The most common electrical faults are short short circuits inside the motor windings and between them, short circuits of the windings to the housing, as well as breaks in the windings or in the external circuit (supply wires and starting equipment).
As a result of the above electric motor malfunctions may occur: inability to start the electric motor; dangerous heating of its windings; abnormal motor speed; abnormal noise (humming and knocking); inequality of currents in individual phases.
Mechanical causes that cause disruption of the normal operation of electric motors are most often observed in the improper operation of bearings: overheating of bearings, oil leakage from them, and the appearance of abnormal noise.
Basic types of faults in electric motors and the reasons for their occurrence.
The asynchronous electric motor does not turn on (fuses blow or protection is triggered). The cause of this in slip ring motors may be shorted positions of the starting rheostat or slip rings. In the first case, it is necessary to bring the starting rheostat to its normal (starting) position, in the second, raise the device that short-circuits the slip rings.
It is also impossible to turn on the electric motor due to a short circuit in the stator circuit. You can detect a short-circuited phase by touch by the increased heating of the winding (feeling should be done by first disconnecting the electric motor from the network); by the appearance of the charred insulation, as well as by measurement. If the stator phases are connected in a star, then the values of currents consumed from the network by individual phases are measured. A phase with short-circuited turns will consume more current than undamaged phases. When connecting individual phases in a triangle, the currents in two wires connected to the defective phase will be greater than in the third, which is connected only to undamaged phases. When taking measurements, use a reduced voltage.
When turned on, the asynchronous electric motor does not move. The reason for this may be a break in one or two phases of the power circuit. To determine the location of the break, first inspect all elements of the circuit supplying the electric motor (check the integrity of the fuses). If during an external inspection it is not possible to detect a phase break, then the necessary measurements are performed with a megger. Why is the stator first disconnected from the supply network? If the stator windings are connected in a star, then one end of the megger is connected to the zero point of the star, after which the other ends of the winding are touched in turn with the second end of the megger. Connecting a megger to the end of a serviceable phase will give a zero reading, connecting to a phase that has an open circuit will show a high resistance of the circuit, i.e. the presence of an open circuit in it. If the star zero point is inaccessible, then the two ends of the megger touch all stator terminals in pairs. Touching the megger to the ends of good phases will show a zero value, touching the ends of two phases, one of which is defective, will show a high resistance, i.e. an open circuit in one of these phases.
If the stator windings are connected in a triangle, it is necessary to disconnect the winding at one point, and then check the integrity of each phase separately.
A phase that has a break is sometimes detected by touch (remains cold). If a break occurs in one of the stator phases while the electric motor is running, it will continue to work, but will begin to hum stronger than under normal conditions. Look for the damaged phase as indicated above.
When an asynchronous motor operates, the stator windings become very hot. This phenomenon, accompanied by a strong hum of the electric motor, is observed when there is a short circuit in any stator winding, as well as when the stator winding is double shorted to the housing.
Working asynchronous electric motor started to buzz. At the same time, its speed and power are reduced. The reason for the malfunction of the electric motor is the failure of one phase.
When the DC motor is turned on, it does not move. The reason for this may be blown fuses, a break in the power supply circuit, or a break in the resistance in the starting rheostat. First, carefully inspect, then check the integrity of the specified elements using a megger or test lamp with a voltage not exceeding 36 V. If it is not possible to determine the location of the break using the indicated method, proceed to checking the integrity of the armature winding. A break in the armature winding is most often observed at the junctions of the commutator with the winding sections. By measuring the voltage drop between the collector plates, the location of the damage is found.
Another reason for this phenomenon may be an overload of the electric motor. This can be checked by starting the electric motor idle, having previously disconnected it from the drive mechanism.
When turned on DC motor fuses blow or maximum protection trips. The shorted position of the starting rheostat may be one of the reasons for this phenomenon. In this case, the rheostat is moved to the normal starting position. This phenomenon can also be observed when the rheostat handle is pulled out too quickly, so when the electric motor is turned on again, the rheostat is pulled out more slowly.
When the electric motor is running, increased heating of the bearing is observed. The reason for increased heating of the bearing may be insufficient clearance between the shaft journal and the bearing shell, insufficient or excess amount of oil in the bearing (check the oil level), oil contamination or the use of inappropriate grades of oil. In the latter cases, the oil is replaced by first washing the bearing with gasoline.
