How to test a 3-phase electric motor with a multimeter. How to check an electric motor

Ideally, in order to check the windings of an electric motor, it is necessary to have special instruments designed for this, which cost a lot of money. Surely not everyone has them in their home. Therefore, it is easier for such purposes to learn how to use a tester that has a different name. Almost every self-respecting home owner has such a device.

Electric motors are manufactured in various versions and modifications, and their malfunctions are also very different. Of course, not every fault can be diagnosed with a simple multimeter, but most often checking the motor windings with such a simple device is quite possible.

Any type of repair always begins with an inspection of the device: the presence of moisture, whether parts are broken, the presence of a burning smell from the insulation and other obvious signs of malfunction. Most often, the burnt winding is visible. Then no checks and measurements are needed. Such equipment is immediately sent for repair. But there are times when there are no external signs of failure, and a thorough check of the motor windings is required.

Types of windings

If you don’t go into details, the motor winding can be imagined as a piece of conductor that is wound in a certain way in the motor housing, and it seems that nothing should break in it.

However, the situation is much more complicated, since the electric motor winding has its own characteristics:

The material of the winding wire must be uniform along its entire length.
The shape and cross-sectional area of the wire must have a certain accuracy.
In industrial conditions, a layer of insulation in the form of varnish must be applied to the wire intended for winding, which must have certain properties: strength, elasticity, good dielectric properties, etc.
The winding wire must provide strong contact when connected.

If there is any violation of these requirements, then the electric current will flow under completely different conditions, and the electric motor will deteriorate its performance, that is, the power, speed will decrease, and may not work at all.

Checking the motor windings of a 3-phase motor . First of all, disconnect it from the circuit. The majority of existing electric motors have windings connected according to circuits corresponding to.

The ends of these windings are usually connected to blocks with terminals that have the appropriate markings: “K” - end, “N” - beginning. There are options for internal connections, the nodes are located inside the motor housing, and different markings (numbers) are used on the terminals.

The stator of a 3-phase electric motor uses windings that have equal characteristics and properties, and the same resistance. When measuring the winding resistances with a multimeter, it may turn out that they have different values. This already makes it possible to assume that there is a malfunction in the electric motor.

Possible faults

It is not always possible to visually determine the condition of the windings, since access to them is limited by the design features of the engine. In practice, you can check the winding of an electric motor using its electrical characteristics, since all motor breakdowns are mainly detected:

A break, when the wire is broken or burnt out, no current will pass through it.
A short circuit caused by damaged insulation between the input and output turns.
A short circuit between turns, with the insulation damaged between adjacent turns. As a result, damaged turns are self-excluded from operation. Electric current flows through the winding, which does not involve damaged turns that do not work.
By breaking through the insulation between the stator housing and the winding.

Methods

Checking the motor windings for open circuits

This is the simplest type of verification. The malfunction is diagnosed by simply measuring the wire resistance value. If the multimeter shows very high resistance, then this means that there is a wire break with the formation of air space.

Checking the motor windings for short circuits

If there is a short circuit in the motor, its power will be cut off by the installed short circuit protection. This happens in a very short time. However, even in such a short period of time, a visible defect in the winding may occur in the form of carbon deposits and metal melting.

If you measure the winding resistance with instruments, you get a small value that approaches zero, since a piece of the winding is excluded from the measurement due to a short circuit.

Checking the motor windings for interturn short circuits

This is the most difficult task in identifying and troubleshooting. To check the motor winding, several measurement and diagnostic methods are used.

Checking the motor windings using an ohmmeter

This device operates on direct current and measures active resistance. During operation, the winding forms, in addition to active resistance, a significant inductive resistance value.

If one turn is closed, then the active resistance will practically not change, and it is difficult to determine it with an ohmmeter. Of course, you can accurately calibrate the device, carefully measure all windings for resistance, and compare them. However, even in this case it is very difficult to detect shorted turns.

