
United States
Prior art keywords
coil
pancake
coils
pancake coils
winding
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Expired - Lifetime
Application number
US857952A
Other languages
English (en)
Inventor
Robert I Van Nice
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Individual
Original Assignee
Individual
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Individual filed Critical Individual
Application granted granted Critical
Publication of US3569883A publication Critical patent/US3569883A/en
Anticipated expiration legal-status Critical
Expired - Lifetime legal-status Critical Current

Links

Images

Classifications

    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
    • H01F27/00—Details of transformers or inductances, in general
    • H01F27/34—Special means for preventing or reducing unwanted electric or magnetic effects, e.g. no-load losses, reactive currents, harmonics, oscillations, leakage fields
    • H01F27/343—Preventing or reducing surge voltages; oscillations

Definitions

  • 336/70 d of h windi are interconnected to provide an interm- Cl 15/14 leaved turn, high series capacitance arrangement wherein the Field of Search 336/69, 70, l t i l i cuit spirals inwardly in the first pancake coil and 186 outwardly in the second.
  • the third and fourth pancake coils 56 f C are also interconnected to provide an interleaved turn, high I 1 Re series capacitance arrangement, but the sequence is reversed UNITED STATES PATENTS to direct the electrical circuit outwardly in the third pancake 3,029,402 4/1962 Highton 336/X coil and inwardly in the fourth to reduce the electrical stress 3,405,378 10/1968 Vincent-Martin 336/70 between the inner turns of the second and third pancake coils.
  • V72 84 70 F TI2 T4 TH T3 TIO T2 T9 Tl T8 TO B. i 86 ⁇ aa T
  • the invention relates in general to electrical inductive apparatus, such as transformers, and more specifically to windings for electrical power transformers of the core-form type, which utilize a plurality of pancake coils disposed in spaced, side-byside relation, and connected to increase the through or series capacitance of the winding.
  • Each pancake coil has a plurality of conductor turns provided by first and second conductors wound together to provide first and second radially interleaved coil sections, with the electrical circuit spiraling through predetermined ,radial sections of the first and second coils of a pair, and then returning to the first coil of the pair to traverse the remaining coil sections of the pair.
  • the art prior to my copending application taught the exchanging of coil sections each time the electrical circuit proceeds from the first to the second pancake coils of the pair, while my copending application teaches reducing the magnitude of voltage oscillations, and hastening their decay, by making each pass of the circuit through the coils of a basic pair via like coil sections. This arrangement substantially reduces stress build up at the midpoint of adjacent coil builds.
  • the present invention is a new and improved twin interleaved winding for electrical inductive apparatus, including a plurality of pancake coils arranged in a stack.
  • Each pancake coil has first and second conductors wound together to provide first and second radially interleaved coil sections, respectively, and the first two pancake coils are connected in the twin interleaved arrangement disclosed in my copending application, whereiri the circuit proceeds through the first and second pancake coils via like coil sections on each of the first and second passes through these coils, with the interconnections between the like coil sections of the two coils being made between the innermost turns of the coil sections.
  • the third and fourth pancake coils are also twin interleaved, with the circuit proceeding through the third and fourth pancake coils via like coil sections on each pass through the coils, but in this instance the interconnections between the like coil sections of the two coils are made between the outermost turns'of the coil sections. This arrangement reduces the voltage between the innermost turns of the second and third pancake coils of the winding to one-half the voltage across one of the pancake coils.
  • FIG. I is a partial sectional elevational view of a magnetic core and high voltage winding assembly of a transformer constructed according to the teachings of the prior art
  • FIG. 2 is a schematic diagram of the high voltage winding shown in FIG. 1;
  • FIG. 3 is a vector diagram which illustrates the electrical stresses between the innermost turns of the second and third pancake coils of the high voltage winding shown in FIG. 1; i
  • FIG. 4 is a diagrammatic view of a twin interleaved high voltage winding constructed according to the teachings of the invention.
  • FIG. 1 a partial sectional elevation of a transformer 10, constructed according to the teachings of the prior art.
  • Transformer 10 is a power transformer ofthe core-form type, which may be single or polyphase. Since each phase of a polyphase transformer would be similar, only one phase is shown in FIG. 1 in order to simplify the drawing.
  • Transformer 10 includes a magnetic core structure 12, which may be of conventional construction, including a winding leg 14 having anaxis 20 about which high and low voltage winding assemblies 16 and 18, respectively, are concentrically disposed.
  • Low voltage winding assembly 18, which may be of conventional construction, has a plurality of conductor turns 22 insulated from the magnetic core 12 and the high voltage winding assembly 16 by electrical insulating means 24.