When starting or during operation of the electric motor, sparks and smoke appear from the gap between the rotor and stator. A possible reason for this phenomenon may be the rotor touching the stator. This occurs when there is significant bearing wear.
When operating a DC motor, sparking is observed under the brushes. The reasons for this phenomenon may be incorrect selection of brushes, weak pressure on the commutator, insufficiently smooth surface of the commutator and incorrect placement of the brushes. In the latter case, it is necessary to move the brushes, placing them on the neutral line.
During operation of the electric motor, increased vibration is observed, which may appear, for example, due to insufficient strength of fastening the electric motor to the foundation plate. If vibration is accompanied by overheating of the bearing, this indicates the presence of axial pressure on the bearing.
Table 1 . Malfunctions of asynchronous electric motors and ways to eliminate them
Malfunction | Possible reason | Remedy |
Brushes spark, some brushes and their fittings become very hot and burn | Brushes are poorly polished | Sand the brushes |
Brushes cannot move freely in the brush holder cage - the gap is small | Set the normal gap between the brush and the holder O.2-O.3 mm |
|
Slip rings and brushes are dirty or oily | Clean the rings and brushes with gasoline and eliminate the causes of contamination |
|
The slip rings have an uneven surface | Grind or grind slip rings |
|
The brushes are pressed weakly against the slip rings | Adjust brush pressure |
|
Uneven current distribution between brushes | Adjust the brush pressure, check the serviceability of the Traverse contacts, conductors, brush holders |
|
Uniform overheating of the stator active steel | Mains voltage is higher than rated | Reduce voltage to nominal; increase ventilation |
Increased local heating of active steel at idle stroke and rated voltage | There are local short circuits between individual active steel sheets | Remove burrs, eliminate short circuits and treat the sheets with insulating varnish |
The connection between the tie bolts and the active steel is broken | Restore the insulation of the tie bolts |
|
The motor with a wound rotor does not develop the rated speed with load | Poor contact in rotor solders | Check all rotor soldering. If there are no malfunctions during external inspection, soldering is checked using the voltage drop method. |
The rotor winding has poor contact with the slip rings | Check the contacts of the conductors at the points of connection with the winding and slip rings |
|
Poor contact in the brush apparatus. The contacts of the mechanism for short-circuiting the rotor are loose | Sand and adjust brush pressure |
|
Poor contact in the connections between the starting rheostat and slip rings | Check the serviceability of the contacts at the points where the connecting wires are connected to the terminals of the rotor and the starting rheostat |
|
An engine with a wound rotor starts running without load - with the rotor circuit open, and when started up with a load it does not develop speed | Short circuit between adjacent clamps of the frontal connections or in the rotor winding | Eliminate contact between adjacent clamps |
The rotor winding is grounded in two places | After determining the short-circuited part of the winding, replace damaged coils with new ones |
|
Squirrel-cage motor does not start | Fuses blown, circuit breaker faulty, thermal relay tripped | Troubleshoot |
When starting the engine, the slip rings overlap with an electric arc. | The slip rings and brush apparatus are dirty | Clean up |
Increased air humidity | Carry out additional insulation or replace the motor with another one suitable for the environmental conditions |
|
A break in the rotor connections and in the rheostat itself | Check the connection is working properly |
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 occurrence of certain problems, The following malfunctions may occur in the operation of the vehicle: inability to start the motor, dangerous heating of the windings, abnormal motor speed, unnatural noise (hum or knock), unequal current 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 electrical circuit of the controller (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.
For various reasons, malfunctions occur in them, which can lead to interruptions in the operation of machines and other production mechanisms. In order for such interruptions to have the least impact on the enterprise’s implementation of production plans, it is necessary to be able to quickly find the cause of the malfunction and eliminate it.The need to quickly eliminate damage is also due to the fact that the operation of an electric motor with minor damage can lead to the development of damage and the need for more complex repairs.
To determine the scope of repair asynchronous electric motor, it is necessary to identify the nature of its malfunctions. Malfunctions of an asynchronous motor are divided into external and internal.
External faults include:
- break of one or more wires connecting the asynchronous motor to the network, or incorrect connection;
- blown fuse link;
- malfunctions of start-up or control equipment, low or high voltage of the supply network;
- overload of asynchronous motor;
- poor ventilation.
