How to ring a three-phase pump. How to check an electric motor and its windings for integrity

In the article I talked about how to check, find and eliminate faults in commutator electric motors, which differ in that they have a brush-commutator assembly. Now I will tell you how to check, find the fault and repair asynchronous electric motor, which is the most reliable and easiest to manufacture of all types of motors. They are less common in everyday life (in a refrigerator compressor or in a washing machine), but they are often found in a garage or workshop: in machine tools, compressors, etc.

Repair or check DIY asynchronous electric motor will not be difficult for most people. The most common failure of asynchronous motors is wear of the bearings, and less often, breakage or dampness of the windings.

Most faults can be identified by external inspection.

Before connecting or if the motor has not been used for a long time, it is necessary to check its insulation resistance with a megger. Or if you don’t know an electrician with a megger, then it wouldn’t hurt to disassemble it for preventive purposes and dry the stator windings for several days.

Before you start repairs electric motor, it is necessary to check the presence of voltage and serviceability of magnetic starters, thermal relays, connection cables and a capacitor, if present in the circuit.

Checking the electric motor by external inspection

Full inspection can only be done after disassembling the electric motor, but do not rush to disassemble it right away.

All work is carried out only after shutdown power supply, checking its absence on the electric motor and taking measures to prevent its spontaneous or erroneous activation. If the device is plugged into a power outlet, then simply remove the plug from it.

If the circuit contains capacitors, then their conclusions must be discharged.

Check before disassembling:

Play in bearings. Read how to check and replace bearings.
Check paint coverage on the body. Burnt or peeling paint in places indicates that the engine is heating up in these places. Pay special attention to the location of the bearings.
Check your paws fastening the electric motor and the shaft together with its connection to the mechanism. Cracks or broken legs must be welded.

For example, the motor from an old washing machine has three outputs. The greatest resistance will be between two points, which includes 2 windings, for example 50 Ohms. If we take the remaining third end, then this will be the common end. If you measure between it and the 2nd end of the starting winding, you will get a value of about 30-35 Ohms, and if between it and the 2nd end of the working winding, about 15 Ohms.

In 380 Volt engines, connected according to the circuit, it will be necessary to disassemble the circuit and ring separately each of the three windings. Their resistance should be the same from 2 to 15 Ohms with deviations of no more than 5 percent.

You definitely need to call all windings are connected to each other and to the housing. If the resistance is not infinitely high, then there is a breakdown of the windings between themselves or to the housing. Such motors must be rewinded.

How to check the insulation resistance of electric motor windings

Unfortunately, Can't check with a multimeter to measure the insulation resistance of the electric motor windings, this requires a 1000-volt megohmmeter with a separate power source. The device is expensive, but every electrician at work who has to connect or repair electric motors has it.

When measuring one wire from the megger is connected to the body in an unpainted place, and the second in turn to each winding terminal. After this, measure the insulation resistance between all windings. If the value is less than 0.5 Megohm, the engine must be dried.

Be careful, To avoid electric shock, do not touch the test clamps while taking measurements.

All measurements are carried out only on de-energized equipment and for a duration of at least 2-3 minutes.

How to find turn-to-turn short circuit

The most difficult thing is to find the interturn closure, in which only part of the turns of one winding is closed to each other. It is not always detected during external inspection, therefore, for these purposes, an inductance meter is used for 380 Volt engines. All three windings must have same value. With an interturn short circuit, the inductance of the damaged winding will be minimal.

When I was in practice 16 years ago at a factory, electricians used a ball bearing with a diameter of about 10 millimeters to search for interturn short circuits in an asynchronous motor with a power of 10 Kilowatt. They took out the rotor and connected 3 phases through 3 step-down transformers to the stator windings. If everything is in order, the ball moves in a circle on the stator, and if there is an interturn short circuit, it is magnetized to the place where it occurs. The check must be short-term and be careful the ball may fly out!

I have been working as an electrician for a long time and check for an interturn short circuit if only a 380 V motor starts to get very hot after 15-30 minutes of operation. But before disassembling, with the motor turned on, I check the amount of current it consumes in all three phases. It should be the same with a slight correction for measurement errors.

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/ 27.07.2018

How to check an electric motor

A malfunction can be detected when the tool body is heated unevenly. When you touch it with your hand, you feel the temperature difference in different places of the case. In this case, the tool must be disassembled and checked with a tester and other methods.

If a short circuit occurs in the stator turns and troubleshooting, first of all we inspect the turns and terminals. As a rule, when a short circuit occurs, the current passing through the windings increases and overheating occurs.

A greater short circuit of turns occurs in the stator windings and the insulation layer is damaged. Therefore, we begin identifying faults by conducting a visual inspection. If no burns or damaged insulation are found, then proceed to the next step.

The cause of the breakdown may be a malfunction of the voltage regulator, which occurs when the excitation currents increase. To detect the problem, the brushes are checked; they must be ground evenly and free of chips and damage. Then you should check using a light bulb and 2 batteries.

Application of a multimeter

Now we need to check the possibility of a break in the stator windings. On the multimeter scale, set the switch to the resistance measurement sector. Without knowing the measurement value, we set the maximum value for your device. We check the functionality of the tester.

We touch each other with our probes. The arrow of the device should show 0. We carry out the work by touching the terminals of the windings. When an infinite value is shown on the multimeter scale, the winding is faulty and the stator should be rewound.

We check the possibility of a short circuit to the housing. Such a malfunction will cause a decrease in the power of the angle grinder, the possibility of electric shock and an increase in temperature during operation. The work is carried out according to the same scheme. Turn on the resistance measurement on the scale.

We place the red probe on the winding terminal, and attach the black probe to the stator housing. If the winding is short-circuited to the housing, the resistance value on the tester scale will be less than on a working one. This malfunction requires rewinding of the stator windings.

It's time to take measurements and check whether there is an interturn short circuit in the stator winding. To do this, the resistance value on each winding is measured. We determine the zero point of the windings by measuring the resistance for each of them. When the device shows the lowest winding resistance, it should be changed.

Checking the motor windings

An electronic rotor tester is a standard digital multimeter. Before you begin testing the circuit, you should check the multimeter and its readiness for use. The switch is set to measure resistance and the probes touch each other. The device should show zeros. Set the maximum measurement value and check:

This completes the rotor check. It is worth recalling the main stages of fault determination once again. Before checking, the angle grinder or any other device should be de-energized. Before taking measurements, you should visually inspect the housings, insulation and the absence of carbon deposits on the stator and rotor.

It is necessary to clean the contact surfaces from blockages with dust and dirt. Contamination causes an increase in current when the motor loses power.

When disassembling the instrument for the first time, write down all your steps. This will allow you to have a hint next time, avoiding the appearance unnecessary details during assembly. If the brush extends beyond the edge of the brush holder by less than 5 mm, such brushes should be replaced. You can check the interturn short circuit with an electronic tester, that is, a multimeter.

Checking the electric motor by external inspection

A full inspection can be carried out only after disassembling the electric motor, but do not rush to disassemble it right away.

All work is carried out only after turning off the power supply, checking that it is not present on the electric motor and taking measures to prevent its spontaneous or erroneous activation. If the device is plugged into a power outlet, then simply remove the plug from it.

If there are capacitors in the circuit. then their conclusions must be deflated.

Check before disassembling:

Play in bearings. Read this article on how to check and replace bearings.
Check the paint coverage on the body. Burnt or peeling paint in places indicates that the engine is heating up in these places. Pay special attention to the location of the bearings.
Check the motor mounting feet and the shaft together with its connection to the mechanism. Cracks or broken legs must be welded.

After disassembling according to these instructions, you need to check:

Part of the winding may burn out and an interturn short circuit will occur (in the picture on the left), or the entire winding (in the right picture). Despite the fact that in the first case the motor will work and overheat, it is still necessary to rewind the windings in any case.

How to ring an asynchronous electric motor

If nothing is revealed during an external inspection, then it is necessary to continue checking using electrical measurements.

How to test an electric motor with a multimeter

The most common electrical measuring instrument in the household is the multimeter. With its help, you can check the integrity of the winding and the absence of a breakdown on the housing.

In 220 Volt engines. It is necessary to ring the starting and working windings. Moreover, the starting resistance will be 1.5 times greater than that of the working one. For some electric motors, the starting and running windings will have a common third terminal. Read more about this here.

