How to test a three-phase motor with a multimeter. Checking the motor windings. Malfunctions and test methods Checking the resistance of the electric motor windings

When a household electrical appliance breaks down, you have to check all its components separately.

And if testing the sensors does not cause any difficulties - it is usually enough to check the resistance, then with the engine everything is not so simple.

This unit is much more complex, and in order to identify its malfunction, you need to know the testing procedure. Next, we’ll talk about how to test an electric motor with a multimeter.

If there is no mechanical damage in the engine, which is usually determined visually, then its malfunction in most cases is due to the following:

there is a break in the internal circuit;
a short circuit has occurred, that is, contact has appeared where there should not be one.

Both defects are detected. Difficulties arise only when checking: in most of them the winding has almost zero resistance and it has to be measured by an indirect method, for which you will need to assemble a simple circuit.

The most popular AC motors are:

Three-phase asynchronous motors also operate with single-phase power.
Asynchronous single- and two-phase capacitors with squirrel-cage rotor. Most household appliance motors belong to this type.
Asynchronous with wound rotor. Such a rotor has a three-phase winding. Motors with a wound rotor are used where it is necessary to regulate the rotation speed and reduce the starting current: in crane equipment, machine tools, etc.
Collector. Used in hand-held power tools.
Asynchronous three-phase with squirrel-cage rotor.

The popularity of motors of the latter type is explained by a number of advantages:

simplicity of design;
strength;
reliability;
low cost;
unpretentiousness (does not require maintenance).

All electric motors consist of two parts: stationary and rotating. The first in AC motors is called the stator, y is called the inductor; the second - respectively, the rotor and armature.

Repair of asynchronous motors

Of the asynchronous motors, the most common are two- and three-phase. They are tested differently. Let's look at each variety in detail.

Three phase motor

The stator winding of such a motor consists of three parts (phases), separated by 120 degrees and connected in a star or delta configuration. The engine operates when the following conditions are met:

winding is done in the correct order;
there is reliable insulation between the turns, as well as between live parts and the housing;
All connections have good electrical contact.

First, the insulation resistance between live parts and the housing is checked. It is more correct to do this with a megger - a tester capable of generating voltages up to 2500 V and measuring resistances up to 300 GOhm. A more common multimeter will also work: it will not allow you to accurately measure resistance, but it can detect breakdown. The measuring range switch is set to the maximum value - 2 or 20 MOhm.

Three-phase asynchronous motors

Measurements are performed in this order:

check the functionality of the device by applying the probes one to the other: normally, the display shows a meager value or a number with two zeros in front;
touch both probes to the motor housing: if there is contact, the multimeter will also show scanty resistance;
Continuing to hold one probe on the body, the second touches the terminals of each phase in turn: normally, the megohmmeter shows 500 - 1000 MOhm or more, the multimeter shows one (symbolizes infinity).

Winding integrity: this operation is convenient to perform by switching the multimeter to the continuity mode. If there is no break in the circuit, the device will sound a sound signal, that is, the user does not have to read the readings on the display. The ends of each winding are located in a terminal box. The absence of a sound signal or a high resistance value on the display indicates an open circuit.
Short-circuited turns: their resistance (a multimeter is enough) must be within certain limits. An overestimated value indicates a break, a low value indicates an interturn short circuit.

Finally, the resistance of the windings is measured. A difference of no more than 1 ohm is allowed.

With a larger mismatch, the winding with less inductance burns out due to the higher current.

Two-phase electric motor

The stator has two windings:

working;
launcher

They measure the resistance of each with a multimeter and compare: normally, the starting resistance is twice as high as that of the working one.

The motor is also checked for short circuits between live parts and the housing - according to the same scheme as a three-phase one.

Checking commutator electric motors

Commutator motors have sections or lamellas where the brushes fit.

Check procedure:

Use a multimeter to determine the resistance between adjacent lamellas. Normally, the values for each pair are the same. In the event of a break (infinitely high resistance) or short circuit (minuscule resistance), the engine tachometer is replaced.
The resistance between the commutator and the rotor housing is measured: normally it is infinitely high.
The stator windings are checked for integrity.
Check the resistance between the stator housing and live parts: normally - infinitely high.

A high-precision low-value resistor (about 20 Ohms) is connected in series with the coil. Resistors with a tolerance of no more than 0.05% are called high-precision. They have a gray stripe in their color marking (not to be confused with silver).
The coil-resistor circuit is connected to a DC source of 12 V or higher. The higher the voltage, the more accurate the measurements. A car battery or computer power supply is used as a 12 V source.
Use a multimeter to measure the voltage drop across the coil. It is important to observe polarity here: the probe connected to the COM port (negative potential) is shorted on the negative or ground side; the second (connects to the “V/Ω” connector) - from the “plus” side.

