Checking the motor windings. Faults and Test Methods

When an electric motor is not working, it is not enough just to look at it to understand the reason. An electric motor stored in a warehouse for a long time may or may not work regardless of its appearance. A quick check can be done with an ohmmeter, much more information is given below in order to correctly assess the condition of the electric motor.

Steps

Part 1

Part 2

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

• There are several types of bearings that are used in electric motors. The two most popular types are brass plain bearings and ball bearings. Many of them have fittings for lubrication, others are lubricated during production ("out of service").

1. Check the bearings. For a cursory inspection of the bearings, place the motor on a firm surface and place one hand on the top of the motor and rotate the rotor with the other hand. Watch carefully, try to feel and hear friction, scratching sounds, uneven rotation of the rotor. The rotor should rotate calmly, freely and evenly.

   Then check the longitudinal play of the rotor, lift, pull the rotor out of the stator by the axle. A small backlash is acceptable (in the most common household engines, the backlash should be no more than 3 mm), but the closer it is to "0", the better. A motor that has bearing problems runs noisily and the bearings overheat, resulting in engine failure.

Part 3

Check motor windings for short to frame. Most household electric motors with closed windings will not work: most likely a fuse will blow or a circuit breaker will trip (motors rated for 380 volts are "ungrounded", so such motors can work with windings closed to the case, and the fuse will not blow).

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

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

1. Make sure the motor windings are not cut off or short-circuited. Many simple single-phase and 3-phase motors (used in household appliances and industry, respectively) can be tested by simply switching the ohmmeter range to the lowest (RX * 1), set the pointer to zero again, and measure the resistance between the motor leads again. Refer to the motor diagram to make sure you are measuring each winding.

   • You can see a very low resistance value. The resistance value can be quite low. Make sure your hands do not touch the ohmmeter probes as this will result in an inaccurate reading. A high resistance value indicates a potential problem with the motor windings that may be breaking. A motor with high resistance windings will not work or its speed controller will not work (this could be with 3-phase motors).

Part 4

1. Check the starting capacitor used to start some motors. Most capacitors are protected from damage by a metal cover on the outside of the motor. The cover must be removed to gain access to the condenser for testing. Visual inspection can help detect condenser oil leaks, openings in the housing, swollen condenser housing, burning or smoke smell - all of which indicate potential problems.

   • The capacitor can be checked with an ohmmeter. Touch the probes to the terminals of the capacitor, the resistance should start at low values ​​and gradually increase, since a small voltage supplied from the ohmmeter batteries gradually charges the capacitor. If the capacitor remains short-circuited or the resistance does not increase, then there is probably a problem with the capacitor and needs to be replaced. The capacitor must be discharged before attempting this test again.

2. Check the rear of the crankcase where the bearing is installed. There, some motors have centrifugal switches used to switch the starting capacitor or to connect the circuits that determine the RPM. Check the relay contacts, whether they are burnt, clean them from dirt and grease. Use a screwdriver to check the switch mechanism, the spring should work freely.

   Check the fan. Type "TEFC" (fully enclosed, air-cooled electric motor). For motors of this type, the fan blades are located behind a metal grill at the rear of the motor. Make sure the fan is securely fastened and not clogged with dirt or other debris. The holes in the metal grill must ensure free air movement, otherwise the engine may overheat and fail.

3. Select the correct engine for the conditions in which it will operate. In humid environments, splash-proof motors are used, and open motors should not be exposed to water or moisture.

   • Splash-proof motors can be installed in damp or humid locations and are designed so that water (or other liquids) cannot enter the engine by gravity or by the flow of water (or other liquid).
   • An open engine, as the name suggests, is completely open. From the ends, these motors have rather large openings, and the stator windings are clearly visible. These openings must not be blocked and these motors must not be installed in wet, dirty or dusty locations.
   • TEFC motors, on the other hand, can be used in all the areas mentioned above, but they also should not be used in conditions for which they are not designed.

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

To find out the cause of the malfunction of the electric motor, it will not be enough just to inspect it, you need to carefully check it. This can be done quickly with an ohmmeter, but there are other ways to check. We will tell you how to check the electric motor below.

