The transformation ratio is a value that shows how many times the input parameter( voltage, current) is less than or greater than the output. If the figure is higher than one, a decrease is performed; on the contrary, a device that increases it is less than one. Accordingly, there are distinguished transformation ratios for voltage or current. Purely practical division, corresponding to the tasks to be solved. The magnetic field induces in the coils of the output winding EMF, the current is not definitely.
There is a complete lack of understanding of the principles of transformer operation. Why a small number of turns is carried out with a thick wire, other questions arise from beginners. Let's start by looking at the cores. Made of ferromagnetic materials. To inside spread the field. That it is the cause of the generation of a secondary winding EMF.Michael Faraday made the core of an experienced transformer( 1831) out of mild steel, due to the severity of the properties, today it is differentHard alloy with up to 1% carbon. Ferromagnetic properties are not clearly expressed, heat losses are falling. First of all - on Foucault's eddy currents. They are induced by an alternating magnetic field in an iron alloy, some other materials. When a transformer operates, losses increase dramatically with increasing frequency, increasing the resistivity by adding silicon is an effective measure to combat this phenomenon. Loss of magnetization reversal is reduced by the use of hard steel. Grades E42, 43, 320, 330, 340, 350, 360. The first digit indicates the percentage of silicon( 3 is about 4.8%), the second characterizes magnetic losses, specific values are given by GOST( for example, 3836), are not defined.
Until the 60s, the cost of transformers was considered for the totality of materials, the losses worried little. But since the 70s, oil prices have grown in order, naturally raising the cost of other energy sources. Previously, hot-rolled steel was replaced with cold-rolled( GOST 21427.2), having an oriented grain structure. The magnetic permeability in the longitudinal direction has naturally increased. The steel itself is cut into plates according to this fact, while the occurrence of eddy currents is blocked. The process is called blending, the layers are separated from each other by a varnish film.
The technology of casting steel, the introduction of new properties are decisive. They answer on a par with active resistance of copper for the losses that occur, which naturally determine the efficiency of the device. Depends on the parameters of the core, the transformation ratio, the magnetic flux incurs some losses, weakens. This fact is fully ignored in the formula that we see in the figure. Where R1 and R2 are the losses in the active resistance of copper, the fact of core reversal is silenced.
Along the way, we analyze the formula. It can be seen: active losses are included in such a way that the transformation ratio increases. It would seem that if you want to reduce the voltage, only on hand, in fact, energy is consumed by the power source, you have to pay the expense. That is why active losses of copper windings tend to be zero. The field does not spread without attenuation, is completely ignored by the formula. To improve the characteristics of the transformer, you have to choose an electrical alloy.
The other side of the coin: reduce active losses, reducing the number of turns. It is required to increase the magnetic induction of the field, which requires the creation of very special steels. Another way to solve the problem was the use of thick wire, dramatically complicating the technology of winding, at the same time significantly increasing the cost, dimensions of the product. Then, at high frequencies, the effectiveness of the method reduces the skin effect, a large cross section creates space for the occurrence of eddy currents. The use of a transposed wire, physically consisting of a large number of thin strands isolated from each other( sometimes strips), partially removes the problem. Insulation with epoxy resin after curing gives the conductors strength.
Concerning transformer steel, in order to solve the loss problem( the emergence of the opportunity to work with high induction), there are three ways:
- Improvement of the orientation of the domains( production process).
- Sheet thickness reduction( today - up to 0.27 mm, thinner steel is rare).
- Surface treatment of steel.
A separate line is the acoustic losses( transformers are buzzing), if the total damage can be reduced, this aspect remains at the level of the middle of the last century. In a general sense, eddy currents, magnetic hysteresis, now introduce equal shares. For this reason, technologists are fighting to reduce the thickness of the sheets, forming an increase in sensitivity to mechanical stress, deformation.