  • High voltage winding 16 includes a plurality of pancake or disc type coils 26, 28, 30 and 32, which are spaced axially apart in a stacked arrangement about the axis 20 of the magnetic core leg 14. Only four pancake coils, each having eight turns, are shown in FIG. 1, and in the remaining figures, in order to adequately illustrate the prior art, and the teachings of the invention. It is to be understood that the winding may have any desired number of pairs of pancake coils, and turns per pancake coil, as required by a specific application.
  • the plurality of pancake coils of winding 16 are constructed and connected to provide a high series capacitance winding, having a single series circuit between the line terminal 34 at the start of the winding 16, and a terminal at the finish end of the winding (not shown) which may be a line terminal or a neutral terminal, depending upon the specific construction and application of the transformer 10.
  • Each of the pancake coils 26, 28, 30 and 32 have a plurality of conductor turns formed of at least first and second electrical conductors or strands spirally wound together about a leaved coil sections I and II of each pancake coil are shown slightly offset from one another in the schematic diagram of FIG. 2, in order to more clearly denote the different radial locations of the first and second coil sections.
  • the innermost turn of the pancake coil is in the first coil section I, and the second coil section II has the next to the innermost turn.
  • the pancake coils of high voltage winding 16 are connected to form a plurality of basic pairs, each having first and second adjacent pancake coils, to provide the basic interleaving arrangement. Then, the basic pairs are interconnected, in order to provide a single series circuit between the start and finish ends of the winding. Since two pancake coils are required to complete the basic interleaving pattern, it is commonly referred to as twin interleaving.
  • Twin interleaving is performed by connecting the inner ends of unlike radial coil sections together in the first and second pancake coils of each basic pair, and by connecting the outer ends of like coil sections in the first and second pancake coils of each basic pair.
  • the circuit enters the outer end of section II of the first pancake coil 26 via conductor 40, which is connected to line terminal 36, and it spirals inwardly via every other turn of the pancake coil.
  • conductor 40 which is connected to line terminal 36, and it spirals inwardly via every other turn of the pancake coil.
  • Each of the turns are identified with the letter C and a number which indicates the position of the turn relative to the line terminal 34.
  • the circuit proceeds to the end of the innermost turn of the first coil section I of the second pancake coil 28, via start-start connection 42, and spirals outwardly via every other turn until reaching the outer end of coil section I.
  • the circuit then goes back to the first pancake coil 26, with the outer end of the first coil section of pancake coil 28 being connected to the outer end of the first coil section of pancake coil 26, via finish-finish connection 44, and the circuit again spirals inwardly via every other turn through pancake coil 26 until reaching the innermost turn of this section.
  • the circuit again proceeds to the second pancake coil 28, with the end of the innermost turn of the first coil section of pancake coil 26 being connected to the end of the innermost turn of the second coil section of pancake coil 28, via start-start connection 46, and the circuit spirals outwardly through pancake coil 28 via every other turn, finally leaving pancake coil 28 via conductor 48, which is connected to the end of the outermost turn of the second coil section of pancake coil 28.
  • This basic twin interleaved arrangement is then repeated for the next basic pairs of pancake coils, such as the second basic pair which comprises pancake coils 30 and 32, with conductor 48 entering the end of the outermost turn of coil section II of pancake coil 30.
  • each conductor turn has a number which identifies its position relative to the line terminal 34, the number of turns between predetermined locations of the winding may be readily ascertained. It will be noted that the difference in the number of turns between the innermost turns of pancake coils 28 and 30, which has been shown by experience to be a weak" location in the winding from the standpoint of possible electrical breakdown, is 24 turns. Since there are 8 turns per pancake coil, the difference may be generalized as being 3N, where N is equal to the number of turns in one pancake coil.
  • the series circuit spirals inwardly in the first pancake coil of the basic pair, and it spirals outwardly in the second pancake coil of the basic pair.
  • the two pancake coils of each pair are wound in opposite circumferential directions, in order to obtain a uniform direction of instantaneous current flow in the pancake coils of the winding. In practice, this may be achieved by winding all the pancake coils in the same circumferential direction, and then breaking down and rewinding by hand alternate coils, in order to achieve the desired result without severing the conductor.
  • Arrows 50 and 52 in FIG. 2 indicate that the series circuit in alternate pancake coils 26 and 30 spirals inwardly
  • the arrows 54 and 56 indicate that the series circuit in the remaining pancake coils 28 and 32 spirals outwardly.
  • FIG. 3 is a vector diagram drawn with respect to the second and third pancake coils 28 and 30, which illustrates why the location between the inner turns of these pancake coils is an area of high electrical stress, which may fail during impulse or transient voltages, if special precautions are not take to increase the insulating clearances at this location.
  • the stress between the innermost turns of pancake coils 28 and 30 is equal to 3N or three times the voltage across one pancake coil, and this stress is indicated by vector 60.