Inspection of the electric motor

First, the check begins with a thorough inspection. In the presence of certain defects in the device, it may fail much earlier than the specified time. Defects can appear due to improper operation of the motor 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 motor. It is printed on a metal nameplate that is attached to the outside of the engine. The type plate contains important information about the technical characteristics of this appliance. As a rule, these are parameters such as:

• information about the engine manufacturing company;
• model name;
• serial number;
• the number of rotor revolutions per minute;
• device power;
• connection diagram of the engine to certain voltages;
• scheme for obtaining a particular speed and direction of movement;
• voltage - voltage and phase requirements;
• dimensions and type of case;
• description of the stator type.

The stator on the electric motor can be:

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

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

For electric motors, the most commonly used types of bearings are:

• brass;
• ball bearings.

Some need to be equipped with grease fittings and some are already lubricated during production.

The bearings should be checked as follows:

• place the engine on a firm surface and place one hand on top of the engine;
• turn the rotor with the other hand;
• try to hear scratching sounds, friction and uneven movement - all this signals a malfunction of the device. A working rotor moves calmly and evenly;
• we check the longitudinal play of the rotor, for this it needs to be pushed over the axle from the stator. Backlash is allowed up to a maximum of 3 mm, but not more.

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

The next stage of verification is checking the motor winding for a short circuit on his body. Most often, a household motor will not work with a closed winding, because a fuse will blow or a protection system will work. The latter is typical for ungrounded devices designed for 380 volts.

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

• set the ohmmeter to resistance measurement mode;
• we connect the probes to the necessary sockets (as a rule, to the common "Ohm" socket);
• choose the scale with the highest multiplier (for example, R * 1000, etc.);
• set the arrow to zero, while the probes should touch each other;
• we find the screw for grounding the electric motor (most often it has a hex head and is colored green). Instead of a screw, any metal part of the case can be suitable, on which the paint can be scraped off for better contact with the metal;
• we press the ohmmeter probe to this place, and we press the second probe in turn to each electrical contact of the engine;
• perfectly the pointer of the measuring device should deviate slightly from the highest resistance indicator.

During work, 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 Mohms. 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 interrupted. Some simple single-phase or three-phase motors are tested by switching the ohmmeter range to the lowest range, then the arrow goes to zero and the resistance between the wires is measured.

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

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

Checking other parts and other potential problems

It is definitely worth checking the starting capacitor, which is needed to start some models of electric motors. Basically, these capacitors are equipped with a protective metal cover inside the motor. And to check the capacitor, you need to remove it. This inspection can reveal signs of a problem such as:

• oil leak from the condenser;
• the presence of holes in the case;
• swollen capacitor body;
• 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 charges 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.

The capacitor must be discharged before retesting.

We move on to the next stage of checking the engine: the back of the crankcase, where the bearings are installed. In this place a number of electric motors are equipped with centrifugal switches that switch starting capacitors or circuits to determine the RPM. You also need to check the relay contacts for burntness. In addition, they should be cleaned of grease and dirt. The switch mechanism is checked with a screwdriver, the spring should work normally and freely.

And the final step is checking the fan. We will take a look at this using the example of checking a TEFC motor fan that is completely enclosed and air-cooled.

Make sure the fan is securely attached and not clogged with dirt or other debris. The openings on the metal grill must be sufficient for free air circulation, if this is not ensured, then engine overheating may occur and subsequently it will fail.

The main thing when choosing an electric motor is its selection in accordance with the conditions where it will be used. For example, for a humid environment, splash-proof devices should be chosen, and open-type devices should never be exposed to liquids. Remember the following:

So, we have listed the most common problems that can happen with household electric motors. Almost all of them can be recognized and certain measures can be taken by checking the device. And how to check it correctly and what details are worth paying attention to first of all, we have considered above.

Various types of electric motors are installed in household appliances and equipment. These differences depend on the operating conditions, purpose and functions performed by them. For example, collector motors are used in electric drills, mixers, food processors, vacuum cleaners, and other devices with frequent changes in shaft rotation speed.

If it is required to provide a long-term stable mode of operation, then in such equipment, asynchronous electric motors are already used, which are most suitable for small home-made machine tools. Nevertheless, in all cases it is often necessary to decide how to check the motor armature at home. Modern services are quite expensive, so many people try to find the malfunction on their own and carry out repairs.

