Definition of kz turns. Device for determining short-circuited turns in coils

If physics was taught well at your school, then you probably remember the experience that clearly explained the phenomenon of electromagnetic induction.

Outwardly, it looked something like this: the teacher came to the classroom, the attendants brought some devices and placed them on the table. After explaining the theoretical material, a demonstration of experiments began, clearly illustrating the story.

To demonstrate the phenomenon of electromagnetic induction, a very large size, a powerful straight magnet, connecting wires and a device called a galvanometer were required.

Galvanometer appearance It was a flat box slightly larger than a standard A4 sheet, and a scale with zero in the middle was placed behind the front wall, covered with glass. Behind the same glass one could see a thick black arrow. All this was quite distinguishable even from the very last desks.

The terminals of the galvanometer were connected to the coil with the help of wires, after which the magnet was simply moved up and down inside the coil by hand. In time with the movements of the magnet, the galvanometer needle moved from side to side, which indicated that current was flowing through the coil. True, after graduation from school, one friend of the physics teacher said that there was a secret handle on the back wall of the galvanometer, which set the arrow in motion by hand if the experiment did not work out.

Now such experiments seem simple and almost unworthy of attention. But electromagnetic induction is now used in many electrical machines and devices. Michael Faraday studied it in 1831.

At that time, there were still no sufficiently sensitive and accurate instruments, so it took many years to guess that the magnet should MOVE inside the coil. Magnets of various shapes and strengths were tried, the winding data of the coils also changed, the magnet was applied to the coil in different ways, but only the variable magnetic flux achieved by the movement of the magnet led to positive results.

Faraday's research proved that electromotive force, arising in a closed circuit (coil and galvanometer in our experience) depends on the rate of change of the magnetic flux, limited by the internal diameter of the coil. In this case, it is absolutely indifferent how the change in the magnetic flux occurs: either due to a change in the magnetic field, or due to the movement of the coil in a constant magnetic field.

The most interesting thing is that the coil is in its own magnetic field created by the current flowing through it. If for some reason the current changes in the circuit under consideration (coil and external circuits), then the magnetic flux that causes the EMF will also change.

Such an emf is called self-induction emf. The remarkable Russian scientist E.Kh. Lenz. In 1833, he discovered the law of the interaction of magnetic fields in a coil, leading to self-induction. This law is now known as Lenz's law. (Not to be confused with the Joule-Lenz law)!

Lenz's law says that the direction of the induction current that occurs in a conducting closed circuit is such that it creates a magnetic field that opposes the change in the magnetic flux that caused the appearance of the inductive current.

In this case, the coil is in its own magnetic flux, which is directly proportional to the current strength: Ф \u003d L * I.

In this formula, there is a proportionality factor L, also called the inductance or self-induction coefficient of the coil. In the SI system, the unit of inductance is called the henry (H). If, with a direct current of 1A, the coil creates its own magnetic flux of 1Wb, then such a coil has an inductance of 1H.

Like a charged capacitor that has a store of electrical energy, a coil through which current flows has a store of magnetic energy. Due to the phenomenon of self-induction, if the coil is connected to a circuit with an EMF source, when the circuit is closed, the current is set with a delay.

In exactly the same way, it does not immediately stop when it is turned off. In this case, the self-induction EMF acts on the coil terminals, the value of which significantly (tens of times) exceeds the EMF of the power source. For example, a similar phenomenon is used in car ignition coils, in line scanning of televisions, as well as in a standard switching circuit. fluorescent lamps. These are all useful manifestations of self-induction EMF.

In some cases, the self-induction EMF is harmful: if the transistor switch is loaded with the coil winding of a relay or electromagnet, then to protect against the self-induction EMF, a protective diode with the polarity of the reverse EMF of the power source is installed in parallel with the winding. This inclusion is shown in Figure 1.

Figure 1. Protection of the transistor switch from self-induction EMF.

Doubts often arise, but are there short-circuited turns in the transformer or motor windings? For such checks, various devices are used, for example, RLC - bridges or home-made devices - probes. However, you can check for short-circuited turns with a simple neon lamp. Any lamp can fit - even from a faulty Chinese-made electric kettle.

To carry out the measurement, a lamp without a limiting resistor must be connected to the winding under study. The winding should have the highest inductance; if it is a mains transformer, then connect the lamp to the mains winding. After that, a current of several milliamps should be passed through the winding. For this purpose, you can use a power supply with a resistor in series, as shown in Figure 2.

Batteries can be used as a power source. If at the moment of opening the supply circuit a lamp flash is observed, then the coil is in good condition, there are no short-circuited turns. (To make the sequence of actions clearer, Figure 2 shows a switch).

Such measurements can be carried out using a pointer avometer as batteries, such as TL-4 in the *1 Ohm resistance measurement mode. In this mode, the specified device gives a current of about one and a half milliamps, which is quite enough to carry out the described measurements. it cannot be used for these purposes - its current is not enough to create the necessary strength of the magnetic field.

