IN modern technology transformers are used quite often. These devices are used to increase or decrease the parameters of alternating electric current. The transformer consists of an input and several (or at least one) output windings on a magnetic core. These are its main components. It happens that the device fails and there is a need to repair or replace it. You can determine whether the transformer is working properly using a home multimeter on your own. So, how to test a transformer with a multimeter?
Basics and operating principle
The transformer itself is an elementary device, and its operating principle is based on the two-way transformation of the excited magnetic field. Typically, a magnetic field can be induced exclusively using alternating current. If you have to work with a constant, you must first transform it.
A primary winding is wound around the core of the device, to which an external alternating voltage with certain characteristics is supplied. Next come one or more secondary windings, in which an alternating voltage is induced. The transmission coefficient depends on the difference in the number of turns and the properties of the core.
Varieties
Today you can find many types of transformer on the market. Depending on the design chosen by the manufacturer, a variety of materials can be used. As for the shape, it is selected solely for the convenience of placing the device in the body of the electrical appliance. The design power is affected only by the configuration and material of the core. In this case, the direction of the turns does not affect anything - the windings are wound both towards and away from each other. The only exception is the identical choice of direction if several secondary windings are used.
To check such a device, a conventional multimeter is sufficient, which will be used as a current transformer tester. No special devices are required.
Check procedure
Testing a transformer begins with identifying the windings. This can be done using markings on the device. Pin numbers, as well as their type designations, should be indicated, which allows you to establish more information in reference books. In some cases there are even explanatory drawings. If the transformer is installed in some kind of electronic device, then the electronic circuit diagram of this device, as well as a detailed specification, can clarify the situation.
So, when all the conclusions are determined, it’s the tester’s turn. With its help, you can identify the two most common faults - a short circuit (to the housing or an adjacent winding) and a winding break. In the latter case, in ohmmeter mode (resistance measurement), all windings are called back one by one. If any of the measurements shows one, that is, infinite resistance, then there is a break.
There is an important nuance here. It is better to check on an analog device, since a digital one can give distorted readings due to high induction, which is especially typical for windings with a large number of turns.
When checking a short circuit to the housing, one of the probes is connected to the terminal of the winding, while the second probe rings the terminals of all other windings and the housing itself. To check the latter, you will need to first clean the contact area from varnish and paint.
Determination of interturn short circuit
Another common failure of transformers is interturn short circuit. It is almost impossible to check a pulse transformer for such a malfunction with just a multimeter. However, if you attract your sense of smell, attentiveness and sharp vision, the problem can well be solved.
A little theory. The wire on the transformer is insulated exclusively with its own varnish coating. If an insulation breakdown occurs, the resistance between adjacent turns remains, as a result of which the contact area heats up. That is why the first step is to carefully inspect the device for streaks, blackening, burnt paper, swelling and a burning smell.
Next, we try to determine the type of transformer. Once this is achieved, you can look at the resistance of its windings using specialized reference books. Next, switch the tester to megohmmeter mode and begin measuring the insulation resistance of the windings. In this case, the pulse transformer tester is a regular multimeter.
Each measurement should be compared with that indicated in the reference book. If there is a discrepancy of more than 50%, then the winding is faulty.
If the resistance of the windings is not indicated for one reason or another, the reference book must provide other data: the type and cross-section of the wire, as well as the number of turns. With their help, you can calculate the desired indicator yourself.
Checking household step-down devices
It is worth noting the moment of checking classic step-down transformers with a multimeter tester. They can be found in almost all power supplies that reduce the input voltage from 220 Volts to the output voltage of 5-30 Volts.
The first step is to check the primary winding, which is supplied with a voltage of 220 Volts. Signs of a primary winding malfunction:
- the slightest visibility of smoke;
- the smell of burning;
- crack.
In this case, the experiment should be stopped immediately.
If everything is normal, you can proceed to measurements on the secondary windings. You can touch them only with the tester contacts (probes). If the results obtained are less than the control ones by at least 20%, then the winding is faulty.
Unfortunately, such a current block can be tested only in cases where there is a completely similar and guaranteed working block, since it is from it that the control data will be collected. It should also be remembered that when working with indicators of the order of 10 ohms, some testers may distort the results.
No-load current measurement
If all tests have shown that the transformer is fully operational, it would not be amiss to conduct another diagnostic - for the no-load current of the transformer. Most often it is equal to 0.1-0.15 of the nominal value, that is, the current under load.
To carry out the test, the measuring device is switched to ammeter mode. Important point! The multimeter should be connected to the transformer under test in a short-circuited manner.
This is important because when electricity is supplied to the transformer winding, the current increases to several hundred times the rated current. After this, the tester probes open and the indicators are displayed on the screen. It is they that display the value of the current without load, the no-load current. In a similar way, indicators are measured on the secondary windings.
To measure voltage, a rheostat is most often connected to the transformer. If you don’t have it at hand, a tungsten spiral or a series of light bulbs can be used.
To increase the load, increase the number of bulbs or reduce the number of turns of the spiral.
As you can see, you don’t even need any special tester to check. A completely ordinary multimeter will do. It is highly desirable to have at least an approximate understanding of the operating principles and structure of transformers, but for successful measurements it is enough just to be able to switch the device to ohmmeter mode.