For example. The motor from an old washing machine has three terminals. The greatest resistance will be between two points, which includes 2 windings, for example 50 Ohms. If we take the remaining third end, then this will be the common end. If you measure between it and the 2nd end of the starting winding, you will get a value of about 30-35 Ohms, and if between it and the 2nd end of the working winding, about 15 Ohms.

In 380 Volt motors connected according to a star or delta circuit, it will be necessary to disassemble the circuit and ring each of the three windings separately. Their resistance should be the same from 2 to 15 Ohms with deviations of no more than 5 percent.

It is imperative to ring all the windings among themselves and on the housing. If the resistance is not infinitely high, then there is a breakdown of the windings between themselves or to the housing. Such motors must be rewinded.

How to check the insulation resistance of electric motor windings

Unfortunately, you cannot check the insulation resistance of the electric motor windings with a multimeter; for this you need a 1000-volt megohmmeter with a separate power source. The device is expensive, but every electrician at work who has to connect or repair electric motors has it.

When measuring, one wire from the megohmmeter is connected to the body in an unpainted place, and the second in turn to each terminal of the winding. After this, measure the insulation resistance between all windings. If the value is less than 0.5 Megohm, the engine must be dried.

Be careful. To avoid electric shock, do not touch the test clamps while taking measurements.

All measurements are carried out only on de-energized equipment and last for at least 2-3 minutes.

How to find turn-to-turn short circuit

The most difficult thing is to find an interturn short circuit. in which only part of the turns of one winding is connected to each other. It is not always detected during external inspection, therefore, for these purposes, an inductance meter is used for 380 Volt engines. All three windings must have the same value. With an interturn short circuit, the inductance of the damaged winding will be minimal.

When I was in practice 16 years ago at a factory, electricians used a ball bearing with a diameter of about 10 millimeters to search for interturn short circuits in an asynchronous motor with a power of 10 Kilowatt. They took out the rotor and connected 3 phases through 3 step-down transformers to the stator windings. If everything is in order, the ball moves in a circle on the stator, and if there is an interturn short circuit, it is magnetized to the place where it occurs. The check should be short-term and be careful the ball may fly out!

Interturn short circuit of the electric motor

Causes of interturn short circuit

If you have read previous articles, then you know that inter-turn short circuit of an electric motor accounts for 40% of electric motor malfunctions. There may be several reasons for an interturn short circuit.

Electric motor overload - the load on the electrical installation exceeds the norm, as a result of which the stator windings heat up and the insulation of the windings is destroyed, which leads to an interturn short circuit. Loading may occur due to improper operation of the equipment. The rated load can be determined from the electrical installation data sheet or read on the electric motor plate. Overload can also occur due to mechanical damage to the electric motor itself. Seized or dry bearings can also cause interturn “shortness”.

The possibility of a factory defect in the windings cannot be ruled out, and if the electric motor was rewound in a handicraft workshop, then there is a high probability that the “mezhvitnyak” is already knocking on your door.

Also, improper operation and storage of the electric motor can cause moisture to get inside the motor; damp windings are also a very common cause of interturn short circuits.

As a rule, with such a short circuit, the electric motor is no longer alive, and will work for a very short time. I think that’s enough of analyzing the reasons, let’s move on to the question “how to determine an interturn short circuit.”

Search for interturn short circuit.

Determining the interturn short circuit is not too difficult, and there are several handy methods for this.

If during operation of the electric motor some part of the stator heats up more than the entire engine, then you should think about stopping it and accurately diagnosing it.

Ordinary current clamps will also help determine the short circuit; we measure the load on each phase in turn and if on one of them it is greater than on the others, then this is a sign that there may be a winding inter-turn. But it should be taken into account that there may be a phase imbalance at the substation in order to verify the incoming voltage with a voltmeter.

You can ring the windings with a tester. To do this, we call each winding separately and compare the obtained resistance results. This method may not work if only a couple of turns are closed, then the discrepancy will be minimal.

It would not be superfluous to test the electric motor with a megger in search of a short circuit to the housing; we apply one probe to the motor housing, and the second to the output of the windings in turn.

If you still have doubts, then you will have to disassemble the electric motor. Having removed the covers and rotor, we visually examine the windings. It is likely that you will see a burnt part.

Well, the most accurate way to check the interturn circuit is to check using a three-phase step-down transformer (36-42 volts) and a bearing ball.

We supply three phases from a step-down transformer to the starter of the disassembled electric motor. With a little acceleration we throw the ball there, if the ball starts to run in a circle inside the stator, then everything is in order. If, after making a couple of revolutions, it sticks to one place, then there is an interturn short circuit.

Instead of a ball, you can use a plate from transformer iron, we apply it inside the stator to the iron and in the place where the interturn it starts to rattle, and where everything is fine the plate will be magnetized.

Be sure to use all of the above methods with a grounded electric motor and strictly using a step-down transformer.

Testing with a ball and a plate at a voltage of 380 volts is prohibited and is very dangerous for your life.

Winding insulation resistance measurement

To test a motor for insulation resistance, electricians use a megger with a test voltage of 500 V or 1000 V. This device measures the insulation resistance of motor windings designed for an operating voltage of 220 V or 380 V.

For electric motors with a rated voltage of 12V, 24V, a tester is used, since the insulation of these windings is not designed for testing under the high voltage of 500 V megger. Typically, the motor data sheet indicates the test voltage when measuring the insulation resistance of the coils.

Insulation resistance is usually checked with a megger

Before measuring the insulation resistance, you need to familiarize yourself with the connection diagram of the electric motor, since some star connections of the windings are connected at a midpoint to the motor housing. If the winding has one or more connection points, delta, star, single-phase motor with starting and running windings, then the insulation is checked between any connection point of the windings and the housing.

If the insulation resistance is significantly less than 20 MΩ, the windings are disconnected and each is checked separately. For a whole motor, the insulation resistance between the coils and metal body must be at least 20 Mohm. If the motor has been operated or stored in damp conditions, then the insulation resistance may be below 20 MΩ.

Then the electric motor is disassembled and dried for several hours with a 60 W incandescent lamp placed in the stator housing. When measuring insulation resistance with a multimeter, set the measurement limit to the maximum resistance, megohms.

How to test an electric motor for broken windings and interturn short circuits

Turn-to-turn short circuits in the windings can be checked with an ohm multimeter. If there are three windings, then it is enough to compare their resistance. The difference in the resistance of one winding indicates an interturn short circuit. The interturn short circuit of single-phase motors is more difficult to determine, since there are only different windings - this is the starting and operating winding, which has less resistance.

There is no way to compare them. You can detect the interturn short circuit of the windings of three-phase and single-phase motors using clamp meters, comparing the winding currents with their passport data. When there is an interturn short circuit in the windings, their rated current increases, and the starting torque decreases, the engine starts with difficulty or does not start at all, but only hums.

Checking the electric motor for open circuit and interturn short circuit of windings

It will not be possible to measure the resistance of the windings of powerful electric motors with a multimeter, because the cross-section of the wires is large and the resistance of the windings is within tenths of an ohm. It is not possible to determine the difference in resistance with such values using a multimeter. In this case, it is better to check the serviceability of the electric motor with a current clamp.

If it is not possible to connect the electric motor to the network, the resistance of the windings can be found by an indirect method. Collect series circuit from a 12V battery with a 20 ohm rheostat. Using a multimeter (ammeter), set the current with a rheostat to 0.5 - 1 A. The assembled device is connected to the winding being tested and the voltage drop is measured.

Testing the electric motor for open circuit and insulation resistance

A lower voltage drop across the coil will indicate an interturn short circuit. If you need to know the winding resistance, it is calculated using the formula R = U/I. A malfunction of the electric motor can also be determined visually, on a disassembled stator, or by the smell of burnt insulation. If a break point is visually detected, it can be eliminated by soldering a jumper, insulating it well and laying it down.

Measurement of the resistance of the windings of three-phase motors is carried out without removing jumpers on the star and delta winding connection diagrams. The resistance of the coils of DC and AC commutator motors is also checked with a multimeter. And if their power is high, the test is carried out using a battery-rheostat device, as indicated above.