The results are much more accurately produced by the bridge method, which measures active resistance. This method is used in a laboratory environment, so ordinary electricians do not use it.

Current measurement in each phase

The phase current ratio will change; if a short circuit occurs between the turns, the stator will heat up. If the engine is fully operational, then the current consumption is the same in all phases. Therefore, by measuring these currents under load, we can confidently say about the actual technical condition of the electric motor.

Checking motor windings with alternating current

It is not always possible to measure the total winding resistance and take into account the inductive reactance. For a faulty motor, you can check the winding with alternating current. For this, an ammeter, a voltmeter and a step-down transformer are used. To limit the current, a resistor or rheostat is inserted into the circuit.

To check the motor winding, a low voltage is applied and the current value is checked, which should not be higher than the nominal value. The measured voltage drop across the winding is divided by the current to obtain the total resistance. Its value is compared with other windings.

The same circuit makes it possible to determine the current-voltage properties of the windings. To do this, you need to take measurements at various current values, then write them down in a table or draw a graph. There should not be large deviations when comparing with other windings. Otherwise, there is an interturn short circuit.

Checking the motor windings with a ball

This method is based on the formation of an electromagnetic field with a rotating effect if the windings are in good condition. They are connected to a symmetrical voltage with three phases, low value. For such tests, three step-down transformers with the same data are used. They are connected separately for each phase.

To limit the load, the experiment is carried out in a short period of time.

Voltage is applied to the stator windings, and a small steel ball is immediately introduced into the magnetic field. When the windings are in good condition, the ball rotates synchronously inside the magnetic circuit.

If there is a short circuit between the turns in any winding, the ball will immediately stop where there is a short circuit. When carrying out the test, the current must not be allowed to exceed the rated value, since the ball can fly out of the stator at high speed, which is dangerous for humans.

Determining the polarity of windings using the electrical method

The stator windings have terminal markings, which sometimes may not be there for various reasons. This creates difficulties during assembly.

To determine the marking, several methods are used:

and an ammeter.
and a voltmeter.

The stator acts as a magnetic circuit with windings operating on the principle of a transformer.

Determining the marking of winding terminals with an ammeter and battery

On the outer surface of the stator there are six wires from three windings, the ends of which are not marked and must be determined by their affiliation.

Using an ohmmeter, find the terminals for each winding and mark them with numbers. Next, mark one of the end and beginning windings, randomly. A dial ammeter is connected to one of the remaining two windings so that the arrow is in the middle of the scale to determine the direction of the current.

The negative terminal of the battery is connected to the end of the selected winding, and the positive terminal briefly touches its beginning.

The pulse in the first winding is transformed into a second circuit, which is closed by an ammeter, and repeats the original shape. If the polarity of the windings coincides with the correct location, then the arrow of the device at the beginning of the pulse will go to the right, and when the circuit opens, the arrow will move to the left.

If the instrument readings are completely different, then the polarity of the winding terminals is reversed and marked. The remaining windings are checked in a similar way.

Determining polarity with a voltmeter and a step-down transformer

The first stage is similar to the previous method: they determine whether the terminals belong to the windings.

The other two windings are connected randomly with two terminals at one point, the remaining pair is connected to a voltmeter and the power is turned on. The output voltage is transformed into other windings with the same value, since they have the same number of turns.

Using a series connection circuit, the 2nd and 3rd windings of the voltage vector are summed up, and the result is shown by a voltmeter. Next, the remaining ends of the windings are marked and control measurements are carried out.

Single-phase motors are low-power electrical machines. In the magnetic circuit of single-phase motors there is a two-phase winding, which consists of a main winding and a starting winding.

The most common motors of this type can be divided into two groups: single-phase motors with a starting winding and motors with a running capacitor.

For engines of the first type, the starting winding is switched on through a capacitor only at the time of start-up and after the engine has developed a normal rotation speed, it is disconnected from the network, after which the engine continues to operate with one working winding. The capacitor capacity is usually indicated on the motor nameplate and depends on its design.

