Electric motors often fail, and the main reason for this is the turn-to-turn 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. Incorrectly operating the electric machine, the worker creates an excessive load on the electric motor.
The normal load can be found in the passport for the equipment, or on the motor plate. Excessive load may occur due to a breakdown of the mechanical part of the electric motor. Rolling bearings can be the cause. They can jam from wear or lack of lubrication, as a result of which the armature coil turns will close.
The closure of the turns also occurs during the repair or manufacture of the engine, as a result of marriage, 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 can penetrate into the motor, the windings will become damp, as a result, a turn circuit will occur.
With a coil circuit, the electric motor does not work properly and for a short time. If the turn-to-turn short circuit is not detected in time, then soon you will have to buy a new electric motor or a completely new electric machine, for example, an electric drill.
When the turns of the motor winding are closed, the excitation current increases, the winding overheats, destroys the insulation, and other turns of the winding close. Due to the increase in current, it can cause the failure of the voltage regulator. The coil circuit is found out by comparing the winding resistance with the norm according to the technical conditions. If it has decreased, the winding must be rewound, replaced.
Closing turns is easy to determine, there are several methods for this. While the motor is running, pay attention to uneven heating of the stator. If one part of it is hotter than the engine housing, then it is necessary to stop work and conduct an accurate diagnosis of the motor.
There are devices for diagnosing the closure of turns, you can check it with current clamps. It is necessary to measure the load of each phase in turn. With a difference in loads on the phases, you need to think about the presence of an interturn circuit. You can confuse the turn circuit with the phase imbalance of the power supply. To avoid misdiagnosis, the incoming supply voltage must be measured.
The windings are checked with a multimeter by dialing. We check each winding with the device separately, compare the results. If only 2-3 turns are closed, then the difference will be imperceptible, the closure will not be detected. Using a megohmmeter, you can ring the electric motor, revealing the presence of a short to the case. We connect one contact of the device to the motor housing, the second to the terminals of each winding.
If there is no confidence in the serviceability of the engine, then it is necessary to disassemble the motor. When parsing, you need to inspect the windings of the rotor, stator, you will probably see the place of the circuit.
The most accurate method for checking the short circuit between winding turns is to check with a step-down transformer on three phases with a ball bearing. We connect three phases from a low-voltage transformer to the stator of the electric motor in disassembled form. We throw the bearing ball into the stator. The ball runs in a circle - this is normal, and if it is magnetized to one place, then there is a short circuit in this place.
Instead of a ball, you can use a plate from the transformer core. It is also carried out inside the stator. In the place where the turns are closed, it will rattle, and where there is no short circuit, it will simply be attracted to the iron. During such checks, one should not forget about the grounding of the motor case, the transformer must be low-voltage. Experiments with a plate and a ball at 380 volts are prohibited, it is life-threatening.
Let's make a choke with our own hands to check the interturn circuit in the motor winding. We need a U-shaped transformer iron. It can be taken, for example, from the old vibration pump "Brook", "Kid". We disassemble its lower part, heat it well. There are coils filled with epoxy resin.
We heat up the epoxy and knock out the coils with the core. With the help of emery or grinder, we cut off the sponges of the core.
These coils are wound just on a U-shaped transformer iron.
No corners required. It is necessary to make a place in which a small and a large anchor can easily fall.
When processing, it must be taken into account that the iron is puff. You can not process it so that the stone lifts it. It is necessary to process in such a direction that the layers lie to each other so that there are no burrs. After processing, remove all chamfers and burrs, as you will have to work with enameled wire, it is undesirable 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 a thick cardboard, 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 draw the non-sharp edge of the scissors along the fold points, so that it is more convenient to bend the cardboard. Cut out the blank for the coil frame. Fold along the fold lines. It turns out the frame of the coil.
Now we make four covers for each side of the coils. We get two cardboard frames for coils.
We calculate the number of turns of the coils according to the formula for transformers.
13200 divided by the cross section of the core in cm 2. The cross section of our core:
3.6 cm x 2.1 cm \u003d 7.56 cm 2.
13200: 7.56 = 1746 turns on two coils. This number is optional, a deviation of 10% in both directions will not play any role. Rounding up, 1800: 2 = 900 turns to be wound on each coil. We have a 0.16mm wire that will work fine for our coils. You can wrap it any way you like. 900 turns can also be wound by hand. If you make a mistake by 20-30 turns, then there will be nothing terrible. Better roll more. Before winding with an awl, we make holes along the edges of the frame to output the wire of the coils.