Similar measurements can be made in exactly the same way if the neon lamp is replaced with one's own fingers: to increase the resolution " measuring device» Fingers should be slightly salivated. With a working coil, you will feel a fairly strong electric shock, of course not fatal, but not very pleasant either.

Figure 2. Detection of shorted turns with a neon lamp.

Probably, many people noticed, when checking the integrity of the windings of electric motors, transformers, chokes using a tester, that if you break the inductor-tester circuit, and then immediately accidentally touch the coil terminals, you can feel a slight electric shock. You can not attach any importance to this effect, you can think that the EMF of the self-induction of the coil is probably manifested, or you can think about it: is it possible to somehow benefit from this?

It turned out that it is possible, because. The self-induced emf of an inductor is a very specific voltage surge, the amplitude of which depends on the supply voltage of the circuit being broken, on the inductance of the coil and on its quality factor. During experimental verification, it turned out that if a neon lamp of the type TN-0.2, TN-0.3, etc., is connected in parallel with the coil under test, then when the circuit is broken, the power source-EMF coil of self-induction of the coil causes flashes of the neon lamp, which are the brighter , the higher the supply voltage of the circuit under test, the inductance of the coil and its quality factor.

It is this condition that meets the network windings of power transformers, simply high-voltage windings of transformers, windings of chokes with significant inductance, windings of electric motors, i.e. it is precisely those units of electrical equipment that are most susceptible to failure due to electrical overloads, leading to overheating of the windings, insulation failure between the turns of the winding and the appearance of short-circuited turns. K.z. coils can also appear due to mechanical damage windings. But in any case, when they appear, the inductor (winding) sharply reduces its quality factor, its resistance to industrial frequency currents decreases and it will heat up above the permissible value, i.e. it will become unsuitable for further use.

It turned out that if we assemble the test circuit shown in the figure, then serviceable inductors when the power circuit is broken (pressing the button) give bright flashes of a neon light bulb. And if there are short-circuited turns in the inductor, then there are no flashes at all, or they are very weak. It is this effect that is useful, because it allows you to identify unusable electrical products that are subject to culling or repair.

It is obvious that windings wound with a thick wire and having a small number of turns, i.e. small inductance, it will not be possible to check this method - even serviceable coils will not flash a neon light bulb. This must be taken into account in order not to draw erroneous conclusions. But for inductors having ohmic resistance direct current of the order of tens to hundreds of ohms or more, this scheme for detecting short-circuited turns is very convenient. Connector X1 can be of any type and is designed to connect a source constant voltage. The value of the supply voltage is not critical and can be in the range of 3 - 24 V, i.e. You can use any batteries or accumulators you have on hand. Toggle switch S1 serves to turn off the device during long breaks in operation. Lamp HL1 can be of any type for a voltage not lower than Epit. It is needed to control the supply voltage to the circuit (to prevent erroneous conclusions about the unsuitability of the tested coil). It is useful to have a known good coil of the same type next to the tested coils for comparative testing. Button S2 can be of any type and serves to break the power circuit when checking the coil. Resistor R1 Tr. (Other) serves to limit the current flowing through the neon bulb HL2. Х2, ХЗ - pins of the LU4 type with clips of the type<крокодил>, which, with flexible conductors soldered to them, are connected directly to the terminals of the inductor being tested.
Assembled without errors, the device does not need to be configured. It can be placed in any small-sized case. I want to draw the attention of novice radio amateurs that this way checks of inductors for the absence or presence of short-circuited turns should in no case be used to test radio-frequency coils, because tuning cores may be demagnetized or even conductors of the coils may burn out.

The circuit of the interturn tester and its operation are quite simple and accessible for assembly even by novice electronics engineers. Thanks to this device, it is possible to test almost any transformers, generators, chokes and inductors with a nominal value from 200 μH to 2 H. The indicator is able to determine not only the integrity of the winding under study, but also perfectly detects an interturn circuit, and in addition, it can be used to check p-n junctions for silicon semiconductor diodes.

It may happen that the wound coil does not contain short-circuited turns, and in the process of operation there is a doubt about its serviceability. How to make sure? Do not disassemble the same transformer to check the coil again. In such cases, another device will help, which allows you to check assembled transformers, chokes and other inductors.