For single-phase asynchronous motors with a running capacitor, the auxiliary winding is permanently connected through a capacitor. The value of the working capacitance of the capacitor is determined by the design of the engine.

If the auxiliary winding of a single-phase motor is starting, it will only be connected for the start time. If the auxiliary winding is a capacitor winding, then its connection will occur through a capacitor. And it remains on while the engine is running.

In most cases, the starting and operating windings of single-phase motors differ in both the cross-section of the wire and the number of turns. The working winding of a single-phase motor always has a larger wire cross-section, and therefore its resistance will be less.

The winding with less resistance is working.

If the motor has 4 terminals, then by measuring the resistance between them, you can determine that the lower resistance is lower for the working winding, and, accordingly, the higher resistance for the starting winding.

Connecting everything is quite simple. Thick wires are supplied with 220V. And one tip of the starting winding, per one of the workers, it doesn’t matter which one, the direction of rotation does not depend on it. It also depends on how you insert the plug into the socket. The rotation will change depending on the connection of the starting winding, namely, by changing the ends of the starting winding.

In the case where the motor has 3 terminals, the measurements will look like this, for example - 10 ohms, 25 ohms, 15 ohms. By measuring, you need to find the tip from which the readings, with two others, will be 15 ohms and 10 ohms. This will be one of the network wires. The tip with 10 ohms is also a network one and the third 15 ohm will be the starting one, it is connected to the second network one through a capacitor. In this case, in order to change the direction of rotation, you need to get to the winding circuit.

The case when measurements, for example, show 10 Ohm, 10 Ohm, 20 Ohm. is also one of the types of windings. for example, in some washing machines and more. In such cases, the working and starting windings are the same (according to the design of three-phase windings). In this case, it does not matter which winding will serve as the working winding and which starting winding. The connection is also made through a capacitor.

Adjustment of asynchronous motors is carried out in the following scope:

Visual inspection;

Mechanical check;

Measuring the insulation resistance of windings relative to the housing and between windings;

Measuring DC winding resistance;

Testing of windings with increased voltage of industrial frequency;

Test run.

External inspection of an asynchronous motor begins with the shield.

The plate should contain the following information:

Name or trademark of the manufacturer,

Type and serial number,

Rated data (power, voltage, current, speed, winding connection diagram, efficiency, power factor),

Year of issue,

Weight and GOST for the engine.

At the beginning of work is mandatory. Then check the condition of the outer surface of the motor, its bearing units, the output end of the shaft, the fan and the condition of the terminal terminals.

If a three-phase motor does not have composite and sectional windings on the stator, then the terminals are designated in accordance with table. 1, and in the presence of such windings, the conclusions are designated by the same letters as simple windings, but with additional numbers in front of the capital letters. For the letters, numbers are placed in front indicating the number of poles of this section.

Table 1

table 2

Note: terminals numbered P - connected to the network, C - free, Z - shorted

The markings of the shields of multi-speed motors and how to turn them on at different speeds can be explained using table. 2.

When externally examining an asynchronous motor, special attention should be paid to the condition of the terminal box and the output ends, in which various insulation faults are very often encountered, and the distance between the live parts and the housing is measured. It should be large enough so that there is no overlap on the surface. No less important is the amount of shaft runout in the axial direction, which according to standards should not exceed 2 mm (1 mm in one direction) for motors up to 40 kW.

The size of the air gap is of great importance, since it has a significant impact on the characteristics of asynchronous motors, therefore, after repairs or in case of unsatisfactory engine operation, the air gap is measured at four diametrically opposite points. Clearances should be equal all around and should not differ at any of these four points by more than 10% from the average.

Asynchronous motors in a number of machine tools, such as thread grinding and gear grinding machines, have special requirements in terms of runout and vibration. Shaft runout and vibration of electrical machines are greatly influenced by the accuracy of processing and the condition of the rotating parts of the machine. Beating and vibration are especially high when the engine shaft is bent.