The multimeter measures voltage much more accurately than resistance - with an accuracy of up to 0.1 mV. This is what the indirect method is based on.

Then the coil resistance is calculated using the formula: Rcat = Ucat * Rres / (12 – Ucat), where

Rcat - coil resistance, Ohm;
Ucat - voltage drop across the coil, V;
Rres - resistor resistance, Ohm;
12 - power supply voltage, V.

Checking DC motors

Testing order:

Checking the winding resistance: such motors have low resistance, so it is also determined indirectly - by voltage and. You will need two multimeters: one used as a voltmeter, the other at the same time as an ammeter. The winding is supplied with power from a battery with a voltage of 4 - 6 V. The resistance is calculated using the formula: R = U / I.
Measuring the resistance of the armature windings and between the collector plates. Normally, the multimeter displays equal values.

For the resistance between the collector plates, the maximum permissible difference is 10%, in the presence of an equalizing winding - 30%.

Features of testing electric motors with additional elements

Electric motors are equipped with additional elements to optimize operation or protection.

Most often used:

Thermal fuses: disconnect the motor from the power supply when it reaches a temperature dangerous for the insulating materials. They are located on the housing (attached with a bracket) or under the winding insulation. In the second case, the test is easier to perform because the conclusions are easily accessible. You can determine which detachable legs are connected to the protective circuit using a multimeter or a phase indicator (similar to a screwdriver with a light bulb). Normally, the resistance between the terminals of the thermal fuse is very small (short circuit).
Thermal relay: often used instead of thermal fuses. Usually they are normally closed, but there are also open ones. To diagnose, using the markings on the relay body, in reference books or on the Internet, find the resistance of its components, then check their actual value with a multimeter. To search on the Internet, type the relay brand in the line followed by “Data Sheet”. If the thermal relay burns out, an analogue is selected based on its parameters.
Three-terminal engine speed sensors. Installed in washing machines. The main element of the sensor is a metal plate, on which a potential difference is formed when small currents are passed through it.

The sensor is powered through the two outer terminals. If you touch them with the probes of a multimeter in ohmmeter mode, it will normally display a negligible resistance.

Checking the third pin is only possible in operating mode when a magnetic field is present. Trying to ring the sensor on the go, that is, with the washing machine turned on, can lead to injury. It is safer to simulate the operating mode by removing the engine and powering the sensor separately. Pulses at the sensor output are formed by rotating the rotor.

A multimeter allows you to identify, if not all, but many breakdowns of an electric motor. Mainly, using continuity testing, breaks and short circuits are detected. Full diagnostics are carried out on special stands; a megohmmeter is required to measure the insulation resistance.

/ 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 body. 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 and avoid the appearance of unnecessary parts 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!

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.

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 makeshift 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 one to the output of the windings in the boron.

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 complete motor, the insulation resistance between the coils and the metal casing must be at least 20 MΩ. 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. Assemble a 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, just like internal combustion engines, electric powertrains can break down, causing problems with vehicle performance. 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 of an electric vehicle 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. In addition, overheating of the bearings, leakage of oil and abnormal operating noise are common problems. The most typical malfunctions of the electrical part include short circuits within 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 the 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 imprecise 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 increased sparking of the brushes is observed on the commutator, then it is worth checking the tightness of their fit, as well as conducting diagnostics for the presence of defects in the working surface of the 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 routine repair work, the power unit is dismantled and worn parts are subsequently partially replaced. Let's look at the order of performing all actions using the example of an asynchronous electric motor.

At the 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. Note! 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 engine will have to be sent for 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 the rated speed 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 engine 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 during inspection it is not possible to detect damage, then it is determined by uneven heating of the rotor winding to the 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 indicated damage, in a motor with a squirrel-cage rotor, the latter may be braked or rotating, and in motors with a wound rotor, the rotor winding may be open. 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 result from 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 gets into it directly as a result of storing the engine in a damp, 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 the operating temperature of the 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.

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 any 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 printed on a metal nameplate, which is attached to the outside of the engine. The marking plate contains important information about the technical specifications of this appliance. 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.

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 need to be equipped with lubrication fittings, and some are already lubricated during the production process.

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.

