INGMAN, Jani (Vinokastie 3B, Salo, FI-24280, FI)
SUORSA, Ilkka (Everstinkuja 4 C 45, Espoo, FI-02600, FI)
INGMAN, Jani (Vinokastie 3B, Salo, FI-24280, FI)
| Claims 1 . An arrangement for reducing capacitive current between a winding (1 10, 210, 310, 410) of an electrical device and another part (1 1 1 , 21 1 , 330, 430) of the elec- trical device, the arrangement comprising an auxiliary winding system in a space between the winding and the other part, characterized in that the auxiliary winding system comprises at least three auxiliary windings (1 12, 1 13, 1 14, 212, 213, 214, 327, 328, 329, 427, 428, 429), a first end of each auxiliary winding being coupled to a fixed electrical potential via an electrical element (1 18, 1 19, 120), and that in a direction (126, 226, 326, 426) of a magnetic axis of the winding, at least one of the auxiliary windings is shorter than a length of the auxiliary winding system. 2. An arrangement according to claim 1 , wherein the other part of the electrical device is another winding (1 1 1 , 21 1 ) that is arranged to encircle the winding. 3. An arrangement according to claim 1 , wherein the other part of the electrical device is a limb (330, 430) of a core element, the limb being encircled by the winding. 4. An arrangement according to claim 1 , wherein at least two of the auxiliary windings (212, 213, 214, 427, 428, 429) are at least partly overlapping with each other in a direction perpendicular to the magnetic axis of the winding. 5. An arrangement according to claim 4, wherein the longest auxiliary winding (212, 427) in the direction of the magnetic axis of the winding is at least partly overlapping with the other auxiliary windings (213, 214, 428, 429) in the direction perpendicular to the magnetic axis of the winding. 6. An arrangement according to claim 5, wherein said other auxiliary windings (213, 214) are located successively in the direction of the magnetic axis of the winding. 7. An arrangement according to claim 1 , wherein the auxiliary winding system comprises three auxiliary windings (212, 213, 214), the first auxiliary winding (213) and the second auxiliary winding (214) being located successively in the direction of the magnetic axis of the winding, and in the direction of the magnetic axis of the winding the third auxiliary winding (212) being overlapping with the first and second auxiliary windings. 8. An arrangement according to claim 1 , wherein the at least three auxiliary windings (212, 213, 214, 427, 428, 429) have mutually different lengths in the direction of the magnetic axis of the winding. 9. An arrangement according to claim 1 , wherein the at least three auxiliary windings (1 12, 1 13, 1 14) are located successively in the direction of the magnetic axis of the winding. 10. An arrangement according to claim 9, wherein there are gaps between adjacent auxiliary windings in the direction of the magnetic axis of the winding. 1 1 . An arrangement according to claim 9, wherein adjacent auxiliary windings are partially overlapping with each other in a direction perpendicular to the magnetic axis of the winding. 12. An arrangement according to claim 1 , wherein the electrical element comprises at least one of the following: an electrical wire, a resistor, a capacitor, and an inductor. 13. An arrangement according to claim 1 , wherein a second end of at least one of the auxiliary windings is connected to the first end of that auxiliary winding via one of the following: a resistor, a capacitor and a snubber circuit. 14. An arrangement according to claim 1 , wherein a second end of at least one of the auxiliary windings is connected to a second end of another auxiliary winding via one of the following: an electrical wire, a resistor, a capacitor and a snubber circuit. 15. An electrical device, comprising: - a primary side (131 , 231 ), - a secondary side (132, 232), - a transformer (103, 203) between the primary side and the secondary side, and - an auxiliary winding system in a space between a winding (1 10, 210) of the transformer and another part (21 1 ) of the electrical device, characterized in that the auxiliary winding system comprises at least three auxil- iary windings (1 12, 1 13, 1 14, 212, 213, 214, 327, 328, 329, 427, 428, 429), a first end of each auxiliary winding being coupled to a fixed electrical potential via an electrical element (1 18, 1 19, 120), and that in a direction (126, 226, 326, 426) of a magnetic axis of the winding, at least one of the auxiliary windings is shorter than a length of the auxiliary winding system. 16. An electrical device according to claim 15, wherein the winding is a primary winding of the transformer and the other part of the electrical device is a secondary winding of the transformer, the primary and the secondary windings being arranged in one of the following manners: the secondary winding is arranged to encircle the primary winding or the primary winding is arranged to encircle the sec- ondary winding. 