Runout is a deviation from the specified (correct) relative position of the surfaces of rotating or oscillating parts such as bodies of revolution. There are radial and axial runouts.

For all machines, beating is undesirable, since this disrupts the normal operation of the bearing units and the machine as a whole. using a dial indicator that allows you to measure beats from 0.01 mm to 10 mm. When measuring shaft runout, the tip of the indicator is placed against a shaft rotating at low speed. By the deviation of the hour indicator hand, the amount of runout is judged, which should not exceed the values specified in the technical specifications for the machine or engine.

The insulation of an electrical machine is an important indicator, since the durability and reliability of the machine depends on its condition. According to GOST, the insulation resistance of windings in MOhm of electrical machines must be no less than

Where U n - rated voltage of the winding, V; P n - rated power of the machine, kW.

The insulation resistance is measured before a test run of the engine, and then periodically during operation, in addition, it is monitored after long breaks in operation and after each emergency shutdown of the drive.

The insulation resistance of the windings relative to the housing and between the windings is measured with cold windings and in a heated state, at a winding temperature equal to the temperature of the nominal mode, immediately before checking the electrical strength of the winding insulation.

If the beginning and end of each phase are identified in the motor, then the insulation resistance is measured separately for each phase relative to the housing and between the windings. For multi-speed motors, the insulation resistance is checked for each winding separately.

For measuring insulation resistance of electric motors voltages up to 1000 V are used at 500 and 1000 V.

The measurement is carried out as follows: the clamp of the “Screen” megohmmeter is connected to the machine body, and the second clamp is connected to the winding terminal with a flexible wire with reliable insulation. The ends of the conductors should be embedded in handles made of insulating material with a metal pin pointed at the end to ensure reliable contact.

The megger handle is rotated at a frequency of approximately 2 rps. Low-power motors have a small capacity, so the arrow of the device is set to a position that corresponds to the insulation resistance of the machine winding.

For new machines, insulation resistance, as practice has shown, fluctuates at a temperature of 20 ° C in the range from 5 to 100 MOhm. Motors of low-responsibility drives of small power and voltages up to 1000 V are not subject to specific requirements for the value of R. From practice, there are cases when motors with resistances of less than 0.5 MOhm were put into operation, their insulation resistance increased and in the future they operated without failure.

A decrease in insulation resistance during operation is caused by surface moisture, contamination of the insulation surface with conductive dust, penetration of moisture into the thickness of the insulation, and chemical decomposition of the insulation. To clarify the reasons for the decrease in insulation resistance, it is necessary to measure using a double bridge, for example R-316, with two directions of current in the controlled circuit. With different measurement results, the most likely reason is the penetration of moisture into the thickness of the insulation.

Specifically question about putting an asynchronous motor into operation should be decided only after testing the windings with increased voltage. Turning on a motor that has a low insulation resistance value without testing it with increased voltage is allowed only in exceptional cases, when the question is decided what is more profitable: to endanger the motor or allow downtime of expensive equipment.

During engine operation, it is possible damage to the insulation, leading to a decrease in its electrical strength below acceptable standards. According to GOST, testing the electrical strength of the insulation of the windings in relation to the housing and to each other is carried out with the engine disconnected from the network for 1 minute with a test voltage, the value of which must be no less than the value given in table. 3.

Table 3

Increased voltage is supplied to one of the phases, and the remaining phases are connected to the motor housing. If the windings are connected inside the motor in star or delta, then the insulation test between the winding and the frame is carried out simultaneously for the entire winding. When performing tests, voltage should not be applied instantly. The test begins with 1/3 of the test voltage, then gradually raises the voltage to the test voltage, and the rise time from half to full test voltage must be at least 10 s.

The full voltage is maintained for 1 minute, after which it is smoothly reduced to 1/3 Usp and the test installation is turned off. The test results are considered satisfactory if during the test there was no breakdown of the insulation or overlaps on the insulation surface, and no sharp shocks were observed on the instruments, indicating partial damage to the insulation.