Internal faults of an asynchronous motor can be mechanical or electrical.
Mechanical damage:
- bearing malfunction;
- deformation or breakage of the rotor shaft (armature);
- loosening of brush holder fingers;
- formation of deep grooves (“tracks”) on the surface of the collector and slip rings;
- loosening of the poles or stator core to the frame; breakage or slipping of wire bands of rotors (anchors);
- cracks in bearing shields or in the frame, etc.
Electrical damage:
- interturn short circuits;
- breaks in the windings;
- breakdown of insulation on the housing;
- insulation aging;
- desoldering connections between the winding and the collector;
- incorrect polarity of poles;
- incorrect connections in coils, etc.
Most common faults asynchronous electric motors :
- Overload or overheating of the electric motor stator - 31%.
- Interturn short circuit - 15%.
- Bearing damage - 12%.
- Damage to stator windings or insulation - 11%.
- Uneven air gap between stator and rotor - 9%.
- Electric motor operation on two phases - 8%.
- Breakage or loosening of the rods in the squirrel cage - 5%.
- Loosening of stator windings - 4%. 9. Electric motor rotor imbalance - 3%. 1
- Shaft misalignment - 2%.
Below is a brief description of some malfunctions in electric motors and possible causes of their occurrence.
The engine does not rotate 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 winding, break 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 rotate, 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.
Engine speed at full load is below rated may be due to low mains voltage, poor contacts in the rotor winding, and also due to high resistance in the rotor circuit of a wound-rotor motor. With high resistance in the rotor circuit, the engine slip increases and its rotation speed decreases.
Resistance in the rotor circuit is increased by poor contacts in the rotor brush device, the starting rheostat, winding connections with slip rings, soldering of the frontal parts of the winding, as well as insufficient cross-section of cables and wires between the slip rings and the starting rheostat.
Bad contacts in the rotor winding can be detected 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 rotates with the wound rotor circuit open. The cause of the malfunction is a short circuit in the rotor winding. When turned on, the engine rotates slowly, and its windings become very hot, since a large current is induced in the short-circuited turns by the rotating stator field. 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 may 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 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 stator active steel occurs due to burnout and melting of steel during short circuits in the stator winding, as well as when 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 winding insulation can occur from prolonged overheating of the motor, moisture and contamination of the windings, exposure to metal dust, shavings, and also 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.
Wetting of the windings occurs in the event of long breaks in the operation of the engine, when water or steam directly enters it as a result of storing the engine in a damp, unheated room, etc. Metal dust trapped inside the machine creates conductive bridges, which can gradually cause short circuits between phases windings and on 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, break of the resistance winding in the starting rheostat or poor contact in the supply wires. A break in the resistance winding in the starting rheostat is detected with a test lamp or megger.
Manufacturers of electric motors in their operating instructions usually provide a list of the main malfunctions that may occur during operation of the electric motor and provide recommendations for eliminating them.
Asynchronous electric motors are more common than others in production and are often found in everyday life. With their help, various machines are set in motion: lathes, milling, sharpening, lifting mechanisms such as an elevator or crane, as well as various types of fans and hoods. This popularity is due to the low cost, simplicity and reliability of this type of drive. But it happens that even simple equipment breaks down. In this article we will look at typical malfunctions of squirrel-cage asynchronous electric motors.
Types of malfunctions of asynchronous motors
Malfunctions can be divided into three groups:
The engine gets hot;
The shaft does not rotate or does not rotate normally;
It makes noise and vibrates.
In this case, the entire engine body or some specific place on it may become hot. And the electric motor shaft may not budge at all, may not develop normal speed, its bearings may overheat, make sounds that are abnormal for its operation, or vibrate.
But first, refresh your memory of its design, and the illustration below will help you with this.
The causes of malfunctions can also be divided into two groups:
Electrical;
Mechanical.
Most faults are detected by comparing phase currents and rated current, and other measuring instruments. Let's look at typical faults.
The electric motor does not start
When voltage is applied, the motor does not start to rotate and does not make any sounds and the shaft does not “try” to move. First of all, check whether power is supplied to the engine. This can be done either by opening the motor board and measuring where the power cable is connected, or by measuring the voltage at the power switch, contactor, starter or circuit breaker.
However, if there is voltage at the motor terminals, then the entire line is normal.