17. An electrical device according to claim 15, wherein the other part of the electrical device is a limb of a core element of the transformer, the limb being encircled by the winding. 18. An electrical device according to claim 15, wherein the fixed potential is a ground potential on the primary side. 19. An electrical device according to claim 15, wherein the electrical device is a switched mode power supply. 20. A method for reducing capacitive current between a winding of an electrical device and another part of the electrical device, characterized in that the method comprises: - using (501 ) at least three auxiliary windings in a space between the winding and the other part, at least one of the auxiliary windings in a direction of a magnetic axis of the winding being shorter than a length of an auxiliary winding system consisting of the at least three auxiliary windings, and - using (502) an electrical element for coupling a first end of each auxiliary winding to a fixed electrical potential. |
Field of the invention
The invention relates to an arrangement and to a method for reducing capacitive current between a winding and another part of an electrical device. The winding and the other part can be, for example, primary and secondary windings of a transformer, or a winding and a core element of an inductive choke element. Furthermore, the invention relates to an electrical device that can be for example a switched mode power supply. Background
Many electrical devices such as for example switched mode power supplies (SMPS) involve a transformer having primary and secondary windings wound on a common core element. A pulsed electrical current fed to the primary winding creates a cyclically changing magnetic field in and around the core element, from which energy is discharged to the secondary winding and eventually to a load. Due to the changing voltages in the windings of the transformer there are capacitive currents generated between the windings and also between the windings and other parts of the electrical device, e.g. the core element. These capacitive currents can be a significant source of electromagnetic interference and other ad- verse effects. The capacitive currents between the primary and secondary windings can be reduced with an auxiliary winding, i.e. a shield winding, that is placed into a space between the primary and secondary windings, and the capacitive current between the windings and the core element can be reduced with an auxiliary winding that is placed into a space between the windings and the core element. One end of the auxiliary winding is typically connected to a fixed electrical potential either directly or via an impedance-creating component such as a resistor, a capacitor, an inductor, or a combination of them. The auxiliary winding can be made of wire or foil. An advantage of auxiliary windings made of wire compared with those made of foil is that auxiliary windings made of wire can be more easily manufactured with an automatic winding machine. A problem associated with auxiliary windings, especially with those made of wire, is that voltage of an auxiliary winding can itself generate significant capacitive currents in an electrical device. An auxiliary winding can be made of branched wire such that voltages induced into different parts of the winding cancel each other at least partly. However, providing a branched auxiliary winding requires manual labour or special manufacturing equipment. These can be a significant cost factor when electrical devices are made in large quantities.
Prior art publications that consider suppressing of capacitive currents between a winding and another part, e.g. another winding or a core element, of an electrical device include for example US7355871 B2 and US2553324. However, none of these cited references provides a solution to the above-described problem that voltage of an auxiliary winding operating as a capacitive shield can itself generate significant capacitive currents in an electrical device. In light of the above-presented, there is a need for technical solutions that are capable of mitigating the above-mentioned problem in a sufficiently cost effective manner.
Summary
In accordance with a first aspect of the invention there is provided a new arrange- ment for reducing capacitive current between a winding of an electrical device and another part of the electrical device. The arrangement comprises an auxiliary winding system in a space between the winding and the other part. The auxiliary winding system comprises at least three auxiliary windings. A first end of each auxiliary winding is coupled to a fixed electrical potential via an electrical element and, in a direction of a magnetic axis of the winding, at least one of the auxiliary windings is shorter than a length of the auxiliary winding system.
The auxiliary winding(s) that is(are) shorter than the length of the auxiliary winding system constitutes a capacitive shield that reduces the capacitive coupling between the winding and the other part of the electrical device without having a high induced voltage that would in turn couple in a capacitive manner.
