The design of a typical transformer is uncomplicated. It consists of a steel core, two coils with wire winding. One winding is called primary, the second - secondary. The appearance of alternating voltage (U1) and current (I1) in the first coil forms a magnetic flux in its core. It creates an EMF directly in the secondary winding, which does not attach to the circuit and has an energy equal to zero.
If the circuit is connected and consumption occurs, this leads to a proportional increase in the current strength in the first coil. This model of communication between the windings explains the process of conversion and redistribution of electrical energy, which is included in the calculation of transformers. Since all the turns of the second coil are connected in series, the total effect of all the EMFs that appears at the ends of the device is obtained.
Transformers are assembled in such a way that the voltage drop in the second winding is a small fraction (up to 2 - 5%), which allows us to accept the assumption that U2 and EMF are equal at its ends. The number U2 will be more / less as much as the difference between the number of turns of both coils - n2 and n1.
The relationship between the number of wire layers is called the transformation ratio. It is determined by the formula (and is denoted by the letter K), namely: K = n1 / n2 = U1 / U2 = I2 / I1. Often this indicator looks like a ratio of two numbers, for example 1:45, which shows that the number of turns of one of the coils is 45 times less than that of the other. This proportion helps to calculate the current transformer.
Electrotechnical cores produce two types: W-shaped, armored, with the branching of the magnetic flux into two parts, and U-shaped - without separation. To reduce probable losses, the core is not made solid, but is made up of separate thin layers of steel isolated from each other by paper. The most common type is cylindrical: the primary winding is superimposed on the frame, then paper balls are mounted, and on top of this, a secondary layer of wire is wound.
The calculation of the transformer can cause some difficulties, but the simplified formulas that are given below will come to the aid of the amateur designer. It is first necessary to determine the levels of voltages and currents individually for each coil. The power of each of them is calculated: P2 = I2 * U2; P3 = I3 * U3; P4 = I4 * U4, where P2, P3, P4 - power (W), incremented by windings; I2, I3, I4 - current strength (A); U2, U3, U4 - voltage (V).
To establish the total power (P) in the calculation of the transformer, you need to enter the sum of the indices of the individual windings, and then multiply by a factor of 1.25, which takes into account losses: P = 1.25 (P2 + P3 + P4 + ...). By the way, the value of P will help calculate the core cross-section (in sq.cm): Q = 1.2 * cor.P.
Then follows the procedure for determining the number of turns n0 per 1 volt according to the formula: n0 = 50 / Q. As a result, the number of turns of the coils is recognized. For the first, taking into account the voltage loss in the transformer, it will be equal to: N1 = 0.97 * n0 * U1
For the rest: N2 = 1.3 * n0 * U2; n2 = 1.3 * n0 * U3 ... The diameter of the conductor of any winding can be calculated by the formula: d = 0.7 * root square. 1 where I is the current strength (A), d is the diameter (mm).
Calculation of the transformer allows you to find the current strength from the total power: I1 = P / U1. The size of the plates in the core remains unknown. To find it, it is necessary to calculate the area of the winding in the core window: Sm = 4 (d1 (sq.) * N1 + d2 (sq.) * N2 + d3 (sq.) * N3 + ...), where Sm is the area (in sq. Mm ), all windings in the window; d1, d2, d3 and d4 - wire diameters (mm); n1, n2, n3 and n4 are the number of turns. Using this formula, the unevenness of the winding, the thickness of the insulation of the wires, the portion occupied by the frame in the lumen of the core window is described. According to the obtained size of the area, a special standard size of the plate is selected for free placement of the coil in its window. And the last thing you need to know is the thickness of the core set (b), which is obtained by the formula: b = (100 * Q) / a, where a is the width of the middle plate (in mm); Q - in sq. see. The most difficult part in this method is to perform the calculation of the transformer (this is a search for a rod element with a suitable size).