By measuring the voltage at the beginning of the line, automatically you will only know that voltage is supplied, but it may not reach the end user as a result of cable breaks, poor connections along its entire length, or due to faulty and low-current circuits.
If you are convinced that voltage is coming to the engine, its further diagnosis consists of testing the windings for a break. You need to check the integrity of the winding, so at the same time you will check for breakdown on the housing. You can ring the windings and, but such a check is not considered accurate.
To check the windings without ringing them or opening the motor board, you can use current clamps. To do this, measure the current in each phase.
If the motor windings are connected by a star and two windings are broken, there will be no current in any of the phases. If there is a break in one of the windings, you will find that there is current in two phases, and it is increased. When connected according to a delta circuit, even if two windings burn out, current will flow in two of the three phase wires.
If there is a break in one of the windings, the engine may not start under load, or it may start, but rotate slowly and vibrate. Below is a device for measuring engine vibrations.
If the windings are in good condition, and the current during measurement is increased and the machine is knocked out or the fuse blows, the shaft or the actuator driven by it is probably jammed. If this is possible, after turning off the power, try to turn the shaft by hand, and you need to disconnect it from the driven mechanism.
When you determine that it is the motor shaft that is not rotating, check the bearings. Electric motors are equipped with either plain or rolling bearings. Worn bushings (sliding bearings) are checked for the presence of lubrication; if the bushings do not have external defects, it is possible to simply lubricate them, having previously cleaned them of dust, chips and other contaminants. But this rarely happens, and this repair method is more relevant for low-power engines of household appliances. In powerful engines, bearings are often simply replaced.
Problems with low speed, heating, shaft immobility and increased bearing wear can be associated with uneven load on the shaft, its misalignment, deformation and bending. If the first two cases can be corrected by correct installation of the shaft or actuator, as well as by reducing the load, then deformation and sagging of the middle part of the shaft requires its replacement or complex repairs. This occurs especially often in powerful electric motors with long shafts.
When one of the bearings wears out, the shaft often “bites.” In this case, as a result of the expansion of the metal due to heating during friction, the shaft may first begin to rotate, but either not reach full speed, and in a particularly advanced case, it will stop altogether.
Rolling bearings also require regular replenishment of lubricant and wear out during operation, especially quickly if there is little lubricant or it is contaminated.
The engine is getting hot
The first reason for engine heating is problems with the cooling system. With such a malfunction, the motor housing becomes completely heated. Most engines are air cooled. For this purpose, the housings are made with fins, and a cooling fan is installed on one side of the shaft, the air flow of which is directed using a casing along the ribs.
If the fan is damaged, or if it, for example, flies off the shaft, an overheating problem arises. Powerful engines use a liquid cooling system. In addition, there are engines without fans - cooled by natural convection.
If the fan is normal, you need to continue diagnostics.
When the engine heats up, check the heating of the bearings. To do this, feel with your hand the surface of the case from the side of the back cover (where there are no protruding rotating shafts - safety is paramount).
If the bearing caps are hotter than other parts of the housing surface, you need to check the presence and condition of the lubricant in them, and when using liners, replace them.
If replacing the grease in the ball bearing does not correct the situation, they should also be replaced.
Local heating of the housing - a situation in which some part of it is clearly hotter than all the others, is observed during interturn short circuits. In such cases, diagnostics are carried out using current clamps - the currents in the phases are compared. If in one of the phases the current clearly exceeds the currents in the other phases, then the malfunction of the motor windings is confirmed. In this case, the repair consists of partial or complete rewinding of the stator.
Increased heating of an asynchronous electric motor can also occur when the stator plates short-circuit.
The engine vibrates, makes noise and makes abnormal noises
Engine noise may also be due to bearing wear. You've probably noticed, like kitchen electrical appliances - this is the reason. Vibrations of the shaft occur during its axial shift and deformation, which we discussed earlier.
Vibrations, noise or overheating of the active steel are also possible if the rotor touches the stator during rotation. This occurs either when the rotor bends or when the stator plates are damaged. In the latter case, it is disassembled and the plates are repressed. The contact point of the plates can be found by the unevenness or it will be polished by the rotor.
