Electric motors often fail, and the main reason for this is interturn short circuit. It accounts for about 40% of all engine breakdowns. What causes a short circuit between the turns? There are several reasons for this.
The main reason is the excessive load on the electric motor, which is higher than the established norm. The stator windings heat up, destroy the insulation, and a short circuit occurs between the turns of the windings. By incorrectly operating an electric machine, an employee creates excessive load on the electric motor.
The normal load can be found in the equipment data sheet or on the motor plate. Excessive load may occur due to a breakdown of the mechanical part of the electric motor. Roller bearings may be the cause. They can jam due to wear or lack of lubrication, resulting in a short circuit in the armature coil turns.
Short circuits of turns also occur during repair or manufacturing of the engine, as a result of defects if the engine was manufactured or repaired in an unsuitable workshop. It is necessary to store and operate the electric motor according to certain rules, otherwise moisture may penetrate inside the motor, the windings will become damp, and as a result, a turn short circuit will occur.
With a turn short circuit, the electric motor does not work fully and does not last long. If the interturn short circuit is not detected in time, you will soon have to buy a new electric motor or a completely new electrical machine, for example, an electric drill.
When the motor winding turns are short-circuited, the excitation current increases, the winding overheats, destroys the insulation, and other winding turns are short-circuited. Due to an increase in current, the voltage regulator may fail. The turn circuit is determined by comparing the winding resistance with the standard according to the technical specifications. If it has decreased, the winding must be rewinded and replaced.
How to find turn-to-turn short circuit
The closure of turns is easy to determine; there are several methods for this. While the electric motor is running, pay attention to uneven heating of the stator. If one part of it heats up more than the motor housing, then it is necessary to stop work and carry out an accurate diagnosis of the motor.
There are devices for diagnosing short circuits of turns; you can check them with current clamps. It is necessary to measure the load of each phase in turn. If there is a difference in loads between the phases, you need to think about the presence of an interturn short circuit. You can confuse a turn short circuit with a phase imbalance in the power supply network. To avoid incorrect diagnosis, it is necessary to measure the incoming supply voltage.
The windings are checked with a multimeter by testing. We check each winding separately with the device and compare the results. If only 2-3 turns are closed, then the difference will not be noticeable, the short circuit will not be detected. Using a megger, you can test the electric motor, identifying the presence of a short circuit to the housing. We connect one contact of the device to the motor housing, the second to the terminals of each winding.
If you are not sure about the serviceability of the engine, then it is necessary to disassemble the engine. When disassembling, you need to inspect the windings of the rotor and stator; the location of the short circuit will probably be visible.
The most accurate method for checking the short circuit between the turns of the windings is to check with a step-down transformer on three phases with a ball bearing. We connect three phases from a transformer with reduced voltage to the disassembled stator of the electric motor. We throw the bearing ball inside the stator. The ball runs in a circle - this is normal, but if it is magnetized to one place, then there is a short circuit in that place.
Instead of a ball, you can use a plate from the transformer core. We also carry it out inside the stator. In the place where the turns are shorted, it will rattle, and where there is no short circuit, it will simply be attracted to the iron. During such checks, we must not forget about grounding the motor frame; the transformer must be low-voltage. Experiments with a plate and a ball at 380 volts are prohibited, it is life-threatening.
Homemade device for determining turn circuits
Let's make a choke with our own hands to check the interturn short circuit in the motor winding. We will need U-shaped transformer iron. It can be taken, for example, from the old vibration pump “Rucheek”, “Malysh”. We disassemble its lower part and heat it well. There are coils filled with epoxy resin.
We heat the epoxy and knock out the coils with the core. Using sandpaper or a grinder, we cut off the jaws of the core.
These coils are wound just on the U-shaped transformer iron.
No need to respect angles. You need to make a place where a small and a large anchor can easily fall.
When processing, it is necessary to take into account that the iron is laminated. You cannot treat it in such a way that the stone lifts it. It is necessary to process in such a direction that the layers lie towards each other so that there are no scuffs. After processing, remove all chamfers and burrs, since you will have to work with enameled wire; it is not advisable to scratch it.