How to deal with transformer windings what's his name connect correctly to the network and not “burn it” and how to determine the maximum currents of the secondary windings???
Many people ask themselves these and similar questions. beginner radio amateurs.
In this article I will try to answer such questions and, using the example of several transformers (photo at the beginning of the article), to understand each of them.. I hope this article will be useful to many radio amateurs.
First, let's recall the general features for armored transformers
- Network winding , as a rule, is wound first (closest to the core) and has the highest active resistance (unless it is a step-up transformer, or a transformer with anode windings).
The network winding may have taps, or consist, for example, of two parts with taps.
- Series connection of windings (parts of windings) for armored transformers is done as usual, beginning with end or terminals 2 and 3 (if, for example, there are two windings with terminals 1-2 and 3-4).
- Parallel connection of windings (only for windings with the same number of turns), the beginning is made as usual with the beginning of one winding, and the end with the end of another winding (n-n and k-k, or pins 1-3 and 2-4 - if, for example, there are identical windings with pins 1-2 and 3-4).
General rules for connecting secondary windings for all types of transformers.
To obtain different output voltages and load currents of windings for personal needs, different from those available on the transformer, can be obtained by various connections of the existing windings to each other. Let's consider all possible options.
The windings can be connected in series, including windings wound with wires of different diameters, then the output voltage of such a winding will be equal to the sum of the voltages of the connected windings (Utotal = U1 + U2... + Un). The load current of such a winding will be equal to the smallest load current of the available windings.
For example: there are two windings with voltages of 6 and 12 volts and load currents of 4 and 2 amperes - as a result, we get a common winding with a voltage of 18 volts and a load current of 2 amperes.
The windings can be connected in parallel, only if they contain the same number of turns
, including those wound with wires of different diameters. The correct connection is checked like this. We connect two wires from the windings together and measure the voltage on the remaining two.
If the voltage is doubled, then the connection was not made correctly, in this case we change the ends of any of the windings.
If the voltage at the remaining ends is zero or so (a difference of more than half a volt is not desirable, the windings in this case will heat up at XX), feel free to connect the remaining ends together.
The total voltage of such a winding does not change, and the load current will be equal to the sum of the load currents of all windings connected in parallel.(Itotal = I1 + I2... + In) .
For example: there are three windings with an output voltage of 24 volts and load currents of 1 ampere each. As a result, we get a winding with a voltage of 24 volts and a load current of 3 amperes.
The windings can be connected in parallel-series (for details for parallel connection, see the paragraph above). The total voltage and current will be the same as in a series connection.
For example: we have two series and three parallel connected windings (examples described above). We connect these two component windings in series. As a result, we get a common winding with a voltage of 42 volts (18+24) and a load current along the smallest winding, that is, 2 amperes.
The windings can be connected back to back, including those wound with wires of different diameters (also parallel and series connected windings). The total voltage of such a winding will be equal to the difference in voltages of oppositely connected windings, the total current will be equal to the smallest winding load current. This connection is used when it is necessary to reduce the output voltage of the existing winding. Also, in order to reduce the output voltage of any winding, you can wind an additional winding on top of all the windings with a wire, preferably of no smaller diameter that winding whose voltage needs to be reduced so that the load current does not decrease. The winding can be wound without even disassembling the transformer if there is a gap between the windings and the core, and turn it on opposite to the desired winding.
For example: we have two windings on a transformer, one is 24 volts 3 amperes, the second is 18 volts 2 amperes. We turn them on oppositely and as a result we get a winding with an output voltage of 6 volts (24-18) and a load current of 2 amperes.
Let's start with a small transformer, adhering to the features described above (left in the photo).
We examine it carefully. All its terminals are numbered and the wires fit to the following terminals; 1, 2, 4, 6, 8, 9, 10, 12, 13, 22, 23, and 27.
Next, you need to test all the terminals with an ohmmeter to determine the number of windings and draw a diagram of the transformer.
The following picture emerges.
Pins 1 and 2 - resistance between them is 2.3 Ohms, 2 and 4 - between them is 2.4 Ohms, between 1 and 4 - 4.7 Ohms (one winding with a middle pin).
Further 8 and 10 - resistance 100.5 Ohms (another winding). Pins 12 and 13 - 26 Ohm (another winding). Pins 22 and 23 - 1.5 Ohm (last winding).
Pins 6, 9 and 27 do not communicate with other pins or with each other - these are most likely screen windings between the network and other windings. These terminals in the finished design are interconnected and attached to the housing (common wire).
Let's carefully inspect the transformer again.
The network winding, as we know, is wound first, although there are exceptions.
It's hard to see in the photo, so I'll duplicate it. A wire coming from the core itself is soldered to pin 8 (that is, it is closest to the core), then a wire goes to pin 10 - that is, winding 8-10 is wound first (and has the highest active resistance) and is most likely network.
Now, based on the data received from the dialing, you can draw a diagram of the transformer.
All that remains is to try to connect the supposed primary winding of the transformer to a 220 volt network and check the no-load current of the transformer.
To do this, we assemble the following chain.
