The high-voltage motor refers to the motor that operates under the power frequency of 50Hz and the rated voltage of 3kV, 6kV and 10kV AC three-phase voltage. There are many classification methods for high-voltage motors, which are divided into four types: small, medium, large and extra large according to their capacity; they are divided into A, E, B, F, H, and C-class motors according to their insulation grades; General-purpose high-voltage motors and high-voltage motors with special structures and uses.
The motor to be introduced in this article is a general-purpose high-voltage squirrel-cage three-phase asynchronous motor.
The high-voltage squirrel-cage three-phase asynchronous motor, like other motors, is based on electromagnetic induction. Under the action of high electromagnetic field and the comprehensive action of its own technical conditions, external environment and operating conditions, the motor will generate electricity within a certain operating period. Various electrical and mechanical failures.
1 Classification of high voltage motor faults
Plant machinery in power plants, such as feed water pumps, circulating pumps, condensation pumps, condensation lift pumps, induced draft fans, blowers, powder dischargers, coal mills, coal crushers, primary fans, and mortar pumps, are all driven by electric motors. verb: move. These machines stop running in a very short period of time, which is enough to cause a reduction in the output of the power plant, or even shutdown, and may cause serious accidents. Therefore, when an accident or abnormal phenomenon occurs in the operation of the motor, the operator should quickly and correctly determine the nature and cause of the failure according to the accident phenomenon, take effective measures, and deal with it in time to prevent the accident from expanding (such as the reduction of the output of the power plant, the power generation of the entire steam turbine). Unit stops running, major equipment damage), resulting in immeasurable economic losses.
During the operation of the motor, due to improper maintenance and use, such as frequent startup, long-term overload, motor damp, mechanical bumps, etc., the motor may fail.
The faults of electric motors can generally be divided into the following categories: ①Insulation damage caused by mechanical reasons, such as bearing wear or bearing black metal melting, excessive motor dust, severe vibration, and insulation corrosion and damage caused by lubricating oil falling on the stator winding, So that the insulation breakdown causes failure; ② the insulation breakdown caused by the insufficient electrical strength of the insulation. Such as motor phase-to-phase short-circuit, inter-turn short-circuit, one-phase and shell grounding short-circuit, etc.; ③ winding fault caused by overload. For example, the lack of phase operation of the motor, the frequent starting and self-starting of the motor, the excessive mechanical load dragged by the motor, the mechanical damage dragged by the motor or the rotor being stuck, etc., will cause the motor winding failure.
2 High voltage motor stator fault
The main auxiliary machines of a power plant are all equipped with high-voltage motors with a voltage level of 6kV. Due to the poor operating conditions of the motors, frequent motor starts, water leakage of water pumps, leakage of steam and dampness installed below negative meters, etc., it is a serious threat. Safe operation of high voltage motors. Coupled with the poor quality of motor manufacturing, problems in operation and maintenance, and poor management, high-voltage motor accidents are frequent, which seriously affects the output of generators and the safe operation of power grids. For example, as long as one side of the lead and blower fails to operate, the output of the generator will drop by 50%.