In an arrangement according to an exemplifying embodiment of the invention, each auxiliary winding is shorter than the length of the auxiliary winding system consisting of the auxiliary windings, and the auxiliary windings are located successively in the direction of the magnetic axis of the winding. In this embodiment of the invention, a sufficient spatial coverage of shielding is reached by locating the auxiliary windings in the successive manner. Depending on mutual positioning of the winding and the other part the capacitive current between which is to be reduced, the auxiliary windings are arranged in one of the following manners: the winding encircles the auxiliary windings or each auxiliary winding encircles the winding. Voltage of each auxiliary winding can be sufficiently low due to the fact that the auxiliary windings are short in the direction of the magnetic axis and hence a number of turns can be low. Therefore, the problem that voltages of the auxiliary windings can themselves generate capacitive currents in an electrical device is mitigated.
In an arrangement according to another exemplifying embodiment of the invention, one of the auxiliary windings is substantially as long as the auxiliary winding system consisting of the auxiliary windings and there are at least two shorter auxiliary windings. In this embodiment of the invention a sufficient spatial coverage of shielding is reached with the long auxiliary winding, and the at least two shorter auxiliary windings are being used for reducing capacitive currents coupled to and from the long auxiliary winding. In the direction of the magnetic axis of the winding, the long auxiliary winding is overlapping with the at least two shorter auxiliary windings in the direction perpendicular to the magnetic axis of the winding. De- pending on mutual positioning of the winding and the other part the capacitive current between which is to be reduced, the auxiliary windings are arranged in one of the following manners: the winding encircles the auxiliary windings or each auxiliary winding encircles the winding. Voltages of the at least two shorter auxiliary windings can be sufficiently low due to the fact that the shorter auxiliary windings are short in the direction of the magnetic axis and hence a number of turns can be low. Therefore, the problem that voltages of the auxiliary windings can themselves generate capacitive currents in an electrical device is mitigated.
In many cases the auxiliary windings of the above-mentioned exemplifying embodiments of the invention can be manufactured with an automatic winding ma- chine. This is advantageous especially where electrical devices are made in large quantities. In certain cases one or more of the auxiliary windings can be used in the associated electrical circuit as a source of auxiliary voltage(s).
In accordance with a second aspect of the invention there is provided a new electrical device that comprises: - a primary side,
- a secondary side,
- a transformer between the primary side and the secondary side, and - an auxiliary winding system in a space between a winding of the transformer and another part of the electrical device, wherein, the auxiliary winding system comprises at least three auxiliary windings, a first end of each auxiliary winding being coupled to a fixed electrical potential via an electrical element, and in a direction of a magnetic axis of the winding, at least one of the auxiliary windings is shorter than a length of the auxiliary winding system.
The electrical device can be, for example, a switched mode power supply.
The windings can generate high frequency resonances inside the transformer as well as transfer them. A small number of turns in the auxiliary windings reduces these resonances in itself. Placing a filtering element over the winding can increase the resonance reduction effect and also reduce the other high frequency signals passing through the transformer.
The generated high frequency resonances depend on the position and winding di- rection of the winding in the direction of the magnetic axis. When many windings are placed in different parts of the magnetic axis they can have the same resonance in different phase. The voltages can be combined directly or through impedance to damp resonances significantly. Combining the voltages directly instead of through the ground is useful because the losses generated by the filtering element are then smaller and also signal integrity is less reduced.
In accordance with a third aspect of the invention there is provided a new method for reducing capacitive current between a winding of an electrical device and another part of the electrical device. The method comprises:
- using at least three auxiliary windings in a space between the winding and the other part, at least one of the auxiliary windings in a direction of a magnetic axis of the winding being shorter than a length of an auxiliary winding system consisting of the at least three auxiliary windings, and
- using an electrical element for coupling a first end of each auxiliary winding to a fixed electrical potential. A number of exemplifying embodiments of the invention are described in the dependent claims. The embodiments of the invention presented in this document are not to be interpreted to pose limitations to the applicability of the appended claims. The verb "to comprise" is used in this document as an open limitation that does not exclude the existence of also unrecited features. The features recited in depending claims are mutually freely combinable unless otherwise explicitly stated.