If a breakdown occurs during testing, find the location and repair the winding. The location of the breakdown can be found by repeatedly applying voltage and then observing the appearance of sparks, smoke, or a slight crackling sound from sparking that is not visible from the outside.

Measuring the direct current resistance of the windings, which is carried out to clarify the technical data of the circuit elements, makes it possible in some cases to determine the presence of short-circuited turns. The temperature of the windings during measurement should not differ from the ambient temperature by more than 5° C.

Measurements are performed using a single or double bridge, using the ammeter-voltmeter method or the microohmmeter method. The resistance values should not differ from the average by more than 20%.

According to GOST, when measuring winding resistance, each resistance must be measured 3 times. When measuring winding resistance using the ammeter-voltmeter method each resistance must be measured at three different current values. The arithmetic mean of three measurements is taken as the actual resistance value.

The ammeter-voltmeter method (Fig. 1) is used in cases where great measurement accuracy is not required. Measurement using the ammeter-voltmeter method is based on Ohm's law:

Where R x - measured resistance, Ohm; U - voltmeter reading, V; I - ammeter reading, A.

The measurement accuracy with this method is determined by the total error of the instruments. So, if the accuracy class of an ammeter is 0.5%, and a voltmeter is 1%, then the total error will be 1.5%.

In order for the ammeter-voltmeter method to give more accurate results, the following conditions must be met:

1. measurement accuracy largely depends on the reliability of the contacts, so it is recommended to solder the contacts before measurement;

2. The source of direct current should be a network or a well-charged battery with a voltage of 4-6 V, in order to avoid the influence of voltage drop across the source;

3. readings from instruments must be carried out simultaneously.

Resistance measurement using bridges is used mainly in cases where it is necessary to obtain greater measurement accuracy. Accuracy reaches 0.001%. The measurement limits of bridges range from 10-5 to 106 Ohms.

A microohmmeter is used to measure a large number of measurements, for example, contact resistances and intercoil connections.

Rice. 1. Circuit for measuring DC winding resistance using the ammeter-voltmeter method

Rice. 2. Scheme for measuring the resistance of the stator winding of an asynchronous motor, connected in a star (a) and in a triangle (b)

Measurements are carried out quickly, since there is no need to adjust the device. The DC winding resistance for motors up to 10 kW is measured no earlier than 5 hours after the end of its operation, and for motors over 10 kW - no less than 8 hours with the rotor stationary. If the motor stator has all six ends of the windings brought out, then the measurement is carried out on the winding of each phase separately.

When internally connecting the windings in a star, the resistance of two series-connected phases is measured in pairs (Fig. 2, a). In this case, the resistance of each phase

With an internal delta connection, the resistance between each pair of output ends of the linear clamps is measured (Fig. 2, b). Assuming that the resistances of all phases are equal, determine the resistance of each phase:

For multi-speed motors, similar measurements are carried out for each winding or for each section.

Checking the correct connection of the windings of alternating current machines. Sometimes, especially after repairs, the water ends of an asynchronous motor turn out to be unmarked, and it becomes necessary to determine the beginnings and ends of the windings. The most common are two methods of determination.

According to the first method, the ends of the windings of individual phases are first determined in pairs. Then assemble the circuit according to Fig. 3, a. The “plus” of the source is connected to the beginning of one of the phases, the “minus” to the end.

Conventionally, C1, C2, C3 are taken as the beginning of phases 1, 2, 3, and C4, C5, C6 as the ends 4, 5, 6. At the moment the current is turned on, an electromotive force with polarity “ is induced in the windings of other phases (2-3). minus" at the beginnings of C2 and C3 and "plus" at the ends of C5 and C6. At the moment the current in phase 1 is turned off, the polarity at the ends of phases 2 and 3 is opposite to the polarity when they are turned on.

After marking phase 1, the direct current source is connected to phase 3, if the needle of the millivoltmeter or galvanometer deviates in the same direction, then all ends of the windings are marked correctly.