  • the stress between the high and low voltage windings l6 and 18 is indicated by the vector 62, which is substantially orthogonal to the direction of the stress between the two pancake coils.
  • the stress between the innermost turns of the pancake coils adds vectorially to the stress between the high and low voltage windings, with the resulting stress being indicated by vector 64. If the insulating clearance in this location between the second and third pancake coils is not great enough, this resultant stress during surge testing or during a steep wavefront surge initiated by lightning or switching, may cause a failure of the insulation at this location, such as a winding tube, establishing a creep path along the surface of the winding tube to ground. This location is especially critical in transformers with high BIL ratings, such as those of I050 KV and above.
  • the stress between the innermost turns of pancake coils 28 and 30 may be reduced in the basic twin interleaving arrangement of FIG. 1 by first entering the outer end of the first coil section I of the first pancake coil, instead of the outer end of the second coil section II, as illustrated, which reduces the stress between the innermost turns to 1N i.e., the voltage across one pancake coil, but this arrangement merely transfers the stress of three times the voltage across one pancake coil (3N) to the next to the innermost turns, which is therefore of little benefit.
  • FIG. 1 Another modification of the basic interleaving arrangement shown in FIG. 1 is to first enter the outer end of the second coil section II of the first pancake coil in alternate pairs of coils, and in the remaining pairs the circuit is first directed into the outer end of the first coil section I of the first pancake coil of a basic pair, which reduces the stress between the innermost turns to 2N, or twice the voltage across one pancake coil, and the stress between the next to the innermost turns is also equal to 2N.
  • the voltage stress between the innermost turns, and between the next to the innermost turns, of the second and third pancake coils in the winding are 1N and 3N, respectively, when the circuit first enters the end of the second coil section II of the first pancake coil of the basic pair, and 3N and 1N, respectively, when the circuit first enters the outer end of the first coil section I of the first pancake coil of the basic pair.
  • the sum of the voltage stress between the innermost turns and the voltage stress between the next to the innermost turns is four times the voltage across one pancake coil, or 4N. It would be desirable to reduce the voltage stress between the innermost turns to less than the voltage across one pancake coil, and to also reduce the sum of the voltage stress between the innermost turns, and between the next to the innermost turns, to less than 4N.
  • FIG. 4 is a diagrammatic view of a high voltage winding 70, which is symmetrical about centerline 72, which may be used to replace the high voltage winding 16 shown in the transformer 10 of FIG. I, to substantially reduce the voltage stress between the innermost turns of the second and third pancake coils from the line end, or ends of the winding.
  • high voltage winding 70 includes a plurality of pancake coils, of which four are shown, referenced 74, 76, 78 and 80, with each having 8 conductor turns. Any desired number of pairs of pancake coils having any desired number of turns may be used, but only four pancake coils, or two pairs, are necessary to illustrate the teachings of the invention.
  • Each of the pancake coils has a plurality of conductor turns formed of first and second conductors wound together to provide first and second radially interleaved coil sections, respectively, each having inner and outer ends, with the innermost turn of a pancake coil being in the first coil section, and the next to the innermost turn being in the second coil section.
  • FIG. 5 is a schematic diagram of high voltage winding 70, with the first and second coil sections being offset slightly to indicate that the innermost turn is in coil section I.
  • the series circuit first enters the outer end of the second coil section of pancake coil 74 via conductor 82, which is connected to line terminal 84, and the circuit spirals inwardly, appearing at every other turn, referenced with the letter T and a number which indicates the location of the turn with respect to the line tenninal 84.
  • the circuit proceeds to the inner end of the second coil section II of the next adjacent pancake coil 76, with a start-start connection 86.
  • This turn of pancake coil 76 is alos referenced T4, as the ends of these turns are at substantially the same potential.
  • the circuit then spirals outwardly, appearing at every other turn in pancake coil 76, and it proceeds back to the first pancake coil 74 of the pair, with a finish-f'mish connection 88 which interconnects the end of the outermost turn of the second coil section of pancake coil 76 with the end of the outermost turn of the first coil section I of pancake coil 74.
  • the series circuit then spirals inwardly through pancake coil 74 again, appearing at every proceeds through the second coil sections of the pancake coils, and then through the first coil sections of the pancake coils, which arrangement reduces the magnitude of transient voltage oscillations produced by a surge potential, as taught by my copending application.
  • the series circuit spirals inwardly in the first pancake coil of the basic pair, and outwardly in the second pancake coil of the basic pair.
  • these two coils are wound in opposite circumferential directions to produce an additive magnetomotive force in the associated magnetic core.
  • the outer end of the first coil section I of pancake coil 76 is connected to the inner end of the first coil section I of pancake coil 78, via finish-start connection 96.