Now we need to make two coils for this core, which we will place on both sides. We measure the thickness and width of the core in the widest places, along the rivets. We take thick cardboard and mark it according to the size of the core. We take into account the size of the groove in the core between the coils. We run the non-sharp edge of the scissors along the bend points to make it easier to bend the cardboard. We cut out the blank for the coil frame. Fold along the fold lines. This creates the frame of the coil.
Now we make four covers for each side of the coils. We get two cardboard frames for the reels.
We calculate the number of turns of the coils using the formula for transformers.
Divide 13200 by the cross-section of the core in cm 2. Section of our core:
3.6 cm x 2.1 cm = 7.56 cm 2.
13200: 7.56 = 1746 turns for two coils. This number is optional; a deviation of 10% in both directions will not play any role. Round up, 1800: 2 = 900 turns need to be wound on each coil. We have 0.16 mm wire, it will fit our coils just fine. You can wind it any way you like. 900 turns can be wound manually. If you make a mistake by 20-30 turns, then nothing bad will happen. Better to wind more. Before winding with an awl, we make holes along the edges of the frame for the output of the coil wires.
We put a heat shrink sleeve on the end of the wire. We insert the end of the wire into the hole, bend it, and begin winding the coil.
The filling turned out to be small, so you can wind it with thicker wire. We solder the wiring with a cambric to the other end and insert it into the hole. Do not wind the coil until the test has been carried out.
Both coils are wound. We put them on the core so that the wires go down and are on one side. The coils are wound exactly the same, the direction of the turns is in the same direction, the ends are brought out in the same way. Now you need to connect one end from one coil and one to the other, and apply 220 volts to the remaining two ends. The main thing is not to get confused and connect the correct wires. To understand the connection order, you need to mentally straighten our U-shaped core into one line, so that the turns in the coils are located in the same direction, moving from one coil to the second. We connect the two ends of the coils. We apply voltage to the two ends.
Let's compare a factory choke and a homemade one.
We check the factory choke with a metal plate for vibration in the place of turn short circuits of the motor armature and mark them with a marker. Now we do the same on our homemade throttle. The results were identical. Our new throttle is working fine.
We remove our coils from the core and secure the windings with electrical tape. We also insulate the solder with tape. We put the finished coils on the core, solder 220 V power to the ends of the wires. The inductor is ready for use.
Interturn closure of armature
To check the armature, we will use a special device, which represents a transformer with a cut-out core. When we place the armature in this gap, its winding begins to act as the secondary winding of a transformer. Moreover, if there is an interturn short circuit on the armature, the metal plate, which will be located on top of the armature, will vibrate or be magnetized to the armature body due to local oversaturation with iron.
We turn on the device. For clarity, we specially closed two lamellas on the collector to show how diagnostics are performed. We place the record on the anchor and immediately see the result. Our record became magnetized and began to vibrate. We turn the armature, the coils shift, and the plate stops vibrating.
Now let's remove the slat short circuit to check. We repeat the check and see that the armature winding is working properly, the plate does not vibrate in any places.
Method No. 2 of checking the armature for a turn short circuit
This method is suitable for those who are not involved in professional repair of power tools. To accurately diagnose an interturn short circuit, a bracket with a coil is required.
Using a multimeter you can only find out if the armature coil is broken. It is better to use an analog tester for this purpose. We measure the resistance between each two lamellas.
The resistance should be the same everywhere. There are cases when the windings are not burned out, the collector is normal. Then the closure of the turns is determined only using a device with a bracket from the transformer. Now we set the multimeter to 200 kOhm, connect one probe to ground, and touch the other to each lamella of the collector, provided that there are no broken coils.
If the armature does not connect to ground, then it is serviceable, or there may be an interturn short circuit.