The next stage of verification is checking the motor winding for short circuit on his body. 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 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 the meter needle should deflect 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 must 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 problems with the motor windings, for example, about a breakup. 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 first be small, 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. In this place a number of electric motors are equipped with centrifugal switches, which switch start 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 with 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 openings on the metal grill must be sufficient for free air circulation; if this is not ensured, then the engine may overheat and subsequently it will 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:

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.

Types of electric motors

The most common electric motors are;

Three-phase asynchronous squirrel-cage motor

— asynchronous three-phase motor with a squirrel-cage rotor. Three motor windings are laid in the stator slots;
— asynchronous single-phase motor with a squirrel-cage rotor. It is mainly used in household electrical appliances in vacuum cleaners, washing machines, hoods, fans, air conditioners;
— brushed DC motors are installed in the electrical equipment of the car (fans, window regulators, pumps);
- AC commutator motor is used in electrical tools. Such tools include electric drills, grinders, hammer drills, meat grinders;
— an asynchronous motor with a wound rotor has a fairly powerful starting torque. Therefore, such motors are installed in lift drives, cranes, and elevators.

Winding insulation resistance measurement

Insulation resistance is usually checked with a megger

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

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

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. Assemble a series circuit from a 12V battery with a 20 ohm rheostat. Using a multimeter (ammeter), a rheostat is used to set the current 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

In this article I want to talk about how to detect a fault in the power supply circuit of a three-phase motor and how to check the motor itself.

Let's start in order.

1. The first thing to do is check for voltage at the circuit breaker(AB) or magnetic starter, i.e. Is there voltage coming from the electrical panel? You can check the voltage using a voltmeter or wherever there is a voltmeter. I do not recommend using a voltage indicator, because... You will determine the presence of input voltage, but not the absence of zero.

2. Check the circuit breaker itself and the magnetic starter for serviceability. Measure the voltage at the input contacts of both devices, and then at the output contacts (the machine must be turned on and the “Start” button pressed, if it is) going to the electric motor. If it is faulty (no voltage), then replace it with one of a similar voltage (220 or 380V) and current (A). If there is no voltage at the output contacts of the magnetic starter, then the contact plates are most likely burnt out. If possible, replace them; if not, then replace the entire starter with a similar one.

Fault: magnetic starter does not work

Check for voltage at the starter coil contacts. It should be remembered that coils are available in 220V and 380V.
If there is no voltage, replace the coil or starter. If voltage is supplied, then it is necessary to “ring” the coil to ensure the integrity of the winding. This can be done using an electrical tester (buzzer) or an electric breaker.
We check the serviceability and integrity of the “Start” and “Stop” buttons.

Button connection diagram:

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3. Checking the integrity of the electrical wire(cable) going to the electric motor.

You can check the integrity of the wire using the buzzer of an electrical tester or. You can also check using a test lamp or voltmeter. Turn off the automatic circuit breaker (AB), disconnect the wires from the electric motor. Then turn on the machine and check for voltage on the wires. Caution, live work!

If there is a possibility that a short circuit has occurred in the cable (soldering and wire breakage), then it is necessary to check the wires for short circuits among themselves. Turn off the machine, disconnect the wires from the electric motor. Using an electrical tester (buzzer) or electric breaker, we check the wires one by one for short circuits with each other.

4. We check the integrity of the windings of the electric motor itself.

Turn off the power supply (automatic).
It is better to disconnect the power wires from the electric motor.
Using an electrical tester (buzzer) or electric breaker, we check the integrity of the stator windings.
Using the same devices, we determine the presence or absence of a “breakdown” in the electric motor housing. One probe of the device is on the housing, the other is on the contact of the output of the electric motor winding. If the buzzer beeped and the needle on the breaker deviated, then a “breakdown” occurred on the electric motor housing - the Khan engine.

You can also check the integrity of the stator windings of an electric motor using a test light. But this is only the case when there are no other devices. We turn off the machine, disconnect the two power phase wires, and leave one. We turn on the machine, check the presence of voltage at all output contacts of the windings. If all the motor windings are intact, the indicator light will light up.

Caution, live work!

Repair of asynchronous motors

Three phase motor

Two-phase electric motor

Checking commutator electric motors

Checking DC motors

Features of testing electric motors with additional elements

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

Related Posts

Winding insulation resistance measurement

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

Basic electric motor malfunctions

Causes of electric motor malfunction

Typical motor problems

Troubleshooting electric motor

Methods for diagnosing faults in asynchronous electric motors

Motor Inspection

Checking other parts and other potential problems

Types of electric motors

Winding insulation resistance measurement

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

Fault: magnetic starter does not work