# Recommendations for the selection of transformers and chokes

When choosing transformers and chokes, it is necessary to take into account that the availability of design documentation for specific types of transformers and chokes is not always sufficient for their selection and in most cases allows you to evaluate this product only from the point of view of interchangeability by nominal parameter values.

The wrong choice of transformers and chokes, as well as their use with deviations from the nominal modes, are the main cause of many failures and damage, both of the transformers and chokes themselves, and of the equipment as a whole. (For how to choose power transformers, see here).
It should be borne in mind that the operating conditions also have a significant influence on the choice of transformers and chokes. Therefore, when choosing transformers and chokes for specific devices, it is necessary to take into account the requirements for resistance to climatic and mechanical influences, as well as requirements for design and electrical parameters.

The choice according to the specified or calculated values of electrical and geometric characteristics, mechanical and climatic influences is always complex. Only this approach will allow making the right decisions, ensuring the selection of transformers and chokes of the optimal design.
The research of the authors made it possible to develop practical recommendations for choosing the optimal geometry of transformers and chokes, ensuring the minimum mass, volume or cost.

Let's take a look at some of them:

1.  The use of a rod transformer with two coils on a magnetic circuit of the PLM type (instead of an armored one) and a frequency of 50 Hz (at the same power) makes it possible to obtain a weight gain of 5–6%.
2. Toroidal transformers with a power of less than 350 VA, with a minimum weight, at a frequency of 400 Hz, compared to rod transformers with two coils of the same weight, give a power gain of 15 to 40%.
3. A rod transformer with two coils, operating at 50 Hz, allows for a smaller volume than an armored transformer. With the same volume of transformers, the former gives a power gain from 6 to 25%.
4. Using a toroidal transformer of less than 500 VA and operating at 400 Hz instead of a rod transformer with two coils at the same power yields a volume gain of 10 to 20%.
5. A rod transformer with two coils and a PLM-type magnetic wire at frequencies of 50 and 400 Hz, compared to a broium transformer at the same cost, gives a power gain of up to 30%.

When choosing transformers and chokes, it is necessary take into account the applied design of the magnetic circuitdetermining their main parameters. So an armored transformer with one coil and four C-shaped magnetic circuits is more technologically advanced in production than a rod transformer with two coils and two C-shaped magnetic circuits. However, a rod transformer with two coils has a higher heat transfer due to the larger cooling surface of the coils, and this allows an increase in the current density.
A rod-type transformer with two coils and a tape magnetic circuit has a power density greater than that of armored transformers with a tape magnetic circuit in terms of mass and volume: at a frequency of 50 Hz - up to 30%, at a frequency of 400 Hz - up to 20%.

Small-sized rod transformers with two coils, in comparison with armored ones, have a lower leakage inductance (our each coil has only half turns and therefore the coil thickness is smaller), a smaller external electromagnetic field (the magnetomotive force in individual coils has the same sign) and a lower susceptibility to extraneous electromagnetic fields ( induced EMF in both coils is subtracted).

When choosing transformers, it should also be taken into account that rod transformers with two coils have a lower fill factor of the window with copper.

Toroidal transformers have the smallest own external electromagnetic field, and less than other transformers are exposed to extraneous electromagnetic fields. However, these transformers are low-tech to manufacture and have a slightly lower fill factor of the window with copper.

The following types of magnetic cores are used:

•   PL - in low-voltage power transformers with the lowest weight at frequencies from 50 to 400 Hz and power over 500 V • A, in chokes with high energy consumption and high-voltage transformers with the lowest weight and cost at frequencies of 50 and 400 Hz. Magnetic cores of PL8 types; PL6.5; SHLO; PL12.5 is used in low-voltage transformers of a simplified design at a frequency of 50 Hz;
• PLV - in high-voltage transformers with a voltage of 20 kV and frequencies of J50 and 400 Hz;
• PLM - in low-voltage transformers with the smallest weight and cost at a frequency of 50 Hz and a power over 1b0 V • A, as well as in the presence of special requirements for leakage inductance;
• ШЛ - in transformers and filter chokes with the smallest mass at a frequency of 400 Hz. Magnetic cores of types 111L25, ShL32, ShL40 are used in saturation chokes;
• ShLM - in transformers with the smallest mass and cost at a frequency of 50 HzG, (with a capacity of approximately up to 100 V • A, depending on the allowable voltage drop) and filter chokes;
SHLO - in low-voltage transformers at frequencies from 1000 to 5000 Hz and high-voltage transformers at frequencies from 50 to 5000 Hz with the smallest mass, volume and cost;
• ШЛП - in transformers and chokes with the smallest volumes at frequencies from 400 to 1000 Hz;
• PLR - in transformers with the lowest cost, designed for a given overheating of the windings;
• ШЛР - in transformers with the lowest cost, designed for the permissible voltage drop in the windings;
• OL - in low-power transformers at frequencies of 50 and 400 Hz.

When choosing small-sized transformers and chokes, the correct choice of the size of the magnetic circuit, the grade of material, winding current and winding methods, which, as a rule, are indicated in the technical specifications for specific types of transformers and chokes, are of great importance. As already noted, when choosing transformers and chokes, it is important to take into account external influencing factors: climatic and mechanical. Under the influence of these factors or after the end of their influence, the products must maintain their parameters within the established norms.
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