There are many reasons for the failure of AC high-voltage motors. For this reason, it is necessary to explore a set of targeted and clear troubleshooting methods for various types of faults, and propose effective preventive measures to eliminate faults in high-voltage motors in a timely manner, so as to reduce the failure rate of high-voltage motors year by year.
What are the common faults of high-voltage motors? How should they be dealt with?
Failure type:
1. Motor cooling system failure
2. Motor rotor failure
3. High-voltage motor stator coil failure
4. Bearing failure
5. Insulation breakdown
1. Motor cooling system failure
Failure analysis:
Due to production needs, high-voltage motors start frequently, have large vibrations, and have large mechanical impulses, which can easily lead to malfunctions of the motor circulation cooling system. This mainly includes the following types:
1, the external cooling pipe of the motor is damaged, resulting in the loss of cooling medium, which in turn reduces the cooling capacity of the high-voltage motor cooling system, and the cooling capacity is blocked, causing the motor temperature to rise;
2. After the cooling water deteriorates, the cooling pipes are corroded and blocked by impurities, causing the motor to overheat;
3, some cooling heat pipes have high requirements for heat dissipation function and thermal conductivity. Objects of different materials leave gaps due to different degrees of shrinkage. Problems of oxidation and rust occur at the combination of the two. Penetration of cooling water will cause a "shooting" accident in the motor, and the motor unit will automatically stop, causing the motor unit to not work properly.
Repair method:
Monitor external cooling piping to reduce the temperature of the fluid in it as much as possible. Improve cooling-water quality to reduce pipe corrosion caused by impurities and the likelihood of blocked cooling passages. Lubricant retained in the condenser reduces its heat-dissipation rate and restricts the flow of liquid refrigerant. For water leaks in aluminum external cooling piping, move the leak detector probe near all potential leak locations. At the locations to be checked, such as connections and welds, run the system again to recirculate the leak-detection agent, and implement a practical repair approach using pressing, plugging and sealing.
When conducting on-site maintenance, glue must be applied to the leakage area of the aluminum external cooling pipe of the high-voltage motor, which can effectively prevent the contact between steel and aluminum and achieve a good anti-oxidation effect.
2. Motor rotor failure
Failure analysis:
During motor starting and overload operation, various forces act on the welded joints between the rotor short-circuit rings and copper bars, gradually loosening the rotor copper bars. In general, because the end ring is not forged from a single piece of copper, poor welding at its joints can cause cracking under thermal stress during operation. An excessively loose fit between the copper bars and iron core allows the bars to vibrate in the slots and may fracture the bars or end rings. In addition, inadequate installation workmanship can cause slight roughening of the bar surfaces. If heat is not dissipated promptly, severe cases can lead to expansion and deformation, intensifying rotor vibration.
Repair method:
First, inspect broken welded joints in the high-voltage motor rotor and carefully remove debris from the core slots. Focus on defects such as broken bars and cracks. Repair fractured welds using copper material, and resume normal operation only after all screws have been tightened. Inspect the rotor winding carefully with an emphasis on prevention, replacing it promptly when necessary to prevent severe core burnout. Check the core clamping bolts regularly, reinstall the rotor and measure core losses when necessary.
3. High-voltage motor stator coil failure
Failure analysis:
Damage to stator-winding insulation accounts for more than 40% of high-voltage motor faults. During rapid starting and stopping or rapid load changes, mechanical vibration produces relative movement between the stator core and windings. Combined with thermal deterioration, this causes insulation breakdown. Higher temperatures accelerate surface deterioration and change the condition of the insulation surface, causing related changes. Oil, moisture and dirt on the winding surfaces lead to discharge between stator phases. At contact areas, the red anti-corona coating on the insulation of high-voltage leads has turned black.
The high-voltage lead part was inspected and it was found that the broken part of the high-voltage lead was at the edge and corner of the stator frame. Continued operation in a humid environment resulted in the aging of the insulation layer of the high-voltage lead wire of the stator winding, resulting in a decrease in the insulation resistance of the winding.