  • the series curcuit spirals outwardly in pancake coil 78, appearing at every other turn until reaching the end of turn T20, where it proceeds via finishfinish connection 98 to the end of the outer turn of the first coil section I of pancake coil 80.
  • the circuit spirals inwardly through pancake coil 80, appearing at every other turn until reaching the end of the innermost turn of the first coil section, and then it proceeds to the end of the innermost turn of the second coil section of pancake coil 78 via start-start connection 100.
  • the series circuit again spirals outwardly through pancake coil 78, until reaching the end of the second coil section, where it proceeds via finish-finish connection 102 to the end of the second coil section in pancake coil 80.
  • the circuit spirals inwardly through pancake coil 80 until reaching the end of the innermost turn of the second coil section at turn T32, and then leaves pancake coil 80 via connection 104.
  • the second basic pair of pancake coils is turned inside out" from its normal connection, with the forward proceeding intercoil connections 98 and 102 being disposed between the outermost turns of like coil sections, instead of between the innermost turns of the coil sections, and the reverse connection is made between the innermost turns of predetermined coil circuit still progresses through like coil sections in each pan cake coil of the basic pair, to preserve the attenuation of transient voltages which such an arrangement provides.
  • the new and improved construction shown in FIG. 4 reduces the voltage stress between the innermost turns to onehalf the voltage across one pancake coil, or N/2, while the voltage stress between the next to the innermost turn is 2.5N, which makes the sum of the stresses between the innermost turns, and the stresses between the next to the innermost turns, equal to 3N.
  • This is a substantial reduction, compared to the prior art arrangements, with'the stress between the innermost turn being only one-half the voltage across one pancake coil, compared to 1N, the lowest stress between the innermost turns available with the prior art arrangements, and the sum of the stresses between the innermost turns, and the next to the innermost turns, is only 3N, compared with 4N in the prior art.
  • the advantage of such a reduction is shown in the vector diagram of FIG.
  • vector 112 which is orthogonal to vector 110, indicates the highlow stress between the high and low voltage windings
  • vector 114 indicates the resultant stress. While the resultant stress 114 is reduced compared with the prior art arrangements, a most important advantage of this arrangement is the fact that the direction of the resultant stress in the new and improved arrangement is more nearly perpendicular to axis 72 of the winding, than in the prior art, which thus stresses the adjacent insulating members more in puncture and less in creep, than does the prior art arrangements, exemplified by the vector diagram shown in FIG. 3.
  • Insulating clearances may be reduced between the second and third pancake coils, and from the pancake coils to the low voltage winding, or the present insulating clearances may be maintained with a higher factor of safety in the design. Further, when the winding must be upgraded to higher BIL ratings, it will not be necessary to in-' crease the insulating clearances at this location by the factor that they would ordinarily have to be increased.
  • the remaining basic pairs of twin interleaved pancake coils may be constructed in the same manner as the first basic pair, comprising pancake coils 74 and 76.
  • the reduction in stress, and the change in the direction of the resultant stress at this location is obtained without a costly change in the overall winding. In fact, it only requires reversing the normal sequence of the third and fourth pancake coils, and making a finish-start connection between the second and third pancake coils, instead of a finish-finish connection.
  • a new and improved high voltage winding for power transformers of the'core-form type which utilize the twin interleaved winding arrangement for increasing the through or series capacitance of the winding, to more uniformly distribute surge potentials across the winding.
  • the disclosed structure reduces the voltage stress adjacent the inner turns of the second and third pancake coils, and changes the direction of the stress at this location, to more efficiently utilize the insulating materials disposed between the high voltage winding in the pancake coil, and between the pancake coils.
  • the disclosed structure reduces the magnitude of oscillatory voltages produced in response to surge potentials, retaining the advantage of the structure disclosed in my copending application.
  • a winding for electrical inductive apparatus comprising: at least first, second, third and fourth pancake coils disposed in spaced, side-by-side relation; each of said pancake coils having a plurality of turns of first and second conductors wound together to provide first and second radially interleaved coil sections, respectively each having inner and outer ends, with the innermost turn being in the first coil section and the next to the innermost turn being in the second coil section; means interconnecting the inner ends of the first coil sections, and the inner ends of the second coil coils; of said first and second pancake coils, means interconnecting the outer ends of the first and second coil sections of said first and second pancake coils respectively;
  • the winding of claim 1 including a line terminal adapted for connection to a source of electrical potential, said line terminal being connected to the outer end of the second coil section of the first pancake coil.