Transformer interturn short circuit
Transformers have a common malfunction - the short circuit of the turns among themselves. It is not always possible to detect this defect with a multimeter. It is necessary to carefully inspect the transformer. The winding wire has varnish insulation; when it breaks down, there is a resistance between the turns of the winding that is not zero. This leads to heating of the winding.
When inspecting the transformer, there should be no burning, charred paper, swelling of the fill, or blackening. If you know the type and brand of the transformer, you can find out what the winding resistance should be. The multimeter is switched to resistance mode. Compare the measured resistance with reference data. If the difference is more than 50%, then the windings are faulty. If the resistance data could not be found in the reference book, then you probably know the number of turns, the type and cross-section of the wire, and you can calculate the resistance using the formulas.
To check with a low voltage output, we connect a voltage of 220 V to the primary winding. If smoke or smell appears, then immediately turn it off, the winding is faulty. If there are no such signs, then we measure the voltage with a tester on the secondary winding. If the voltage is reduced by 20%, there is a risk of failure of the secondary winding.
If there is a second serviceable transformer, then by comparing the resistances the serviceability of the windings is determined. To check in more detail, use an oscilloscope and a generator.
Stator interturn short circuit
Often a faulty motor has an interturn short circuit. First, check the stator winding for resistance. This is an unreliable method, since the multimeter cannot always accurately show the measurement result. This also depends on the motor rewinding technology and the age of the iron.
Clamps can also measure resistance and current. Sometimes they check by the sound of a running motor, provided that the bearings are in good condition, lubricated, and the drive gearbox is in good condition. They also check the turn-to-turn short circuit with an oscilloscope, but they are more expensive and not everyone has this device.
Externally inspect the engine. There should be no traces of oil, smudges, or smell. The current measured by phase must be the same. A good tester checks the windings for resistance. If the difference in measurements is more than 10%, there is a possibility of a short circuit in the winding turns.
Write comments, additions to the article, maybe I missed something. Take a look at, I will be glad if you find something else useful on mine.
The proposed indicator was developed to check for the presence of short-circuited (short-circuited) turns of the windings of various electrical devices - transformers, direct and alternating current machines, magnetic amplifiers, etc. To reduce material costs, their magnetic cores are often made of soft magnetic materials with relatively large specific losses. For this reason, it is often impossible to obtain reliable information about the presence of short-circuit turns in the traditional way - by disrupting the oscillations of a low-power generator, which is possible not only due to the presence of short-circuit turns, but also due to losses due to hysteresis and eddy currents in the magnetic circuit.
The principle of operation of the proposed device is based on recording the reaction of the shock excitation circuit formed by the built-in capacitor and the tested coil to a voltage pulse: if there are no short-circuited turns, then when a charged capacitor is connected to it, damped oscillations appear in the circuit, and if there are such turns, aperiodic ones occur.
The indicator diagram is shown in Fig. 1. It contains a capacitor C2, which, together with the tested coil L x, forms a shock excitation circuit; a switch on an assembly of field-effect transistors VT1, the operation of which is controlled by the SB1 button; An RS trigger on the elements of the DD1 microcircuit, which serves to suppress the bounce of the button contacts, a pulse shaper on the VT2 field-effect transistor and a binary counter on the DD2 chip. LED HL1 indicates the status of the counter "two or more".
The device works as follows. After turning on the power, the output of the RS trigger (pin 4 of element DD1.2) is set to a log level. Oh, so transistor VT1.1 is open and VT1.2 is closed. Through the open transistor VT1.1, capacitor C2 is charged to the voltage of the power source. Since it is greater than the threshold voltage of transistor VT2, the latter opens, connecting the CP input of the DD2.1 meter to the common wire. The counter triggers are set to an arbitrary state when the power is turned on.
