Power transformers

From the practice of repair and design

A. Zyzyuk, Lutsk

 

It would seem that everything, or almost everything, has already been said about network transformers. However, in the practice of repair and design, there are very interesting and extraordinary situations, in terms of practical application, on which you should pause in order to consider them more closely.

 

This article provides tips on the use and manufacture of network transformers, designed for self-repetition by any radio amateur. Since in our time the issues of energy saving come to the main positions, special attention is also paid to these issues.

In recent years, switching mains power supplies have become more common - this is not surprising. The gain is obtained in at least two positions: efficiency and weight and dimensions. These, of course, are positive factors that incline radio amateurs to the side of the widespread use of switching power supplies.

Do not forget about the disadvantages of switching power supplies (SMPS). Should a radio amateur, for the sake of prestige, try to use an SMPS in all its designs? The answer to the question lies on the surface, if we recall the maintainability of the SMPS design, not to mention the materials and time spent on the manufacture of the SMPS.

So, consider the situations where the networked SMPS can serve as an alternative to the network transformer (ST).

Of course, in low-voltage equipment (with supply voltages of several tens of volts), especially with high current consumption, it is advantageous to use so-called choppers (pulse step-down voltage converters). With high-voltage voltage converters (PN), which are network SMPS, the situation is somewhat different. The popularization of circuitry of low-power (from units to several tens of watts) network SMPS often does not bring the expected results. The situation can change dramatically when the SMPS is designed for hundreds of watts or more.

Let us consider the case when the power consumed from the network is in the range of 1 ... 30 W.

The circuitry of network SMPS is the simplest only for expensive specialized microcircuits. Making a pulse transformer for a network SMPS is not the easiest and most rewarding task. Simple coil units are used more often in low-voltage SMPS circuits than in networked SMPS.

When it is required to solve specific problems in the above power range, it is possible to do without a network SMPS, especially if there is no time to experiment with an SMPS and to repair and manufacture it. An ST can serve as a reliable substitute for the network SMPS. The reliability of the fabrication of networked PTs is not something difficult to implement. With high-quality manufacturing, a ST is more reliable than an SMPS containing dozens of radio components, often operating in a stressful mode.

All equipment sooner or later fails, and recently there has been a real flurry of sudden failures of a wide variety of electronic equipment, for example, mobile phones (the ever-increasing installation density and the dubious origin of components cannot but affect the decrease in quality and reliability). The conclusion is unambiguous: the radio amateur needs to avoid complicating the circuitry everywhere, wherever possible. It is possible to protect yourself from global "chipization", avoiding the heap of circuitry, for example, in network power supplies (PSU).

 

A low-voltage, high-power SMPS is easier to manufacture than a high-power network-based SMPS. In addition, components for a low-voltage high-power SMPS are widespread and inexpensive compared to a powerful network-based SMPS. Let us delve into the consideration of the most important problematic issues on the use of ST, as well as the most effective and simplest in practice methods for resolving typical problems.

The reliability of ST, first of all, depends on the used winding wire, the type of its insulation and the method of winding (in bulk, which is now common among manufacturers of low-power STs, or layer by layer, with insulation between layers), insulation material, magnetic core, on the mode work of CT, etc. The topic of insulating material could be bypassed, but the prevailing circumstances (for example, illiterate articles on the Internet) predispose to the opposite.

When it is advised to use ordinary paper or cardboard as an insulating material between the primary (network) and secondary windings of the CT, then such advice should be treated with caution. I can say, relying primarily on my practical experience, that it is possible to avoid problems with insulation of dubious quality, which inevitably arise over time (due to the gradual degradation of dielectric parameters), only by choosing the right type of insulating material. In no way should ordinary paper be used as insulating materials between the primary and secondary windings, especially in toroidal CTs.

If the CT is supposed to be operated in conditions of high humidity, for example, as part of a charger in a garage, then the insulation between the primary and secondary windings of the CT must be sufficiently reliable. In sealed structures, water condensation may appear as the temperature drops. The combined effect of moisture and temperature determines the so-called thermal-humidity aggressiveness of the climate, which causes accelerated aging of materials. And dielectrics are no exception. Its conductivity ultimately depends on hygroscopicity (the ability to absorb moisture from the environment) and moisture permeability of a particular insulating material. Insulation conductivity leads to problems in the operation of the PT, to the need for its replacement or repair.