Conclusion
We looked at a number of electric motor malfunctions, how to eliminate them and the causes of their occurrence. Operation of an overheating motor is fraught with premature failure of the winding insulation. After a long period of inactivity, you cannot start the engine without measuring the resistance between the windings and the housing using a megohmmeter.
An insulation resistance of about 1 MOhm per 1 kV of supply voltage is considered normal. That is, a motor with a winding insulation resistance of no less than 0.5 MOhm can be considered suitable for operation in a network with a voltage of 380 V. Otherwise you risk damaging it. If the insulation resistance is less, dry the engine by frequently removing the casing or back cover. During operation, the winding resistance gradually increases due to the evaporation of moisture during heating.
Subject to the operating mode, operating and maintenance rules, as well as normal power supply, an asynchronous motor lasts a long time, often reusing its resource several times over. In this case, the main repair consists of lubrication and replacement of bearings.
During improper transportation, installation and operation, the electric motor may fail. Breakdowns are possible if the rules of technical operation are not followed and due to wear of parts.
In the first case, to eliminate malfunctions, you need to quickly find the cause and eliminate it by carrying out minor repairs. But the technology for repairing electric motors in the second case is a complex process: it is a major overhaul. But it would be optimal to constantly monitor the engine’s operation - the so-called preventive inspection.
Look for the reason in poor-quality work on soldering the contacts in the rotor circuit, so inspect the quality of all soldering of the winding - re-solder the faulty ones, and also re-solder those that cause concern.
Stator overheating
If uniform overheating of the stator active steel occurs, while it has a nominal value, then the cause may be the mains voltage, which may be higher than the nominal one, or a fan malfunction. The cause of the malfunction can be eliminated quite easily: by reducing the load or strengthening the motor that is on the fan. To do this, you need to repair the fan by removing the protective casing. But if the overheating is uneven, then there may be several reasons:
- a breakdown in the stator winding or a short circuit to the housing, which leads to burnout of the teeth, as well as to their melting;
- a short circuit occurred between some plates, which could have been caused by burrs, as well as contact of the rotor with the stator housing.
In order to repair the stator of an electric motor , you need to cut out faulty elements and remove burrs. Then insulate the sheet from one another using mica or special cardboard, the insulation of which is insulating varnish. Separate the sheets that are connected and varnish them. If there is too much damage, then re-mixing is carried out with re-insulation of all steel sheets and the stator is rewinded. If the stator windings are uniformly overheated, its windings may be incorrectly connected: when they are connected not together - with a “star”, but in series - with a “delta”; there may be engine overload or improper ventilation operation; a low value of the stator voltage at the input leads to an overcurrent of the motor. To eliminate this, we reduce the load, increase the voltage values to the nominal value or reduce the load current values to the nominal value. We solder the stator windings together - into a “star”. When the stator winding is intensely heated, the circuit may be closed. Disconnect the winding, probe it, find the fault and repair that part of the circuit. If necessary, rewind the entire winding or the part that is damaged.
Rotor malfunction.
If the rotor overheats, humming and braking, or if there are asymmetrical current readings in the phases, look for the cause in poor-quality soldering of the rotor circuit, therefore, before you start repairing the electric motor rotor, inspect the quality of all soldering of its windings - re-solder the faulty ones, and also re-solder those that give rise to concern . If Rotorimmovableand open, and the three rings have the same voltages, look for the reason in a break in the wires that connect the rotor to the starting rheostat. This is possible due to wear of the liners, shifting of the bearing shields, which causes a powerful attraction of the rotor to the stator. Rrepair of asynchronous electric motors in this case, it consists of replacing the liners and adjusting the bearing shields.
Sparking andnon-standardheataniyebrushes and commutator.
Reasons: the brush has become unusable or is installed incorrectly, or the dimensions of the brush do not correspond to the dimensions of the holder cage, or the brush is poorly connected to the fittings. You just need to accurately position the brushes and holders.
Increased vibration.
This can happen due to an unbalanced rotor, clutch or pulley. Vibration can also occur due to inaccurate centering of the device shafts or when the connecting coupling halves are bent. The rotor must be balanced. To do this, you need to balance the coupling halves and pulleys. The engine must be centered. Install the coupling half in the correct position; to do this, first remove it. Find the point of poor connection or break and eliminate the fault.
Knock in bearingsteachings.
It can appear due to broken tracks and destroyed rolling elements. Just replace the bearing with a good one.