To check the inductor L x connected to terminals X1 and X2, press and hold the SB1 button in this state. In this case, the RS trigger changes its state - a log level appears at the output (pin 4) of the DD1.2 element. 1. At the moment the RS trigger is switched, a short pulse appears at the output of element DD1.3 (pin 11), resetting counters DD2.1 and DD2.2. A high level at the gate closes transistor VT 1.1, disconnecting the charged capacitor C2 from the power source, and opens VT1.2, connecting the coil being tested in parallel with it. In the absence of short-circuited turns in the circuit L x C2, damped harmonic oscillations arise with a frequency depending on the capacitance and inductance of its elements. When capacitor C2 is recharged, transistor VT2 periodically opens, generating pulses that are sent to the input of counter DD2.1. As soon as the voltage amplitude in the circuit becomes less than the threshold voltage of transistor VT2, the flow of pulses to the counter input stops and at least one of the counter outputs is set to a log level of 1, so the HL1 LED lights up, signaling the serviceability of the tested coil. After releasing the button, the device returns to its original state. The counter is reset to zero again by a reset pulse from the output of element DD1.3.
If there are short-circuited turns in the coil, only one pulse is received at the counter input, and since output 1 (pin 3) of the DD2.1 counter is not connected to the OR element on diodes VD1-VD5, the HL1 LED does not respond to it. Circuit R3VD1-VD4 protects the gate of transistor VT2 from static electricity.
There are no special requirements for most parts of the probe: resistors and capacitors can be of any type, diodes - any low-power silicon, LED HL1 - any, preferably with increased brightness. The main requirement for transistor VT2 is a low threshold voltage. For transistors of the KP504 series, it does not go beyond 0.6...1.2 V, so you can use a transistor with any letter index. You can use the KP505G transistor (it has a threshold voltage of 0.4...0.8 V).
The device is assembled on a fragment of a universal breadboard measuring 50x30 mm. To facilitate installation of the VT1 transistor assembly (it is available in an SO-8 package with a lead pitch of 1.27 mm), an adapter board was made. To do this, a fragment was cut out of a breadboard for microcircuits with planar leads (Fig. 2), designed for mounting four pins with a pitch of 1.27 mm. A cut is made in the foil of the wide printed conductor on the opposite side of the fragment to create a gap between pins 5, 6 and 7, 8 of the assembly. The terminals of the adapter board are pieces of tinned copper wire with a diameter of 0.7 mm soldered to the resulting pads for pins 5-8 and soldered into round pads that end the printed conductors for pins 1-4. By bending the leads of the adapter board at the desired angle, it can be mounted either parallel to the main board or perpendicular to it. Unused inputs of the DD1 chip (pins 8, 9) should be connected either to the positive power line or to a common wire.
The assembled device, together with a power battery made up of four AAA-size elements connected in series, is placed in a housing, which can conveniently be used as a plastic soap dish. The position of the board in the case is fixed with pieces of foam rubber, and the halves of the case are fastened to one another with miniature self-tapping screws. The device does not require setup.
As the test showed, the indicator confidently detects the presence of short-circuit turns in transformers with a power ranging from several watts (a transformer from a network adapter) to several kilowatts (a welding transformer), and when connected to both the primary and secondary windings (the short-circuit turn was created artificially, by closing a piece of mounting wire passed through the window of the magnetic circuit). In devices with a branched magnetic circuit (three-phase transformers, magnetic amplifiers, etc.), it is necessary to check the windings on each rod. In AC machines, due to the different spatial orientation of the windings, the check should also be carried out winding by winding. In most cases, electric motors with a squirrel-cage rotor can be checked without disassembly - apparently, the air gap between the rotor and stator creates sufficient magnetic resistance, weakening the influence of short-circuited rotor turns (the need for disassembly arose only in cases where the device showed the presence of short-circuited turns in all windings). Motors of very different designs and power were tested - from low-power single-phase (EDG of various modifications, KD-3.5) to three-phase imported power of 3.5 kW (from a woodworking machine). Commutator motors must be checked at different armature positions.
Literature
1. Krivonos A. Determination of short-circuited turns in the windings of transformers and chokes. - Radio, 1968, No. 4, p. 56.