The winding resistance of these motors is checked separately on the stator and rotor. On the rotor, it is better to check the resistance directly on the brushes by turning the rotor. In this case, it is possible to determine whether the brushes are not tightly attached to the rotor lamellas. Remove carbon deposits and irregularities on the collector lamellas by grinding them on a lathe.

It is difficult to do this operation manually; this malfunction may not be eliminated, and the sparking of the brushes will only increase. The grooves between the slats are also cleaned. A fuse or thermal relay can be installed in the windings of electric motors. If there is a thermal relay, check its contacts and clean them if necessary.

Basic electric motor malfunctions

Every year, gasoline engines are increasingly being replaced by electric motors installed in a new type of car called electric vehicles. However, like engines internal combustion, electric powertrains can break down, causing problems in the functioning of the vehicle. The majority of electric motor malfunctions occur due to severe wear of mechanism parts and aging of materials, which is reinforced by improper operation of such a vehicle. There can be many reasons for the appearance of characteristic problems, and we will now tell you about some (the most common) ones.

Causes of electric motor malfunction

All possible malfunctions An electric vehicle's engine can be divided into mechanical and electrical. The causes of mechanical problems include distortions of the electric motor housing and its individual parts, loosening of fastenings and damage to the surface of the constituent elements or their shape. Besides, common problem is overheating of the bearings, oil leakage from them and the appearance of abnormal operating noise. To the most typical malfunctions The electrical part is attributed to short circuits inside the windings of the electric motor, as well as between them, short circuits of the windings to the housing and breaks in the windings or in the external circuit, that is, in the supply wires and starting equipment.

As a result of the appearance of certain malfunctions, the following malfunctions may be observed in the operation of the vehicle: inability to start the motor, dangerous heating of the windings, abnormal rotation speed of the electric motor, unnatural noise (hum or knock), unequal current strength in individual phases.

Typical motor problems

Let's look at electric motor breakdowns in more detail, identifying their possible causes.

AC motor

Problem: when connected to the power supply, the electric motor does not develop the rated speed and makes unnatural sounds, and when the shaft is turned by hand, uneven operation is observed. The reason for this behavior is most likely a break in two phases when connecting the stator windings with a triangle, or a break when connecting a star.

If the engine rotor does not rotate, makes a strong hum and heats up above the permissible level, we can say with confidence that the stator phase is to blame. When the engine hums (especially when trying to start), and the rotor rotates at least slowly, the cause of the problem is often a break in the rotor phase.

It happens that with a rated load on the shaft, the electric motor operates stably, but its rotation speed is slightly lower than the rated one, and the current in one of the stator phases is increased. As a rule, this is a consequence of a phase failure when connecting the windings with a delta.

If at idle speed of the electric motor there is local overheating of the active steel of the stator, this means that due to damage to the inter-sheet insulation or burnout of the teeth due to damage to the winding, the sheets of the stator core are closed to each other.

When the stator winding overheats in certain places, when the engine cannot develop the rated torque and hums strongly, the cause of this phenomenon should be sought in a turn short circuit of one phase of the stator winding or an interphase short circuit in the windings.

If the entire electric motor overheats evenly, then the fan of the ventilation system is faulty, and overheating of the plain bearings with ring lubrication is due to one-sided attraction of the rotors (due to excessive wear of the liner) or poor fit of the shaft to the liner. When a rolling bearing overheats and produces abnormal noise, it is likely that the cause is contamination of the lubricant, excessive wear of the rolling elements and races, or inaccurate alignment of the unit shafts.

Knocking in the plain bearing and in the rolling bearing is explained by serious wear of the liner or destruction of the tracks and rolling elements, and increased vibration is a consequence of imbalance of the rotor due to interaction with pulleys and couplings, or the result of inaccurate alignment of the unit shafts and misalignment of the connecting coupling halves.

A DC electric motor may also have its own characteristic faults:

Under serious load, the machine’s armature may not rotate, and if you try to turn it by external force, the engine will run “staggered.” Reasons: poor contact or complete break in the excitation circuit, interturn or short circuits inside the independent excitation winding. Under conditions of rated values of the network voltage and excitation current, the armature rotation speed may be less or more than the established norm. In this case, the culprits for this situation are the brushes, shifted from the neutral position in the direction of rotation of the shaft or against it.

It may also be that the brushes of one sign spark a little stronger than the brushes of another sign. Perhaps the distances between the rows of brushes around the circumference of the commutator are not the same, or there is an interturn short circuit in the windings of one of the main or additional “pluses”. If the sparking of the brushes is also accompanied by blackening of the commutator plates, which are located at a certain distance from each other, then the culprit for this situation is most likely poor contact or a short circuit in the armature winding. Also, do not forget about the possibility of a break in the armature coil connected to the blackened plates.

In cases where only every second or third plate of the collector darkens, the cause of the malfunction may be a weakened compression of the collector or protruding micanite of the insulating tracks. Brushes can spark even with normal heating of the motor and a fully functional brush apparatus, which is explained by unacceptable wear of the commutator.

The reasons for increased sparking of brushes, overheating of the commutator and darkening of most of it are usually the insulation tracks (they say the commutator “beats”). When the motor armature rotates in different directions, the brushes also spark with different intensities. There is only one reason - the displacement of the brushes from the center.

If there is increased sparking of the brushes on the commutator, then it is worth checking the tightness of their fit, as well as conducting diagnostics for the presence of defects work surface brushes In addition, the reason may be uneven pressure of the brushes or their jamming in the brush holder. Naturally, if any of the listed problems are detected, it must be properly eliminated, but quite often only highly qualified specialists can do this.

Troubleshooting electric motor

High-quality overhaul of electric motors can only be carried out at specialized enterprises. During the current repair work the power unit is disassembled and subsequent partial replacement worn parts. Let's look at the order of performing all actions using the example of an asynchronous electric motor.

On initial stage Using a screw puller, remove the pulley or coupling half from the electric motor pulley. After this, you need to unscrew the bolts securing the fan casing and remove it. Next, using the same screw puller, you need to unscrew the locking screw and remove the fan itself. If necessary, the same tool can be used to remove the bearings from the motor shaft, and then, by unscrewing the fastening bolts, remove their covers.

After this, you should unscrew the bolts securing the bearing shields and remove these shields with light blows of a hammer through a wooden spacer. To avoid damaging the steel and windings, a cardboard spacer is placed in the air gap, onto which the rotor is lowered. Reassembling the electric motor is carried out in the reverse order.

After repair work is completed (the specifics depend on the nature of the breakdown), the electric motor should be tested. To do this, simply rotate the rotor by holding the pulley, and if the assembly is done correctly, the unit should rotate easily. If everything is normal, the motor is installed in place, connected to the network and checked for operation in idle mode, after which the motor is connected to the machine shaft and tested again. Let's look at options for troubleshooting an electric motor using the example of some typical breakdowns.

So, let's imagine that the motor does not start due to a lack of voltage in the network, the machine is turned off or the fuses are blown. The presence of voltage can be checked using a special device - an AC voltmeter with a 500 V scale, or using a low-voltage indicator. The problem can be resolved by replacing the blown fuses. Pay attention! If at least one fuse blows, the engine will make a characteristic hum.

A phase break in the stator winding can be detected using a megger, but before doing this, all ends of the motor windings must be freed. If a break is detected inside the winding phase, the motor will have to be sent to professional repair. The acceptable norm for reducing the voltage at the motor terminals when starting it is considered to be 30% of the nominal value, which is caused by losses in the network, insufficient power of the transformer or its overload.

If you notice a decrease in voltage at the electric motor terminals, you need to replace the supply transformer or increase the cross-section of the supply line wires. Lack of power supply contact in one of the stator windings (phase loss) causes an increase in current in the element windings and a decrease in the number of revolutions. If you leave the motor running on two windings, it will simply burn out.

In addition to the listed electrical problems, electric motors can also suffer from mechanical problems. Thus, excessive heating of the bearings is often caused by improper assembly of these parts, poor alignment of the motor, contamination of the bearings, or excessive wear of the balls and rollers.

In any case, before proceeding to direct action, you should conduct a complete diagnosis of the electric motor and the parts interacting with it. The inspection procedure begins with checking the battery, and if it is in good condition, then the next step is to check the power supply to the controller circuit (the computer that controls the rotation speed of the electric motor). It is quite possible that you will find a broken wire along the path from the battery to the board. The breakdown of an electronic board is not a frequent occurrence, but if there is even the slightest doubt about its serviceability, then it is better to immediately visually assess the condition of the part. If there has been strong heating of the board elements, you will immediately notice blackened and swollen areas with possible leaks.

