A toroidal transformer is an electrical voltage or current transducer whose core is bent and closed by a ring. The profile of the section is different from the round, the name is still used in the absence of the best.
Differences between toroidal transformers
The author of toroidal transformers is Michael Faraday. It is possible to find a utopian idea in the domestic literature( especially of communist times): first collected a similar Yablochkov comparing the indicated date - usually 1876 - with early experiments on electromagnetic induction( 1830).The conclusion is sought: England has outstripped Russia by half a century. Those interested in the details will refer to the law of electromagnetic induction. Provides detailed information about the design of the world's first toroidal transformer. The product is distinguished by the shape of the core. In addition to the toroidal form it is customary to distinguish:
- Armor. Differ in ferromagnetic alloy redundancy. To close the field lines( to pass inside the material) the yoke covers the windings from the outside. As a result, the input and output are wound around a common axis. One on top of another or near.
- Stem. The core of the transformer passes inside the winding turns. Spatial input and output spaced out. The yoke absorbs a small part of the lines of magnetic field strength, passing outside the turns. Actually need to connect rods.
A beginner has a hard time, it is useful to explain in more detail. The core is the part of the core that passes inside the coils. Wire is wound on the core. Yoke is the part of the core connecting the rods. Need to transmit the magnetic field lines. The yoke closes the core, forming a seamless structure. Closure is required for free propagation inside the material of the magnetic field.
Theme Magnetic induction shows that the field inside a ferromagnet is greatly enhanced. The effect forms the basis of the functioning of transformers.
The yoke is part of the core core with a minimum composition. The armor covers additional windings outside along the length, as if protecting. From the analogy came the name. Michael Faraday chose a torus rather intuitively. Formally, it is possible to call the core core, although the guide axis of symmetry of the windings is an arc.
Horse horseshoe became the support of the first magnet( 1824).Perhaps the fact gave the direction of the flight of creative thought of the scientist correct azimuth. Use Faraday other material, the experience will end in failure.
Thor wound in a single tape. Such cores are called spiral, in contrast to the armor and core, which appear in the literature for the term lamellar. This is misleading. Once again it should be said: the toroidal core, being wound with separate plates, is called spiral. It is necessary to break up parts when there is no tape. This is due to purely economic reasons.
To summarize: in its original form, the Faraday toroidal transformer had a round core. Today the form is unprofitable, it is impossible to provide mass production with appropriate technology. Although the deformation of the wire at the bend angles unambiguously leads to deterioration of the product characteristics. Mechanical stresses increase the ohmic resistance of the winding.
Toroidal Transformer Cores
The toroidal transformer is named for the core shape. Michael Faraday made a bagel using a solid piece of mild steel of round section. The design is inappropriate at the present stage for several reasons. The focus is on minimizing losses. The solid core is unprofitable, eddy currents are induced, strongly heating the material. It turns out the melting induction furnace, easily turning steel into a liquid.
To avoid wasting energy and heating the transformer, the core is cut into strips. Each is isolated from the next, for example, varnish. In the case of toroidal cores, they are wound in a single helix or strip. Steel is usually on one side has an insulating coating thickness of a micrometer unit.
These steels are used to design current transformers, which are often toroidal in design. Those interested can familiarize themselves with GOST 21427.2 and 21427.1.For cores( as the name of the documents suggests), anisotropic cold-rolled sheet steel is more commonly used today. The name is laid down: the magnetic properties of the material are not identical along different axes of coordinates. The field flux vector must coincide with the direction of rolling( in our case it moves in a circle).Previously used another metal. The cores of high-frequency transformers can be made of steel 1521. Within the framework of the site, the features of the materials used are discussed( see transformation ratio).Steel is marked differently, the following information is included in the designation:
- First place is given to the figure characterizing the structure.3.
- is used for anisotropic steels. The second digit indicates the percentage of silicon:
- is less than 0.8%.
- 0.8 - 1.8%.
- 1.8 - 2.8%.