2. Dmitriev V. Device for determining interturn short circuits. - Radio, 1969, No. 2, p. 26.
3. Pozdnikov I. Probe for testing inductors. - Radio, 1990, No. 7, p. 68, 69.
Publication date: 16.01.2014
Readers' opinions
- Alexander0107 / 06.23.2016 - 22:22
IMHO, it is better to make a source follower instead of a shaper on the KP504 and IE10 counters, instead of a push-button control - a pulse generator with an adjustable period, and observe the oscillations at the follower output on an oscillator, then everything will be visible clearly and unmistakably. And the probe from Radio 1990 #7 actually generates even if there is an artificial short circuit. - Dmitry / 12/30/2015 - 15:54
The device does not operate using the method of detecting disruption of oscillations, since there is no master oscillator here at all. Shock excitation of the circuit is used on the test coil and the reference capacitor. Then the damped oscillations are counted until their amplitude reaches a certain minimum limit, at which the KP504 field switch stops opening. The counter counts them, and if it counts 2 or more impulses, it says “good”, less - bad. The problem is the opening threshold of the transistor and its low steepness. That is, it does not work well as a threshold device. I tried 2N7002. A comparator is asking for it instead - it should work much better. - Yuri / 03.08.2015 - 13:59
Have you tried to assemble it, we assembled it and it didn’t work for us, do you happen to have any typos in the diagram? We have a field effect transistor BSS 129 analogue of KP 503 since we didn’t find KP 504, do you have a printed circuit board, we really want to assemble it. Or write to me by email [email protected] - Sergey / 05/25/2014 - 11:58
The author is confusing something. There are a bunch of simple and reliable circuits, even those produced by industry, that work not to disrupt oscillations, but to change their parameters. A breakdown is usually when the winding is complete.
In addition to checking for a break, you must also check the coil for the absence of short-circuited turns inside it. It is impossible to check for a short circuit inside the winding using an ohmmeter without first disassembling it. Therefore, to identify such a defect, it is better to use a simple device, the diagram of which is shown in Fig. 40.
Using this device, you can determine the presence of short-circuited turns inside inductors or windings of small transformers, the internal diameter of which does not exceed 35 mm. In some cases, the device is able to detect short-circuited turns in coils of larger diameter. It should be noted that the device can be adapted to test coils of various sizes; for this it is only necessary to provide for the use of replaceable coils wound on rods of the appropriate diameter.
Diagram and principle of operation of the device. The device is assembled on a transistor, which makes it small-sized and very convenient to use. The HF oscillation generator is assembled on a P11A type transistor, but any other transistor that has the same parameters can be used. In the case of using p-p-p transistors, the polarity of connecting the generator to the power system must be reversed. The device is powered by a KBS-0.5 battery. Inductors L1—L3 are wound on a ferrite rod and have the following data: L1 contains 110 turns of PEL 0.15 wire; L2 - 210 turns of PEL wire 0.15; L3—55 turns of PEL wire 0.12—0.17. When assembling the device, the coils must be installed so that part of the ferrite rod (35-50 mm) is located above the upper part of the device body, since the test coil is placed on this part of the rod during testing. The operation of the device is based on the principle of absorbing vibration energy induced by a high-frequency generator in coil L3 when installed on a coil rod with short-circuited turns.
Change in induced e. d.s. is fixed by an indicator, with which you can determine the presence of defects in the coil. The device can use any microammeter of a magnetoelectric system with a total deviation current of 50-100 µA. Devices of the types M4204, M494, M49 are most suitable for this purpose (the latter type of device can be recommended in cases where the dimensions of the device are not critical, for example, when operating the device in stationary conditions).
The resistance of the additional resistor R2 should be selected experimentally when setting up the device, depending on the sensitivity of the indicator used. It is necessary to pay attention to the fact that if there is no test coil on the ferrite rod, the angle of deflection of the indicator needle would be at least 3/4 of the entire scale. This will allow you to clearly monitor changes in the indicator readings in the case when a defective coil is placed on the rod.
