1, the general method of electrical equipment maintenance
Maintenance methods are closely related to the type of electrical equipment, technical requirements, working conditions and practical tools. According to the common points of various maintenance methods, it can be concluded that there are six kinds of simplest and most commonly used methods: watching, listening, smelling, touching, measuring, and doing.
Look: 1. Observe the appearance of the components of the electrical equipment. For example, is the fuse blown, the fasteners loosened, and the insulator carbonized? 2. Observe the value indicated by the monitoring instrument or the status of the indicated device.
Listen: Listen to changes in sound when the electrical equipment is operating to determine the operating conditions. For example, an asynchronous motor can not be started individually, and at the same time it emits a “click†sound; when the motor bearing is damaged, it emits a “rustle†sound, and so on.
Smell: Smell the odor emitted when electrical equipment is operating. When the temperature rise exceeds the limit due to a short circuit, overload, or other fault in the electrical equipment, a pungent smell may appear.
Touch: By touching the temperature of the shell of the electrical device, roughly determine whether the operating condition of the low-level insulation device or the general device is normal.
Measurement: Test various electrical equipment operating parameters and insulation resistance values ​​with common measuring instruments. Do: Perform regular cleaning, maintenance, inspection, and maintenance in accordance with the requirements of the electrical equipment maintenance cycle.
2, the three-phase asynchronous motor common fault analysis
Three-phase asynchronous motors are the most widely used and most used high-power electrical equipment in coal mining enterprises. Scientifically and rationally maintaining and managing these three-phase asynchronous motors makes them in a normally available state of technology. This is of crucial importance. The premise of finding faults and resolving faults in time is the in-depth understanding of the root causes of the faults. As an example, a simple analysis of the root causes of common faults in three-phase asynchronous motors is provided.
2.1 Three-phase asynchronous motor single operation
Two thermal relays are commonly used in the electric drive system for overload protection and single protection to prevent the asynchronous motor from running in a single operation. Since the thermal relay cannot accurately set the operating value, a single-phase and operational fault of the three-phase asynchronous motor often occurs, causing the motor to overheat or burn out. The cause of this failure can be analyzed from both the motor failure and the main circuit is not normal. When one phase of the motor armature winding is broken, the lead wire is broken, or the terminal screw is loose, it will cause asynchronous motor single-wire operation or V-shaped three-phase operation.
From the point of view of the main circuit, if the power supply is missing when the fuse is blown or the contact of the main contactor contacts is poor, the motor will be connected to the single-phase power supply.
When a three-phase asynchronous motor is in operation and one phase is powered off, it does not stop. In general, a three-phase asynchronous motor can only bear (60-70)% of the rated load when it is in single-phase operation. Therefore, if the thermal relay malfunctions or is not set correctly, the motor will run in a single-phase overload, and a slightly longer time will cause the motor to run. Serious fever. The single-phase operation fault shows that the stator three-phase current is seriously unbalanced, the running sound is abnormal, and the motor does not appear to have “strengthâ€; when the motor is turned on and then turned on, it cannot start and emit a buzzing sound.
During maintenance, the setting value of the thermal relay should be carefully checked and adjusted so that it can be used as overload protection during single-phase operation; during the inspection, the temperature rise and sound of the motor should be monitored to see if the sound is normal. Single-phase operation failure; always check the contact of the main circuit contactor in the starter cabinet. When the electrical appliance is in motion, the three-phase contacts should be able to contact reliably. A multimeter can be used to check single-phase operation failures.
2.2 Stator winding short circuit
Asynchronous motor stator winding short-circuit phase short circuit and inter-turn short circuit two.
2.2.1 Stator winding phase-to-phase short circuit
The insulation resistance of any two phases of a normal three-phase asynchronous motor shall not be less than 0.5MΩ. When the insulation resistance is zero or close to zero, it indicates that the insulation between the phases is damaged, and a phase-to-phase short-circuit fault has occurred.
