Broadband eddy current probe

Description

[0001] The subject matter disclosed herein relates to eddy current probes, in particular, to an eddy current probe useable over a wide range of frequencies.

[0002] Eddy current probes are a form of nondestructive testing devices that can be used to inspect test objects, such as tubes or pipes, to detect and analyze defects in the objects. Nondestructive testing allows an inspection technician to maneuver an eddy current probe through a test object in order to scan for defects.

[0003] In an eddy current probe, a magnetic field is used to induce an electrical current in the test object. The magnetic field is typically generated by one or two electrically conductive coils, or windings, in the probe. During operation of the probe an electrical current is sent through the coil, or coils, which generates a magnetic field that passes through the test object and induces an electrical current in the test object called an eddy current. The eddy current travels through a tube in a circular pathway along a circumference of the tube in a plane that is perpendicular to an axis of the tube.

[0004] If the induced eddy current passes through a flaw or defect in the test object, for example, erosion and pitting of the inside diameter of a tube, or if the inside diameter of the tube fluctuates as the probe travels through the tube, the induced eddy current is perturbed and the coils will detect this as a varying impedance. In response to detecting the varying impedance, the coils will generate electrical signals that represent physical characteristics of the defect. By analyzing these electrical signals, various characteristics of the defect (e.g., location, size) can be determined. Each of the coils will detect a different impedance variance because the coils are located at a different distance from the defect. The impedance difference detected by the coils can be converted into a two-dimensional impedance data display.

[0005] Typical eddy current probes have a limited testing frequency range because the two coils in the probe are balanced in a wheatstone bridge configuration against fixed resistors. The fixed resistors are typically located in a remote test instrument.

[0006] US-A-2 104 646 concerns a testing instrument for testing a hollow axle of a car wheel. The instrument includes a testing rod having a core with lateral flanges that form a pair of grooves at the outer free end of the testing rod. Four inductive windings are provided, each with the same number of turns about the core. One pair of windings is wound side-by-side in one of the grooves, and the other pair of windings is wound side-by-side in the other groove.

[0007] The present invention provides an eddy current probe for detecting a defect in a test object as defined in appended claim 1, and a method of making the eddy current probe as defined in appended claim 7.

[0008] The discussion above is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter.

[0009] An eddy current probe is constructed with four coils in a wheatstone bridge configuration. An advantage that may be realized in the practice of some disclosed embodiments of the eddy current probe is that a wider range of frequencies can be employed during an inspection operation, thereby revealing higher resolution of defects in the objects undergoing inspection.

[0010] In one embodiment, an eddy current probe for detecting a defect in a test object is disclosed. The eddy current probe comprises a first coil and a second coil both electrically connected to a first voltage terminal, the first voltage terminal for receiving a drive voltage, a third coil and a fourth coil both electrically connected to a second voltage terminal, the second voltage terminal for receiving a ground, and wherein the first coil is electrically connected to the third coil at a first bridge terminal, the second coil is electrically connected to the fourth coil at a second bridge terminal, and wherein a voltage difference between the first and second bridge terminals indicates the defect in the test object when the four coils of the probe are inside the test object.

[0011] In another embodiment, a method of making an eddy current probe for inspecting a test object is disclosed. The method comprises winding a first, second, third, and fourth coil around a bobbin, electrically connecting the first coil and the second coil to a first voltage terminal, electrically connecting the third coil and the fourth coil to a second voltage terminal, electrically connecting the first coil and the third coil to a first bridge terminal, and electrically connecting the second coil and the fourth coil to a second bridge terminal.

[0012] In yet another embodiment, a system for analyzing a defect in a test object is disclosed. The system comprises an eddy current probe, the probe comprising a first coil and a second coil both electrically connected to a first voltage terminal, the first voltage terminal for receiving a drive voltage, a third coil and a fourth coil both electrically connected to a second voltage terminal, the second voltage terminal for connecting to a ground source, and wherein the first coil is electrically connected to the third coil at a first bridge terminal, wherein the second coil is electrically connected to the fourth coil at a second bridge terminal, and wherein a voltage difference between the first and second bridge terminals indicates a presence of the defect in the test object, and a test instrument comprising a drive voltage source electrically connected to the first terminal for providing the drive voltage, a ground source electrically connected to the second terminal for providing the ground, and a receiver amplifier electrically connected to the first and second bridge terminals for detecting a magnitude of the voltage difference.