In the case where the car owner has at least minimal knowledge in the field of electronics, he can independently check fuses, semiconductor parts (like diodes and transistors), all contacts, capacitances and soldering quality.

When the ECU output has operating voltage in the on state, then, as a rule, the cause of the malfunction should be sought in the electric motor itself. The complexity of repairing the unit depends on the specific malfunction and type of mechanism. So, when examining AC electric motors with rotary power, first of all, it is necessary to check the contact brushes, because they are most often the cause of breakdowns of motors of this type. After this, you should check the windings for breaks or short circuits. In the event of a break, the tester will not show any resistance value, and in the event of a short circuit, the resistance indicator will correspond to zero or one Ohm.

Having discovered a malfunction, it, of course, needs to be eliminated. This can be done either by repairing and replacing failed parts (for example, a brush), or by replacing the entire motor with a working analogue.

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Methods for diagnosing faults in asynchronous electric motors

The engine does not turn over when starting or its rotation speed is abnormal. The causes of this malfunction may be mechanical or electrical problems.

Electrical problems include: internal breaks in the stator or rotor windings, breaks in the supply network, disruption of normal connections in the starting equipment. If the stator winding breaks, a rotating magnetic field will not be created in it, and if there is a break in two phases of the rotor, there will be no current in the winding of the latter that interacts with the rotating field of the stator, and the engine will not be able to operate. If a winding break occurs while the motor is running, it may continue to operate at rated torque, but the rotation speed will be greatly reduced and the current will increase so much that without maximum protection, the stator or rotor winding may burn out.

If the motor windings are connected in a triangle and one of its phases is broken, the motor will begin to turn around, since its windings will be connected in an open triangle, in which a rotating magnetic field is formed, the current strength in the phases will be uneven, and the rotation speed will be lower than the nominal one. With this fault, the current in one of the phases in the case of the rated load of the motor will be 1.73 times greater than in the other two. When the motor has all six ends of its windings removed, a phase break is determined with a megohmmeter. The winding is disconnected and the resistance of each phase is measured.

The engine rotation speed at full load is lower than rated due to low mains voltage, poor contacts in the rotor winding, and also due to high resistance in the rotor circuit of a wound-rotor motor. With high resistance in the rotor circuit, the motor slip increases and its rotation speed decreases.

Resistance in the rotor circuit is increased by poor contacts in the rotor brush device, the starting rheostat, winding connections with slip rings, soldering of the frontal parts of the winding, as well as insufficient cross-section of cables and wires between the slip rings and the starting rheostat.

Poor contacts in the rotor winding can be identified if a voltage equal to 20-25% of the rated voltage is applied to the motor stator. The locked rotor is slowly turned by hand and the current strength in all three phases of the stator is checked. If the rotor is in good condition, then in all its positions the current strength in the stator is the same, and if there is a break or poor contact it will vary depending on the position of the rotor.

Poor contacts in the solders of the frontal parts of the phase rotor winding are determined by the voltage drop method. The method is based on increasing the voltage drop in places of poor-quality soldering. In this case, the voltage drop values are measured at all connections, after which the measurement results are compared. Solders are considered satisfactory if the voltage drop in them exceeds the voltage drop in solders with minimum values by no more than 10%.

Rotors with deep slots may also experience breakage of the rods due to mechanical overstressing of the material. The rupture of the rods in the groove part of the squirrel-cage rotor is determined as follows. The rotor is pushed out of the stator and several wooden wedges are driven into the gap between them so that the rotor cannot turn. A reduced voltage of no more than 0.25 Un is supplied to the stator. A steel plate is placed in turn on each groove of the protruding part of the rotor, which should overlap the two teeth of the rotor. If the rods are intact, the plate will be attracted to the rotor and rattle. If there is a gap, the attraction and rattling of the plate disappears.

The engine turns when the wound rotor circuit is open. The cause of the malfunction is a short circuit in the rotor winding. When turned on, the motor turns slowly, and its windings become very hot, since a large current is induced in the short-circuited turns by the rotating field of the stator. Short circuits occur between the clamps of the frontal parts, as well as between the rods when the insulation in the rotor winding is broken down or weakened.

This damage is determined by a thorough external inspection and measurement of the insulation resistance of the rotor winding. If damage cannot be detected during inspection, it is determined by uneven heating the rotor windings are felt by touch, for which the rotor is braked and a reduced voltage is applied to the stator.

Uniform heating of the entire engine above the permissible norm can result from prolonged overload and deterioration of cooling conditions. Increased heating causes premature wear of the winding insulation.

Local heating of the stator winding, which is usually accompanied by a strong hum, a decrease in the motor rotation speed and uneven currents in its phases, as well as the smell of overheated insulation. This malfunction can occur as a result of incorrect connection of the coils to each other in one of the phases, a short circuit of the winding to the housing in two places, a short circuit between two phases, a short circuit between the turns in one of the phases of the stator winding.

When there is a short circuit in the motor windings, the rotating magnetic field in the short-circuited circuit will induce e. d. s, which will create a large current, depending on the resistance of the closed circuit. A damaged winding can be found by the value of the measured resistance, while the damaged phase will have less resistance than the good ones. Resistance is measured using a bridge or ammeter-voltmeter method. The damaged phase can also be determined by measuring the current in the phases if a reduced voltage is supplied to the motor.

When connecting the windings in a star, the current in the damaged phase will be greater than in the others. If the windings are connected in a triangle, the line current in the two wires to which the damaged phase is connected will be greater than in the third wire. When determining the specified damage to an engine with squirrel cage rotor the latter may be braked or rotating, and in slip-wound rotor motors the rotor winding may be open-circuited. Damaged coils are determined by the voltage drop at their ends: on damaged coils the voltage drop will be less than on healthy ones.

Local heating of the active steel of the stator occurs due to burnout and melting of the steel during short circuits in the stator winding, as well as when the steel sheets are shorted due to the rotor touching the stator during engine operation or due to the destruction of insulation between individual sheets of steel. Signs of the rotor touching the stator are smoke, sparks and a burning smell; active steel in places of contact takes on the appearance of a polished surface; a humming sound appears, accompanied by engine vibration. The cause of contact is a violation of the normal gap between the rotor and stator as a result of wear of bearings, improper installation, large bending of the shaft, deformation of the stator or rotor steel, one-sided attraction of the rotor to the stator due to turn short circuits in the stator winding, strong vibration of the rotor, which determined with a probe.

Abnormal engine noise. A normally running engine produces a uniform hum, which is characteristic of all AC machines. An increase in humming and the appearance of abnormal noise in the engine may be a consequence of a weakening of the press-fit of the active steel, the packages of which will periodically be compressed and weakened under the influence of the magnetic flux. To eliminate the defect, it is necessary to repress the steel packages. Strong humming and noise in the machine can also be the result of an uneven gap between the rotor and stator.

Damage to the winding insulation can occur from prolonged overheating of the motor, moisture and contamination of the windings, metal dust and shavings getting on them, as well as as a result of natural aging of the insulation. Damage to the insulation can cause short circuits between phases and turns of individual winding coils, as well as short circuits of the windings to the motor housing.

Humidification of the windings occurs in the event of long breaks in the operation of the engine, when water or steam directly enters it as a result of storing the engine in damp conditions. unheated room etc.

Metal dust that gets inside the machine creates conductive bridges, which can gradually cause short circuits between the phases of the windings and to the housing. It is necessary to strictly observe the timing of inspections and scheduled preventive maintenance of engines.

The insulation resistance of motor windings with voltages up to 1000 V is not standardized; insulation is considered satisfactory with a resistance of 1000 ohms per 1 V of rated voltage, but not less than 0.5 MΩ at operating temperature windings

A short circuit of the winding to the motor body is detected with a megohmmeter, and the location of the short circuit is detected by “burning” the winding or by feeding it with direct current.

The “burning” method is that one end of the damaged phase of the winding is connected to the network, and the other to the housing. When current passes at the point where the winding is shorted to the housing, a “burn-through” is formed, smoke and the smell of burnt insulation appear.