- 2.8 - 3.8%.
- 3.8 - 4.8%.
- The third digit indicates the main characteristic. There may be specific losses, the magnitude of the magnetic induction at a fixed field strength.
- Type of steel. With increasing numbers, specific losses are lower. Depends on the technology of metal production.
When transporting, the steel structure is inevitably damaged. We will eliminate the defects by special annealing at the assembly site. It is done without fail for measuring current transformers where accuracy of readings is important. The core is wound in a single piece or cutting strips on a cylindrical or oval-shaped mandrel. If necessary, the tape can be cut from a single sheet( economically often impractical).Each should be at least six and a half of the winding radius. To achieve the desired length is allowed to connect the individual strip spot welding. Charging( breaking up in thin layers) eliminates the phenomenon of eddy currents. The loss of magnetization reversal varies little, making up a small fraction of the previously mentioned parasitic effect.
Loses the relative position of the end and beginning of the tape. To the spiral is not unwound, the last turn is welded to the previous spot welding. Winding is conducted with tension, collected from several strips of tape is usually not possible to fit tightly, the weld is overlapped. Sometimes the torus is cut into two parts( split core), in practice it is required relatively rarely. Halves when assembling are tightened with a bandage. In the manufacturing process, the finished toroidal core is cut with a tool, the ends are ground. The coils of the helix are sealed with a binder so that it does not unwind.
Closed-Core Transformer. Winding of
toroidal transformers. As a rule, additional insulation of the toroidal core from the windings is performed, even if varnished wire is used. Electrotechnical cardboard( GOST 2824) with a thickness of up to 0.8 mm( other variants are possible) is widely used. Common Cases:
- Cardboard is wound with the seizure of the previous turn on the toroidal core. The method is characterized as half-full( half width).The end is glued or fastened with a keeper tape.
- At the ends, the core is protected by cardboard washers with notches with a depth of 10 - 20 mm, with a pitch of 20-35 mm, overlying the thickness of the torus. The outer, inner edge is covered with stripes. Technologically, the washers are collected last, the cut teeth are bent. On top of the spiral wound tape.
- Cuts can be made on strips, then they are taken with a margin to allow for more height of the torus, rings are strictly in width, overlapping over the bends.
- Thin stripes, textolyte rings are attached to the toroidal core with glass fiber ribbons with a full lap.
- Sometimes rings are made of electrical plywood, getinax, thick( up to 8 mm) textolite with a margin of outer diameter of 1-2 mm. The outer and inner edge is protected by cardboard stripes with a bend along the edges. Between the first turns of the winding, the air gap remains the core. The gap under the cardboard is needed in case the edges under the wire are rubbed. Then the current carrying part never touches the toroidal core. On top of the reel tape. Sometimes the outer edge of the rings is smoothed, so that the winding of the corners goes smoothly.
- There is a type of insulation similar to the previous one, from the inside along the rings on the outer edges there are grooves to the core where the strips lie. Elements are made of PCB.On top of the reel tape.
Winding is usually performed concentric( one above the other), or alternating( as in the first experiment of Michael Faraday in 1831), sometimes called disc. In the latter case, a sufficiently large number of them can be wound up through one, alternately: either high voltage or low. Pure electrotechnical copper( 99.95%) with a specific resistance of 17.24 - 17.54 mW m is used. Due to the high cost of the metal, refined aluminum is taken to produce toroidal transformers of low and medium power. For other cases, there are limitations in conductivity and plasticity.
In high-power transformers, copper wire has a rectangular cross-section. It is done to save space. The vein must be thick, passing a considerable current in order not to melt, the circular cross section will lead to an excessive increase in size. The gain of the uniform distribution of the field over the material would be reduced to zero. Thick rectangular wire is quite convenient to lay, which can not be said to thin. As for the rest( according to design features), the winding is performed exactly the same way as in the case of a conventional transformer. Coils are made cylindrical, screw, single-layer, multi-layer.