[0013] This brief description of the invention is intended only to provide a brief overview of subject matter disclosed herein according to one or more illustrative embodiments, and does not serve as a guide to interpreting the claims or to define or limit the scope of the invention, which is defined only by the appended claims. This brief description is provided to introduce an illustrative selection of concepts in a simplified form that are further described below in the detailed description. This brief description is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter. The claimed subject matter is not limited to implementations that solve any or all disadvantages noted in the background.

[0014] So that the manner in which the features of the invention can be understood, a detailed description of the invention may be had by reference to certain embodiments, some of which are illustrated in the accompanying drawings. It is to be noted, however, that the drawings illustrate only certain embodiments of this invention and are therefore not to be considered limiting of its scope, for the scope of the invention encompasses other equally effective embodiments. The drawings are not necessarily to scale, emphasis generally being placed upon illustrating the features of certain embodiments of the invention. In the drawings, like numerals are used to indicate like parts throughout the various views. Thus, for further understanding of the invention, reference can be made to the following detailed description, read in connection with the drawings in which:

FIG. 1 is a schematic diagram of an exemplary eddy current testing system;

FIG. 2 is a diagram of a bobbin used for winding conductive wire to form four coils thereon; and

FIG. 3 is an example probe package for the probe configuration disclosed herein.

[0015] FIG. 1 illustrates a test system 100 comprising an eddy current probe circuit 112, in a wheatstone bridge configuration, connected to a test instrument 113. The eddy current probe circuit 112 comprises a first coil 101 and a second coil 102 electrically connected to a first voltage terminal 105 which, in turn, is electrically connected to a drive voltage source 109, for example a drive amplifier, in the test instrument 113. The eddy current probe circuit 112 further comprises a third coil 103 and a fourth coil 104 electrically connected to a second voltage terminal 106 which, in turn, is electrically connected to a ground source 111 in the test instrument 113. The drive voltage source 109 provides a drive voltage to the first, second, third and fourth coils 101, 102, 103, 104 through the first voltage terminal 105. The ground source 111 provides a ground to the first, second, third and fourth coils 101, 102, 103, 104 through the second voltage terminal 106. The ground source 111 is typically located in the test instrument 113 and is connected to second voltage terminal 106 via a coaxial cable. The ground source 111 need not be located in the test instrument 113, and the second voltage terminal 106 can be connected to a more convenient source of ground.

[0016] The first coil 101 and the third coil 103 are electrically connected together at a first bridge terminal 107, and the second coil 102 and the fourth coil 104 are electrically connected together at a second bridge terminal 108. Both the first and the second bridge terminals 107, 108 are electrically connected to a receiver amplifier 110 in the test instrument 113.

[0017] FIG. 2 is a diagram of a bobbin 200 comprising a first winding section 201 and a second winding section 202. As an example method of forming the four coils 101-104 of the eddy current probe circuit 112 of FIG. 1, conductive wire is wound around first winding section 201 to form third coil 103. Additional conductive wire is wound around first winding section 201 over the winding of the third coil 103 therein to form first coil 101. To form the other two coils, conductive wire is then wound around second winding section 202 to form fourth coil 104. Additional conductive wire is wound around second winding section 202 over the winding of the fourth coil 104 therein to form second coil 102.

[0018] The first coil 101 and the third coil 103 in first winding section 201 are electrically connected together at first bridge terminal 107 which, in turn, is electrically connected to receiver amplifier 110 in the test instrument 113, as shown in FIG. 1, when the test system 100 is in operation. This equivalent to forming a center tap of a stacked coil in the first winding section 201 to separate the stacked coil into the two coils 101, 103, and electrically connecting the center tap, which is first bridge terminal 107, to receiver amplifier 110 in the test instrument 113.

[0019] The second coil 102 and the fourth coil 104 in second winding section 202 are electrically connected together at second bridge terminal 108 which, in turn, is electrically connected to receiver amplifier 110 in the test instrument 113, as shown in FIG. 1, when the test system 100 is in operation. This is equivalent to forming a center tap of a stacked coil in the second winding section 202 to separate it into two coils 102, 104, and electrically connecting the center tap, which is second bridge terminal 108, to receiver amplifier 110 in the test instrument 113.