In series with the intended primary winding of the transformer (for us these are pins 8-10), we connect an ordinary incandescent lamp with a power of 40-65 watts (for more powerful transformers 75-100 watts). In this case, the lamp will play the role of a kind of fuse (current limiter), and will protect the transformer winding from failure when connected to a 220 volt network, if we have chosen the wrong winding or the winding is not designed for a voltage of 220 volts. The maximum current flowing in this case through the winding (with a lamp power of 40 watts) will not exceed 180 milliamps. This will protect you and the transformer being tested from possible troubles.
And in general, make it a rule that if you are not sure about the correct choice of the network winding, its switching, or the installed winding jumpers, then always make the first connection to the network with an incandescent lamp connected in series.
Being careful, we connect the assembled circuit to a 220 volt network (I have a slightly higher network voltage, or rather, 230 volts).
What do we see? The incandescent lamp does not light.
This means that the network winding has been selected correctly and further connection of the transformer can be made without a lamp.
We connect the transformer without a lamp and measure the no-load current of the transformer.
The no-load current (OC) of the transformer is measured as follows; a similar circuit is assembled that we assembled with a lamp (I won’t draw it anymore), only instead of the lamp an ammeter is turned on, which is designed to measure alternating current (carefully inspect your device for the presence of such a mode). The ammeter is first set to the maximum measurement limit, then, if there is a lot of it, the ammeter can be transferred to a lower measurement limit. Being careful, we connect to a 220 volt network, preferably through an isolation transformer. If the transformer is powerful, then at the moment the transformer is connected to the network, it is better to short-circuit either with an additional switch, or simply short-circuit with each other, since the starting current of the primary winding of the transformer exceeds the no-load current by 100-150 times and the ammeter may fail. After the transformer is connected to the network, the ammeter probes are disconnected and the current is measured.
The no-load current of the transformer should ideally be 3-8% of the rated current of the transformer. It is considered normal that the current is 5-10% of the rated value. That is, if a transformer with a calculated rated power of 100 watts, the current consumption by its primary winding is 0.45 A, then the XX current should ideally be 22.5 mA (5% of the nominal) and it is desirable that it does not exceed 45 mA (10 % of face value).
As you can see, the no-load current is a little more than 28 milliamps, which is quite acceptable (well, maybe a little too high), since this transformer looks like it has a power of 40-50 watts.
We measure the open-circuit voltage of the secondary windings. It turns out at terminals 1-2-4 17.4 + 17.4 volts, terminals 12-13 = 27.4 volts, terminals 22-23 = 6.8 volts (this is at a network voltage of 230 volts).
Next we need to determine the capabilities of the windings and their load currents. How it's done?
If it is possible and the length of the winding wires suitable for the contacts allows, then it is better to measure the diameters of the wires (roughly up to 0.1 mm - with a caliper and accurately with a micrometer).
If it is not possible to measure the diameters of the wires, then proceed as follows.
We load each of the windings in turn with an active load, which can be anything, for example, incandescent lamps of various power and voltage (an incandescent lamp with a power of 40 watts at a voltage of 220 volts has an active resistance of 90-100 Ohms in a cold state, a lamp with a power of 150 watts - 30 Ohm), resistance wires (resistors), nichrome spirals from electric stoves, rheostats, etc.
We load until the voltage on the winding decreases by 10% relative to the no-load voltage.
After measure load current
.
This current will be the maximum current that the winding is capable of delivering for a long time without overheating.
The voltage drop is conventionally accepted to be up to 10% for a constant (static) load in order to prevent the transformer from overheating. You may well take 15%, or even 20%, depending on the nature of the load. All these calculations are approximate. If the load is constant (lamp intensity, for example, Charger), then a smaller value is taken, if the load is pulsed (dynamic), for example ULF (except for mode “A”), then a higher value can be taken, up to 15-20%.
I take the static load into account, and I succeeded; winding 1-2-4 load current (with a decrease in winding voltage by 10% relative to the no-load voltage) - 0.85 amperes (power about 27 watts), winding 12-13 (pictured above) load current 0.19-0, 2 amperes (5 watts) and winding 22-23 - 0.5 amperes (3.25 watts). The rated power of the transformer is about 36 watts (rounded to 40)
Other transformers are checked in the same way.
The photo of the second transformer shows that the leads are soldered to contact blades 1, 3, 4, 6, 7, 8, 10, 11, 12.
After dialing, it becomes clear that the transformer has 4 windings.
The first is on pins 1 and 6 (24 Ohm), the second is 3-4 (83 Ohm), the third is 7-8 (11.5 Ohm), the fourth is 10-11-12 with a tap from the middle (0.1+0.1 Ohm) .
Moreover, it is clearly visible that windings 1 and 6 are wound first (white leads), then comes winding 3-4 (black leads).
24 Ohms of active resistance of the primary winding is quite enough. For more powerful transformers, the active resistance of the winding reaches several Ohms.
The second winding is 3-4 (83 Ohms), possibly boosting.
Here you can measure the diameters of the wires of all windings, except for winding 3-4, the terminals of which are made of black, stranded, mounting wire.
Next we connect the transformer through an incandescent lamp. The lamp does not light up, the transformer looks like it has a power of 100-120, we measure the no-load current, it turns out 53 milliamps, which is quite acceptable.