The causes of phase-to-phase short circuits in three-phase asynchronous motors are:
1 When the motor windings are severely overheated, especially in the downhole environment (heating during operation and moisture absorption during shutdown), electrical breakdown occurs due to the weak insulation between the stator windings;
2 Double-layer winding motors, in which some of the upper and lower sides of the slots belong to two-phase windings, may cause electrical breakdown due to weak interlayer insulation;
3 Interphase short-circuit faults are manifested as motor running sound, stator current unbalance, protective relay action or fuse blowout, and even winding burnout.
2.2.2 Stator Winding Interturn Short Circuit
A three-phase asynchronous motor stator winding inter-turn short circuit is a short-circuit occurring between turns of a coil of a phase winding. This short circuit is caused by the insulation damage of the conductor skin in the coil, which causes adjacent conductors to come into contact with each other.
At the beginning of a short circuit in the turn, only two wires may be in contact due to insulation wear at the overlap. As a result of the circulation flow in the short circuit turns, the coil quickly heats up, further damaging the insulation of the adjacent wires, causing the number of short circuits to increase and the failure to increase. When there are enough short circuit turns, the fuse will be blown and even the winding burns.
When a three-phase winding has a phase-to-turn short circuit, the number of turns of the phase winding is reduced, and the stator three-phase current is unbalanced. An unbalanced three-phase current causes the motor to vibrate while emitting an abnormal sound. The average torque of the motor drops significantly and it becomes weak when dragging the load.
The causes of short circuits in the turn are:
1 In the dismantling and maintenance of the motor, due to improper operation, the end of the wound wound insulation, so that the wires are in contact with each other;
2 The motor is overloaded for a long time, the motor is overheated, and the insulation damage of the coil is weak.
3When the stator goes offline, individual wires overlap in the slot. After long-term operation, due to the effect of electromagnetic force, the insulation at the intersection will be damaged and a short circuit between turns will be developed. Use a visual inspection or a short-circuit detector to determine the short circuit point.
2.2.3 stator winding ground fault
When the insulation resistance between the stator winding conductor and the iron core is zero or close to zero, it is considered that a stator winding ground fault has occurred in the motor.
The main causes of stator winding ground faults are: motor insulation aging, loss of insulation performance; stator notch insulation damage, conductor and core contact; winding end insulation damage and touch the end cap;
Stator winding leads to cable insulation damage and bumps. After the stator windings are grounded, if the motor base is not grounded well, the base will become live, threatening the safety of the operator;
Short-circuit faults occur when the stator winding is grounded at multiple points. Therefore, when the stator winding is grounded, it must be carefully checked and eliminated. Ground faults can be checked with a megohmmeter.
2.2.4 The motor overheated, exceeding the allowable temperature
Asynchronous motor overheating is a relatively common fault, the reasons for which are more complex and can be analyzed from the aspects of power supply, motor, control equipment and load.
1 When the power supply voltage is too high, it can be known from U≈4.44f1w1kw1 that the magnetic flux will increase and the magnetic circuit of the motor will become saturated. At this time, the stator current drastically increases and the motor temperature rises. When the power supply voltage is too low, if the load torque is fixed, the reduction of the magnetic flux will inevitably lead to an increase in the rotor current. At this time, the stator current increases and the temperature rise of the motor increases.
2 Three-phase asymmetrical supply voltage. A three-phase asynchronous motor needs to work under a three-phase symmetrical voltage, and its three-phase voltage asymmetry should be less than 5% of the rated voltage. When the three-phase voltage value difference is large, the three-phase current of the stator of the asynchronous motor will be unbalanced. Under the rated load, the current of a certain phase winding will exceed the rated value, so that the phase winding is overheated and the stator winding of the asynchronous motor partially overheats. malfunction.
3 control lines. If the control line is poorly maintained, contact with the contacts is not good, and the single-phase operation of the motor will also increase the motor current. In some devices, the drag motor has a brake device. The brake device does not cooperate well with the action, and the motor is locked in a serious state, which will cause the motor to overheat. In addition, the number of motor starts per hour is too much, or the motor over-quota operation has an effect on the heating of the stator.
4 load reasons. The motor is running under overload for a long time and the protection device is not reliable and can not be operated in time, so that the stator current of the motor exceeds the rated value; the mechanical connection of the motor and the dragged device is not good, the gear box is dirty or the coupling eccentricity makes the motor No-load loss increases; the motor is subjected to an undue impact load; the motor stalls due to a load failure.