[0020] Typical inductance values of the coils 101-104 are approximately 80 µH. For optimal performance of the probe, first coil 101 should have an inductance substantially equal to that of third coil 103, and second coil 102 should have an inductance substantially equal to that of fourth coil 104. The better balanced the coils 101-104 are with respect to their inductance values, the cleaner the defect detection signal from the probe will be. With respect to the magnetic field, when it's perturbed by a flaw during inspection, it will be detected differently by the top coils (first coil 101 and second coil 102) and the bottom coils (third coil 103 and fourth coil 104). The perturbation as detected by the first coil 101 is subtracted out from the perturbation as detected by the third coil 103 which provides a sharp resolution of the flaw. Because all the coils 101-104 of the probe circuit are located in the same probe housing, or probe package 300 (FIG. 3), any ambient temperature changes, or changes in the conditions of the testing environment during inspection, are less likely to disturb the balance of the four coils 101-104 because they are all closely packaged together. This is in contrast with other types of eddy current probe designs wherein some of the wheatstone bridge circuit elements are remotely located in the test instrument 113.

[0021] FIG. 3 is an eddy current probe package 300 that includes a probe head 301, a first centering foot 302 and a second centering foot 304. Located between the first centering foot 302 and the second centering foot 304 is the bobbin housing portion 303. Within the bobbin housing portion 303 there is disposed the eddy current probe circuit 112 comprising the first, second, third, and fourth coils 101, 102, 103, 104, first and second voltage terminals 105, 106, and the first and second bridge terminals 107, 108. Connecting portion 305 attaches to a shaft or cable, or multiple cables, that connects to the test instrument 113, which shaft, cable, or cables, includes electrical lines for electrically connecting first voltage terminal 105 to drive voltage source 109, second voltage terminal 106 to a ground source 111, and first and second bridge terminals 107, 108 to receiver amplifier 110, as previously described above. First centering foot 302 and second centering foot 304 should substantially match the inside diameter of a test object (e.g., tube or pipe) being inspected to maintain a uniform distance between the probe coils 101-104 and test object. A typical test object can include a carbon steel tube with aluminum fins.

[0022] In operation, probe package 300 is connected to test instrument 113 via a cable or cables, as described above. The drive voltage is applied to first voltage terminal 105 by the drive voltage source 109 in test instrument 113, and the ground is applied to the second voltage terminal 106 by the ground source 111 in test instrument 113. As the probe package 300 is moved through an inside diameter of a test object under inspection, the receiver amplifier 110 detects a magnitude of any voltage difference between the first and second bridge terminals 107, 108 caused by defects in or on the test object.

[0023] In view of the foregoing, embodiments described herein provide an eddy current probe having a wide frequency range. It can operate at 400 Hz in carbon steel tubes with aluminum fins (i.e. tubes made from magnetic materials) and can be used with non-magnetic tubes such as brass, at 20 KHz test frequency, and stainless steel, at 300 KHz test frequency, as examples. A technical effect is to double the frequency range performance typical of common probes. Higher testing frequency provides cleaner signals and better data quality. It also provides an inspector a wider selection of lower frequencies to perform a back-up confirmation test of a test object undergoing inspection.

[0024] This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims.

Claims

1. An eddy current probe (112) for detecting a defect in a test object, the eddy current probe comprising:

a bobbin (200);

a first coil (101) and a second coil (102) wound around the bobbin (200), the first coil (101) and the second coil (102) both electrically connected to a first voltage terminal (105) for receiving a drive voltage; and

a third coil (103) and a fourth coil (104) wound around the bobbin (200), the third coil (103) and the fourth coil (104) both electrically connected to a second voltage terminal (106) for receiving a ground,

wherein the first coil (101) is wound over the third coil (103), and

wherein the first coil (101) is electrically connected to the third coil (103) at a first bridge terminal (107), the second coil is electrically connected to the fourth coil (104) at a second bridge terminal (108), and wherein a voltage difference between the first and second bridge terminals (107, 108) indicates the defect in the test object when the four coils of the probe are inside the test object.