We measure the open-circuit voltage of the windings. It turns out 3-4 - 233 volts, 7-8 - 79.5 volts, and winding 10-11-12 at 3.4 volts (6.8 with the middle terminal). We load winding 3-4 until the voltage drops by 10% of the no-load voltage, and measure the current flowing through the load.
The maximum load current of this winding, as can be seen from the photograph, is 0.24 amperes.
The currents of other windings are determined from the current density table, based on the diameter of the winding wire.
Winding 7-8 is wound with 0.4 wire and filament with 1.08-1.1 wire. Accordingly, the currents are 0.4-0.5 and 3.5-4.0 amperes. The rated power of the transformer is about 100 watts.
There is one more transformer left. It has a contact strip with 14 contacts, the top 1, 3, 5, 7, 9, 11, 13 and bottom are even, respectively. It could switch to different mains voltages (127,220,237); it is quite possible that the primary winding has several taps, or consists of two half-windings with taps.
We call, and we get this picture:
Pins 1-2 = 2.5 Ohm; 2-3 = 15.5 Ohm (this is one winding with a tap); 4-5 = 16.4 ohms; 5-6 = 2.7 Ohm (another winding with tap); 7-8 = 1.4 Ohm (3rd winding); 9-10 = 1.5 Ohm (4th winding); 11-12 = 5 Ohm (5th winding) and 13-14 (6th winding).
We connect to pins 1 and 3 a network with an incandescent lamp connected in series.
The lamp burns at half intensity. We measure the voltage at the terminals of the transformer, it is 131 volts.
This means they didn’t guess right and the primary winding here consists of two parts, and the connected part at a voltage of 131 volts begins to enter saturation (the no-load current increases) and therefore the lamp filament becomes hot.
We connect pins 3 and 4 with a jumper, that is, two windings in series and connect the network (with a lamp) to pins 1 and 6.
Hurray, the lamp is not on. We measure the no-load current.
The no-load current is 34.5 milliamps. Here, most likely (since part of winding 2-3, and part of the second winding 4-5 have greater resistance, then these parts are designed for 110 volts, and parts of windings 1-2 and 5-6 are 17 volts each, that is, the total for one part 1278 volts) 220 volts was connected to pins 2 and 5 with a jumper on pins 3 and 4 or vice versa. But you can leave it the way we connected it, that is, all parts of the windings in series. This is only better for the transformer.
That's it, the network has been found, further actions are similar to those described above.
Rod transformers, features
There are also core transformers, they look like this
Quite common trances, by the way, they were used in many televisions of “tube” times...
What are their main features:
Rod transformers usually have two symmetrical coils, and the mains winding is divided into two coils, that is, 110 (127) volt turns are wound on one coil, and on the other. The numbering of the terminals of one coil is similar to the other; the terminal numbers on the other coil are marked (or conventionally marked) with a stroke, i.e. 1", 2", etc.
The mains winding is usually wound first (closest to the core).
The network winding may have taps, or consist of two parts (for example, one winding - pins 1-2-3; or two parts - pins 1-2 and 3-4).
In a rod transformer, the magnetic flux moves along the core (in a “circle, ellipse”), and the direction of the magnetic flux of one rod will be opposite to the other, therefore, to connect the two halves of the windings in series, contacts of the same name or beginning to beginning (end to end) are connected on different coils. , i.e. 1 and 1", the network is supplied to 2-2", or 2 and 2", the network is then supplied to 1 and 1".
For a serial connection of windings consisting of two parts on one coil, the windings are connected as usual, beginning to end or end to beginning, (n-k or k-n), that is, pin 2 and 3 (if, for example, there are 2 windings with pin numbers 1-2 and 3-4), also on the other coil. Further serial connection of the resulting two half-windings on different coils, see the paragraph above.
For parallel connection of windings ( only for windings with the same number of turns ) on one coil the connection is made as usual (n-n and k-k, or pins 1-3 and 2-4 - if, for example, there are identical windings with pins 1-2 and 3-4). For different coils, the connection is made as follows, k-n-tap and n-k-tap, or connect terminals 1-2" and 2-1" - if, for example, there are identical windings with terminals 1-2 and 1"-2" .
Once again, I remind you to follow safety precautions, and it is best to have an isolation transformer at home for experiments with a voltage of 220 volts (a transformer with 220/220 volt windings for galvanic isolation from an industrial network), which will protect against electric shock if you accidentally touch the bare end of the wire .
Notes and additions:
*author of the article Nikolay Petrushov
*Material from the site To help the radio amateur
The word “transformer” is derived from the English word "transform"- transform, change. I hope everyone remembers the movie “Transformers”. There, cars were easily transformed into transformers and back. But... our transformer is not converted according to appearance. It has an even more amazing property - converts AC voltage of one value to AC voltage of another value! This property of a transformer is very widely used in radio electronics and electrical engineering.
Types of transformers
Single-phase transformers
These are transformers that convert single-phase AC voltage of one value into single-phase AC voltage of another value.
Basically single-phase transformers have two windings, primary And secondary. One voltage value is applied to the primary winding, and the voltage we need is removed from the secondary winding. Most often in everyday life you can see the so-called network transformers, in which the primary winding is designed for mains voltage, that is, 220 V.
