Choosing a transformer: purpose and principle of operation, methods of selecting a transformer by power

Every radio amateur is faced with the choice of a transformer for powering various circuits and devices, as well as for creating power supplies. For this purpose, you can use ready-made options or calculate and make a transformer with your own hands. It is necessary to understand the main purpose, the principle of operation, as well as navigate and calculate the necessary parameters. Several methods are used to select a transformer.

Basic concepts about transformers

The main purpose of the transformer (T) is to convert the alternating voltage (U) into the required ratings. T has been widely used as the simplest converter of AC U, although DC can also be converted, but this method is economically disadvantageous. T works only from variable U, and this is due to the principle of its action.

Transformer (T) - converter of variable input U into the required rating or ratings for powering consumers. Most consumers are powered by direct current, which is obtained by converting AC U to DC using a diode bridge or some other rectifier. This variable U converter is primitive in its design, however, there are some structural features.

T consists of a magnetic circuit and coils on which a copper insulated wire is wound. The magnetic core is made of special steel, which has ferromagnetic properties and is called ferromagnet. The main difference between ferromagnets and ordinary steel is the presence of atoms with constant spin and orbital angular momentum (CuOM). SiOM depend on temperature and magnetic field, and due to this, the T windings do not overheat during operation due to the absence of Foucault currents. A special transformer steel with ferromagnetic properties reduces the formation of Foucault currents to a minimum, which is not enough to overheat the windings.

The most common materials for the manufacture of a magnetic circuit are electrical transformer steel (ETS) and permalloy. ETS differs from ordinary steel in its physicochemical properties, since it contains a significant mass fraction of silicon (Si), which at using special technologies provided at the factory, it combines with carbon under the action of high temperature and pressure.

This ETS manufacturing technology widespread, since it is used in almost all T. Another type of ferromagnet for the manufacture of a magnetic circuit is permalloy, which is a compound of an alloy of nickel and iron, used to make T of low power. The area of ​​the magnetic circuit is influenced by the power (P) T.

The windings are coils with wound insulated wire with a special varnish coating. The wire diameter and the number of turns depends on U and current (I), and this also affects the P of the transformer. The number of coils must be at least 2, but one coil is allowed, provided that 2 windings are wound on it (one of which is primary).

Principle of operation

The principle of operation of T is quite simple and is based on finding a conductor with the number of turns n in an alternating magnetic field. An alternating magnetic field (PMF) is a field, the value and direction of the magnetic flux lines (Ф) of which changes according to the law of change in the values ​​of the variable I, which generates it over time. When current passes through the turns of the primary winding coil (KPO), F is formed, which also penetrates the secondary winding coil (KVO).

Due to the closed structure of the magnetic circuit, the F lines are closed. To reduce the loss of electrical energy, the coils are placed as close to each other as possible. It is optimal to use one coil with 2 or more windings. However, this option is not allowed in older welding machines. In this case, the coils must be separate to increase heat transfer during welding. In addition, oil-cooled T are used at power substations, but their windings are structurally located on different coils.

AT the law of electromagnetic induction applies, at which there is a change in Ф and induction of the electromotive force (EMF) of self-induction in the KPO, and the EMF that occurs in the KVO is called the EMF of mutual induction.

At T there are 2 modes of operation: idle and active (load). At idle, consumption I takes place from 3 to 10% of the nominal value (Iн). In the active mode, I arises in the KVO, and, consequently, a magnetomotive force (MDF) appears. In this case, it is possible to calculate the main parameter T, which is called the transformation ratio k: I1 / I2 = w2 / w1 = 1 / k, where I1, I2 - I KPO and KVO, respectively, and w2 and w1 are the number of turns of KVO and KPO.

From the definition of k follows another relationship between the EMF of the windings (e1 and e2) and currents: e1 * I1 = e2 * I2 = 1. Based on this ratio, it can be concluded that the power (P = e * I) consumed by the KPO is equal to the power consumption of the KVO under load. Power T is measured in volt-amperes and is abbreviated as "VA".

The connection between the EMF in the windings is directly proportional to the number of turns. Based on Lenz's law, the windings T are pierced by the same Ф, and this fact allows us to calculate k in another way. Based on the law of induction for instantaneous EMF values, we obtain the following equalities for KPO and KVO:

1. e1 = - w1 * dF / dt * E-8.
2. e2 = - w2 * dF / dt * E-8.

The ratio dФ / dt is the magnitude of the change in Ф per unit time (according to the law describing the variable U). Based on the expressions for the instantaneous values ​​of the EMF, the dependence of the EMF for each winding on the number of turns is derived: e1 / e2 = w1 / w2. This statement is also true not for instantaneous indicators, it follows that e1 = U1, e2 = U2. Changing the quantities, the following relations are obtained: e1 / e2 = U1 / U2 = w1 / w2 = k.

According to the transformation ratio, T are divided into decreasing and increasing. To search for k you need to use several ways:

1. According to the passport.
2. In a practical way.
3. Application of the Schering bridge.
4. Use of UICT.

Very often radio amateurs use a practical definition of this parameter. Although not entirely accurate. To calculate the power supply, this method is quite enough.

