Nuances of designing pulse transformers on a toroidal core

Toroidal magnetic systems are composed of toroidal (ring) magnetic circuits, the number of which can be from one to several dozen. Each magnetic core is manufactured by winding a tape on an annular mandrel (see Fig. 1), i.e. the technology is basically the same as that of twisted rods. For winding, a tape made from transformer steel grades 3421-3425 or from permalloy is used. The magnetic cores made of transformer steel, due to the sintering of the insulating coating of the tape, have a sufficiently high mechanical strength and allow the arrangement of a pulse transformer without special protective parts. The mechanical stresses arising during the fastening of the magnetic cores do not significantly affect the magnetic properties of the magnetic system. When assembling, preference is given to the vertical installation of the magnetic cores, since this provides better oil convection at the end parts of the magnetic cores and, therefore, facilitates heat removal. Horizontal installation is used in cases when, to increase the cross-sectional area, the magnetic system is recruited from a large number of magnetic cores. In this case, the non-uniformity of the cross section is large: b> a. When the magnetic cores are installed vertically, the "long side" of the turns is horizontal, which leads to their sagging. When installed horizontally, on the contrary, the "long side" of the turns is vertical and sagging is excluded. Unlike magnetic cores made of transformer steel, magnetic cores made of permalloy are easily deformed and their magnetic properties change significantly under various mechanical influences.

   For these reasons, permalloy magnetic cores are usually placed in protective shells made of non-magnetic metal or mechanically strong dielectric (see Fig. 2). Metal shells can be made of 
Импульсный трансформатор на торе

copper, aluminum, stainless steel. The advantage of copper and aluminum shells is their high thermal conductivity, which facilitates cooling of the magnetic system. However, the insufficient mechanical strength of shells made of these metals makes it difficult to manufacture and assemble large-sized structures. Therefore, in the latter, it is advisable to use stainless steel shells, which are mechanically stronger. The disadvantage of such shells is a significantly lower thermal conductivity. Shells made of insulating materials - glass epoxy, organic glass, plastics - have low thermal conductivity and therefore are used to protect small magnetic cores, usually less than 250 mm in diameter. The space between the shell and the magnetic core is usually filled with an elastic shock-absorbing compound, for example, CIATIM-201 silicone grease or the like. The relative structural complexity and high cost of permalloy magnetic circuits makes their use expedient only for very short pulses, when it is necessary to reduce eddy current losses by reducing the thickness of the tape of the magnetic material. It was previously noted that permalloy has no advantages over transformer steel either in terms of induction gain or in terms of electrical resistivity. Therefore, taking into account the significant structural complexity of the permalloy magnetic circuit, their use is justified only when the tape thickness should be less than 0.05 mm, i.e. when the magnetic circuit cannot be made of electrical steel. For this reason, the magnetic core of transformers designed to operate at short duration and high pulse repetition rates must be made of permalloy. However, thin strips of permalloy do not always help to avoid large losses, and the thermal mode of operation of the magnetic system turns out to be excessively stressed. In such cases, it is necessary to use water cooling of the magnetic circuit. For this purpose, the shells, preferably copper or aluminum, are made with water jackets. The main technological difficulty in the manufacture of magnetic cores from permalloy is the need to anneal them in special furnaces - in an atmosphere of hydrogen, inert gases, or in a vacuum. Usually, these are small furnaces and are suitable for annealing magnetic circuits of a relatively small diameter, no more than 600 mm. For this reason, it is necessary to use magnetic circuits from a large number of small diameter magnetic circuits, which leads, on the one hand, to a large non-sided cross-section of the magnetic circuit, on the other, to a significant limitation of the power and energy of a pulse transformer designed for short pulses. At the same time, magnetic cores from transformer steel can be manufactured with a diameter of up to several meters, since 3421-3425 steels are supplied in an annealed state and after the magnetic core is made, it is necessary to perform not annealing, but only sintering of the tape. Thus, the diameter of the magnetic circuit made of transformer steel is limited only by the capabilities of the technological equipment for winding the tape and the difficulties in transporting large-size pulse transformers with toroidal magnetic circuits. The most important limiting factor is the rail gauge of 2.77 m.