In the diagrams, a single-phase transformer is designated as follows:
The primary winding is on the left, and the secondary winding is on the right.
Sometimes many different voltages are required to power different devices. Why put your own transformer on each device if you can get several voltages at once from one transformer? Therefore, sometimes there are several pairs of secondary windings, and sometimes even some windings are derived directly from the existing secondary windings. Such a transformer is called a transformer with multiple secondary windings. In the diagrams you can see something like this:
Three-phase transformers
These transformers are mainly used in industry and are most often larger in size than simple single-phase transformers. Almost all three-phase transformers are considered power transformers. That is, they are used in circuits where powerful loads need to be powered. This could be CNC machines and other industrial equipment.
In the diagrams, three-phase transformers are designated like this:
The primary windings are designated in capital letters, and the secondary windings are in small letters.
Here we see three types of winding connections (from left to right)
- star-star
- star-triangle
- triangle-star
In 90% of cases, star-star is used.
Transformer operating principle
Let's look at this picture:
1 – primary winding of the transformer
2 – magnetic circuit
3 – secondary winding of the transformer
F– direction of magnetic flux
U1– voltage on the primary winding
U2– voltage on the secondary winding
The picture shows the most common single-phase transformer.
The magnetic core consists of special steel plates. Magnetic flux F flows through it (shown by arrows). This magnetic flux is created by the alternating voltage of the primary winding of the transformer. The voltage is removed from the secondary winding of the transformer.
But how is this possible? We don’t have any connection between the primary and secondary windings, do we? How can current flow through an open circuit? It's all about the magnetic flux that the primary winding of the transformer creates. The secondary winding “catches” this magnetic flux and converts it into alternating voltage with the same frequency.
Currently, transformers are created in a different design. This design has its advantages, such as the convenience of winding the primary and secondary windings, as well as smaller dimensions.
Transformer formula
So what does the voltage that the transformer gives us on the secondary winding depend on? And it depends on the turns that are wound on the primary and secondary windings!
Where
N 1 – number of turns of the primary winding
N 2 – number of turns of the secondary winding
I 1 – current strength of the primary winding
I 2 – current strength of the secondary winding
The transformer also observes the law of conservation of energy, that is, whatever power enters the transformer, such power leaves the transformer:
This formula is valid for ideal transformer. A real transformer will produce slightly less power at the output than at its input. The efficiency of transformers is very high and sometimes even reaches 98%.
Types of transformers by output voltage
A step-down transformer
This is a transformer that lowers the voltage. Let's say 220 V goes to the primary winding, and we get 12 V to the secondary winding. That is, we converted a higher voltage into a lower voltage.
Step-up transformer
This is a transformer that increases the voltage. Here, too, everything is painfully simple. Let's say we supply 10 Volts to the primary winding, and remove 110 V from the secondary winding. That is, we have increased our voltage several times.
Matching transformer
Such a transformer is used for matching between stages of circuits.
Isolation or isolation transformer (transformer 220-220)
Such a transformer is used for electrical safety purposes. Basically, this is a transformer with the same number of windings at the input and output, that is, its voltage on the primary winding will be equal to the voltage on the secondary winding. The neutral terminal of the secondary winding of such a transformer is not grounded. Therefore, if you touch a phase on such a transformer, you will not be hit electric shock. You can read about its use in the article about.
How to check a transformer
Short circuit of windings
Although the windings fit very tightly to each other, they are separated by a varnish dielectric, which covers both the primary and secondary windings. If it occurs somewhere, the transformer will get very hot or make a strong hum during operation. In this case, it is worth measuring the voltage on the secondary winding and comparing it so that it matches the passport value.
Transformer winding break
If there is a break, everything is much simpler. To do this, we use a multimeter to check the integrity of the primary and secondary windings.
In the photo below I am checking the integrity of the primary winding, which consists of 2650 turns. Is there resistance? So everything is OK. The winding is not broken. If it were broken, the multimeter would show “1” on the display.
In the same way we check the secondary winding, which consists of 18 turns.
Transformer operation
Operation of a step-down transformer
So, our guest is a transformer from a wood burning device:
Its primary winding is numbers 1, 2.
Secondary winding – numbers 3, 4.
N 1– 2650 turns,
N 2– 18 turns.
Its insides look like this:
We connect the primary winding of the transformer to 220 Volts
We set the multimeter to measure alternating current and measure the voltage on the primary winding (mains voltage).
We measure the voltage on the secondary winding.
It's time to test our formulas
1.54/224=0.006875 (voltage ratio coefficient)
18/2650=0.006792 (winding ratio)
Let's compare the numbers... the error is actually a penny! The formula works! The error is associated with heating losses of the transformer windings and magnetic circuit, as well as the measurement error of the multimeter. There is a simple rule about current strength: By lowering the voltage, we increase the current, and vice versa, by increasing the voltage, we lower the current.
Transformer at idle
No-load operation of the transformer means operation of the transformer without load on the secondary winding.
Our guinea pig will be a different transformer
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There are two pairs of secondary windings here, but we will use only one.
The two red wires are the primary winding of the transformer. We will supply voltage to these wires from a 220 V network.
