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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Power transformers for a frequency of 50 Hz. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Power Supplies Power transformers are part of a large number of circuits for stabilizing and non-stabilizing power supplies for the secondary power supply of household and amateur radio-electronic equipment operating from the AC mains. A transformer is a static electromagnetic device. With the help of a transformer, alternating current electrical energy with some parameters is converted into electrical energy with other parameters. Thus, with the help of a transformer, it is possible to lower and increase voltage and current, and also electrical isolation of the output power supply channels from the network and from each other occurs. The work of the transformer is based on the interaction of the electromagnetic field of the primary winding of the transformer on the secondary windings. The primary (mains) winding is connected to the AC network U1 with a current frequency of 50 Hz or from 400 to 5000 Hz, and consumers of electrical energy (load) are connected to the secondary windings. Figure 1, a shows a simplified diagram of the transformer, and Fig. 1, b - a diagram of its inclusion. The windings of the transformer are placed on a common magnetic circuit made (for better magnetic coupling) of a ferromagnetic material.
The magnetic flux Fo closes along the magnetic circuit and induces EMF in the primary and secondary windings, respectively, E1 and E2. Taking into account the voltage drop across the active resistance r1 of the primary and r2 secondary windings, E1=U1 - r1I1, and E2=U2+r2I2. But part of the magnetic flux closes and dissipates in the air, the so-called Fras, which acts only on the turns of the primary winding. They try to reduce this Fras, thus increasing the efficiency of the transformer. This is the operating mode of the transformer for the rated load. There are also idle and short circuit modes. Thus, the main element of the transformer is the magnetic circuit (core). For the core of transformers operating at a frequency of 50 Hz, special electrical hot-rolled steel grades 1511, 3412 are mainly used. Steel of these grades is produced in the form of sheets with a thickness of 0,35 and 0,5 mm. Cold-rolled steel grades 3411 and 3412 are also used. It is produced in the form of sheets with a thickness of 0,35; 0,5 mm and in the form of tapes with a thickness of 0,28; 0,3; 0,35; 0,5 mm. Cold-rolled steels have a higher magnetic induction than hot-rolled steels, so cold-rolled steel transformers lei are smaller in size and weight with the same power. For transformers operating at a frequency of 50 Hz, electrical steel with a thickness of 0,15-0,5 mm is used, at a frequency of 400 to 5000 Hz - steel with a thickness of 0,05-0,08 mm. Depending on the requirements for the transformer (power, cost, specific characteristic), a lamellar or tape magnetic circuit is used. The main types and sizes of transformer plates are shown in Fig. 2, where: a - E-shaped; b - W-shaped plates of various types: W - with h> 2,5 ... 3l1; Shu - with a moderate base and h>3l1; Sha - with h>l1; Shb - with h l1; e - U-shaped plates: Mon - with h>l1 and Pu - with h>1l1.
Thus, depending on the design of the magnetic core, transformers are divided into lamellar armored (Ш-shaped) and lamellar (П-shaped). They are shown in Fig.3.
Magnetic circuits for power transformers are assembled only with an overlap (overlapping). Tape cores are also used for transformers. Such transformers have a much smaller stray field, i.e. create less interference on the surrounding circuit elements and details of the device being created. This allows them to be located near the functional units of highly sensitive radio equipment. The core of a W-shaped transformer is designated by the name of the type and numbers that determine the width of the middle rod l (Fig. 3, a) or side (Fig. 3, b) and the thickness B of the magnetic circuit. Structural parameters of the magnetic core Minimum bar cross-sectional area Sc=B(l-∆l), where B is the thickness of the set; l - set width; ∆l - limit deviation. Minimum set window area Sok \u1d lXNUMX (h-∆h), where l1 is the width of the dialing window; h-window height; ∆h-limit deviation. For magnetic cores ШI, ШШ, ШП (Fig. 4, a-g)
The average length of the magnetic line of the force field of the core Iср=h-1[h+2l1+1,18(H-h) + 0,4I/H-I For the magnetic circuit SHU (Fig. 4, e) lav=2(h+l1)+1,57l. For the magnetic circuit PN, PU (Fig. 4, e) lcp=2(h+l1)+1,57(Hh); lо=2l+2В+2,5l1+8δк, where lо is the average length of the conductor of the electric current of the core; δk is the total value of the gap and the thickness of the transformer frame (within 0,55-1,5 mm). To facilitate manufacturing, tape magnetic circuits are made split. The junction is well polished and pulled together well during assembly so that there is no loss of magnetic flux, and so that the transformer does not buzz. Continuous tape magnetic cores have more (by 20-30%) high magnetic induction, i.e. have magnetic losses. But winding such transformers is much more difficult. The winding of continuous transformers is done on special machines or at home using a shuttle. Tape cores of transformers are divided into rod (Fig. 5, a), armored (Fig. 5, b) and ring (Fig. 5, c), where a is the thickness of the winding; b - tape width; c - window width; h - window height; R - inner radius (from 5 to 2 mm depending on the thickness of the tape). Rod structures are divided into submarines - U-shaped tape; PLM - U-shaped tape with a reduced ratio of the window width to the thickness of the winding (c / a <1); PLR - U-shaped tape with geometry dimensions with the lowest cost of the transformer.