2. The eddy current probe of claim 1, further comprising at least one centering foot (302, 3 or 4) for centering the probe within an inside diameter of the test object as it travels through the test object.

3. The eddy current probe of claim 1 or claim 2, wherein an inductance of the first coil (101) and the third coil (103) are substantially equal.

4. The eddy current probe of claim 1, 2 or 3, wherein an inductance of the second coil (102) and the fourth coil (104) are substantially equal.

5. The eddy current probe of any preceding claim, wherein the second coil (102) is wound over the fourth coil (104).

6. A system for analyzing a defect in a test object, the system comprising:

the eddy current probe (112) of any preceding claim, and

a test instrument (113) comprising

a drive voltage source (109) electrically connected to the first terminal for providing the drive voltage,

a ground source (111) electrically connected to the second terminal for providing the ground, and

a receiver amplifier (110) electrically connected to the first and second bridge terminals (107, 108) for detecting a magnitude of the voltage difference.

7. A method of making an eddy current probe (112) for inspecting a test object, the method comprising:

winding a first (101), second (102), third (103), and fourth (104) coils around a bobbin (200), wherein the steps of winding the first coil (101) and winding the third coil (103) comprise winding the first coil (101) over the third coil (103);

electrically connecting the first coil (101) and the second coil (102) to a first voltage terminal (105);

electrically connecting the third coil (103) and the fourth coil (104) to a second voltage terminal (106);

electrically connecting the first coil (101) and the third coil (103) to a first bridge terminal (107); and

electrically connecting the second coil (102) and the fourth coil (104) to a second bridge terminal (108).

8. The method of claim 7, further comprising electrically connecting a drive voltage to the first voltage terminal (105) and electrically connecting a ground to the second voltage terminal (106).

9. The method of claim 7 or claim 8, further comprising electrically connecting an amplifier to the first and second bridge terminals (107, 108).

10. The method of claim 7, 8 or 9, further comprising disposing the first coil (101), the second coil (102), the third coil (103), the fourth coil (104), and the bobbin (200) within a probe package (300), the probe package comprising a size for traveling through an inside diameter of the test object.

11. The method of any one of claims 7 to 10, further comprising providing at least one centering foot (302, 304) on a probe package (300) for centering the probe within the inside diameter of the test object as it moves through the test object.

12. The method of any one of claims 7 to 11, wherein the steps of winding the second coil (102) and winding the fourth coil (104) comprise winding the second coil (102) over the fourth coil (104).

Ansprüche

1. Wirbelstromsonde (112) zum Detektieren eines Defekts in einem Prüfobjekt, wobei die Wirbelstromsonde umfasst:

einen Spulenkörper (200);

eine erste Spule (101) und eine zweite Spule (102), die um den Spulenkörper (200) gewickelt sind, wobei die erste Spule (101) und die zweite Spule (102) beide elektrisch mit einer ersten Spannungsklemme (105) verbunden sind, um eine Antriebsspannung zu empfangen; und

eine dritte Spule (103) und eine vierte Spule (104), die um den Spulenkörper (200) gewickelt sind, wobei die dritte Spule (103) und die vierte Spule (104) beide elektrisch mit einer zweiten Spannungsklemme (106) verbunden sind, um eine Masse zu empfangen,

wobei die erste Spule (101) über die dritte Spule (103) gewickelt ist, und

wobei die erste Spule (101) an einer ersten Brückenklemme (107) elektrisch mit der dritten Spule (103) verbunden ist, wobei die zweite Spule an einer zweiten Brückenklemme (108) elektrisch mit der vierten Spule (104) verbunden ist, und wobei eine Spannungsdifferenz zwischen der ersten und der zweiten Brückenklemme (107, 108) den Defekt des Prüfobjekts anzeigt, wenn sich die vier Spulen der Sonde innerhalb des Prüfobjekts befinden.

2. Wirbelstromsonde nach Anspruch 1, ferner umfassend mindestens einen Zentrierfuß (302, 3 oder 4) zur Zentrierung der Sonde innerhalb eines Innendurchmessers des Prüfobjekts, während diese sich durch das Prüfobjekt bewegt.