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We will remove the voltage from the secondary winding from two blue wires.
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In order to take measurements, we will need to set the knob to measure alternating voltage. If you do not know how to measure alternating voltage and current, I recommend reading this article.
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We measure the voltage on the primary winding of the transformer, where we supply 220 V.
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The multimeter shows 230 V. Well, it happens).
Now we measure the voltage on the secondary winding of the transformer
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We got 22 Volts.
I wonder how much current our transformer consumes from the outlet during idle mode?
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The multimeter showed 60 milliamps. This is understandable, because our transformer is not ideal.
As you can see, there is no load on the secondary winding of the transformer, but it still “eats” the current strength, and therefore electrical energy from the network. If we calculate the power, we get P=IU=230×0.06=13.8 Watts. And if we leave it turned on for at least an hour, then it will consume 13.8 Watt*hour of electricity or 0.0138 kWh*hour. How much does one kilowatt of electricity cost now? In Russia 4-5 rubles. A kopeck saves the ruble. Therefore, it is not recommended to leave electrical appliances with a transformer power supply connected to the network.
Transformer under load
Experience No. 1
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I wonder if the current on the primary winding will change if we load the secondary winding with our light bulbs? The lights lit up, and the current strength on the primary winding also changed ;-)
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When we measured without load, we had 60 milliamps in the primary circuit. Our secondary winding circuit was open since we did not connect any load. As soon as we connected incandescent lamps to the secondary winding of the transformer, they immediately began to consume current. But by the way, the current increased in the primary winding circuit to the level of 65.3 milliamps. This begs the conclusion:
If the current in the secondary winding circuit of the transformer increases, then the current in the primary winding circuit also increases.
Experience No. 2
Let's do another experiment. To do this, we measure the voltage without load on the secondary winding of the transformer, the so-called idle mode.
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Now we connect our light bulbs and measure the voltage again
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Wow, the voltage dropped by 0.2 V.
Let's measure the current in the secondary winding with light bulbs
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We got 105 milliamps.
We carry out all the same similar operations for a powerful one with a nominal value of 10 Ohms and a dissipation power of 10 Watts. We measure the voltage on the secondary winding when the resistor is turned on
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We got 18.9 V. Did you see how much the voltage dropped? If at idle it was 22.2 V, now it is 18.9 V!
I wonder how much current flows in the secondary circuit in which the resistor is connected
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Wow, almost 2 Amps.
Conclusion: when the load is turned on, a voltage drop occurs. The voltage drops the more, the more current the load consumes. Another important factor also plays a role here - transformer power. The greater the power of the transformer, the less voltage drop will be. The power of a transformer depends on its dimensions. The larger the dimensions, the larger its core size. Consequently, such a transformer can deliver a decent amount of current in the secondary winding with minimal voltage drop.
Before connecting the transformer to the network, you need to determine primary winding of the transformer, test its primary and secondary windings with an ohmmeter.
several primary windingsIn step-down transformers, the resistance of the mains winding is much greater than the resistance of the secondary windings and can differ by a hundred times.
There can be several primary (network) windings, or a single winding can have taps if the transformer is universal and designed for use at different mains voltages.
In two frame transformers on core magnetic cores, the primary windings are distributed over both frames.
protected by fuse When testing transformers, you can use the diagram below. At wrong, The FU fuse will protect the network from short circuits and the transformer from damage.
Video: A simple way to diagnose a power transformer
When the type of power transformer is unknown, especially since we do not know its passport data, an ordinary pointer tester and a simple device in the form of an incandescent lamp come to the rescue.
How to choose a fuse for a transformer
We calculate the fuse current in the usual way:
I – current for which the fuse is designed (Ampere),
P – overall power of the transformer (Watt),
U – mains voltage (~220 Volts).
35 / 220 = 0.16 Ampere
The closest value is 0.25 Ampere.
determination of transformer primary voltage Circuit for measuring the no-load current (IO) of the transformer. The XX current of the transformer is usually measured to exclude the presence of short-circuited turns or to ensure that the primary winding is connected correctly.
When measuring the XX current, you need to gradually increase the supply voltage. In this case, the current should increase smoothly. When the voltage exceeds 230 volts, the current usually begins to increase more sharply. If the current begins to increase sharply at a voltage significantly less than 220 Volts, it means that either you have chosen the primary winding incorrectly, or it is faulty.
| Power, W) | Current XX (mA) |
| 5 — 10 | 10 — 200 |
| 10 -50 | 20 — 100 |
| 50 — 150 | 50 — 300 |
| 150 — 300 | 100 — 500 |
| 300 — 1000 | 200 — 1000 |
Approximate currents of XX transformers depending on power.
It should be added that the currents of XX transformers, even of the same rated power, can differ greatly. The higher the induction values included in the calculation, the greater the XX current.
Connection diagram for determining the number of turns per volt. You can select a ready-made transformer from among the standardized VT types,
TA, TNA, Chamber of Commerce and Industry and others. And if you need to wind or rewind
transformer for the required voltage, what should I do then?
Then you need to select a power transformer suitable for power
from an old TV, for example, a transformer and the like.
It must be clearly understood that how more quantity turns in the primary winding the greater its resistance and therefore the less heating and second, the thicker the wire, the more current can be obtained, but it depends on the size of the core - whether you can accommodate the winding.