The device is assembled on two transistors and is a low-frequency generator. The occurrence of oscillations occurs as a result of positive feedback between cascades. The depth of feedback depends on whether there are short-circuited turns in the coil being tested, or they are absent. In the presence of closed coils, the generation breaks down. In addition, the circuit has negative feedback, which is regulated by potentiometer R5. It allows you to select the desired mode of operation of the generator when testing coils with different inductances.
To control the voltage of the generator in the circuit there is an AC voltmeter. It consists of a milliammeter and two rectifier diodes. AC voltage is applied through capacitor C5. This capacitor also serves as a limiter, allowing you to set a certain deviation of the milliammeter needle. Here it is desirable to use a milliammeter with a low deviation current (1 mA, 0.5 mA) so that the measuring circuit does not affect the operation of the generator.
As rectifier diodes, diodes of the type D1, D2 with any letter index. When the generator is running, select the capacitance of the capacitor C5 so that the milliammeter needle deviates to the middle of the scale. If this fails, put a resistor in series with the milliammeter and select its resistance according to the required arrow deviation.
Transistors take type MP39-MP42 (P13-P15) with an average gain (40-50). Resistors can be of any type with a power of 0.12 W. Buttons, switch, terminals, you can also take any.
The device is powered by a Krona battery or any other source with a voltage of 7-9 V.
Use a suitable sized wooden, metal or plastic box to assemble the instrument. On the front panel, fix the control knobs and milliammeter, and on top of the terminals for connecting the coils under test.
How to use the device? Turn on the VK toggle switch. The arrow of the milliammeter should deviate approximately to the middle of the scale. Connect the terminals of the tested coil to the terminals "Lx" and press the Kn1 button. Between the base of the transistor T1 and the power plus, the capacitor C1 will be connected, which with the capacitor C2 will form a voltage divider, which sharply reduces the connection between the stages. If there are no short-circuited turns in the winding under test, then the milliammeter readings may increase or decrease slightly. In the presence of even one short-circuited coil, the oscillations of the generator break down, and the needle returns to zero.
The position of the slider of the variable resistor R5 depends on the inductance of the tested coil. If this is, for example, the winding of a power transformer or a rectifier choke, which have a large inductance, the engine should be in the far right position according to the diagram. With a decrease in the inductance of the tested coil, the amplitude of the oscillations of the generator decreases, and with very small inductances, generation may not occur at all. Therefore, with a decrease in inductance, the variable resistor slider must be moved to the left according to the circuit. This allows you to reduce the depth of negative feedback and thereby increase the voltage between the emitter and collector of transistor T1
When testing coils of very low inductance - the circuit of receivers with ferrite cores, the inductance of which is from 3 to 15 mH, it is additionally necessary to increase the depth of positive feedback. To do this, just press the button Kn2. The device can test coils with inductance from 3 mH to 10 H.

Attention!

If you cannot find a 1.2 kΩ variable resistor, assemble the circuit section near R5 according to the following scheme:

100Ω R5 1kΩ 100Ω To R3 (---[___]----[___]----[___]---) to R7 | To R6

The variable resistor must be single-turn and non-inductive, such as SP0, SP3, SP4 (or a foreign equivalent). The main thing is that the track should be graphite, not wire.

100 Ω resistors should be soldered to the R5 terminals, then put on them with cambric or heat shrink tubing.

Transistors are suitable for any of the series: MP39B, MP40 (A / B), MP41, MP41B, MP42, MP42B (or analogues). If you change the layout of the board, you can install transistors KT361 (except KT361A), KT209D or any other low-power P-N-P with Ku \u003d 40 ... 50.

Printed circuit board:


(download in Sprint-Layout 5 format)

The scheme is taken from the brochure "The first steps of a radio amateur - issue 4/1971", spread printed circuit board- Alexander Tauenis.

ATTENTION! 05/13/2013 updated board layout, a new version available at the same link. In addition to the original version for transistors MP39-42, the .lay file also includes versions with transistors KT361 (conventional mounting) and KT361 (surface mounting, size 0805). The SMD version includes 1kΩ resistors, so you can use a regular 1kΩ variable resistor R5 without any 1960s twists and turns.

Collected today and checked. Works.
R not less than 20 kOhm... on the board 10 kOhm. R2, R5, R6 at 470 ohms.
R1 10ohm
R2, R5, R6 820 ohms ... less is possible, but then R is needed with more resistance.
R3 47 kOhm
R4 365 Ohm
R7 10k
C1 - C3 30 nF
C4 0.5nF
L1 5 ohm 360 turns with 0.13 wire insulated
L2 10 ohm 460 turns with 0.09 mm wire insulated
Wound on 5 mm spools. I wound it by 10 mm and with a larger section and more coils. there were no smaller ones at hand.
The distance between the centers of the coils is 27 mm (important).
VD1 any diode
VD2 LED. Or 2 different or 2 color.
VT1 - VT5 any low-frequency transistor (in this case kt361). It is better to use not those on the board, but modern analogues.

S1 switch.
Power supply 3V.
The generator frequency should be 34.5 kHz .... there was nothing to check ... because. the oscilloscope was written off and dismantled, there is no personal money.

r.s. on the diagram, with a green marker, he marked what he drew on the printed circuit board.

the rosin was not washed off. this is a test instrument.
in the future I plan to do the same on a transistor assembly or common logic.
I drew the board in SL 6.0.