Transformation Ratio In terms of reducing the thickness of the sheets, the prospect is seen in the use of amorphous steel. The main limitation is imposed by magnetostriction( changing the geometric dimensions of the material by the action of the field).The effect reduces the gain on the secondary winding, similar to hysteresis. However, despite the brittleness, the complexity of annealing in the technological cycle, it is possible to obtain sheets with a thickness of a few hundredths of a mm. Experts call the main obstacle to the use of high cost, not mentioned above features.
The main use segment is within wound magnetic circuits. Here( as opposed to shaking) the core is not composed of strips, it is one whole piece that forms a closely twisted spiral. With regards to other assembly techniques, hope is given by the fact that losses are independent of the direction along the crystal lattice. Since there are no oriented domains, the requirements for surface treatment of steel sheets are eliminated.
In view of the described features of amorphous steel, it becomes possible to assemble transformers with an acceptable transfer ratio of high-frequency signals.
Circulation currents, transformation ratio, short circuit parameters
More often transformers at a substation are connected in parallel for obvious reasons. Consumption is too large for a single product to withstand the load. It would seem that there are no features here; in practice, the technical characteristics of transformers even for one factory batch are different. Standards are selected according to GOST 14209, IEC 905. It is considered acceptable to install together the specified deviations of the transformation ratio:
- For products with a transformation ratio of 3 or less, on a non-main branch - 1%( both sides).
- For products with a transformation ratio above 3, on the main branch - 0.5% in each direction.
At substations where there are products with different transformation ratios, equalizing currents between them occur in the absence of a load. The load situation exacerbates. Currents are distributed inversely with the short-circuit resistances. There are requirements for other parameters. The permissible deviation of the short-circuit voltage is limited to within 19%, giving preference to transformers of the same bank.
In three-phase networks, the requirements for the coefficient apply only to the windings within a separate phase. If the values differ, the current begins to circulate. Even if there is no load. Sometimes a phenomenon called equalization, equalizes the voltage drop of two parallel-connected branches( windings).In the formula for the dependence of the amplitude of this current on the transformation ratio: in the numerator on the right side is the relative difference( see list above), the denominator is formed by twice the relative voltage( short circuit).The left part of the equality contains the ratio of the circulation current to the nominal.
Here we explain: short circuit voltage is taken as a percentage of nominal. The value is established empirically. A certain voltage is applied to the primary winding, the secondary is short-circuited. Achieve compliance with the current working. Adjust the amplitude of the input voltage. The value at which the above conditions are achieved, hereinafter referred to as short circuit voltage. Usually expressed as a percentage of nominal, which is reflected by the formula.
The ratio shows: at Uk% = 5, the difference between the transformation ratios of 1% of the circulating currents will reach 10% of the nominal. It will cause the windings to heat up and aggravate the heat loss situation at the site. If the short circuit voltages differ for two transformers, it is supposed to use instead of doubling the summing operation. In addition, the nominal power is different - bring the numbers to a common denominator. To do this( optional), one digit is divided by its own power, multiplied by the nominal power of another transformer.
Sometimes there are less errors if we use absolute values instead of relative ones. Here, U is the phase voltage from the side of the HH winding;Zk1, Zk2 - complex resistances( short circuit impedance) of products.k1, k2 - the transformation ratios of both products, and the letter of the Greek alphabet delta indicates the difference. Currents in different directions tend to balance the potential difference through a voltage drop. The complexity of the resistance is reminiscent of the inductive component, since the winding is a coil.
When the number of transformers more than two formula becomes more complicated. The image is given, since the physical meaning of each quantity is clear from what was said earlier. The current formula is total, for each parallel winding is less than the number of times equal to the transformation ratio. The dot above the symbol means: the number is complex.
The presence of special voltage regulating devices sensibly improves the situation. In this case, the number of turns changes, and the transformation coefficients are aligned. Under load, the currents are unevenly distributed. In the ideal case, the value is inversely proportional to the input impedance of the product. If the inductances are different, it is possible to use reactors, in any case, it is clear that, with parallel connection, the parameters of both transformers should not diverge too much. It is gratifying that for the load mode an exact calculation of the coefficients is not required. .. because an obvious difference brings the system into emergency mode. Specificity is not important. The main thing - to avoid the final failure of products.