To determine the beginnings and ends using the second method, the motor windings are connected in a star or triangle (Fig. 3, b), and a single-phase reduced voltage is supplied to phase 2. In this case, between the ends C1 and C2, as well as C2 and C3, a voltage appears that is slightly greater than the supplied one, and between the ends C1 and C3 the voltage turns out to be zero. If the ends of phases 1 and 3 are connected incorrectly, then the voltage between the ends C1 and C2, C2 and C3 will be less than the supplied one. After mutual determination of the markings of the first two phases, the third is similarly determined.

Initial switching on of an asynchronous motor. To determine the complete serviceability of the engine, it is tested at idle and under load. First, check again the condition of the mechanical parts and the filling of bearings with grease.

The ease of movement of the engine is checked by turning the shaft manually, and no cracking, grinding or similar sounds should be heard indicating contact between the rotor and stator, as well as the fan and the casing, then check the correct direction of rotation; for this, the engine is turned on briefly.

The duration of the first activation is 1-2 s. At the same time, the magnitude of the starting current is observed. It is advisable to repeat the short-term engine start 2-3 times, gradually increasing the duration of activation, after which the engine can be turned on for a longer period. While the engine is idling, the service technician must ensure that the running parts are in good condition: no vibrations, no current surges, no heating of the bearings.

If the results of the test runs are satisfactory, the engine is turned on together with the mechanical part or tested on a special stand. The time for checking engine operation ranges from 5 to 8 hours, while monitoring the temperature of the main components and windings of the machine, the power factor, and the lubrication condition of the bearings of the components.

Types of electric motors

The most common electric motors are;

Three-phase asynchronous squirrel-cage motor

Asynchronous three-phase motor with squirrel-cage rotor. Three motor windings are laid in the stator slots;
- asynchronous single-phase motor with squirrel-cage rotor. It is mainly used in household electrical appliances in vacuum cleaners, washing machines, hoods, fans, air conditioners;
- DC commutator motors are installed in the electrical equipment of the car (fans, window lifters, 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

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.

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.

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 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 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.

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 influence of the electromagnetic field, the second is stationary 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 is 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, the cooling impeller, and check the temperature by touching metal surfaces with your hand. 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. Control board elements 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.

Household appliance motors often use a design with a starting winding whose resistance exceeds 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.

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 issue, 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 testing the windings and if they are intact, experienced craftsmen recommend connecting 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 in 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 position of the rotor through the 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. The service center technician will tell you over the phone how to check the motor winding with a multimeter. 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 the service, you need to use a washing machine; mechanical components are decisive. Motor shaft misalignment is a special case of problems with drum rotation.

/ 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 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 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

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

Testing the electric motor for open circuit and insulation resistance

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 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 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.

The short circuit of the winding to the motor body is detected with a megohmmeter, and the location of the short circuit is detected by the method of “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.

A large number of 220 V electrical appliances that everyone uses contain electric motors. These include various types of power tools and electrical appliances used in the kitchen and apartment - washing machines, dishwashers, vacuum cleaners, etc., etc. All these motors perform mechanical work and this makes our lives much easier. Therefore, their malfunctions are, as they say, like a bolt from the blue.

Suddenly the importance of the electric motor and its serviceability become clear. To prevent such a nuisance, it is recommended to periodically check the engines of household electrical appliances and power tools. Moreover, the checks must correspond to the operating load - the longer the electrical appliance is used, the more frequent checks are necessary. In this regard, we will further tell our readers how to check the electric motor themselves.

What to remember when checking

We do not recommend that our readers independently check electric motors, or any other electrical appliances, without a certain, even small, amount of knowledge in electrical engineering. Although such testing does not require detailed technical descriptions and knowledge of a large number of formulas, there is always a risk of electric shock. For this reason, it is best to entrust electrical equipment inspections and repairs to trained personnel. And without certain knowledge, one wrong touch with a screwdriver in the wrong place can ruin either the engine or something else.