Repair method:
According to on-site working conditions, first wrap the high-voltage lead section of the motor winding with insulating tape. Using the coil-lifting technique commonly employed by maintenance electricians, slowly lift only the upper slot side of the faulty coil 30–40 mm away from the inner wall of the stator core and secure it in place. Initially clamp the newly wrapped insulation with a simple heated compression fixture. Half-lap wrap the straight section of the upper coil side with 10–12 layers of mica paper tape for insulation to ground. Then wrap the insulation to ground at the two end noses of the adjacent-slot coil. Apply high-resistance semiconductive paint over a length of 12 mm to the sloping sections of the coil ends.
It is best to heat and cool twice each. Tighten the die screws again before heating for the second time.
4. Bearing failure
Failure analysis:
Deep groove ball bearings and cylindrical roller bearings are most commonly used in high-voltage motors. The main reasons for motor bearing failure are unreasonable installation and failure to install according to corresponding regulations. If the lubricant is unqualified, if the temperature is abnormal, the performance of the grease will also change greatly. These phenomena make the bearings prone to problems and lead to motor failure. If the coil is not firmly fixed, the coil and the iron core will vibrate, and the positioning bearing will bear excessive axial load, which will cause the bearing to burn out.
Repair method:
Special bearings for motors include open and closed types, and the specific selection should be based on the actual situation. For bearings, special clearance and grease need to be selected. When installing the bearing, attention should be paid to the selection of lubrication. Sometimes grease with EP additives is used. A thin layer of grease can be applied on the inner sleeve to increase the operating life of the motor bearing. Correctly select bearings and use bearings accurately to reduce the radial clearance of the bearing after installation and use a shallow outer ring raceway structure to prevent it.
When assembling the motor, it is also necessary to carefully check the matching dimensions of the bearing and the rotor shaft when installing the bearing.
5. Insulation breakdown
Failure analysis:
A damp environment and poor electrical and thermal conduction performance can cause excessive motor temperature rise, leading to deterioration or even peeling of rubber insulation. This can loosen leads and cause breakage or even arcing. Axial vibration causes friction between the coil surface, spacer blocks and core, wearing away the semiconductive corona protection layer on the outside of the coil. In severe cases, it directly damages the main insulation and causes insulation breakdown. Moisture in a high-voltage motor can prevent the resistance of its insulating materials from meeting the specified requirements, resulting in motor failure. After excessively long service, poor contact between the corona protection layer and the stator core can cause arcing and winding breakdown, ultimately causing motor failure. Oil contamination penetrating the main insulation inside a high-voltage motor can readily cause turn-to-turn short circuits in stator coils. Poor internal contacts can also easily cause motor failure.
Repair method:
Insulation technology is one of the important process technologies in motor manufacturing and maintenance. In order to ensure the stability of the motor for a long time, the heat resistance of the insulation must be improved. A shielding layer of semiconductor material or metal material is placed inside the main insulation to improve the voltage distribution along the surface. A complete grounding system is one of the important measures for the system to resist electromagnetic interference.
What is the most serious failure of high-voltage motors?
1. Common faults of high-voltage motors
Electromagnetic faults:
1, phase-to-phase short circuit of stator winding
Phase-to-phase short circuit of the stator winding is the most serious fault of the motor. It will cause serious damage to the winding insulation of the motor itself and burn the iron core. It will also cause a reduction in the grid voltage, affecting or destroying the normal power consumption of other users. Therefore, it is required to remove the faulty motor as soon as possible.
2, one-phase winding inter-turn short circuit motor
When a phase winding is short-circuited between turns, the fault phase current increases, and the degree of current increase is related to the number of short-circuited turns. The inter-turn short circuit destroys the symmetrical operation of the motor and causes serious local heating.
