How to wind a transformer? Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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How to wind a transformer? Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Ham Radio Technologies

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Radio amateurs are increasingly asking themselves such a burning question, and the point is not only that there are fewer winding wires, their prices have increased, and the quality has deteriorated. Where did, for example, the wires of the PELSHO, PELBO and other brands that were sold in Soviet times in sets and coils go? The first of the above wires is used for winding loop coils on low-frequency bands, chokes, transformers on ferrite rings, etc. The second is necessary for winding windings of powerful power transformers. The advantage of such wires over conventional ones with a varnish coating is great.

First of all, this is the winding pitch created by the braiding of the wire. In powerful network transformers, the voltage difference in the windings between adjacent conductors is 1 V or more, thin varnish insulation, when heated and vibrated with the network frequency, gradually becomes thinner (wears off from friction between vibrating turns) and crumbles. As a result, interturn short circuits occur.

To illustrate, I will give a simple calculation. Let's take transformer iron with core section area S=10 cm2. Based on a simple estimate, Pr=S2, we determine that the overall power of the future transformer will be approximately 100 watts. Number of turns per 1 V:

w1 =50/S=50/10=5(вит./В),

Accordingly, the interturn voltage:

U1=1/5=0.2(V)

If the transformer iron is with a cross-sectional area S=50 cm2, the overall power of the transformer in this case is Pg=2500 W, and w1=50/50=1 (vit./V), which is equal to the interturn voltage in the windings. With a further increase in the overall power, the turn-to-turn voltage increases, the risk of insulation breakdown increases, and the reliability of the transformer naturally decreases.

How to get out of this situation? It should be remembered that the wires are not only winding. To wind the transformer, you can use a mounting wire in fluoroplastic insulation (MGTF) with a cross section corresponding to the required current. Since in such wires it is customary to indicate not the diameter, but the cross section (along the core), then you should use the conversion formula

d=2• (Sp/3.14)^0,5

where Sp - wire section, mm2; d - wire diameter, mm. For example, the MGTF-0.35 wire has d-0,66 mm. The wire diameter, depending on the required current I (A), is determined by the formula:

d = 0,8•I0,5.

Then the current in the winding wire:

I \u0.8d (d / 2) ^ 0.68 \uXNUMXd XNUMX (A)

The excellent quality of the insulation of MGTF wires makes it possible to do without interlayer spacers during winding, and its heat resistance allows winding transformers operating at elevated temperatures (fluoroplastic insulation does not melt or char)

Sometimes for balanced circuits it is required to wind a transformer with strictly identical windings.

This can be done by taking a flat cable as winding wires, for example, used in computer connecting cables. Having separated the required number of conductors from the cable, they wind the winding with them, which is then used as several identical ones, isolated from each other. The insulation of the flat cable is sufficiently thermally stable.

To obtain high currents, the secondary windings of power supply transformers are wound with sufficiently thick wires and tires. This work, it must be said, requires not only material (monetary), but also physical costs, since it is necessary to bend the elastic copper bus (wire) to tightness, trying to lay it coil to coil. As an alternative, I suggest using an acoustic cord, which is usually

connect the amplifier to the speakers. The acoustic cord has a large cross section of the core and. being double, ensures that the half-windings are identical for a full-wave rectifier with a mid-point. Little attention is paid to the identity of these half-windings, and this entails an increase in the background to which modern high-quality equipment is so sensitive.

The identity of the windings can be ensured in another way, for example, by winding them with a microphone cord (with a stereo cord we get three windings). In this way it is possible to wind the winding(s) with an electrostatic screen. To do this, the shielding braid of the microphone cord is connected (on one side) to the common wire.

A coaxial cable, due to the large difference in the cross sections of the inner core and braid, is not very suitable for symmetrical windings, but can be used as a winding wire when the screen and the inner core are interconnected. The inner core of the cable can also be used for measuring purposes. In all cases, one should not forget about the thermal stability of the wire insulation. The increased relative lacquer thickness of the wire insulation, on the one hand, reduces the number of winding turns that can be placed in the window of the transformer core, on the other hand, makes the use of interlayer insulation (up to interwinding insulation) unnecessary, which speeds up the manufacture of the transformer, and with heat-resistant wire insulation it increases reliability transformers.

Author: V.Besedin, Tyumen

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Torentol
In some cases, it is useful to wind a high-frequency transformer or choke for transistor devices on a ferrite ring so that the last layer of the winding completely covers the previous layers. The beginning of the winding should be connected to the transistor, and the end to the power bus, which is always connected to the common wire by alternating current. In this case, the transformer or inductor will be, as it were, shielded by the last turns of the winding.

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