Collector motors and basic armature malfunctions

Collector electric motors are designed to operate from household networks, with a voltage of 220V. Almost all of them are synchronous units. Unlike induction motors, collector devices consist of a stationary stator and a rotating winding on the shaft - an armature. The voltage is supplied to them using a brush-graphite device, which is the collector.

The main reason for checking the armature and other parts is sparks. Active sparking indicates wear on the brushes and the manifold assembly or broken contacts. In addition, sparks can appear as a result, that is, a short circuit of the windings in the collector. The appearance of such violations requires high-quality diagnostics, starting with a visual inspection and ending with a check with a multimeter.

An initial inspection allows you to identify broken or burnt out windings, as well as burnout at the points of their connection. Therefore, first of all, attention should be paid to the condition of the windings and the integrity of the turns. If the windings are completely or partially blackened, this already indicates certain problems with the armature. Sometimes sniffing the insulation is enough to detect the characteristic burning smell.

More accurate information can be obtained by checking the armature with a multimeter. The call is performed in stages, capturing all the elements of the engine:

• First, the pairwise conclusions of the stator windings are called to the lamellas. The resistances on each of them must have the same value.
• Next, the resistance between the lamellas and the armature body is checked. Normally, it should be infinite.
• The integrity of the winding is checked by continuity of the terminals.
• After that, the condition of the circuit between the stator housing and the terminals of the armature winding is checked. If there is a breakdown on the case, it is strictly forbidden to connect the household device to voltage. In this case, mandatory repair or complete replacement of defective parts is required.

After repairing the collector motor, you need to connect all the elements together and connect the device to a 220V power supply. If the unit is working properly, then the repair has been done correctly.

Induction motor check

In addition to collector motors, in everyday life you can also find asynchronous motors installed in some models of washing machines or refrigerators compressors. More often they are used in compressors, pumps, various machine tools and other equipment. Despite their high reliability, these motors are also susceptible to breakdowns and malfunctions. In these structures, the stator windings play the role of an anchor, therefore, a visual inspection should be started with them.

Often, the windings stop working when they are damp or if the turns are broken. Therefore, if the motor has not been used for a very long time, it is necessary to check the insulation resistance with. In the absence of a megaohmmeter, it is recommended to disassemble and dry the stator windings for preventive purposes for several days.

It is possible that the cause of the malfunction does not lie in the electric motor itself, but is associated with some other factors. Therefore, before starting to repair the unit itself, you should make sure that there is voltage, check magnetic starters, connection cables, and a thermal relay. If there is a capacitor in the circuit, it must also be checked. If all the listed elements are in good working order, you can start disassembling the engine for initial inspection. The check should be carried out with a complete power failure. It is necessary to prevent spontaneous or erroneous switching on of the unit.

During the inspection, besides other parts, the stator windings are especially carefully checked. They must be intact, with no protruding or torn wires. Particular attention should be paid to black spots that indicate possible burning of the wires. In good condition, the conductors are dark red. Blackening occurs when the electrical insulating varnish applied to their surface burns out. On examination, complete or partial burnout of the winding and interturn short circuit can be detected. If partially burned out, the engine will run and heat up quickly. Therefore, the winding is rewound completely anyway.

If an external examination did not give results, further diagnostics should be carried out using measuring instruments. Most often, a multimeter is used for these purposes, which makes it possible to determine the integrity of the winding, the presence or absence of a breakdown to the case.

In 220V motors, the starting and working windings are ringing. The resistance of the starting should be 1.5 higher than that of the working one. In 380V electric motors connected with a star or delta, the circuit is disassembled, after which each winding is called in turn. The resistance on each of them should be the same, with a deviation of no more than 5%. Also, all the windings are sure to ring among themselves and to the case. If the resistance value is not infinite, this indicates the presence of a breakdown of the windings to the case or to each other. In this case, their complete rewind is required.