Mains powered version of the device. To sort coils under production conditions, you can use a simpler device, in which an incandescent light bulb is used instead of a dial indicator. The diagram of such a device is shown in Fig. 41. A light bulb (6.3 V, 0.1 A) is connected to the collector circuit of a transistor amplifier. The operating mode of the transistors is set using resistors R1 and R2.
It should be borne in mind that if, when setting up the device, a lack of generation is detected, then the ends of the coil L1 or L2 must be changed. The presence of generation can be judged by the deflection of the instrument needle or by the brightness of the light bulb.
The device is easy to manufacture and is made from standard parts. For the second device it is necessary to make a rectifier. To do this, you can use any low-power power transformer, from the secondary winding of which you can remove 12-15 V.
The operating mode and output voltage of the stabilizer, which includes diode D808 and transistor P201, are set using resistor R5.
It may happen that the wound coil does not contain short-circuited turns, and during operation doubt arises about its serviceability. How can you be sure of this? Do not disassemble the transformer to check the coil again. In such cases, another device will help, which allows you to check transformers, chokes and other inductors in assembled form.
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 the cascades. The depth of feedback depends on whether there are short-circuited turns in the coil being tested or whether they are absent. In the presence of closed turns, generation is interrupted. In addition, the circuit has negative feedback, which is regulated by potentiometer R5. It allows you to select the desired operating mode of the generator when testing coils with different inductances.
To monitor the generator voltage, the circuit contains an AC voltmeter. It consists of a milliammeter and two rectifier diodes. Alternating voltage is supplied through capacitor C5. This capacitor also serves as a limiter, allowing you to set a certain deviation of the milliammeter needle. Here it is advisable to use a milliammeter with a low deflection current (1 mA, 0.5 mA) so that the measuring circuit does not affect the operation of the generator.
Diodes of type D1, D2 with any letter index are suitable as rectifier diodes. When operating the generator, select the capacitance of capacitor C5 such that the milliammeter needle deviates to the middle of the scale. If this fails, place a resistor in series with the milliammeter and select its resistance according to the required needle deflection.
Take transistors like MP39-MP42 (P13-P15) with an average gain (40-50). Resistors can be of any type with a power starting from 0.12 W. You can take any buttons, switch, terminals too.
The device is powered by a Krona battery or any other source with a voltage of 7-9 V.
To assemble the device, use a wooden, metal or plastic box of suitable dimensions. On the front panel, attach the control knobs and a milliammeter, and on top there are terminals for connecting the coils under test.
How to use the device? Turn on the Vk toggle switch. The milliammeter needle should deflect approximately to the middle of the scale. Connect the terminals of the coil being tested to the “Lx” terminals and press the Kn1 button. Between the base of transistor T1 and the power plus, capacitor C1 will be connected, which, together with capacitor C2, will form a voltage divider, sharply reducing the coupling between the stages. If there are no short-circuited turns in the winding being tested, then the milliammeter readings may increase or decrease slightly. If there is even one short-circuited turn, the oscillations of the generator are disrupted and the needle returns to zero.
The position of the variable resistor R5 slider depends on the inductance of the coil being tested. If this is, for example, the winding of a power transformer or rectifier choke, which have high inductance, the motor should be in the extreme right position according to the diagram. As the inductance of the coil being tested decreases, the oscillation amplitude of the generator decreases, and with very small inductances, generation may not occur at all. Therefore, as the inductance decreases, the variable resistor slider needs to 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 - circuits 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 Kn2 button. 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 diagram:
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 is graphite and not wire.
100 Ω resistors should be soldered to the terminals of R5, then a cambric or heat-shrinkable tube should be placed on them.
Any of the following transistors are suitable: MP39B, MP40(A/B), MP41, MP41B, MP42, MP42B (or analogues). If you change the board layout, you can install transistors KT361 (except KT361A), KT209D or any other low-power P-N-P with Ku = 40...50.
Printed circuit board: 
(download in Sprint-Layout 5 format)
The circuit is taken from the brochure “The First Steps of a Radio Amateur - Issue 4/1971”, the printed circuit board was laid out by Alexander Tauenis.