Various embodiments of the invention both as to constructions and to methods of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. Brief description of the figures
Figure 1 a shows a block diagram of an electrical device having an arrangement according to an embodiment of the invention for reducing capacitive current between primary and secondary windings of a transformer of the electrical device, figure 1 b shows a section view of the transformer of the electrical device shown in figure 1 a, figure 1 c shows a section view of another transformer that can be used in the electrical device shown in figure 1 a, figure 2a shows a block diagram of an electrical device having an arrangement according to an embodiment of the invention for reducing capacitive current be- tween primary and secondary windings of a transformer of the electrical device, figure 2b shows a section view of the transformer of the electrical device shown in figure 2a, figure 3 shows a section view of a transformer of an electrical device having an arrangement according to an embodiment of the invention for reducing capacitive current flowing via a winding of the transformer, figures 4a and 4b show section views of transformers of electrical devices having an arrangement according to an embodiment of the invention for reducing capacitive current flowing via a winding of a transformer, and figure 5 is a schematic diagram of a method according to an embodiment of the invention for reducing capacitive current between a winding of an electrical device and another part of the electrical device. Description of embodiments of the invention
Figure 1 a shows a block diagram of an electrical device according to an embodiment of the invention. Main signal propagation directions are illustrated with arrows. The electrical device comprises a primary side 131 , a secondary side 132, and a transformer 103 between the primary and secondary sides. A pair of input pins 101 is connected to an input part 102 that typically comprises ordinary input part functionalities like protection (e.g. a fuse), filtering, and full-wave rectification. A main current path on the primary side leads from the input part 102 through a primary winding 1 10 of the transformer 103 to a switch part 104, the task of which is to periodically close and open the connection from the main current path to ground on the primary side. The pulses that control the operation of the switch part 104 come from a pulse generation part 105. A secondary winding 1 1 1 of the transformer 103 is connected to a rectifier part 106 on the secondary side, from which there is a connection to a waveform smoothing and filtering output part 107 and further to output pins 108. A control part 109 is configured to monitor output characteristics such as output voltage and current, and send a feedback signal back to the pulse generation part 105 on the primary side.
The electrical device comprises an arrangement according to an embodiment of the invention for reducing capacitive current between the primary and secondary windings 1 10 and 1 1 1 of the transformer 103. The arrangement for reducing capacitive current comprises auxiliary windings 1 12, 1 13, and 1 14 in a space between the primary and secondary windings of the transformer. A first end of the auxiliary winding 1 12 is coupled to a fixed electrical potential via an electrical element 1 18, a first end of the auxiliary winding 1 13 is coupled to the fixed electrical potential via an electrical element 1 19, and a first end of the auxiliary winding 1 14 is coupled to the fixed electrical potential via an electrical element 120. In the embodiment of the invention illustrated in figure 1 a, the fixed electrical potential is a ground potential on the primary side. Each of the electrical elements 1 18, 1 19, and 120 can be, for example, a mere electrical wire wherein the first end of each auxil- iary winding is directly connected to the fixed electrical potential. It is also possible that each of the electrical elements 1 18, 1 19, and 120 is an impedance-creating component such as, for example, a resistor, a capacitor, an inductor, or a combination of them. A second end of each auxiliary winding can be open-ended or the second end can be connected to the first end of that auxiliary winding via an ad- mittance-creating component such as a resistor, a capacitor, or a snubber circuit. A snubber circuit can be e.g. a series connection of a resistor and a capacitor. Blocks 1 15, 1 16, and 1 17 represent the possible admittance-creating components between the first and second ends of the auxiliary windings. The second end of an auxiliary winding may alternatively be connected to the second end of another auxiliary winding via an electrical wire, or a component such as a resistor, a ca- pacitor, or a snubber circuit. For example, the second end of the auxiliary winding 1 12 may be connected to the second end of