The engine does not start as a result of blown fuses in the armature winding, breakage of the resistance winding in the starting rheostat, or broken contact in the supply wires. A break in the resistance winding in the starting rheostat is detected with a test lamp or megger.

Using a multimeter and several devices, without particularly understanding the principle of operation of electric motors, you can check:

Winding insulation test

Regardless of the design, the electric motor must be checked using a megger for insulation breakdown between the windings and the housing. Testing with a multimeter alone may not be sufficient to detect insulation damage, so use high voltage.

megohmmeter for measuring insulation resistance

The electric motor passport must indicate the voltage for testing the winding insulation for electrical strength. For engines connected to a 220 or 380 V network, 500 or 1000 Volts are used when testing them, but in the absence of a source, you can use the mains voltage.

asynchronous motor passport

The insulation of the winding wires of low-voltage motors is not designed to withstand such overvoltages, so when checking, you need to check the passport data. Sometimes, on some electric motors, the output of the star-connected windings can be connected to the housing, so you should carefully study the connection of the taps when doing a check.

Checking windings for open circuits and interturn short circuits

To test the windings for a break, you need to switch the multimeter to ohmmeter mode. An interturn short circuit can be identified by comparing the winding resistance with the passport data or with measurements of the symmetrical windings of the motor being tested.

It must be remembered that in powerful electric motors the cross-section of the winding wires is quite large, so their resistance will be close to zero, and ordinary testers do not provide such measurement accuracy of tenths of an ohm.

Therefore, you need to assemble a measuring device from a battery and a rheostat (approximately 20 Ohms) setting the current to 0.5-1A. The voltage drop across a resistor connected in series to the circuit of the battery and the measured winding is measured.

To check with the passport data, you can calculate the resistance using the formula, but you don’t have to do this - if identical windings are required, then the coincidence of the voltage drop across all measured terminals will be sufficient.

Measurements can be made with any multimeter

Digital multimeter Mastech MY61 58954

Below are algorithms for checking electric motors that have a necessary condition performance is the symmetry of the windings.

Checking asynchronous three-phase motors with squirrel-cage rotor

In such motors, only the stator windings can be ringed, the electromagnetic field of which induces currents in the short-circuited rotor rods that create a magnetic field that interacts with the stator field.

Malfunctions in the rotors of these electric motors are extremely rare, and to identify them, it is necessary special equipment.

motor rotor

To check a three-phase motor, you need to remove the terminal block cover - there are terminals for connecting the windings, which can be connected in a star type.

or "triangle". The dialing can be done without even removing the jumper -

It is enough to measure the resistance between the phase terminals - all three ohmmeter readings must match.

If the readings do not match, it will be necessary to disconnect the windings and check them separately. If the calculated resistance of one of the windings is less than that of the others, this indicates the presence of an interturn short circuit, and the electric motor must be rewinded.

Checking capacitor motors

To check a single-phase asynchronous motor with a squirrel-cage rotor, by analogy with a three-phase motor, it is necessary to ring only the stator windings.

But single-phase (two-phase) electric motors have only two windings - working and starting.

The resistance of the working winding is always less than that of the starting winding

Thus, by measuring the resistance, it is possible to identify the terminals if the plate with the diagram and symbols is worn out or lost.

Often, such motors have the working and starting windings connected inside the housing, and a common terminal is drawn from the connection point.

The affiliation of the terminals is identified as follows - the sum of the resistances measured from the common tap must correspond to the total resistance of the windings.

Checking brushed motors

Since AC and DC commutator motors have a similar design, the dialing algorithm will be the same.

First check the stator winding (in DC motors it can be replaced by a magnet). Then they check the rotor windings, the resistance of which should be the same, by touching the commutator brushes or opposite contact terminals with probes.

It is more convenient to check the rotor windings at the brush terminals by rotating the shaft, ensuring that the brushes contact only one pair of contacts - in this way you can detect burns at some contact pads.

Checking wound rotor motors

An asynchronous motor with a wound rotor differs from a conventional three-phase electric motor in that the rotor also has phase windings,

star-connected

which are connected using slip rings on the shaft. To check the rotor windings, you need to find the leads from these rings and make sure that the measured resistances match. Often such engines are equipped mechanical system turning off the rotor windings when the speed increases, so the lack of contact may be due to a breakdown in this mechanism.

The stator windings are checked as with a conventional three-phase motor.

Photos borrowed from the site http://zametkielectrika.ru

infoelectrik.ru

How to check an electric motor: testing steps and troubleshooting

To find out the cause of an electric motor problem, it will not be enough to simply inspect it; you need to check it thoroughly. This can be done quickly using an ohmmeter, but there are other ways to check. We will tell you how to check the electric motor below.

Motor Inspection

First, the inspection begins with a thorough inspection. If there are certain defects in the device, it may fail much earlier than the scheduled time. Defects may appear due to improper operation of the engine or its overload. These include the following:

broken stands or mounting holes;
the paint in the middle of the engine has darkened due to overheating;
the presence of dirt and other foreign particles inside the electric motor.

The inspection also includes checking the markings on the electric motor. It is applied to a metal nameplate, which is attached to the outside of the engine. The label contains important information O technical specifications of this device. As a rule, these are parameters such as:

information about the engine manufacturing company;
model name;
serial number;
number of rotor revolutions per minute;
device power;
diagram of connecting the motor to certain voltages;
scheme for obtaining one or another speed and direction of movement;
voltage – requirements in terms of voltage and phase;
dimensions and type of housing;
description of the stator type.

The stator on an electric motor can be:

closed;
blown by a fan;
splash-proof and other types.

How to check electric motor bearings?

After inspecting the device, you can begin to check it, and this should be done starting with the engine bearings. Very often, electric motor malfunctions occur due to their breakdown. They are needed to ensure that the rotor moves smoothly and freely in the stator. Bearings are located at both ends of the rotor in special niches.

The most commonly used types of bearings for electric motors are:

brass;
ball bearings.

Some require lubrication fittings and some are already lubricated during production.

Bearings should be checked as follows:

Place the engine on a hard surface and place one hand on its top;
turn the rotor with your second hand;
try to hear scratching sounds, friction and uneven movement - all this indicates a malfunction of the device. A working rotor moves calmly and evenly;
we check the longitudinal play of the rotor; to do this, it needs to be pushed by the axis from the stator. A maximum play of 3 mm is allowed, but no more.

If there are problems with the bearings, the electric motor runs noisily, they themselves overheat, which can lead to failure of the device.

How to check the winding of an electric motor?

The next stage of testing is to check the motor winding for a short circuit to its housing. Most often, a household motor will not work with a closed winding, because the fuse will blow or the protection system will trip. The latter is typical for ungrounded devices designed for a voltage of 380 volts.

An ohmmeter is used to check resistance. You can use it to check the motor winding in this way:

set the ohmmeter to resistance measurement mode;
we connect the probes to the required sockets (usually to the common “Ohm” socket);
select the scale with the highest multiplier (for example, R*1000, etc.);
set the arrow to zero, and the probes should touch each other;
we find a screw for grounding the electric motor (most often it has a hex head and is painted in green). Instead of a screw, any metal part of the case can be used, on which the paint can be scraped off for better contact with the metal;
We press the ohmmeter probe to this place, and press the second probe in turn to each electrical contact of the engine;
ideally an arrow measuring instrument should deviate slightly from the highest resistance value.

While working, make sure that your hands do not touch the probes, otherwise the readings will be incorrect. The resistance value should be shown in millions of ohms or megohms. If you have a digital ohmmeter, some of them do not have the ability to set the device to zero; for such ohmmeters, the zeroing step should be skipped.

Also, when checking the windings, make sure that they are not short-circuited or broken. Some simple single-phase or three-phase electric motors are tested by switching the ohmmeter to the lowest range, then setting the needle to zero and measuring the resistance between the wires.

To make sure that each of the windings is measured, you need to refer to the motor diagram.

If the ohmmeter shows a very low resistance value, it means that it either exists, or you touched the probes of the device. And if the value is too high, then this indicates a problem with the motor windings, for example, a break. If the resistance of the windings is high, the entire motor will not work, or its speed controller will fail. The latter most often concerns three-phase motors.