It is not always possible to find a passport to T. To determine k, 2 voltmeters are used (1 for KPO, 2 for KVO), then U is measured several times on the windings. After that, k is calculated for several values ​​and its average value is taken.

Power calculation

To select T as a power source, the permissible power of the consumer or consumer group should be calculated. There are 2 options for collecting T: selection according to the table and calculation. Finding out the power of the transformer is quite simple, you need to use the formula for determining the power: P = U * I. The most accurate option is to calculate T as a power source.

There is a T available with a total power of 180 VA. It is necessary to find out whether it can be used as a 160 VA power supply. This method allows you to select a transformer by power according to the table.

Load factor T: kz = Sр / Str. Sр - total design power: Sp = P / cosph = 180 / 0.8 = 225 VA. The cosph factor is taken equal to 0.8. Power power T Str = 160 VA. Based on this, kz = 225/160 = 1.4 (> 1). If we take T with a power of 250 VA, then kz = 225/250 = 0.9 (<1).

The maximum permissible loads at a temperature of 30 degrees should be selected from table 1.

Table 1 - Permissible maximum loads T in the summer period at a temperature of 30 degrees:

Basically, T will not work for a long time under load, and in idle mode it consumes about 65% of the rated power: S = 225 * 0.65 = 146.25 VA.

Coefficient K1 is calculated as follows: K1 = 146.25 / 160 = 0.91 (initial load T). According to the table at K1 = 0.9 at Str = 160 VA T will work for 0.5 hours. This option is not valid. Therefore, it is necessary to choose T with a power reserve of 250 VA.

Self-calculation

To manufacture T of the required power, you must perform the calculation yourself. If you know U and the maximum I that the KVO should generate, then the P of the secondary circuit is calculated using the following formula: P2 = U2 * I2. With a coefficient of efficiency (efficiency = 0.8) T, the power of the KPO is calculated in the following way: P1 = P2 / 0.8 = 1.25 * P2.

The transfer of power from one winding to another is carried out using Ф in the magnetic circuit, therefore, depends on P1 cross-sectional area of ​​the core S, which is equal to the square root of P1 (in watts): S = sqrt (P1) (centimeters square). Based on the value of S, the number of turns w per 1 V is determined: w = 50 / S. The number of turns of the windings is calculated by the formulas: w1 = w * U1 and w2 = w * U2 + (w * U2 * 0.1).

Primary current is calculated using the formula: I1 = P1 / U1. The diameters (d) of the wire of the windings are determined by the values ​​of the currents, and the permissible current density for T is 2 A / sq.mm: d = 0.8 * sqrt (I). The cross-sectional area is calculated using the formula: S = 3.1416 * d * d / 4.

An example of calculating basic parameters

It is necessary to manufacture and calculate T for the power supply.

T must have the following parameters:

1. Primary winding 220 V.
2. Secondary windings: 660 V - 100 mA and 6 V - 5 A.
3. Total power of KVO: P2 = 660 * 0.1 + 6 * 5 = 96 W.
4. Primary power: P1 = 1.25 * 96 = 120 W.
5. Core section area: S = sqrt (120) = 10.95 = 11 sq. cm.
6. The number of turns per 1 V: w = 50/11 = 4.54 = 5.
7. Primary current: I1 = 96/220 = 0.436 A.

Number of turns and d:

• KPO: w1 = 5 * 220 = 1100 and d = 0.8 * sqrt (0.436) = 0.53 sq. mm.
• KVO - 660 V: w2 = 5 * 660 + (5 * 660 * 0.1) = 3300 + 330 = 3630 and d = 0.8 * sqrt (I) = 0.25 sq. mm.
• KVO - 6 V: w2 = 5 * 6 + (5 * 6 * 0.1) = 30 + 3 = 33 and d = 1.79 sq. mm.

The cross-sectional area of ​​the core (selection of a ready-made one) is Sm = 5 * 4 = 20 sq. cm = 2000 sq. mm. Checking winding placement, based on the parameters of the magnetic circuit:

1. For KPO, the diameter, taking into account insulation: d1iz = 0.63 mm.
2. For KVO: d2iz = 0.35 mm and d2iz = 1.89 mm.

For KPO: S = 0.8 * 0.63 * 1100 = 554.4 sq. mm. For KVO: S = 0.8 * (0.35 * 3630 + 33 * 1.89) = 1332.87 sq. mm. Total area S = 554.4 + 1332.87 = 1887.27 sq. mm. Checking the fulfillment of the inequality: Sm> S: 2000> 1887.27 (it is fulfilled, therefore, the magnetic circuit is suitable for T).

Thus, the choice of a transformer in terms of power for solving a specific task can be done using a table or calculated and manufactured independently. The latter option allows a more flexible and high-quality approach to the choice of T for any consumer. However, the approach of choosing a ready-made T saves a lot of time.