the auxiliary winding 1 13, the second end of the auxiliary winding 1 13 may be connected to the second end of the auxiliary winding 1 14, and the second end of the auxiliary winding 1 14 may be connected to the second end of the auxiliary winding 1 12, via snubber circuits. Figure 1 b shows a section view of the transformer 103. A core element 121 of the transformer is of a so-called EE type, consisting of two E-shaped halves 121 a and 121 b. The middle limb of each half is slightly shorter than the side limbs of that half, leaving a magnetic gap in the middle limb of the assembled core element. The primary winding 1 10 encircles the middle limb in a loop-shaped form. An ar- row 126 denotes the direction of a magnetic axis of the primary winding. The secondary winding 1 1 1 is wound so that the secondary winding is magnetically concurrent with the primary winding 1 10 and encircles the primary winding. The term "magnetically concurrent" means here that the magnetic axes of the primary and secondary windings are substantially parallel with each other. In some cases it is as well possible that the primary winding encircles the secondary winding. The auxiliary windings 1 12, 1 13, and 1 14 are wound so that they encircle the primary winding in a tubular space between the primary and secondary windings 1 10 and 1 1 1 . Each of the auxiliary windings is shorter in the direction of the magnetic axis 126 than a length of an auxiliary winding system consisting of the auxiliary wind- ings 1 12, 1 13, and 1 14. Therefore, a capacitive shield between the primary and secondary windings consists of three successively placed auxiliary windings each of them being shorter than the primary and secondary windings. A sufficient spatial coverage of the capacitive shielding in the direction of the magnetic axis 126 is reached by locating the auxiliary windings 1 12, 1 13, and 1 14 in the successive manner. Voltage of each auxiliary winding can be sufficiently low due to the fact that the auxiliary windings are short in the direction of the magnetic axis and hence a number of turns in each auxiliary winding can be low. Therefore, the problem that voltages of the auxiliary windings can themselves generate capacitive currents in the transformer is mitigated. In the transformer illustrated in figure 1 b, ad- jacent auxiliary windings, e.g. 1 12 and 1 13, can be side-by-side or there can be a small gap between them in the direction of the magnetic axis 126. It is also possible that adjacent auxiliary windings overlap each other as will be illustrated with the aid of figure 1 c. The mutual arrangement of the auxiliary windings (side-by- side, gaps, or overlapping) can be determined e.g. on the basis of aspects related to manufacture, e.g. tolerances of a winding machine. The transformer 103 further comprises an insulating layer 122 between the primary winding 1 10 and the mid- die limb of the core element 121 , an insulating layer 123 between the primary winding and the auxiliary windings 1 12, 1 13, and 1 14, an insulating layer 124 between the secondary winding 1 1 1 and the auxiliary windings, and an insulating layer 125 around the secondary winding.
Figure 1 c shows a section view of another transformer that can be used in the electrical device shown in figure 1 a. The core element 121 of the transformer is of a so-called El type, consisting of an E-shaped part 121 a and an l-shaped yoke 121 b. The middle limb of the E-shaped part is slightly shorter than the side limbs of that part, leaving a magnetic gap in the middle limb of the assembled core element. The primary winding 1 10 encircles the middle limb in a loop-shaped form. A direction of a magnetic axis of the primary winding is denoted with an arrow 126. The secondary winding 1 1 1 is wound so that the secondary winding encircles the primary winding 1 10. The auxiliary windings 1 12, 1 13, and 1 14 are wound so that they encircle the primary winding in a tubular space between the primary and secondary windings 1 10 and 1 1 1 . Each of the auxiliary windings is shorter in the di- rection of the magnetic axis 126 than a length of an auxiliary winding system consisting of the auxiliary windings 1 12, 1 13, and 1 14. Adjacent auxiliary windings, e.g. 1 12 and 1 13, are partially overlapping with each other in the direction perpendicular to the magnetic axis.
In the exemplifying cases illustrated in figures 1 a, 1 b and 1 c there are three suc- cessively placed auxiliary windings. It should be noted, however, that the number of the auxiliary windings can be more than three depending on physical dimensions and/or electrical characteristics. The electrical device illustrated in figure 1 a is a switched mode power supply (SMPS) but the arrangements according to above-presented embodiments of the invention (figures 1 b and 1 c) can be used for reducing capacitive currents also in other electrical devices having a transformer.