The insulation resistance of the motor windings is checked separately. In this case, the multimeter will not help; you will need a 1000V megohmmeter connected to a separate power source. When performing measurements, one wire of the device touches the motor housing in an unpainted place, and the other wire is alternately connected to each winding terminal. If the insulation resistance is less than 0.5 MΩ, the motor needs to be dried. When taking measurements, be careful not to touch the test leads. The measured equipment must be de-energized, the measurement duration is at least 2-3 minutes.

The greatest difficulty is the search for turn-to-turn closure. It cannot be detected by visual inspection. For use special inductance meters, which normally show the same value on all windings. In the presence of damage, the inductance of such a winding will be the lowest.

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

Let's start in order.

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

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

Fault: the magnetic starter does not work

• Check the presence of voltage at the contacts of the starter coil. It should be remembered that there are 220V and 380V coils.
• If there is no voltage, then replace the coil or starter. If voltage is applied, it is necessary to "ring out" the coil for winding integrity. This can be done using an electric tester (buzzer) or an electric breaker.
• We check the health and integrity of the "Start" and "Stop" buttons.

Button connection diagram:

Use adsense clicker on your websites and blogs or YouTube

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

You can check the integrity of the wire using the buzzer of the electric tester or. You can also check with a test lamp or voltmeter. We turn off the machine (AB), disconnect the wires from the electric motor. Then we turn on the machine and check the presence of voltage on the wires. Caution, live work!

If there is a possibility that a short circuit has occurred in the cable (soldering and wire breakage), then it is necessary to check the wires for a short circuit among themselves. We turn off the machine, disconnect the wires from the electric motor. With the help of an electric tester (buzzer) or an electric breaker, we check in turn the wires for a short circuit among themselves.

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

• We turn off the power supply (automatic machine).
• It is better to disconnect the power leads from the motor.
• Using an electric tester (buzzer) or an electric breaker, we check the integrity of the stator windings.
• Using the same devices, we determine the presence or absence of "breakdown" on the motor housing. One probe of the device is on the case, the other is on the contact of the output of the winding of the electric motor. If the buzzer sounded, and the arrow deviated on the brehunka, then there was a "breakdown" on the body of the electric motor - the "khan" engine.

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

Caution, live work!

Types of electric motors

The most common electric motors are;

Asynchronous three-phase squirrel-cage motor

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

Measurement of winding insulation resistance

To check the motor for insulation resistance, electricians use a megohmmeter with a test voltage of 500 V or 1000 V. This device measures the insulation resistance of the windings of motors 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 at a high voltage of 500 V megohmmeter. Usually, the test voltage is indicated in the passport for the electric motor when measuring the insulation resistance of the coils.

Insulation resistance is usually checked with a megohmmeter

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

If the insulation resistance is significantly less than 20 MΩ, the windings are disconnected and each tested separately. For a whole motor, the insulation resistance between the coils and the metal case 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 the insulation resistance with a multimeter, set the measurement limit to the maximum resistance, at megohms.

How to ring an electric motor for an open winding and interturn short circuit

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

There is no way to compare them. It is possible to identify the turn-to-turn closure of the windings of three-phase and single-phase motors with a measuring clamp, comparing the currents of the windings with their passport data. With a turn-to-turn closure in the windings, their rated current increases, and the starting torque decreases, the motor hardly starts 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 work 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 at such values ​​with a multimeter. In this case, it is better to check the serviceability of the electric motor with a clamp meter.

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

Electric motor ringing for open circuit and insulation resistance

A smaller coil voltage drop will indicate an interturn short circuit. If you want to know the resistance of the winding, it is calculated by the formula R = U / I. Motor failure can also be identified visually, on a disassembled stator, or by the smell of burnt insulation. If the breakage point is visually detected, it can be eliminated, the jumper can be soldered, well insulated and laid.

The measurement of the resistances of the windings of three-phase motors is carried out without removing the jumpers on the connection diagrams of the “star” and “triangle” windings. The resistance of the coils of the DC and AC collector motors is also checked with a multimeter. And with their high power, the test is carried out using a battery device - a rheostat, as indicated above.

The resistance of the windings 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 rotating the rotor. In this case, it is possible to determine the loose fit of the brushes to the rotor blades. Eliminate carbon deposits and irregularities on the collector lamellas by grinding them on a lathe.

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