We put on a heat-shrinkable cambric at the end of the wire. We insert the end of the wire into the hole, bend it, and start winding the coil.
The filling turned out to be small, so you can wind the wire thicker. At the second end, solder the wiring with cambric 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 in exactly the same way, the direction of the turns is in the same direction, the ends are brought out 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 unbend our U-shaped core into one line so that the turns in the coils are located in one direction, moving from one coil to the second. We connect the two beginnings of the coils. Apply voltage to both ends.
Compare the choke factory and homemade.
We check the factory throttle with a metal plate for vibration of the place of the coil 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 works fine.
We remove our coils from the core, fix the windings with electrical tape. We also isolate the solder with tape. We put the finished coils on the core, solder 220 V power to the ends of the wires. The choke is ready for operation.
To check the armature, we use a special device, which is a transformer with a cut core. When we put an armature in this gap, its winding starts to work as a secondary winding of a transformer. In this case, if there is an inter-turn 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 specifically closed two lamellas on the collector to show how the diagnostics are performed. We place the plate on the anchor and immediately see the result. Our record became magnetized and began to vibrate. We turn the anchor, the turns are displaced, and the plate stops vibrating.
Now let's remove the lamella closure for verification. We repeat the check and see that the armature winding is working, the plate does not vibrate in any places.
This method is suitable for those who are not engaged in professional repair of power tools. For accurate diagnosis of an interturn circuit, a bracket with a coil is required.
With a multimeter, you can only find out a break in the armature coil. It is better to use an analog tester for this purpose. Between each two lamellas we measure the resistance.
The resistance should be the same everywhere. There are cases when the windings did not burn 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, we close one probe to ground, and with the other we touch each collector lamella, provided that there is no break in the coils.
If the anchor does not ring to ground, then it is serviceable, or there may be an inter-turn short circuit.
Transformers have a common fault - the circuit of the turns between themselves. It is not always possible to detect this defect with a multimeter. You need to carefully inspect the transformer. The winding wire has lacquer insulation; when it breaks down, there is a resistance between the turns of the winding, which is not equal to zero. It also leads to the heating of the winding.
When examining the transformer, it should not have burning, charred paper, swelling of the filling, blackening. If you know the type and brand of the transformer, you can find out what the resistance of the windings 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 the number of turns, the type and cross section of the wire are probably known, 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 there is smoke, smell, 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 underestimated 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 health of the windings is determined. To check in more detail, use an oscilloscope and a generator.
Often a faulty motor has an inter-turn short circuit. First, check the stator winding for resistance. This is an unreliable method, since the multimeter cannot always accurately show the measurement result. It also depends on the technology of rewinding the engine, on the old 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, the drive gearbox is working. They also check the interturn circuit with an oscilloscope, but they are more expensive, not everyone has this device.
Externally inspect the engine. There should be no traces of oil, smudges, smell. The current measured in phases must be the same. A good tester checks the windings for resistance. With a difference in measurements of more than 10%, there is a possibility of closing the turns of the windings.
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-circuit) turns of the windings of various electrical devices - transformers, DC and AC machines, magnetic amplifiers, etc. To reduce material costs, their magnetic circuits 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 hysteresis and eddy current losses in the magnetic circuit.
The principle of operation of the proposed device is based on registering the response of the shock excitation circuit formed by the built-in capacitor and the coil under test to a voltage pulse: if there are no short-circuited turns, then when a charged capacitor is connected to it, damped oscillations occur in the circuit, and if there are such turns, aperiodic ones.
The indicator scheme 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 the assembly of field-effect transistors VT1, the operation of which is controlled by the button SB1; 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 microcircuit. LED HL1 indicates the state of the counter "two or more".
The device works as follows. After turning on the power, the output of the RS flip-flop (pin 4 of the DD1.2 element) is set to the log level. Oh, so the transistor VT1.1 is open, and VT1.2 is closed. Through an open transistor VT1.1, capacitor C2 is charged to the voltage of the power source. Since it is greater than the threshold voltage of the transistor VT2, the latter opens by connecting the input of the CP counter DD2.1 to a common wire. The counter triggers are set to an arbitrary state when the power is turned on.