3. Wirbelstromsonde nach Anspruch 1 oder Anspruch 2, wobei eine Induktivität der ersten Spule (101) und der dritten Spule (103) jeweils im Wesentlichen gleich ist.

4. Wirbelstromsonde nach Anspruch 1, 2 oder 3, wobei eine Induktivität der zweiten Spule (102) und der vierten Spule (104) jeweils im Wesentlichen gleich ist.

5. Wirbelstromsonde nach einem der vorhergehenden Ansprüche, wobei die zweite Spule (102) über die vierte Spule (104) gewickelt ist.

6. System zum Analysieren eines Defekts in einem Prüfobjekt, wobei das System umfasst:

die Wirbelstromsonde (112) nach einem der vorhergehenden Ansprüche, und

ein Prüfinstrument (113), umfassend

eine Antriebsspannungsquelle (109), die elektrisch mit der ersten Klemme verbunden ist, um die Antriebsspannung bereitzustellen,

eine Massequelle (111), die elektrisch mit der zweiten Klemme verbunden ist, um die Masse bereitzustellen, und

einen Empfangsverstärker (110), der elektrisch mit der ersten und der zweiten Brückenklemme (107, 108) verbunden ist, um eine Größe der Spannungsdifferenz zu erfassen.

7. Verfahren zum Herstellen einer Wirbelstromsonde (112) zum Untersuchen eines Prüfobjekts, wobei das Verfahren umfasst:

Wickeln einer ersten (101), einer zweiten (102), einer dritten (103) und einer vierten (104) Spule um einen Spulenkörper (200), wobei die Schritte des Wickelns der ersten Spule (101) und des Wickelns der dritten Spule (103) das Wickeln der ersten Spule (101) über die dritte Spule (103) umfassen;

elektrisches Verbinden der ersten Spule (101) und der zweiten Spule (102) mit einer ersten Spannungsklemme (105);

elektrisches Verbinden der dritten Spule (103) und der vierten Spule (104) mit einer zweiten Spannungsklemme (106);

elektrisches Verbinden der ersten Spule (101) und der dritten Spule (103) mit einer ersten Brückenklemme (107); und

elektrisches Verbinden der zweiten Spule (102) und der vierten Spule (104) mit einer zweiten Brückenklemme (108).

8. Verfahren nach Anspruch 7, ferner umfassend das elektrische Verbinden einer Antriebsspannung mit der ersten Spannungsklemme (105) und das elektrische Verbinden einer Masse mit der zweiten Spannungsklemme (106).

9. Verfahren nach Anspruch 7 oder Anspruch 8, ferner umfassend das elektrische Verbinden eines Verstärkers mit der ersten und der zweiten Brückenklemme (107, 108).

10. Verfahren nach Anspruch 7, 8 oder 9, ferner umfassend das Anordnen der ersten Spule
(101), der zweiten Spule (102), der dritten Spule (103), der vierten Spule (104) und des Spulenkörpers (200) innerhalb eines Sondenpakets (300), wobei das Sondenpaket eine Größe umfasst, mit der es sich durch einen Innendurchmesser des Prüfobjekts bewegen kann.

11. Verfahren nach einem der Ansprüche 7 bis 10, ferner umfassend das Bereitstellen mindestens eines Zentrierfußes (302, 304) an einem Sondenpaket (300) zum Zentrieren der Sonde innerhalb des Innendurchmessers des Prüfobjekts, während diese sich durch das Prüfobjekt bewegt.

12. Verfahren nach einem der Ansprüche 7 bis 11, wobei die Schritte des Wickelns der zweiten Spule (102) und des Wickelns der vierten Spule (104) das Wickeln der zweiten Spule (102) über die vierte Spule (104) umfassen.