Checking other parts and other potential problems

You should definitely check the starting capacitor, which is needed to start some electric motor models. Basically these capacitors are equipped with a protective metal cover inside the motor. To check the capacitor you need to remove it. Such an inspection may reveal signs of problems such as:

oil leak from the condenser;
presence of holes in the body;
swollen capacitor housing;
unpleasant odors.

The capacitor is also checked using an ohmmeter. The probes should touch the terminals of the capacitor, and the resistance level should be small at first, and then gradually increase as the capacitor is charged with voltage from the batteries. If the resistance does not increase or the capacitor is short-circuited, then most likely it is time to change it.

Before re-testing, the capacitor must be discharged.

We move on to the next stage of engine testing: the rear part of the crankcase, where the bearings are installed. At this point, a series of electric motors are equipped with centrifugal switches that switch starting capacitors or circuits to determine the number of revolutions per minute. You also need to check the relay contacts for burnt marks. In addition, they should be cleaned of grease and dirt. The switch mechanism is checked using a screwdriver; the spring should work normally and freely.

And the final stage is checking the fan. We will look at this using the example of testing a TEFC engine fan, which is completely enclosed and air-cooled.

Make sure the fan is securely attached and not clogged with dirt or other debris. The holes on the metal grill must be sufficient for free air circulation; if this is not ensured, the engine may overheat and subsequently fail.

The main thing when choosing an electric motor is to select it in accordance with the conditions where it will be used. For example, splash-proof devices should be chosen for a humid environment, and open-type devices should absolutely not be exposed to liquids. Remember the following:

Splash-proof motors can be used in damp and damp places. Their design is such that liquid cannot enter the device under the pressure of gravity or water flow;
an open engine means that all its parts will be in plain sight. The devices have huge holes at the ends and the stator windings are clearly visible. These holes must absolutely not be blocked, and electric motors of this type cannot be used in wet, dirty or dusty areas;
TEFC type motors can be used everywhere, except for those conditions for which they are not designed, which can be read in the user manual for the device.

So, we have listed the most common problems that can occur with household electric motors. Almost all of them can be recognized and certain measures taken by checking the device. We discussed above how to check it correctly and what details you should pay attention to first of all.

stanok.guru

How to check an electric motor with a multimeter: step-by-step instructions and recommendations

The question often arises of how to check an electric motor after failure, as well as after repair, if it does not spin. There are several ways to do this: external inspection, a special stand, “testing” the windings with a multimeter. The last method is the most economical and universal, but it does not always give the correct results. For most constants, the winding resistance is practically zero. Therefore, an additional circuit for measurements will be required.

Motor design

To quickly learn how to check an electric motor, you need to clearly understand the structure of the main parts. All motors are based on two parts of the structure: the rotor and the stator. The first component always rotates under the action electromagnetic field, the second is motionless and just creates this vortex flow.

To understand how to check an electric motor, you will need to disassemble it at least once with your own hands. Different manufacturers have different designs, but the principle of diagnosing the electrical part remains unchanged for now. There is a gap between the rotor and stator in which small metal shavings can accumulate when the housing becomes depressurized.

When bearings wear out, they can produce excessive current readings, as a result of which the protection will be knocked out. When dealing with the question of how to check an electric motor, do not forget about mechanical damage to the moving parts and where the contacts are located.

Difficulties in diagnosis

Before checking the electric motor with a multimeter, you should conduct an external inspection of the housing, cooling impeller, check the temperature by touching your hand metal surfaces. A heated case indicates excessive current due to problems with the mechanical part.

You will need to analyze the condition of the insides of the boron, check the tightness of the bolts or nuts. If the connection of live parts is unreliable, failure of the windings can occur at any time. The surface of the engine must be free of contaminants and there must be no moisture inside.

If we consider the question of how to check an electric motor with a multimeter, then you need to take into account several nuances:

In addition to a multimeter, you will need pliers for non-contact measurement of the current passing through the wire.
A multimeter can only measure slightly high resistances. To check the condition of the insulation (where the resistance is from kOhm to MOhm) use a megohmmeter.
To draw conclusions about the suitability of the motor, you will need to disconnect the mechanical components (gearbox, pump and others) or you need to be sure that these components are in full working order.

Switching equipment

To start the rotation of the windings, a board or relay is used. To begin to understand the question of how to check the winding of an electric motor, you need to disconnect the supply circuit. The elements of the control board can “ring” through it, which will introduce an error in the measurements. With the wires folded back, you can measure the incoming voltage to be sure that the electronic circuit is working properly.

Motors in household appliances often use a design with starting winding, the resistance of which exceeds the value of the operating inductance. When taking measurements, take into account the fact that current-collecting brushes may be present. Carbon deposits often appear at the point of contact with the rotor; after cleaning it, you need to restore the reliability of the brushes during rotation.

Washing machines use small-sized motors with one working winding. The whole essence of diagnostics comes down to measuring its resistance. The current is measured less frequently, but by reading the characteristics at different speeds, conclusions can be drawn about the serviceability of the motor.

Electrical diagnostic details

Let's look at how to check the serviceability of an electric motor. First of all, inspect the contact connections. If there are no visible damages, then open the junction of the wires with the engine and disconnect them. It is advisable to determine the type of motor. If it is a collector type, then there are lamellas or sections where the brushes attach.

It is required to measure the resistance between each adjacent lamellas with an ohmmeter. It should be the same in all cases. If short-circuited sections or their breakage are observed, the motor tachometer needs to be replaced. If you “ring” the rotor coil itself, then 12 V of the multimeter may not be enough. To accurately assess the condition of the winding, an external power source will be required. It can be a PC unit or a battery.

To measure small resistance values, a resistor of a known value is installed in series with the winding being measured. It is enough to select a resistance of about 20 ohms. After applying power from an external source, the voltage drop across the winding and resistor is measured. The resulting value is obtained from the formula R1 = U1*R2/U2, where R2 is the resistor, U2 is the voltage drop across it.

Diagnostics of asynchronous motors

Industrial washing machines can use powerful three-phase electric motors. Their rotor is often made in the form of stacked plates with a magnetic core. Phase windings are often stationary and located in the stator.
It is much easier to check such a motor with a multimeter. Use an ohmmeter to measure the resistance of each winding. It should be the same. Do not forget to check the breakdown on the case by measuring the resistance on the case. However, it is more reliable to check the insulation with a megohmmeter.

When answering the question of how to check the windings of an electric motor with a tester, it should be noted that “phase imbalance” in an asynchronous motor is not allowed. The resistance difference should not exceed one ohm. Otherwise, the current at the smaller inductance increases, which leads to burning of the winding.

If the DC motor

These motors have very low winding resistance and measurements are carried out using two instruments. At the same time, take readings from the ammeter and voltmeter. A battery with a voltage of 4-6 V is selected as a source. The resulting value is determined by the formula R = U/I.

Check all available resistance of the armature windings, measure the values between the collector plates. All multimeter readings must be equal. From this comparison, we can draw conclusions on how to check the armature of an electric motor.

The difference in resistance readings between adjacent collector plates is allowed no more than 10%. When the design provides an equalizing winding, the operation of the motor will be normal with a difference in values of 30%. Multimeter readings do not always give an accurate forecast of the condition of the washing machine motor. Additionally, an analysis of the operation of the motor on a calibration stand is often required.

Checking the Direct Drive Motor

If we consider the question of how to check the electric motor of a washing machine, then we should take into account the type of connection of the drum to the shaft. The type of design of the electrical part depends on this. A multimeter is used to test the windings and draw conclusions about their integrity.

The performance check is carried out after replacing the Hall sensor. This is what fails in most cases. After checking the windings if they are intact experienced craftsmen It is recommended to connect the motor directly to a 220 V network. As a result, uniform rotation is observed; to change its direction, you can re-plug the plug into the socket, turning it with other contacts.

This simple method helps identify a common problem. However, the presence of rotation does not guarantee normal operation in all modes that differ during spinning and rinsing.

Diagnostic sequence

First of all, it is recommended to immediately pay attention to the condition of the brushes and wiring. Carbon deposits on live parts indicate abnormal operating conditions of the engine. The current collectors themselves must be smooth, without chips or cracks. Scratches also lead to sparking, which is detrimental to the motor windings.