5 The motor itself is malfunctioning. The motor stator windings are short circuited, grounded or broken by one phase; the stator windings of the motor after the repair are incorrectly wired; the motor rotor breaks, the end ring is open; the motor cooling has obstacles, such as fan damage, air blockage, excessive dirt on the surface, etc. : Mechanical assembly is poor, shaft bending deformation, bearing damage, stator and rotor phase wipe.
3, analysis of the causes of difficult faults in several major electrical equipment
Electrical equipment may have many kinds of failure phenomena, and any kind of electrical failure may be caused by one or more reasons. That is to say, a variety of reasons may lead to the occurrence of the same failure phenomenon. Among the numerous causes of failures, some are known to electrical management personnel, or they are general common sense. They are limited in length and are not what this article describes. The following section focuses on the analysis of difficult problems and causes of several major electrical equipments for reference by maintenance personnel and technicians.
3.1 Thermal Relay Troubleshooting and Analysis of Causes
Symptom 1: The power device is operating normally, but the thermal relay frequently operates, or the electrical device is burned and the thermal relay does not move.
Analysis of the reasons: 1 thermal relay adjustable component fixed anchor loose, not in the original set point; 2 thermal relay through a huge short-circuit current, the bimetal element has been permanently deformed; 3 thermal relay has not been tested, dust accumulation, or Rusty, or action mechanism stuck, wear, bakelite parts deformation; 4 thermal relay adjustable parts damage. (Conventional reason: The thermal relay external wiring screws are not tightened or the setting current value is too low for a frequent action; the setting current value is too high together with no protection).
Symptom 2: The thermal relay moves quickly and slowly.
Analysis of the reasons: 1 The internal mechanism of the thermal relay has loose parts; 2 The bimetal is bent during inspection; 3 The external connection screws are not tightened.
3.2 Automatic Switch Troubleshooting and Analysis
Fault phenomenon 1: electric operation automatic switch, contact can not be closed.
Cause analysis: 1 solenoid rod stroke is not enough; 2 motor operation positioning switch failure; 3 controller rectifier or capacitor damage.
Symptom 2: The manual operation of the automatic switch, the contact can not be closed.
Analysis of the reasons: 1 voltage release or voltage coil coil burnout; 2 energy storage spring deformation or fracture, resulting in reduced or not closing force closing; 3 counter force spring force is too large; 4 agencies can not reset the trip.
Symptom 3: Automatic switch temperature rise is too high.
Analysis of the reasons: 1 contact pressure is too low; 2 loosening of the connecting screws of two conductive parts; 3 excessive burning of the contact surface or poor contact.
Symptom 4: Loss of voltage release is noisy.
Analysis of reasons: 1 too much reaction spring force; 2 iron core work surface oil; 3 short circuit ring fracture. (The conventional reason is an example: the starter motor is automatically switched off immediately - the instantaneous setting of the overcurrent release is too small).
3.3 Three-phase asynchronous motor trouble and failure analysis
Symptom 1: The motor has a low speed and looks powerless after it starts.
Analysis of the reasons: 1 overload; 2 single operation, barely overloaded after starting; 3 stator windings should be connected to "â–³" shape and wrongly connected to the "Y"; 4 squirrel cage rotor bar or end ring fracture or open welding.
Symptom 2: The temperature rise of the motor is too high.
Analysis of the reasons: 1 load is too heavy, and the protection device fails; 2 stator windings are short-circuited or grounded; 3 under heavy load single-phase operation; 4 motor mechanically inflexible, no-load loss;
Symptom 3: The motor is noisy during operation.
Analysis of the reasons: 1 single-phase operation; 2 stator winding lead incorrectly; 3 fixed, rotor phase friction (ie, sweeping hall); 4 bearing damage is seriously lack of grease; fan blade deformation collision shell. Of course, the range of electrical equipment is relatively wide. Here, only a summary of conventional electrical equipment and common faults is provided for the reference of the personnel engaged in electrical equipment maintenance and technical management personnel in mines.
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