Landscapes

  • Engineering & Computer Science (AREA)
  • Power Engineering (AREA)
  • Coils Of Transformers For General Uses (AREA)
  • Insulating Of Coils (AREA)
US857952A 1969-09-15 1969-09-15 Electrical winding Expired - Lifetime US3569883A (en)

Applications Claiming Priority (1)

Application Number Priority Date Filing Date Title
US85795269A 1969-09-15 1969-09-15

Publications (1)

Publication Number Publication Date
US3569883A true US3569883A (en) 1971-03-09

Family

ID=25327100

Family Applications (1)

Application Number Title Priority Date Filing Date
US857952A Expired - Lifetime US3569883A (en) 1969-09-15 1969-09-15 Electrical winding

Country Status (3)

Country Link
US (1) US3569883A (fr)
JP (1) JPS513369B1 (fr)
FR (1) FR2061430A5 (fr)

Cited By (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
DE2841592A1 (de) * 1978-09-25 1980-03-27 Transformatoren Union Ag Stufenwicklung fuer transformatoren und drosseln fuer sehr grosse stroeme
US5764122A (en) * 1991-09-26 1998-06-09 Siemens Aktiengesellschaft Winding arrangement of a coil
US6271743B1 (en) * 1994-11-07 2001-08-07 Siemens Aktiengesellschaft Winding arrangement of a coil
US6275365B1 (en) * 1998-02-09 2001-08-14 American Superconductor Corporation Resistive fault current limiter
US6472966B1 (en) * 1997-05-15 2002-10-29 Magnet-Motor Gesellschaft Fur Magnetmotorische Technik Mbh Superconducting heavy-current disconnector

Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3029402A (en) * 1958-10-16 1962-04-10 Ferranti Ltd Inductive windings
US3405378A (en) * 1965-06-17 1968-10-08 Alsthom Savoisienne High voltage coil with flat disks
US3477052A (en) * 1967-11-30 1969-11-04 Westinghouse Electric Corp High series capacitance winding for electrical inductive apparatus

Family Cites Families (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
FR1129757A (fr) * 1955-08-10 1957-01-25 Materiel Electrique S W Le Enroulement de transformateur et de bobine d'induction
FR1237442A (fr) * 1958-10-16 1960-07-29 Ferranti Ltd Perfectionnements aux enroulements inductifs
BE583931A (fr) * 1958-10-25 1960-02-15 Klement Gottwald Villamossagi Enroulement permettant de réduire les différences de tension dues aux ondes de choc
GB1040013A (en) * 1962-03-06 1966-08-24 English Electric Co Ltd Improvements in or relating to inductive windings
US3528046A (en) * 1966-11-22 1970-09-08 Gen Electric Interlaced disk winding with improved impulse voltage gradient

Patent Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3029402A (en) * 1958-10-16 1962-04-10 Ferranti Ltd Inductive windings
US3405378A (en) * 1965-06-17 1968-10-08 Alsthom Savoisienne High voltage coil with flat disks
US3477052A (en) * 1967-11-30 1969-11-04 Westinghouse Electric Corp High series capacitance winding for electrical inductive apparatus

Cited By (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
DE2841592A1 (de) * 1978-09-25 1980-03-27 Transformatoren Union Ag Stufenwicklung fuer transformatoren und drosseln fuer sehr grosse stroeme
US5764122A (en) * 1991-09-26 1998-06-09 Siemens Aktiengesellschaft Winding arrangement of a coil
US6271743B1 (en) * 1994-11-07 2001-08-07 Siemens Aktiengesellschaft Winding arrangement of a coil
US6472966B1 (en) * 1997-05-15 2002-10-29 Magnet-Motor Gesellschaft Fur Magnetmotorische Technik Mbh Superconducting heavy-current disconnector
US6275365B1 (en) * 1998-02-09 2001-08-14 American Superconductor Corporation Resistive fault current limiter

Also Published As

Publication number Publication date
FR2061430A5 (fr) 1971-06-18
JPS513369B1 (fr) 1976-02-03

Similar Documents

Publication Publication Date Title
US3528046A (en) Interlaced disk winding with improved impulse voltage gradient
US3392326A (en) Coil winding buffer conductors having impedance means
US3348182A (en) Winding transposition
US3246270A (en) Graded insulation for interleaved windings
US3569883A (en) Electrical winding
US3359518A (en) Interleaved windings effecting a uniformly distributed surge potential
US3899763A (en) Interleaved winding for electrical inductive apparatus
US4270111A (en) Electrical inductive apparatus
US3466584A (en) Winding for a stationary induction electrical apparatus
US3845437A (en) Series-multiple transformer having non-integer series-parallel voltage ratio
US3766504A (en) Interleaved transformer winding having three parallel connected conductors
US3538473A (en) Stranded winding for high current electric apparatus
US3144628A (en) Transformer with winding sections connected in series or parallel
US3644786A (en) Electrical windings
US3380007A (en) Shielded arrangements for electrical transformers
EP3400602A1 (fr) Transformateur à enroulements multicouches
US3611229A (en) Electrical winding with interleaved conductors
US3564470A (en) Electrical winding structures
US3621428A (en) Electrical windings and method of constructing same
US3719910A (en) Electrical inductive apparatus
US3702451A (en) Electrical inductive apparatus
US3221282A (en) Inductive windings having decreased interleaving in pancake sections
US3710292A (en) Electrical windings
US3278879A (en) Interleaved series sections in pancake coils
US3673530A (en) Electrical windings