Figure 2a shows a block diagram of an electrical device according to an embodiment of the invention. Main signal propagation directions are illustrated with arrows. The electrical device comprises a primary side 231 , a secondary side 232, and a transformer 203 between the primary side and the secondary side. The electrical device comprises an arrangement according to an embodiment of the in- vention for reducing capacitive current between primary and secondary windings 210 and 21 1 of the transformer 203. The arrangement comprises auxiliary windings 212, 213, and 214 in a space between the primary and secondary windings. A first end of each auxiliary winding is coupled to a fixed electrical potential directly or via an impedance-creating component such as for example a resistor, a capacitor, an inductor, or a combination of them. In the embodiment of the invention illustrated in figure 2a, the fixed electrical potential is a ground potential on the primary side. A second end of each auxiliary winding can be open-ended or connected to the first end of that auxiliary winding via an admittance-creating component such as a resistor, a capacitor, or a snubber circuit. A second end of an auxiliary winding can alternatively be connected to a second end of another auxiliary winding via an electrical wire, a resistor, a capacitor, or a snubber circuit. One or more of the auxiliary windings can also be used as a source of auxiliary voltage. In the electrical device shown in figure 2a, the auxiliary winding 212 is used as a source of aux- iliary voltage U aux for a pulse generation part 205, the task of which is to periodically close and open the connection from the primary winding to the ground on the primary side. The auxiliary windings 213 and 214 are located successively in the direction of the magnetic axis of the primary winding 210, and are overlapping with the auxiliary winding 212 in a direction perpendicular to the magnetic axis of the primary winding. The second end of the auxiliary winding 213 is connected to the second end of the auxiliary winding 214.
Figure 2b shows a section view of the transformer 203 of the electrical device shown in figure 2a. A core element of the transformer is of a so-called EE type, consisting of two E-shaped halves 221 a and 221 b. The primary winding 210 encir- cles the middle limb in a loop-shaped form. A direction of a magnetic axis of the primary winding is denoted with an arrow 226. The secondary winding 21 1 is wound so that the secondary winding encircles the primary winding 210. The auxiliary winding 212 is wound so that it encircles the primary winding in a tubular space between the primary and secondary windings, and the auxiliary windings 213 and 214 are wound so that they encircle the auxiliary winding 212 in the said tubular space. The auxiliary winding 212 is longer than the auxiliary windings 213 and 214 in the direction of the magnetic axis of the primary winding. The auxiliary windings 213 and 214 have the same length and are located successively in the direction of the magnetic axis of the primary winding. The shorter auxiliary wind- ings 213 and 214 are overlapped with the longer auxiliary winding 212 in a direction perpendicular to the magnetic axis. A sufficient spatial coverage of capacitive shielding in the direction of the magnetic axis 226 is reached with the longer auxil- iary winding 212, and the shorter auxiliary windings 213 and 214 are being used for reducing capacitive currents coupled to and from the longer auxiliary winding. Voltages of the shorter auxiliary windings 213 and 214 can be sufficiently low due to the fact that the shorter auxiliary windings are short in the direction of the mag- netic axis 226 and hence a number of turns can be low. Therefore, the problem that voltages of the auxiliary windings can themselves generate capacitive currents is mitigated.
The auxiliary winding 212 is wound evenly, filling the whole winding window. This winding has a benefit of small leakage inductance between other windings in the transformer. The auxiliary windings 213 and 214 are wound so that one auxiliary winding fills mostly one half of the winding window and another auxiliary winding fills mostly the second half of the window. In the direction of the magnetic axis of the winding the magnetic gap in the middle limb is located between the first and second auxiliary windings. In figures 2a and 2b, the auxiliary windings 213 and 214 are short-circuited to each other, but they could also be connected through an electrical circuit, such as a capacitor-resistor circuit. Another benefit of the arrangement illustrated in figures 2a and 2b is that the transformer generated oscillations, which in turn can generate emissions in the charger, can be damped.
In the exemplifying case illustrated in figures 2a and 2b there are three auxiliary windings, two of which having the same length. It should be noted, however, that there can be more than three auxiliary windings depending on physical dimensions and/or electrical characteristics. It should also be noted, that auxiliary windings can have same or mutually different lengths. For example, in an arrangement according to an embodiment of the invention, the at least three auxiliary windings have mutually different lengths in the direction of the magnetic axis of the winding, and the shorter auxiliary windings are overlapped with the longest auxiliary winding in a direction perpendicular to the magnetic axis of the winding.