The rotor of washing machines often warps, causing the lamellas to chip or break. The control board constantly monitors the rotor position through a Hall sensor or tachogenerator, adding or decreasing the voltage applied to the working winding. This results in strong noise during rotation, sparking, and disruption of operating modes during spinning.

This phenomenon can only be noticed during the spin cycle, and the washing cycle is stable. Diagnosing the operation of a machine does not always involve analyzing the state of the electrical part. Mechanics may be the cause of malfunction. Without load, the engine can spin quite evenly and gain speed steadily.

If he still knocks out the defense?

After the measurements have been taken, in the event of floating faults, it is not recommended to connect to the network for testing. You can permanently damage the motor without knowing there is a problem. A technician will tell you how to check the winding of an electric motor with a multimeter. service center by phone. Under his guidance, it will be easier to determine the type of design and procedure for diagnosing a faulty washing machine.

However, even experienced craftsmen often fail to repair complex cases where the fault is floating. To check in the service you need to use washing machine, mechanical components are critical. Motor shaft misalignment is a special case of problems with drum rotation.

fb.ru

How to test an electric motor with a multimeter

Not everyone has expensive special-purpose appliances at home. Therefore, you need to know how to test an electric motor with a multimeter: as a rule, you should have such a device at home.

Electric motors come in various modifications, and their malfunctions also vary. Not every breakdown can be diagnosed with a regular tester, but the vast majority are quite possible.

Any repair begins with a visual inspection: are there any broken parts, is the engine flooded, is there a smell of burnt insulation, and much more. Often burnt windings are visible to the naked eye, and in this case any measurements will be unnecessary: such a device is immediately sent for rewinding. But sometimes a more painstaking check is required.

Regular asynchronous
Other models

Regular asynchronous

Asynchronous electric motors are most often used in the two most common versions: three- and two-phase. Each of these models has its own nuances that need to be dealt with.

For three phases

Any, even the most complex unit, has only two malfunctions: lack of contact where it should be, and its presence in a place where it should not be. An AC motor, three-phase, consists of three coils connected in either a star or delta connection. The performance of such an electrical machine depends on correct winding, reliable contacts and high-quality insulation.

In the absence of a megohmmeter, it will not be possible to check the short circuit to the housing qualitatively, but it is still approximately possible. To do this, you need to set the resistance measurement values on the device to the maximum - megohms. Of course, this is not 500 or 1000 volts, however, with a “dead” ground it will be visible even at low voltage.

Make sure the motor is de-energized - trying to measure the resistance in the circuit connected to the electric motor. network will lead to failure of the device. Next, calibration is necessary: set the arrow to the zero position (with the probes closed).

Before any resistance measurement, it is important to briefly connect the probes to each other to ensure that the device is working properly and that all settings are set correctly.

We connect one of the probes to the engine housing. We make sure that there is a contact by connecting another contact of the ohmmeter to the body and observing the readings of the device. If everything is normal, use this probe to touch the output of each of the three phases in turn. With good insulation, such a test should reveal very high resistance - hundreds, or even thousands of megohms.

Someone may argue that according to the rules, an insulation resistance of no higher than 0.5 MOhm is allowed. This is true in relation to a megohmmeter with a power source of at least 500 V. We take measurements with a conventional tester with batteries that have an EMF of no higher than 9V. At what voltage will our engine operate? At 380 or 220 volts, therefore, you need to understand this difference and remember that, according to Ohm's law, the resistance value also depends on the voltage.

At the next stage, we make sure that each of the three windings is intact. For this purpose, it is enough to simply ring the three ends going into the boron el. engine. We don’t yet have the task of making any serious measurements: if the winding is broken, what’s the point of checking anything else? We need to repair this damage and only then move on.

Now you can try to identify short-circuited turns. Sometimes this is visible visually. If not, we define it differently. When the winding is short-circuited, there will be an asymmetrical current consumption from the electric circuit. networks. With a star connection, if winding A3 is damaged, the ammeter will determine an increased value in circuit A3, as can be seen in the figure. If we have a “triangle”, the greater value will be in A1 and A3 - the ends connected to the faulty section.

Two-phase

An electric motor with two coils is more often called single-phase, since it is connected to a regular electric motor. networks. Using an ohmmeter you need to ring the starting and operating windings. The starting resistance is 1.5 times higher than that of the working one - you need to build on this.

Let's take as an example washing machine old style. Its single-phase motor has three outputs. The highest resistance between the ends indicates that there are two coils connected in series. All that remains is to use an ohmmeter to find the midpoint - this way the ends of each of the coils will be determined separately.

Do not forget about the resistance to the body - there should be no breakdown. If the resistance is small, then the stator must be rewinded. Still, if there is such a possibility, it is better to do such a measurement with a megohmmeter with a voltage of 500 or 1000 volts.

Other models

Single-phase collector electric. engines can also be measured using the device.

Using a device switched on to units of ohms, we measure the resistance of the collector lamellas in pairs. The data obtained must be the same.
Now we measure the resistance between the commutator and the armature body. It should tend to infinity.
The next stage is checking the stator winding of a single-phase device.
As we did with the armature, we measure the resistance between the terminals and the stator housing. It should be as large as possible.

The interturn short circuit cannot be determined using a conventional device. It can be detected with a special device designed to detect armature faults.

A DC electric motor is a complex electric motor. car. The resistance of its excitation and additional poles is low, so they are checked either with a microohmmeter or with a double bridge.

The anchor can be measured special method voltmeter-ammeter. To do this, use a probe with springs and good insulation.

The brushes are removed from the armature, and a low constant voltage of 4–6 volts is applied to its plates. A millivoltmeter measures the voltage drop between these plates. Resistance is calculated using the formula: R=U*10-3 /I. The value on other plates is measured in the same way. They should differ from each other by no more than 10%.

Most of the faults that occur in electrical engines are diagnosed with a conventional multimeter. However, for more serious diagnostics of the performance of devices, special devices are used that are too expensive for household use, but having enough knowledge and experience, sometimes you can do without them.

When an electric motor does not work, it is often not enough to simply look at it to understand the reason. An electric motor that has been stored for a long time may or may not work regardless of its appearance. A quick check can be done using an ohmmeter, much more information is given below to correctly assess the condition of the electric motor.

Steps

Part 1

External inspection

Part 2

Bearing check

Start by checking the motor bearings. Many electric motor failures are caused by faulty bearings. Bearings allow the shaft (rotor) to rotate freely and smoothly in the stator. The bearings are located at both ends of the engine rotor shaft in bell-shaped niches.

There are several types of bearings that are used in electric motors. The two most popular types are brass plain bearings and ball bearings. Many of them have fittings for lubrication, while others have lubrication included during manufacture (“non-serviceable”).

Perform a bearing check. To perform a quick bearing inspection, place the motor on a hard surface and place one hand on top of the motor while rotating the rotor with the other hand. Watch carefully, try to feel and hear friction, scratching sounds, and uneven rotation of the rotor. The rotor should rotate calmly, freely and evenly.

Then check the longitudinal play of the rotor, push it, pull the rotor by the axis from the stator. A small backlash is acceptable (in the most common household engines the backlash should be no more than 3 mm), but the closer it is to “0”, the better. An engine that has problems with bearings runs noisily, the bearings overheat, which leads to engine failure.

Part 3

Checking the motor windings

Check the motor windings for short circuit to frame. Most household electric motors with closed windings will not work: most likely the fuse will blow or the circuit breaker will trip (motors designed for 380 Volts are “ungrounded”, so such motors can operate with windings shorted to the frame without the fuse blowing).

Use an ohmmeter to check resistance. Set the ohmmeter to resistance measurement mode, connect the probes to the appropriate sockets, usually the “common” and “Ohm” sockets (check the meter's instruction manual if necessary). Select the scale with the highest multiplier (R*1000 or similar) and set the needle to “0” with the probes touching each other. Find the screw intended for grounding the motor (they are often green, with a hex head) or any metal part of the case (if necessary, scrape off the paint to make good contact with the metal) and press one ohmmeter probe to this place, and the other probe to each of the electrical motor contacts. Ideally, the ohmmeter needle should barely deviate from the highest resistance. Make sure your hands do not touch the probes as this will cause inaccurate measurements.