Let us remind our readers that the operation of each electric motor is based on the interaction of the stator and rotor.

A stator that is static, i.e. motionless, part of the body fixed or resting on a supporting base.
The rotor rotates and is therefore consonant with the English word rotate, which means “to turn.” Basically the rotor is located inside the stator. But there are designs of electric motors in which the stator is largely covered by the rotor. Such engines were used, for example, in electric gramophone record players. They can also be found in some models of washing machines, fans and more.

Checking the bearings

The movement of the rotor relative to the stator is possible thanks to bearings. They can be structurally implemented on one of the principles:

slip,
rolling.

The ease of rotation of the shaft and rotor of the electric motor is the first point of checking any engine. To put it into practice, you need to:

disconnect the motor being tested from the power source or electrical network;
holding the shaft with your hand, shake it back and forth or turn the rotor.

But since motors are often part of an electric drive with a gearbox, you need to know for sure that the shaft you are holding is part of the rotor, not the gearbox. Some gear reducers, with a certain force, still allow their shaft to be rotated, and in this way the condition of the bearings can be assessed. But many globoids and worms do not. In this case, you must try to gain access to the motor shaft inside the gearbox. Better yet, if possible, disconnect the gearbox from the engine.

If rotation is difficult, then the bearing is faulty for the following reasons:

its service life has expired due to wear of the working elements;
There is either too little lubrication or no lubrication at all. But it may also be that a lubricant that does not meet the operating conditions was used. For example, some of its varieties become so thick at temperatures below zero that they slow down rotation. In this case, the bearings are washed with gasoline and the lubricant is replaced with another one suitable for these conditions.
The gaps between the rubbing elements of the bearing are clogged with dirt. It is also possible that small foreign objects may enter.

We check the engines visually

If the bearings are in good condition, holding the shaft with your hand and rocking it from side to side, you will not feel any play. At the same time, when the engine is running, there is no noise coming from the bearing. And, conversely, in a worn bearing both play and significant noise are noticeable, especially if it is a rolling bearing. For an asynchronous motor, regardless of whether it is three-phase or single-phase, the lack of normal performance is most often associated with the bearings.

In such engines these are the only parts that mechanically wear out over time. The exception is asynchronous motors with rings. They also contain synchronous electric motors. The rings and brushes sliding on them are subject to wear and, along with the bearings, are inspected to check their normal performance. The surfaces of rings that are in good and serviceable condition are smooth and free of scratches. The brushes must be ground into the surface of the rings and pressed securely against them.

But for most readers, the most common problems will be related to commutator motors. They are basic in all electrical appliances and power tools. And their wearing parts are also bearings and brushes. But the brushes slide not along the rings, but along the commutator. Its surface is non-uniform, which significantly accelerates the wear of the brushes, which then turn into graphite dust.

It settles on all surfaces of the engine and body of the electrical appliance, creating conditions for the appearance of electrical circuits. Therefore, when checking such electrical appliances, it is important to promptly identify a critical level of contamination with graphite dust and perform high-quality cleaning of it from both the engine itself and all other surfaces.

How to test an electric motor with a multimeter

But inspection of risky elements of electric motors is usually insufficient. Moreover, in this way it is impossible to identify a fault in the windings. Therefore, you need to know how to ring an electric motor with a multimeter or tester. Such a continuity check of the windings of a three-phase, single-phase and direct current electric motor will allow you to understand some faults and identify the need to rewind the damaged winding.

It usually makes no sense to measure the winding resistance, since the resistance of the windings of most engines is very small in value. Moreover, the higher the power and, accordingly, the cross-section of the winding wires, the lower the ohmic resistance. By the way, this is also typical for transformers. Therefore, checking the windings when characteristic faults appear in electric motors comes down to calling them with a tester.