Figure 3 shows a section view of a transformer of an electrical device according an embodiment of the invention. The transformer comprises a core element 321 , a primary winding 310, and a secondary winding 31 1 . The primary winding 310 encircles a middle limb 330 of the core element in a loop-shaped form. A direction of a magnetic axis of the primary winding is denoted with an arrow 326. The secondary winding 31 1 is wound so that the secondary winding encircles the primary winding 310. The transformer comprises an arrangement according to an em- bodiment of the invention for reducing capacitive current between the primary winding 310 and the middle limb 330 of the core element. The arrangement com- prises auxiliary windings 327, 328, and 329 that are wound so that they circulate the middle limb 330 in a tubular space between the primary winding 310 and the middle limb. Each of the auxiliary windings is shorter, in the direction of the magnetic axis 326, than a length of an auxiliary winding system consisting of the auxil- iary windings 327, 328, and 329. Therefore, a capacitive shield between the primary winding and the middle limb consists of three successively placed auxiliary windings each of them being shorter than the primary winding. A sufficient spatial coverage of the capacitive shielding in the direction of the magnetic axis 326 is reached by locating the auxiliary windings 327, 328, and 329 in the successive manner. Voltage of each auxiliary winding can be sufficiently low due to the fact that the auxiliary windings are short in the direction of the magnetic axis and hence a number of turns in each auxiliary winding can be low. Therefore, the problem that voltages of the auxiliary windings can themselves generate capacitive currents in the transformer is mitigated. In the transformer illustrated in figure 3, adja- cent auxiliary windings, e.g. 327 and 328, can be side-by-side or there can be a small gap between them in the direction of the magnetic axis 326. It is also possible that adjacent auxiliary windings overlap each other. The mutual arrangement of the auxiliary windings (side-by-side, gaps, or overlapping) can be determined e.g. on the basis of aspect related to manufacture. Figure 4a shows a section view of a transformer of an electrical device according an embodiment of the invention. The transformer comprises a core element 421 , a primary winding 410, and a secondary winding 41 1 . The primary winding 410 encircles a middle limb 430 of the core element in a loop-shaped form. A direction of the magnetic axis of the primary winding is denoted with an arrow 426. The sec- ondary winding 41 1 is wound so that the secondary winding encircles the primary winding 410. The transformer comprises an arrangement according to an embodiment of the invention for reducing capacitive current between the primary winding 410 and the middle limb 430 of the core element. The arrangement comprises auxiliary windings 427, 428, and 429. The auxiliary winding 429 is wound so that it encircles the middle limb 430 in a tubular space between the primary winding and the middle limb, the auxiliary winding 428 is wound so that it encircles the auxiliary winding 429 in the said tubular space, and the auxiliary winding 427 is wound so that it encircles the auxiliary windings 428 and 429 in the said tubular space. The auxiliary windings 427, 428, and 429 have mutually different lengths in the direction of the magnetic axis 426. The auxiliary windings are overlapping each other in a direction perpendicular to the magnetic axis 426. A sufficient spatial coverage of capacitive shielding in the direction of the magnetic axis 426 is reached with the longest auxiliary winding 427. Voltage of the shortest auxiliary winding 429 can be sufficiently low due to the fact that the shortest auxiliary winding is short in the direction of the magnetic axis 426 and hence a number of turns can be low. The auxiliary windings 428 and 429 are being used for reducing ca- pacitive currents coupled to and from the longest auxiliary winding 427. Similarly, the shortest auxiliary winding 429 is being used for reducing capacitive currents coupled to and from the auxiliary winding 428. Therefore, the problem that voltages of the auxiliary windings can themselves generate capacitive currents is mitigated. Figure 4b shows a section view of a transformer in which there is a different arrangement of auxiliary windings 427a, 428a, and 429a than in the transformer illustrated in figure 4a.