The ohmmeter should indicate a resistance value in the millions of ohms (or "MΩ"). Sometimes the value can be as low as a few hundred thousand ohms (500,000 or so). This may be acceptable, but the higher the resistance value, the better.
Many digital ohmmeters do not offer the option to set the meter to “0”, so skip the “zero” if you have a digital ohmmeter.

Make sure that the motor windings are not torn or short-circuited. Many simple single-phase and 3-phase motors (used in household appliances and in industry, respectively) can be checked by simply switching the ohmmeter range to the lowest (RX*1), set the needle to zero again and measure the resistance between the motor wires again. Refer to the motor diagram to make sure you measure each winding.
- You can see a very low resistance value. The amount of resistance can be quite low. Make sure that your hands do not touch the ohmmeter probes, as this will cause inaccurate readings. Great value resistance indicates a potential problem with the motor windings, which may be open. A motor with high winding resistance will not run or its speed controller will not operate (this can happen with 3-phase motors).

Part 4

Troubleshooting other potential problems

Check the starting capacitor used to start some motors. Most capacitors are protected from damage by a metal cap on the outside engine. The cover must be removed to access the capacitor for inspection. A visual inspection will help detect oil leaks from the condenser, holes in the housing, a swollen condenser housing, a burning or smoking smell - all of these indicate potential problems.
- Testing the capacitor can be done using an ohmmeter. Touch the leads of the capacitor with the probes; the resistance should start low and gradually increase as the small voltage supplied from the ohmmeter batteries gradually charges the capacitor. If the capacitor remains shorted or the resistance does not increase, then there is likely a problem with the capacitor and will need to be replaced. The capacitor must be discharged before attempting this test again.
Check the rear of the engine crankcase where the bearing is installed. There, some engines have centrifugal switches used to switch the starting capacitor or to connect circuits that determine the number of revolutions per minute. Check the relay contacts to see if they are burnt, clean them of dirt and grease. Using a screwdriver, check the switch mechanism; the spring should work freely.

Check the fan. Type "TEFC" (totally closed, with air cooled electric motor). This type of engine has the fan blades behind a metal grille at the rear of the engine. Make sure the fan is secure and free of dirt and other debris. The holes in the metal grill must allow free air movement, otherwise the engine may overheat and fail.
Select the right engine for the conditions in which it will operate. In wet environments, splash-proof motors are used, and open motors should not be exposed to water or moisture.
- Splashproof motors can be installed in damp or damp locations and are designed so that water (or other liquids) cannot penetrate the motor due to gravity or the flow of water (or other liquid).
- An open engine, as the name suggests, is completely open. These motors have fairly large openings at the ends, and the stator windings are clearly visible. These openings should not be blocked and these motors should not be installed in damp, dirty or dusty locations.
- TEFC motors, on the other hand, can be used in all the applications mentioned above, but they should also not be used in conditions for which they are not designed.

This is not to say that it is uncommon for motor windings to be both "open" and "short" at the same time. At first glance, this may seem like an oxymoron, but in fact it is not. An example would be an "open" circuit caused by a foreign object that has entered the motor, or excessive supply voltage that literally causes the wires in the windings to melt and causes an open circuit. If the end of the molten copper wire comes into contact with the motor frame or another grounded part of the motor, it will cause a "short circuit". This doesn't happen often, but it can happen.
A NEMA Quick Reference This link provides you with typical seats and sizes of electric motors.

Measurement of the insulation resistance of the windings relative to the machine body and between the windings is carried out in order to check the insulation condition and the suitability of the machine for subsequent tests. It is recommended to measure:

in a practically cold state of the test machine - before the start of its testing according to the appropriate program;

regardless of the temperature of the windings - before and after testing the insulation of the windings for electrical strength relative to the machine body and between the windings with alternating voltage.

Measuring the insulation resistance of the windings should be carried out: at a rated winding voltage of up to 500 V inclusive - with a 500 V megohmmeter; at a rated winding voltage of over 500 V - with a megohmmeter of at least 1000 V. When measuring the insulation resistance of windings with a rated voltage over 6000 V, which have significant capacitance relative to the housing, it is recommended to use a 2500 V megohmmeter with a motor drive or with a static rectification circuit alternating voltage.

Measurement of insulation resistance relative to the machine body and between the windings should be carried out in turn for each circuit that has separate terminals, with the electrical connection of all other circuits to the machine body.

Measurement of the insulation resistance of three-phase current windings tightly connected in a star or triangle is carried out for the entire winding in relation to the housing.

Insulated windings and protective capacitors, as well as other devices permanently connected to the machine body, must be disconnected from the machine body while measuring their insulation resistance.

Measuring the insulation resistance of windings with direct water cooling should be carried out with a megohmmeter having internal shielding; in this case, the megohmmeter clamp connected to the screen should be connected to drainage collectors, which should not have a metal connection with external system powering the windings with distillate.

After measuring the insulation resistance of each circuit, it should be discharged electrical connection with a grounded machine body. For windings with a rated voltage of 3000 V and above, the duration of connection to the housing should be:

for machines with power up to 1000 kW (kVA) - at least 15 s;

for machines with a power of more than 1000 kW (kVA) - at least 1 min.

When using a 2500 V megohmmeter, the duration of connection to the housing must be at least 3 minutes, regardless of the power of the machine.

Measurement of the insulation resistance of embedded resistance thermal converters should be carried out with a 500 V megohmmeter.

Measurement of the insulation resistance of insulated bearings and shaft oil seals relative to the housing should be carried out at ambient temperature with a megohmmeter with a voltage of at least 1000 V.

Table 2.

Table 3.

Table 4.

Insulation resistance R from is the main indicator of the insulation condition of the stator and rotor of the electric motor.

Simultaneously with measuring the insulation resistance of the stator winding, the absorption coefficient is determined. Measurement of rotor insulation resistance is carried out for synchronous electric motors and electric motors with a wound rotor with a voltage of 3 kV and higher or a power of more than 1 MW. The rotor insulation resistance must be at least 0.2 MOhm.

The absorption coefficient in operation must be determined only for electric motors with a voltage above 3 kV or a power greater than e 1MW.

Prepare measuring instruments:

Check the charge level of the battery or accumulator for the MIC-2500 type megohmmeter.

Set the test voltage value.

If using an ESO202 type pointer device, install it horizontally.

For ES0202, set the required measurement limit, the instrument scale and the value of the megger test voltage.

Check the functionality of the megohmmeter. To do this, you need to close each other test leads and start rotating the generator handle at a speed of 120¸140 rpm. The instrument needle should show “0”. Open the measuring probes and begin to rotate the generator handle at a speed of 120-140 rpm. The arrow of the device should show “10 4 MOhm”.

Before taking measurements, it is necessary to open the input device of the electric motor (boron), wipe the insulators from dust and dirt and connect the megohmmeter according to the diagram shown in the figure.

Drawing. Measuring the insulation resistance of electric motor windings.

Figure A shows a diagram for connecting a megohmmeter to the electric motor under test, the windings of which are connected in a star or triangle inside the housing and it is impossible to disconnect it in a boron. In this case, a megohmmeter is connected to any terminal of the stator of the electric motor and the insulation resistance is measured for the entire winding immediately relative to the housing.

In Figure B, the insulation resistance is measured on the electric motor for each part of the winding separately, while the other parts of the winding (which are not currently being processed) are short-circuited and connected to ground.

When measuring insulation resistance, megohmmeter readings are taken every
15 seconds and the result is the resistance counted 60 seconds after the start of the measurement, and the ratio of the readings R 60 /R 15 is considered the absorption coefficient.

For electric motors with a rated voltage of 0.4 kV (electric motors up to 1000 V), a one-minute insulation measurement with a 2500 V megohmmeter is equivalent to a high-voltage test.

For synchronous electric motors, when measuring the insulation resistance of the stator windings (stator windings), it is necessary to short-circuit and ground the rotor winding. This must be done to eliminate the possibility of damage to the rotor insulation.

Today’s article is an answer to a reader’s question.

There will be questions and new articles.

Checking the electric motor by external inspection

How to check the insulation resistance of electric motor windings

How to find turn-to-turn short circuit

How to check an electric motor

Application of a multimeter

Checking the motor windings

Checking the electric motor by external inspection

How to ring an asynchronous electric motor

How to test an electric motor with a multimeter

How to check the insulation resistance of electric motor windings

How to find turn-to-turn short circuit

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