Armor structures are divided into ShL - W-shaped tape; ShLM - Ш-shaped tape with a reduced ratio of the width of the window to the thickness of the winding; ShLO - W-shaped tape with an increased ratio of the width of the tape to the thickness of the winding (b / a> 3); ShLR - W-shaped tape with the geometry of the lowest cost of the transformer. We choose the core for the transformer in order to obtain the lowest cost, volume and mass: type PL - for low-voltage transformers with a power of more than 500 V A; type PLM - for low-voltage transformers with a power of more than 100 V A and when the smallest stray field is required; type ShLM - for a power of 100 V A and with a limited voltage drop across the windings. Ideal, of course, is a transformer with a tape ring core. It has very little leakage flux, low magnetic resistance and little sensitivity to external magnetic fields. The transformer has three modes of operation: no-load, rated load and short circuit. In the X.X. Ix flows through the primary winding w1 (Fig. 1) and creates the main magnetic flux Fx in the core. The useful power given by the transformer is equal to zero. Active power is consumed from the network, which is determined only by losses (depending on the core material) in the core of the transformer itself. Ix also has a reactive component, which leads to a deterioration in the power factor cosϕ of the supply network. This mode is not dangerous for the transformer. The short circuit mode (short circuit or low load in the secondary circuit) is dangerous and can cause damage (heating and even ignition) of the transformer. In the rated load mode, the voltage on the secondary winding is a complex value and depends on the value and nature of the load resistance. The windings of armored and rod-type transformers are usually made on frames, but frameless (sleeve) is also used. Windings of ring cores are made on ring frames or on a magnetic circuit wrapped with some kind of insulation. Frames are made of electric cardboard, plastic, just cardboard. It is desirable to impregnate frames with special varnishes or moisture-proof compounds. The windings are placed one above the other or one next to the other. Low power transformers are usually made on lamellar or tape cores of an armor structure. The windings in this case are placed on the middle rod. In the manufacture of transformers of medium and high power, it is better to use core magnetic cores. The windings are placed on the frames of two side rods. The network winding (primary) is usually wound first on the frame. Next, the secondary windings are wound. It is desirable to place an electrostatic shield between the primary and secondary windings. It is made either with an insulated wire in one layer, or with one open turn of foil. One end of such an electrostatic shield is connected to the chassis or to the common wire of the device, which makes it possible to reduce interference and interference penetrating through the interturn and interwinding capacitances from the network and vice versa. This is very relevant at the present time, since in our reality there are usually a lot of different radio and electrical devices that interfere with the mains. Especially a lot of interference is given by switching power supplies of modern consumer radio equipment. When winding transformers on the "ring", the windings must be evenly spaced around the circumference of the core. Windings with a midpoint are best wound with two wires at once. Next, connect the beginning of one winding to the end of the other to get a midpoint. This results in good winding symmetry. The wound windings must be insulated from each other. This is done using cable paper, varnished cloth, fluoroplastic tape, just paper, etc. In the manufacture of high-voltage windings, they must be insulated every 2-3 layers. A polyethylene terephthalate film with a thickness of up to 59 microns is very good for these purposes. The windings of household transformers are wound with copper (rarely aluminum) insulated round (rarely rectangular) wires. Round wires with high-strength (viniflex) insulation such as PEV-1, PEV-2 are very well suited for this purpose. Wire type PEL (insulation with oil-resin varnish) is currently used less often. Wire brand PEV-1, PEV-2 is produced with a diameter of 0,03 to 2,5 mm. The breakdown voltage of these wires, depending on the diameter, is from 600 to 2500 V. A wire of increased heat resistance such as PET and PETV is also used. The degree of filling the window cores with copper is determined by the filling factor of the window Kok = Sm / Sok. This is the ratio of the total shadi section of the copper wire windings to the area of the core window. For household appliances, the value of Kok in the calculations is taken as follows: This is for PEL, PEV, PET, PETV winding wires of round cross section. When determining the heating temperature of the transformer, it is necessary to take into account the current density in the windings J and the heat-radiating surface of the transformer windings. Required wire diameter for winding windings (without insulation): dm = 1,13(I/J)1/2, where I is the effective current in the winding; J is the given current density. When winding the windings turn to turn, the tight fit of the turns to each other will never work, therefore, it is necessary to take into account the stacking coefficient Cook. For wires with a diameter of 0,05 to 0,1 mm it is 0,83-0,85, for a diameter of 0,1 to 0,56 mm it is 0,92-0,93, and above it is 0,95. It is also necessary to take into account the swelling coefficient Kraz due to insufficient wire tightness. So, for a wire with a diameter of up to 0,5 mm Kraz \u1,05d 1,07 ... 0,5, and more than 1,1 mm Kraz \u1,12d XNUMX ... XNUMX. Calculation of the transformer Determine the overall power of the transformer for windings with a midpoint
where Квi is a coefficient taking into account the type of rectifier (0,71 for full-wave rectification, 1 for bridge rectifier circuits and with voltage doubling); n is the number of secondary windings of the transformer; Pn.tr - the total power of the secondary windings; htr depends on Рn.tr. (Fig. 6, where 1 - ring; 2 - rod and armored magnetic core)
where Ui, Ii are the voltage and current of the secondary windings.
If windings without midpoint Рg=0,5 Рn.tr(1+1/htr) For a half-wave rectifier circuit Рg=0,5 Рn.tr(1+Q.i); Sq.i=(1-I2d)1/2, where Id is the ratio of the average current in the load to the effective current of the winding. After finding Pr, the product of the core window, which is occupied by the windings, is determined by the cross-sectional area of the steel: ScSok=[Rg(1+htr)102/4KfsBJKsKokhtr] where Kf is the voltage curve shape factor (1,11 for the sinusoidal shape); The Kc-factor of filling the core with steel is 0,8-.95 (the lower value corresponds to a thinner sheet or strip of electrical steel). Author: O.G. Rashitov
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