Possible options for the arrangement of magnetic circuits in toroidal magnetic systems

Layout of magnetic systems with vertical installation вертикальная установка тороидального трансформатораtwisted magnetic circuits are shown in the figure, where 1 - toroidal magnetic circuit; 2 - cheek made of mechanically strong insulating material, for example glass epoxy (cheeks can also be metal, for example cast from cast iron); 3 - foundation slab serving as the structural basis of the transformer; 4 - glass tape bandage. The magnetic cores are fixed in the groove on the inner side of the cheeks. The outer side of the cheeks slopes towards the inner side. This makes it relatively easy to implement tapered windings in a pulse transformer. With cylindrical windings, the outer and inner sides of the cheeks are parallel. In this or similar way, it is possible to assemble magnetic cores from magnetic cores up to 1.5 m in diameter. The layout of magnetic cores with vertical installation of magnetic cores in the shell is shown in the figure, where 1 - magnetic core placed in shells - 2; 3 - a square welded to the shells for attaching the magnetic circuit to the base plate 4. Bandages are also applied to the shells with magnetic circuits in several places. Such or similar arrangements of magnetic systems are advisable in high-voltage imported transformers, since they allow the simplest way to implement a secondary winding with an input in the middle. A significant drawback of both vertical arrangements is the low mechanical strength of the structure, as a result of which the assembled pulse transformer is not transportable with large diameters of magnetic systems. For this reason, pulse transformers of such designs have to be assembled at the site of their permanent installation.
Layout of magnetic systems with horizontal installation of twisted magnetic circuitsгоризонтальная установка тороидального трансформатора shown in the figure, where 1 is a toroidal magnetic circuit, 2 is a shock-absorbing gasket; 3 - clamping flange. The flanges are equipped with projections through which the magnetic system is pulled together by pins. The windings are placed in the sectors between the projections. To increase the rigidity of the magnetic circuit, the flanges in the region of the protrusions can be reinforced with stiffeners. The arrangement of magnetic systems with horizontal installation of magnetic cores in the shell is shown in the figure where 1 - magnetic core; 2 - shock-absorbing pad; 3 - shell; 4 - a rib welded to the shell, by means of which the magnetic system is attached to the tank cover 5. It should be noted that in the case of a horizontal arrangement, the peripheral part of the magnetic system is divided by protrusions or ribs. As a result, the windings of the pulse transformer have to be sectioned, and the number of sections is equal to the number of sectors between the projections or ribs. If windings with a central entry are used, then the number of sections is doubled. This leads to a decrease in the length of each section and, therefore, makes it difficult to obtain a high longitudinal dielectric strength of the pulse transformer windings. It is more expedient to design a magnetic system with three sectors instead of the four shown in the figures, since this increases the length of each section. If the magnetic core is of a large diameter, then the noted circumstances are not of particular importance, but with small diameters, it is difficult to design a transformer for high voltages with such an arrangement. In this respect, the advantage of vertical installation, when the magnetic circuit has only one sector, which makes it possible to increase the length of the section and thereby increase the longitudinal dielectric strength.
A significant disadvantage of the horizontal installation also consists in the practical impossibility of arranging cooling oil channels, which makes it difficult to remove heat from the magnetic circuit. Layout of the magnetic system with vertical installation of magnetic cores in the shell магнитопровод в обечайке ,установленный вертикальноshown in the figure, where 1 - magnetic core placed in the shell - 2; 3 - a square welded to the shells for attaching the magnetic circuit to the base plate 4. Treads are also applied to the shells with magnetic circuits in several places. Such or similar arrangements of magnetic systems are advisable in high-voltage pulse transformers, since they allow the simplest way to implement a secondary winding with a central bushing. A significant drawback of both vertical layouts is the low mechanical strength of the structure, as a result of which, with large core diameters, the assembled transformer is not transportable. For this reason, transformers of such designs have to be assembled at the site of their permanent installation. The layout of the magnetic circuit with horizontal installation of twisted magnetic circuits is shown in the following figure, where 1 - toroidal magnetic circuit; 2 - shock-absorbing pad; 3 - clamping flange. The flanges are equipped with protrusions through which the magnetic circuit is pulled together by pins. The windings are placed in sectors between the projections. To increase the rigidity of the magnetic system, the flanges in the region of the projections can be reinforced with stiffeners.
Layout of the magnetic system with horizontal installation of magnetic cores in the shell горизонтальная установка тороидального трансформатора в обечайкеshown in the figure, where 1 - magnetic circuit; 2 - shock-absorbing pad; 3 - shell; 4 - a rib welded to the shell, by means of which the magnetic circuit is attached to the tank lid 5. It should be noted that in the case of a horizontal arrangement, the peripheral part is divided by projections or ribs. As a result, the windings of the pulse transformer have to be sectioned, and the number of sections is equal to the number of sectors between the projections or ribs. If windings with a central entry are used, then the number of sections is doubled. This leads to a decrease in the length of each section and, therefore, makes it difficult to obtain a high longitudinal dielectric strength of the transformer windings. It is more expedient to design a magnetic circuit with three sectors instead of the four shown in the figures, since this increases the length of each section. If the magnetic cores are large, then these circumstances are not of particular importance, but with small core diameters, it is difficult to design pulse transformers for high voltages with such an arrangement. In this respect, the advantage of vertical installation is obvious, when the magnetic system has only one sector, which makes it possible to increase the length of the section and thereby increase the longitudinal dielectric strength.
A significant drawback of horizontal installation also lies in the practical impossibility of arranging cooling oil channels, which makes it difficult to remove heat from the magnetic system.

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