The easiest to manufacture (for a manufacturer of household appliances) are ST types TS-180, TS-200 or TS-270, in which interlayer and interwinding special paper insulation is used, which provides reliable long-term insulation. The reliability of transformers, in this case, is ensured by the use of specially processed paper.

In the past, varnished papers were used as interwinding and interlayer insulation up to temperatures below 130 ° C, which were replaced by synthetic films with increased electrical strength.

Impregnation, enveloping and pouring are used to protect ST from the effects of the external environment. These highly effective, but at the same time, laborious and expensive technological solutions for ensuring increased reliability of CTs are used by amateurs only by force, for example, in high-voltage CTs, where, without such methods of isolation, breakdown between the windings occurs very quickly.

Poor quality insulation alone can ruin the PT. Moreover, at first, such a ST can work without any complaints. Over time, the CT begins to "beat" the current, pinching the hands. Gradually, the insulation only gets worse and over time, the operation of the CT becomes extremely unpleasant, and soon - life-threatening.

To avoid electric shock, not to mention re-winding the secondary winding (to replace the insulation, you must first remove the secondary winding), you must use special insulating materials, such as varnish cloth or (even better) glass cloth. These materials are most suitable for toroidal CTs, as they provide the highest dielectric strength with a minimum thickness of the insulating film (up to 60 kV / mm).

Good results are provided by capacitor paper (up to 20 kV / mm), laid in several layers. For toroidal transformers, it is better to use varnished cloth. It should be noted right away that for the degradation of a dielectric it is not at all necessary to have a large temperature difference and high humidity. If the material for insulation is chosen unsuccessfully, then the room conditions are quite enough to eventually make sure of everything that has been said and realize how important this issue is.

Another very important circumstance is the obligatory high voltage test of the interwinding insulation of the CT. But this must be done in such a way as to ensure the conditions of non-destructive testing, avoiding the occurrence (introduction) of new defects in the interwinding insulation. This is achieved by limiting the current in the test voltage circuit at the level of several tens of microamperes, .. not more than 100 μA. The value of the test voltage is determined from the ratio:

 

Uncn=1000+2Upa6,

 

where Uslave - operating voltage.

The insulation breakdown voltage should be 1.5-2 times higher than the test voltage. Therefore, the TU for testing indicates the value of 3850 V of the test voltage for the interwinding insulation of the CT, which it must withstand for 5 minutes.

Do not be surprised that many of the home-made STs (and not only home-made ones!) Will not be able to pass this test. If the material of interwinding insulation of the proper quality is used in the CT, then there is no need to alter the CT, even if it withstands a reduced test voltage, but the CT is operated exclusively in room conditions, i.e. the stability of the insulation parameters is ensured. The author of the article uses as a test voltage a constant voltage of 3000V from a portable device containing a voltage converter and a microammeter to measure the current along the circuit 0 ... 3000 V.

It is the gradual increase in the test voltage on the CT with mandatory current limitation that provides a non-destructive test method. The actual measuring device was made to test, first of all, high-voltage transistors of line-box TV sets, as well as to test diodes and capacitors.

An increase in temperature will shorten the life of the insulation. That is why it is so important that the CT heats up less, i.e. by reducing the load factor of the CT, we will certainly increase the reliability of the CT.

The "no-load" current (1xx) should be insignificant if we want to approach the network SMPS in terms of efficiency (efficiency). In modern CT losses "no-load" are from 0.1 to 2% of their rated power, and 1xx - from 0.5 to 10% of the rated current of the primary winding. Large values refer to PTs of low power, i.e. for low-power STs, the 1xx currents are clearly overestimated. Unfortunately, for low-power industrial CTs, 1xx can significantly exceed 10% from the maximum current of the primary winding or from the maximum current used in a particular situation (device design). With large-scale production, each gram of consumables is saved, especially copper wire, especially now, when the prices for copper have sharply increased (for example, 1 m of enameled wire with a diameter of 1 mm costs 85 kopecks). It is not necessary to talk about Asian STs, since many readers got to know them from personal experience.

 

So, in the light of the rise in electricity prices, it becomes very important to minimize the 1xx of any ST, especially those with too much current. Let's take a quick look at the three main ways to reduce the 1xx value.