3, single phase to ground short circuit
The power supply network of high-voltage motors is generally a neutral point non-directly grounded system. When a single-phase ground fault occurs in a high-voltage motor, if the grounding current is greater than 10A, the motor stator core will be burned. In addition, a single-phase ground fault may develop into a turn-to-turn short circuit or a phase-to-phase short circuit. Depending on the size of the ground current, the faulty motor can be removed or an alarm signal can be issued.
4, one phase of power supply or stator winding is open circuit
An open circuit of one phase of the power supply or the stator winding causes the motor to operate with phase loss, the conduction phase current increases, the motor temperature rises sharply, the noise increases, and the vibration increases. Stop the machine as soon as possible, otherwise the motor will burn out.
5, the power supply voltage is too high or too low
If the voltage is too high, the magnetic circuit of the stator core will be saturated, and the current will increase rapidly; if the voltage is too low, the motor torque will decrease, and the stator current of the motor running with load will increase, causing the motor to heat up, and in severe cases, the motor will burn out.
Mechanical failure:
1, bearing wear or lack of oil
Bearing failure can easily cause the temperature of the motor to rise and the noise to increase. In severe cases, the bearings may lock up and the motor may burn out.
2, poor assembly of motor accessories
When assembling the motor, the screw handles are uneven and the inner and outer small covers of the motor rub against the shaft, causing the motor to become hot and noisy.
3, poor coupling assembly
The transmission force of the shaft increases the temperature of the bearing and increases the vibration of the motor. In severe cases, it will damage the bearings and burn the motor.
2. Protection of high-voltage motors
1, phase-to-phase short circuit protection
That is, current quick-break or longitudinal difference protection reflects the phase-to-phase short circuit fault of the motor stator. Motors with a capacity less than 2MW are equipped with current quick-break protection; motors with a capacity of 2MW and above or less than 2MW but the current quick-break protection sensitivity cannot meet the requirements, and important motors with six outlet wires can be equipped with longitudinal difference protection. The phase-to-phase short-circuit protection of the motor acts on tripping; for synchronous motors with automatic demagnetization devices, the protection should also act on demagnetization.
2, negative sequence current protection
As a protection for motor inter-turn, phase failure, reversed phase sequence and large voltage imbalance, it can also be used as a backup for the main protection of three-phase current imbalance and inter-phase short circuit fault of the motor. Negative sequence current protection operates on trip or signal.
3, single-phase ground protection
The power supply network of high-voltage motors is generally a small current grounding system. When a single-phase grounding occurs, only the grounding capacitor current flows through the fault point, which is generally less harmful. Only when the grounding current is greater than 5A should the installation of single-phase grounding protection be considered. When the grounding capacitor current is 10A and above, the protection can operate in tripping with a time limit; when the grounding capacitance current is 10A or below, the protection can operate in tripping or signaling. The wiring and setting of motor single-phase ground fault protection are the same as those of line single-phase ground fault protection.
4, low voltage protection
When the power supply voltage decreases for a short period of time or is restored after an interruption, many motors start at the same time, which may cause the voltage to recover for a long time or even fail to recover. In order to ensure the self-starting of important motors, for unimportant motors or process or safety reasons, it is not allowed to install low-voltage protection on self-starting motors with delayed action before tripping.
5, overload protection
Long-term overloading will cause the motor temperature to rise beyond the allowable value, causing the insulation to age and even cause failure. Therefore, motors that are prone to overload during operation should be equipped with overload protection. Depending on the importance of the motor and the conditions under which overload occurs, the action can be set to signal, automatic load reduction or tripping.
6, long startup time protection
The reaction motor starting time is too long. When the actual starting time of the motor exceeds the set allowable time, the protection will trip.
7, overheating protection
It responds to an increase in the positive sequence current of the stator or the occurrence of a negative sequence current caused by any reason, causing the motor to overheat, and the protection operates to alarm or trip. Overheating prohibits restarting.
8, stall protection (positive sequence overcurrent protection)
If the motor is blocked during starting or running, the protection action will trip. For synchronous motors, out-of-step protection, loss of excitation protection and asynchronous impact protection should also be added.
Source: Internet
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