The transformers shown in figures 3, 4a and 4b further comprise windings also in a tubular space between the primary and secondary windings. These windings can be used for providing an arrangement according to an embodiment of the invention for reducing capacitive current between the primary and secondary windings.
In the above-described examples, an arrangement according to an embodiment of the invention is used for reducing capacitive currents flowing between windings or via a winding of a transformer. It should be noted that arrangements according to embodiments of the invention can be used as well in conjunction with other inductive components. For example, an arrangement according to an embodiment of the invention can be used for reducing capacitive current between a winding of an inductive choke element and a core element of that inductive choke element in a similar manner as for reducing capacitive current between a winding of trans- former and a core element of that transformer.
Figure 5 is a schematic diagram of a method according to an embodiment of the invention. In the text below, the numbers in parentheses refer to examples shown in figures 1 a-1 c, 2a, 2b, 3, 4a, 4b, and 5. The method is aimed for reducing capacitive current between a winding (1 10, 210, 310, 410) of an electrical device and another part (1 1 1 , 21 1 , 330, 430) of the electrical device, and comprises:
- using (501 ) at least three auxiliary windings (1 12, 1 13, 1 14, 212, 213, 214, 327, 328, 329, 427, 428, 429) in a space between the winding and the other part, at least one of the auxiliary windings in a direction of a magnetic axis of the winding being shorter than a length of an auxiliary winding system consisting of the at least three auxiliary windings, and
- using (502) an electrical element (1 18, 1 19, 120) for coupling a first end of each auxiliary winding to a fixed electrical potential. In a method according to an embodiment of the invention, the other part of the electrical device is another winding (1 1 1 , 21 1 ) that is arranged to encircle the winding.
In a method according to an embodiment of the invention, the other part of the electrical device is a limb (330, 430) of a core element, the limb being encircled by the winding.
In a method according to an embodiment of the invention, at least two of the auxiliary windings (212, 213, 214, 427, 428, 429) are at least partly overlapping with each other in a direction perpendicular to the magnetic axis of the winding.
In a method according to an embodiment of the invention, the longest auxiliary winding (212, 427) in the direction of the magnetic axis of the winding is at least partly overlapping with the other auxiliary windings (213, 214, 428, 429) in the direction perpendicular to the magnetic axis of the winding.
In a method according to an embodiment of the invention, said other auxiliary windings (213, 214) are located successively in the direction of the magnetic axis of the winding.
In a method according to an embodiment of the invention, the at least three auxiliary windings (212, 213, 214, 427, 428, 429) have mutually different lengths in the direction of the magnetic axis of the winding.
In a method according to an embodiment of the invention, the at least three auxil- iary windings (1 12, 1 13, 1 14) are located successively in the direction of the magnetic axis of the winding. Adjacent auxiliary windings may be side-by-side, or there may be gaps between adjacent auxiliary windings in the direction of the magnetic axis of the winding, or adjacent auxiliary windings may partially overlap with each other in a direction perpendicular to the magnetic axis of the winding. In a method according to an embodiment of the invention, the electrical element for coupling the first end of a certain auxiliary winding to the fixed electrical poten- tial comprises at least one of the following: a mere electrical wire, a resistor, a capacitor, and an inductor.
In a method according to an embodiment of the invention, a second end of at least one of the auxiliary windings is connected to the first end of that auxiliary winding via one of the following: a resistor, a capacitor and a snubber circuit.
While there have been shown and described and pointed out fundamental novel features of the invention as applied to embodiments thereof, it will be understood that various omissions and substitutions and changes in the form and details of the devices and methods described may be made by those skilled in the art without departing from the scope of the inventive idea defined in the independent claims. For example, it is expressly intended that all combinations of those elements and/or method steps which perform substantially the same function in substantially the same way to achieve the same results are within the scope of the invention. Moreover, it should be recognized that structures and/or elements and/or method steps shown and/or described in connection with any disclosed form or embodiment of the invention may be incorporated in any other disclosed or described or suggested form or embodiment as a general matter of design choice. The specific examples provided in the description given above should not be construed as limiting. Therefore, the invention is not limited merely to the embodiments described above, many variants being possible without departing from the scope of the inventive idea defined in the independent claims.
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