Revendications

1. Sonde à courant de Foucault (112) pour détecter un défaut dans un objet de test, la sonde à courant de Foucault comprenant :

une bobine (200) ;

un premier enroulement (101) et un deuxième enroulement (102) enroulés autour de la bobine (200), le premier enroulement (101) et le deuxième enroulement (102) étant tous les deux connectés électriquement à une première borne de tension (105) pour recevoir une tension de commande ; et

un troisième enroulement (103) et un quatrième enroulement (104) enroulés autour de la bobine (200), le troisième enroulement (103) et le quatrième enroulement (104) étant tous les deux connectés électriquement à une seconde borne de tension (106) pour recevoir une masse,

dans laquelle le premier enroulement (101) est enroulé sur le troisième enroulement (103) et

dans laquelle le premier enroulement (101) est connecté électriquement au troisième enroulement (103) au niveau d'une première borne de pont (107), le deuxième enroulement est connecté électriquement au quatrième enroulement (104) au niveau d'une deuxième borne de pont (108) et dans laquelle une différence de tension entre les première et deuxième bornes de pont (107, 108) indique le défaut dans l'objet de test lorsque les quatre enroulements de la sonde se situent à l'intérieur de l'objet de test.

2. Sonde à courant de Foucault selon la revendication 1, comprenant en outre au moins un pied de centrage (302, 3 ou 4) pour centrer la sonde à l'intérieur d'un diamètre intérieur de l'objet de test lorsqu'il se déplace à travers l'objet de test.

3. Sonde à courant de Foucault selon la revendication 1 ou la revendication 2, dans laquelle une inductance du premier enroulement (101) et du troisième enroulement (103) sont sensiblement égales.

4. Sonde à courant de Foucault selon la revendication 1, 2 ou 3, dans laquelle une inductance du deuxième enroulement (102) et du quatrième enroulement (104) sont sensiblement égales.

5. Sonde à courant de Foucault selon une quelconque revendication précédente, dans laquelle le deuxième enroulement (102) est enroulé sur le quatrième enroulement (104).

6. Système d'analyse d'un défaut dans un objet de test, le système comprenant :

la sonde à courant de Foucault (112) selon une quelconque revendication précédente et

un instrument de test (113) comprenant

une source de tension de commande (109) connectée électriquement à la première borne pour fournir la tension de commande,

une source de masse (111) connectée électriquement à la deuxième borne pour fournir la masse et

un amplificateur de récepteur (110) connecté électriquement aux première et deuxième bornes de pont (107, 108) pour détecter une amplitude de la différence de tension.

7. Procédé de fabrication d'une sonde à courant de Foucault (112) pour inspecter un objet de test, le procédé comprenant :

l'enroulement de premier (101), deuxième (102), troisième (103) et quatrième (104) enroulements autour d'une bobine (200), dans lequel les étapes d'enroulement du premier enroulement (101) et du troisième enroulement (103) comprennent l'enroulement du premier enroulement (101) sur le troisième enroulement (103) ;

la connexion électrique du premier enroulement (101) et du deuxième enroulement (102) à une première borne de tension (105) ;

la connexion électrique du troisième enroulement (103) et du quatrième enroulement (104) à une seconde borne de tension (106) ;

la connexion électrique du premier enroulement (101) et du troisième enroulement (103) à une première borne de pont (107) ; et

la connexion électrique du deuxième enroulement (102) et du quatrième enroulement (104) à une seconde borne de pont (108).

8. Procédé selon la revendication 7, comprenant en outre la connexion électrique d'une tension de commande à la première borne de tension (105) et la connexion électrique d'une masse à la deuxième borne de tension (106).

9. Procédé selon la revendication 7 ou la revendication 8, comprenant en outre la connexion électrique d'un amplificateur aux première et deuxième bornes de pont (107, 108).

10. Procédé selon la revendication 7, 8 ou 9, comprenant en outre la disposition du premier enroulement (101), du deuxième enroulement (102), du troisième enroulement (103), du quatrième enroulement (104) et de la bobine (200) à l'intérieur d'un boitier de sonde (300), le boitier de sonde présentant une taille pour passer à travers un diamètre intérieur de l'objet de test.

11. Procédé selon l'une quelconque des revendications 7 à 10, comprenant en outre la fourniture d'au moins un pied de centrage (302, 304) sur un boîtier de sonde (300) pour centrer la sonde à l'intérieur du diamètre intérieur de l'objet de test lorsqu'il se déplace à travers l'objet de test.

12. Procédé selon l'une quelconque des revendications 7 à 11, dans lequel les étapes d'enroulement du deuxième enroulement (102) et d'enroulement du quatrième enroulement (104) comprennent l'enroulement du deuxième enroulement (102) sur le quatrième enroulement (104).

Drawing