ATTENTION! 05/13/2013 the board layout was updated, the new version is available via the same link. In addition to the original version for transistors MP39-42, the .lay file also includes versions with transistors KT361 (regular mounting) and KT361 (surface mounting, size 0805). The SMD version includes 1KΩ resistors, so you can use a regular 1KΩ variable resistor R5 without unnecessary distortions a la the 1960s.
If physics was taught well at your school, then you probably remember an experiment that clearly explained the phenomenon of electromagnetic induction.
Outwardly, it looked something like this: the teacher came to the class, the attendants brought some instruments 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.
The galvanometer in appearance was a flat box slightly larger in size than a standard A4 sheet, and behind the front wall, covered with glass, there was a scale with a zero in the middle. Behind the same glass one could see a thick black arrow. All this was quite distinguishable even from the very last desks.
The galvanometer leads were connected to a coil using wires, after which a magnet inside the coil was simply moved up and down 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 graduating from school, one physics teacher I knew told me that on the back wall of the galvanometer there was a secret handle, which was used to move the needle by hand if the experiment was unsuccessful.
Now such experiments seem simple and almost unworthy of attention. But electromagnetic induction is now used in many electrical machines and devices. In 1831, Michael Faraday studied it.
At that time there were not yet sufficiently sensitive and accurate instruments, so it took many years to figure out that the magnet should MOVE inside the coil. Magnets of different 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 alternating magnetic flux achieved by moving the magnet led to positive results.
Faraday's research proved that the electromotive force arising in a closed circuit (coil and galvanometer in our experiment) depends on the rate of change of the magnetic flux limited by the inner diameter of the coil. In this case, it makes absolutely no difference how the magnetic flux changes: 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 the current in the circuit under consideration (coil and external circuits) changes for some reason, then the magnetic flux causing the EMF will also change.
Such an EMF is called self-induced EMF. The remarkable Russian scientist E.Kh. studied this phenomenon. Lenz. In 1833, he discovered the law of 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 states that the direction of the induction current arising in a conducting closed circuit is such that it creates a magnetic field that counteracts the change in the magnetic flux that caused the appearance of the induction current.
In this case, the coil is in its own magnetic flux, which is directly proportional to the current strength: Ф = L*I.
In this formula there is a proportionality factor L, also called the inductance or self-inductance coefficient of the coil. The SI 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.
Just like a charged capacitor 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 established with a delay.
In exactly the same way, it does not immediately stop when disconnected. In this case, a self-inductive emf acts at 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 scans of televisions, as well as in the standard circuit for switching on fluorescent lamps. These are all useful manifestations of self-induced emf.
In some cases, the self-induction EMF is harmful: if the transistor switch is loaded with the winding of a relay coil or electromagnet, then to protect against self-induction EMF, a protective diode is installed parallel to the winding with the polarity of the back EMF of the power source. This inclusion is shown in Figure 1.
Figure 1. Protection of the transistor switch from self-induction EMF.
Doubts often arise as to whether there are short-circuited turns in the transformer or motor windings? For such checks, various devices are used, for example, RLC bridges or homemade probes. However, you can check for short-circuited turns using a simple neon lamp. Any lamp can be used - 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 test. The winding should have the highest inductance; if it is a mains transformer, then connect the lamp to the mains winding. After this, a current of several milliamps should be passed through the winding. For this purpose, you can use a power source with a resistor in series, as shown in Figure 2.
Batteries can be used as a power source. If at the moment the supply circuit is opened, a flash of the lamp 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).
Similar measurements can be carried out using a pointer avometer, such as TL-4, as batteries in the *1 Ohm resistance measurement mode. In this mode, the specified device produces 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 magnetic field strength.
Similar measurements can be carried out exactly the same way if the neon lamp is replaced with your own fingers: to increase the resolution of the “measuring device”, the fingers should be slightly wet. If the coil is working properly, you will feel a fairly strong electric shock, of course not fatal, but not very pleasant either.
Figure 2. Detection of shorted turns using a neon lamp.