Unfortunately, it is not possible to ring the winding in this way in order to prevent a malfunction. This way you can only deal with problems that have already arisen. And in engines they influence the correct rotation of the rotor. At the same time, the rotation speed decreases, the body heats up noticeably more, and the sound of the running engine changes noticeably. This is especially noticeable by ear in commutator engines. They operate with a characteristic buzzing sound, which is associated with a magnetostrictive effect.

If the connection of one or more windings is broken, they do not create sound vibrations, and the pitch of the sound decreases. To find damage, you need a tester set to measure resistance in ohms. There are pairs of plates located on the collector, one opposite the other. Therefore, you need to touch any collector plate with one probe and find a paired plate from the diametrically opposite side with another probe.

The device will show a certain resistance value on it. It should be small in size, and its value decreases as the power of the motors increases. If the desired plate is either not located or is located away from the diametral line passing through the first plate, and this arrangement is no longer repeated for other plates similar to the first, then

or a break in the plate-winding-plate circuit;
or the insulation inside the winding is broken and an electrical circuit appears due to its damage.

The rotor will need repair. During the test, a dot mark is applied to the examined plates, for example, with nail polish. But first you need to test the varnish. Once dry and hardened, it should come off easily from the surface. In collector motors operating from a 220 V network, the stator winding is used. It is more difficult to check it with a tester, since to compare the measured resistance values you need another identical motor. But since the no-load current value must be specified for the engine, it can be measured with a tester.

Observing safety precautions, you must connect the electrical circuit to a de-energized outlet (for example, by making a disconnect on the panel). The engine must be securely fastened to resist the starting force. Then voltage is applied, and the current strength is shown on the device display and compared with the passport data. If there is a short circuit in the stator winding, the current strength will be greater than that indicated in the technical data sheet.

Similar problems with the stator occur in asynchronous motors. When there is a short circuit between the turns or to the housing, the rotor rotation speed always decreases. In such cases, you need to take a tester and ring the asynchronous electric motor using the insulation resistance table (if it is given in the technical documentation). In a working engine, each winding is reliably isolated both from other windings and from the housing, as the device will show during testing.

Other faults

But in addition to the already mentioned problems, which mainly occur during the operation of engines, there are also exotic malfunctions.

For example, damage to the “squirrel cage” in asynchronous models. With this malfunction, the stator is in perfect order, but the engine still does not produce full power. Since the damage is internal, the easiest way is to replace the rotor with a good one.

Wound windings are used only if there are rings in the rotor. If it rotates with the chain of rings open, it means that there is a short circuit between the turns. And the engine “unauthorized” turned into an asynchronous model with a squirrel-cage rotor.
Uncharacteristic noises. The reasons may be disturbances in the structure of the core plates. Also, if the rotor touches the stator, it will not only be audible, but may cause heating and smoke. This is always a consequence of wear or sudden failure of the bearings.

Compliance with the recommended operating conditions and scheduled inspections will allow you to use equipment with engines for as long as possible and without problems. Follow the instructions and get the most out of your electrical appliances.

How to test a 3-phase electric motor with a multimeter. How to check an electric motor - simple tips for electricians

Types of windings

However, the situation is much more complicated, since the electric motor winding has its own characteristics:

Possible faults

Methods

Checking the motor windings for open circuits

Checking the motor windings for short circuits

Checking the motor windings for interturn short circuits

Checking the motor windings using an ohmmeter

Current measurement in each phase

Checking motor windings with alternating current

Checking the motor windings with a ball

Determining the polarity of windings using the electrical method

To determine the marking, several methods are used:

Determining the marking of winding terminals with an ammeter and battery

Determining polarity with a voltmeter and a step-down transformer

Types of electric motors

Winding insulation resistance measurement

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

Checking other parts and other potential problems

Motor design

Difficulties in diagnosis

Switching equipment

Electrical diagnostic details

Checking the Direct Drive Motor

Diagnostic sequence

If he still knocks out the defense?

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

What to remember when checking

Checking the bearings

We check the engines visually

How to test an electric motor with a multimeter

Other faults