 The first method consists in a proportionally increased number of turns of all windings and has no peculiarities. The number of turns of all windings is increased 1.2-1.4 times higher than the calculated value. But there are some nuances here. To ensure smaller voltage drops on the secondary windings, select with a margin. The value of 1xx with this design of ST, depending on the coefficient of increasing the number of turns (1.2-1.4), decreases several times.

 

The second way to reduce 1xx is to slightly reduce the voltage on the primary winding of the CT due to an incandescent lamp (LN) connected in series with it. As a rule, LVs are used, designed for an operating voltage of 220 V. Often, LVs are used for other operating voltages. The use of 220 V LN is more preferable in situations where effective protection of the CT is required in modes close to the short circuit in the secondary winding of the CT, as well as in case of an emergency increase in the mains voltage to 300 ... 380 V. The maximum permissible voltage for the considered tandem from the LV 220 V and CT for 220 V is at least 440 V. Such a system reliably ensures the operation of the CT in almost all abnormal situations for the CT. The method is attractive precisely because it does not require significant material costs and work performed, but at the same time it is effective not only in low-power STs per unit of watts, but can be successfully applied at high powers, from tens to hundreds of watts.

The increase in the efficiency and reliability of the CT is achieved by decreasing 1хх due to a drop in some voltage across the LN. At the same time, in the CT, if necessary, the secondary winding is wound up (only the secondary one, which is much easier and faster to do than in the case of reworking the primary winding). Repeatedly it happened so that it was not required to wind up the secondary winding, despite the voltage drop across the LV of several tens of volts. For example, a rectifier on two diodes (full-wave, with a midpoint, with a tap from the CT) can be converted into a bridge.Сетевые силовые трансформаторы-ремонт и исследование      

Due to the double use of the alternating voltage by the bridge rectifier, in comparison with the factory version on two diodes, where the entire winding ended up with half the constant voltage, the bridge rectifier made it possible to have an output voltage margin so that it could be used to reduce, but already by primary winding CT (Fig. 1).

Previously, phasing the windings replaced their serial connection with a parallel one, i.e. at the same load current, the voltage dips were reduced both at the input of the rectifier and at its output. When it is required to increase the efficiency of the entire system as a whole, the bridge is powered from the entire serially connected winding. Having almost double the reserve for the rectified output voltage, it is possible to connect the LN in series to significantly facilitate the CT operation mode. As a result, the current 1xx decreased several times.

When it is not possible to wind the secondary winding, the CTs replace the CTs of another type, which has the voltage on the secondary winding when the LV is connected in series as needed.

The type and power of the LN needs to be selected already for each specific situation. There is nothing complicated in this event. With the help of LATR, the device connected to the CT can work within the required limits of the mains voltage, as a rule, 180 ... 260 V. It is pleasantly surprising that with the LN it is easier to expand the range, especially towards higher values, up to 270 V, or even more.

             
  Надежность силовых трансформаторов

There are very pleasant prospects for a sharp increase in the reliability of the ST, since the correct choice of the LN will actually ensure the practically trouble-free operation of the MV in any emergency situation for it (from the "idle" mode with an emergency increase in the mains voltage and to the situation of the short circuit mode of the secondary windings). Here, the LN already performs the functions of a fuse, but it does it flexibly, without blowing like a fuse, without disconnecting the equipment from the network.

In a similar way, it is possible to save STs of various foreign devices: from the cheapest, including common universal lamps (with voltage converters for powering LDS, with a radio receiver, flashing lights and music chips), with their chargers, and to more expensive products, both industrial and and self-manufacturing.

 

The third way to improve the efficiency of ST. The essence of this option is the simultaneous use of two copies of the same type of ST (Fig. 2), i.e. two CTs of the same type must work together from the same mains for one common load. For this purpose, the mains windings of both CTs are connected in series (i.e., phased). With the secondary windings, they either act in the same way, or turn them on in parallel.

As a result, a composite ST is obtained, which is similar in load characteristics to a single version of ST, but in some characteristics it greatly surpasses its prototype. So, for example, the maximum allowable mains voltage is more than 400 V. The 1xx current can be reduced when operating in a 220 V network by at least 4 times. The disadvantage of this option is that voltage dips appear on the CT windings. To eliminate them, a voltage stabilizer is used after the rectifier.

A source: www.electrician.com.ua

Total comments: 0

leave a comment

Your email will not be published.

en_GBEN