What do we do next if the number of turns per volt is unknown?
For this you need a LATR, a multimeter (tester) and a measuring device alternating current —
ammeter. We wind the winding over the existing one at your discretion,
the diameter of the wire is any, for convenience we can wind it and simply install it
insulated wire.
Formula for calculating transformer turns
50/S
Related formulas:
P=U2*I2 (transformer power)
Sheart(cm2)= √ P(va) N=50/S
I1(a)=P/220 (primary winding current)
W1=220*N (number of turns of the primary winding)
W2=U*N (number of turns of the secondary winding)
D1=0.02*√i1(ma) D2=0.02*√i2(ma)
K=Swindow/(W1*s1+W2*s2)
50/S is an empirical formula where S is the area of the transformer core in cm2 (width x thickness), and is believed to be valid up to a power of about 1kW.
Having measured the area of the core, we estimate how much is needed
wind turns on 10 volts, if it is not very difficult, without disassembling
transformer we wind the control winding through the free
space (gap).
We connect the laboratory autotransformer to
primary winding and apply voltage to it, turn it on in series
control ammeter, gradually increase the voltage of the LATR-ohm until
appearance of no-load current.
If you plan to wind the transformer with enough
a “hard” characteristic, for example, it could be a power amplifier
transmitter in SSB, CW mode, where quite sharp
load current surges at high voltage (2500 -3000 V), for example,
then we set the no-load current of the transformer to about 10% of
maximum current, at maximum transformer load. Having measured
the resulting voltage of the wound secondary control winding, we make
calculation of the number of turns per volt.
Example: input voltage 220 volts, measured voltage of the secondary winding 7.8 volts, number of turns 14.
Calculate the number of turns per volt
14/7.8=1.8 turns per volt.
If you don’t have an ammeter at hand, you can use it instead
voltmeter, measuring the voltage drop across a resistor connected to the gap
applying voltage to the primary winding, then calculate the current from
obtained measurements.
An electrical transformer is a fairly common device used in everyday life to solve a number of problems.
And breakdowns can occur in it, which can be identified by a device for measuring electric current parameters - a multimeter.
From this article you will learn how to test a current transformer with a multimeter (ring), and what rules should be followed when doing this.
As you know, any transformer consists of the following components:
- primary and secondary coils (there may be several secondary ones);
- core or magnetic circuit;
- frame.
Thus, the list of possible breakdowns is quite limited:
- The core is damaged.
- A wire in one of the windings has burned out.
- The insulation is broken, resulting in electrical contact between the turns in the coil (turn-to-turn short circuit) or between the coil and the housing.
- Coil terminals or contacts are worn.
Current transformer T-0.66 150/5a
Some of the defects are determined visually, so the transformer must first be carefully inspected. Here's what you should pay attention to:
- cracks, chips of insulation or its absence;
- condition of bolted connections and terminals;
- swelling of the fill or its leakage;
- blackening on visible surfaces;
- charred paper;
- characteristic smell of burnt material.
If there is no obvious damage, you should check the device for functionality using instruments. To do this, you need to know which windings all its conclusions belong to. On larger transducers, this information can be presented in graphical form.
If there is none, you can use a reference book in which you can find your transformer by marking. If it is part of an electrical appliance, the data source may be a specification or a circuit diagram.
Methods for checking a transformer with a multimeter
First of all, you should check the insulation condition of the transformer. To do this, the multimeter must be switched to megger mode. After this, measure the resistance:
- between the housing and each of the windings;
- between the windings in pairs.
The voltage at which such a test should be carried out is indicated in the technical documentation for the transformer. For example, for most high-voltage models, insulation resistance measurements are prescribed to be carried out at a voltage of 1 kV.
Checking the device with a multimeter
The required resistance value can be found in the technical documentation or in the reference book. For example, for the same high-voltage transformers it is at least 1 mOhm.
This test is not capable of detecting interturn short circuits, as well as changes in the properties of wire and core materials. Therefore, it is imperative to check the performance characteristics of the transformer, for which the following methods are used:
Not all devices perceive a voltage of 220 volts. Reduces voltage to allow the use of electrical appliances.
How to check a varistor with a multimeter and what a varistor is needed for, read.
You can familiarize yourself with the rules for checking the voltage in an outlet with a multimeter.
Direct method (testing the circuit under load)
This is the one that first comes to mind: you need to measure the currents in the primary and secondary windings of a working device, and then, by dividing them by each other, determine the actual transformation ratio. If it corresponds to the passport, the transformer is working, if not, you need to look for a defect. This coefficient can be calculated independently if you know the voltage that the device should produce.
For example, if it says 220V/12V, then we have a step-down transformer, therefore, the current in the secondary winding should be 220/12 = 18.3 times higher than in the primary (the term “step-down” refers to voltage).
Scheme for testing a single-phase transformer by direct measurement of primary and secondary voltages using a standard transformer
The load must be connected to the secondary winding so that currents flow in the windings at least 20% of the rated values. When you turn it on, be on your guard: if you hear a crackling sound, there is a burning smell, or you see smoke or sparking, the device must be turned off immediately.