To check the inductor L xconnected to terminals X1 and X2, press and hold the SB1 button in this state. In this case, the RS-flip-flop changes its state - a log level appears at the output (pin 4) of the DD1.2 element. 1. At the moment of switching the RS-flip-flop, a short pulse appears at the output of the element DD1.3 (pin 11), resetting the counters DD2.1 and DD2.2. The transistor VT 1.1 closes at a high level at the gate, disconnecting the charged capacitor C2 from the power source, and VT1.2 opens, connecting the coil under test in parallel. In the absence of short-circuited turns in it, damped harmonic oscillations arise in the L x C2 circuit with a frequency depending on the capacitance and inductance of its elements. When the capacitor C2 is recharged, the transistor VT2 periodically opens, generating pulses that are fed to the input of the counter DD2.1. As soon as the voltage amplitude in the circuit becomes less than the threshold voltage of the transistor VT2, the flow of pulses to the input of the counter stops and at least one of the outputs of the counter is set to log 1, so the HL1 LED lights up, signaling the health of the coil under test. When the button is released, the device returns to its original state. The counter is again reset by a reset pulse from the output of the element DD1.3.
If there are short-circuited turns in the coil, only one pulse enters the counter input, and since output 1 (pin 3) of the DD2.1 counter is not connected to the OR element on the VD1-VD5 diodes, the HL1 LED does not respond to it. Circuit R3VD1-VD4 protects the gate of the 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 the 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 with dimensions of 50x30 mm. To facilitate the installation of the transistor assembly VT1 (it is available in the SO-8 package with a lead pitch of 1.27 mm), a riser 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 leads 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 the leads 5, 6 and 7, 8 of the assembly. Adapter board pins - pieces of tinned copper wire with a diameter of 0.7 mm are soldered to the resulting pads for pins 5-8 and soldered into round pads that terminate the printed conductors for pins 1-4. By bending the riser pins 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 battery, made up of four AAA-sized cells connected in series, is placed in a case, which can be conveniently 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 one to the other with miniature self-tapping screws. The device does not require adjustment.
As the test showed, the indicator confidently detects the presence of short-circuit turns in transformers with power from a few watts (transformer from a network adapter) to several kilowatts (welding transformer), moreover, when connected to both the primary and secondary windings (the short-circuit coil was created artificially, by closing a piece of the 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 test should also be carried out 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 the stator creates sufficient magnetic resistance, which weakens the effect of short-circuited turns of the rotor (the need for disassembly arose only in those cases when the device showed the presence of short-circuit turns in all windings). Motors of very different design and power were tested - from low-power single-phase (EDG of various modifications, KD-3.5) to three-phase imported with a power of 3.5 kW (from a woodworking machine). Collector 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
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.
The galvanometer in appearance 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 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 the electromotive force that occurs 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 the standard circuit for switching on 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 exactly the same if the neon lamp is replaced with one's own fingers: to increase the resolution of the "measuring device", the 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.
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 with an ohmmeter without first disassembling it. Therefore, to detect such a defect, it is better to use a simple device, the diagram of which is shown in Fig. 40.
With 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 can 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 interchangeable coils wound on rods of the appropriate diameter.
Scheme and principle of operation of the device. The device is assembled on a transistor, which made it small-sized and very convenient to use. The RF oscillation generator is assembled on a P11A type transistor, however, any other transistor with the same parameters can be used. In the case of using transistors of the p-p-p type, 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 wire 0.15; 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 above the upper part of the device case, since the tested coil is put on this part of the rod during testing. The operation of the device is based on the principle of absorption of the energy of vibrations induced by a high-frequency generator in the L3 coil when a coil with short-circuited turns is installed on the rod.
Change of induced e. d.s. is fixed by an indicator, with the help of which it is possible to establish the presence of marriage in the coil. Any microammeter of the magnetoelectric system with a total deflection current of 50-100 μA can be used in the device. Instruments of types M4204, M494, M49 are most suitable for this purpose (the latter type of instrument can be recommended when the dimensions of the instrument are not critical, for example, when operating the instrument in stationary conditions).
The resistance of the additional resistor R2 should be selected empirically when setting up the device, depending on the sensitivity of the indicator used. It is necessary to pay attention to the fact that in the absence of the tested coil on the ferrite rod, the angle of deviation of the indicator needle would be at least 3/4 of the entire scale. This will allow you to clearly monitor the change in the indicator readings in the case when a defective coil is put on the rod.
Mains powered version. For sorting coils under production conditions, you can use a simpler device in which an incandescent light bulb is used instead of a dial indicator. A diagram of such a device is shown in Fig. 41. A light bulb (6.3 V, 0.1 A) is included in the collector circuit of the transistor amplifier. The operating mode of the transistors is set by 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 deviation of the arrow of the device or by the brightness of the light bulb.
The device is easy to manufacture, made of 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 12-15 V can be removed.
The mode of operation and the output voltage of the stabilizer, which includes the D808 diode and the P201 transistor, are set using the resistor R5.