If the transformer under test has several secondary windings, then those that are not connected to the load should be short-circuited. In an open secondary coil, when the primary coil is connected to an alternating current source, high voltage may appear, which can not only damage the equipment, but also kill a person.
Serial connection of transformer windings using a battery and a multimeter
If we are talking about a high-voltage transformer, then before turning it on you need to check whether its core needs to be grounded. This is indicated by the presence of a special terminal marked with the letter “Z” or a special icon.
The direct method of checking a transformer allows you to fully assess the condition of the latter. However, it is not always possible to turn on the transformer with a load and make all the necessary measurements.
If due to safety requirements or other reasons this cannot be done, the condition of the device is checked indirectly.
Indirect method
This method includes several tests, each of which displays the state of the device in one aspect. Therefore, it is advisable to carry out all these tests together.
Determining the reliability of winding terminal markings
To carry out this test, the multimeter must be switched to ohmmeter mode. Next, you need to “ring” all available conclusions in pairs. Between those of them that belong to different coils, the resistance will be equal to infinity. If the multimeter shows a specific value, then the terminals belong to the same coil.
You can immediately compare the measured resistance with that given in the reference book. If there is a discrepancy of more than 50%, then an interturn short circuit or partial destruction of the wire has occurred.
Connecting a transformer to a multimeter
Please note that on coils with high inductance, that is, consisting of a significant number of turns, the digital multimeter may erroneously show an overestimated resistance. In such cases, it is advisable to use an analog device.
The windings should be checked with direct current, which the transformer cannot transform. When using an alternating voltage, an EMF will be induced in other coils and it is quite possible that it will be quite high. So, if an alternating voltage of only 20 V is applied to the secondary coil of a 220/12 V step-down transformer, then a voltage of 367 V will appear at the primary terminals and if they are accidentally touched, the user will receive a strong electric shock.
Next, you need to determine which terminals should be connected to the current source and which to the load. If it is known that the transformer is a step-down transformer, then the coil with the largest number of turns and the highest resistance must be connected to the current source. With a step-up transformer the opposite is true.
All methods for measuring electric current
But there are models that have both step-down and step-up coils among the secondary coils. Then the primary coil can, with a certain degree of probability, be recognized by the following characteristics: its terminals are usually attached away from the rest, and the coil can also be located on the frame in a separate section.
The development of the Internet has made this method possible: you need to take a photo of the transformer and write a request with the attached photo and all available information (brand, etc.) to one of the online thematic forums.
Perhaps one of its participants has dealt with such devices and can tell you in detail how it needs to be connected.
If the secondary coil has intermediate taps, it is necessary to recognize its beginning and end. To do this, you need to determine the polarity of the terminals.
Determining the polarity of winding terminals
As a meter, you should use a magnetoelectric ammeter or voltmeter, the polarity of the terminals of which is known. The device must be connected to a secondary coil. It is most convenient to use those models in which the “zero” is located in the middle of the scale, but in the absence of one, the classic one with the “zero” location on the left will do.
If there are several secondary coils, the others need to be bypassed.
Checking the polarity of phase windings of AC electrical machines
A small direct current must be passed through the primary coil. An ordinary battery can serve as a source, but a resistor must be included in the circuit between it and the coil to prevent a short circuit. An incandescent lamp can serve as such a resistor.
There is no need to install a switch in the primary coil circuit: just follow the multimeter needle to close the circuit by touching the wire from the lamp to the coil output, and immediately open it.
If the same poles from the battery and the multimeter are connected to the terminals of the coils, that is, the polarity is the same, then the arrow on the device will move to the right.
For a multi-polar connection - to the left.
At the moment the power is turned off, the opposite picture will be observed: with a unipolar connection, the arrow will move to the left, with a multi-polar connection - to the right.
On a device with a “zero” at the beginning of the scale, the movement of the needle to the left is more difficult to notice, since it almost immediately bounces off the limiter. Therefore you need to watch carefully.
Using the same scheme, the polarities of all other coils are checked.
A multimeter is a very necessary device for measuring current strength, which is used to identify malfunctions of certain devices. – read useful tips optionally.
Instructions for checking diodes with a multimeter are presented.
Removing the magnetization characteristic
To be able to use this method, you need to prepare ahead of time: while the transformer is new and known to be in good working order, its so-called current-voltage characteristic (volt-ampere characteristic) is measured. This is a graph showing the dependence of the voltage at the terminals of the secondary coils on the magnitude of the magnetizing current flowing through them.
Schemes for measuring magnetization characteristics
Having opened the circuit of the primary coil (so that the results are not distorted by interference from nearby power equipment), alternating current of varying strength is passed through the secondary, measuring the voltage at its input each time.
The power of the power supply used for this must be sufficient to saturate the magnetic circuit, which is accompanied by a decrease in the slope of the saturation curve to zero (horizontal position).
Measuring instruments must belong to an electrodynamic or electromagnetic system.
Before and after the test, the magnetic circuit must be demagnetized by increasing the current in the winding in several steps and then reducing it to zero.
As you use the device, you need to take the current-voltage characteristic at certain intervals and compare it with the original one. A decrease in its steepness will indicate the appearance of an interturn short circuit.
Video on the topic