2.1 Common faults are as follows
①Due to frequent starting and stopping, long starting time, and starting with load, the aging of the stator insulation is accelerated, resulting in insulation damage during the starting process or during operation, and the motor is burned; ②The quality of the motor is poor, and the connection wire at the end of the stator winding is poorly welded. The mechanical strength is not enough, the stator slot wedge is loose, and the insulation is weak. Especially outside the notch, after repeated starts, the connection is broken, and the insulation at the end of the winding falls off, resulting in a short circuit of motor insulation breakdown or a short circuit to ground, and the motor is burned; The cannon caught fire and damaged the motor. The reason is that the lead wire specification is low, the quality is poor, the running time is long, the number of starts and stops is many, the metal is mechanically aged, the contact resistance is large, the insulation becomes brittle, and the heat is generated, causing the motor to burn out. Most of the cable joints are caused by the maintenance personnel’s irregular operation and careless operation during the repair process, causing mechanical damage, which develops to motor failure; ④The mechanical damage causes the motor to be overloaded and burns out, and the bearing damage causes the motor to sweep the chamber, causing the motor to burn out; The poor maintenance quality and disrepair of the electrical equipment cause the three-phase closing at different times, resulting in operating overvoltage, which causes insulation breakdown and burns out the motor; ⑥ The motor is in a dusty environment, and dust enters between the stator and rotor of the motor. The incoming material causes poor heat dissipation and serious friction, which causes the temperature to rise and burns the motor; ⑦ The motor has the phenomenon of water and steam entering, which causes the insulation to drop, resulting in short-circuit blasting and burning the motor. Most of the reason is that the operator does not pay attention to washing the ground, causing the motor to enter the motor or the equipment leaks and the steam leakage is not detected in time, which causes the motor to burn; Motor damage due to overcurrent; ⑨ motor control circuit failure, overheating breakdown of components, unstable characteristics, disconnection, loss of voltage in series, etc.; In particular, the zero-sequence protection of low-voltage motors is not installed or replaced with a new large-capacity motor, and the protection setting is not changed in time, resulting in a large motor with a small setting, and multiple starts are unsuccessful; 11The switches and cables on the primary circuit of the motor are broken and phase is missing Or grounding causes motor burnout; 12 wound motor stator and rotor switch time limit is improperly matched, causing the motor to burn out or fail to reach the rated speed; 13 motor foundation is not firm, the ground is not fastened well, causing vibration and shaking Exceeding the standard will damage the motor.
In the motor manufacturing process, a small number of stator coil lead heads (segments) have serious defects, such as cracks, cracks and other internal factors, and due to different working conditions during the motor operation, (heavy load and frequent starting of rotating machinery, etc.) only plays an accelerated fault. effect that occurs. At this time, the electromotive force is relatively large, which causes strong vibration of the connection line between the stator coil and the pole phase, and promotes the gradual expansion of the residual crack or crack in the lead end of the stator coil. The result is that the current density of the unbroken part at the defect of the turn reaches a considerable degree, and the copper wire at this place has a sharp drop in stiffness due to the rise of temperature, resulting in burnout and arcing. A coil wound by a single copper wire, when one of them breaks, the other one is usually intact, so it can still be started, but each subsequent start breaks first. , both may flashover burn another adjacent copper wire that has increased a considerable current density.
It is recommended that the manufacturer strengthen the process management, such as the winding process of the winding, the cleaning and sanding process of the lead tip of the coil, the binding process after the coil is embedded, the connection of the static coil, and the bending of the lead tip before the welding head (flat bending makes bending ) finishing process, it is best to use silver welded joints for high-voltage motors above medium size. On the operating site, the newly installed and overhauled high-voltage motors shall be subjected to withstand voltage test and direct resistance measurement using the opportunity of regular minor repairs of the unit. The coils at the end of the stator are not tightly bound, the wooden blocks are loose, and the insulation is worn, which will cause breakdown and short-circuit of the motor windings, and burn the motor. Most of these faults occur at the end leads. The main reason is that the wire rod is poorly formed, the end line is irregular, and there are too few end binding rings, and the coil and the binding ring are not tightly attached, and the maintenance process is poor. Pads often fall off during operation. Loose slot wedge is a common problem in various motors, mainly caused by poor coil shape and poor structure and process of coil in the slot. A short circuit to ground causes the coil and iron core to burn out.
3 High voltage motor rotor failure
Common faults of high-voltage cage-type asynchronous motors are: ①The rotor squirrel cage is loose, broken and welded; ②The balance block and its fixing screws are thrown out during operation, which will damage the coil at the end of the stator; ③The rotor core is loose during operation, and the deformation, Unevenness causes sweep and vibration. The most serious of these is the problem of squirrel cage bars breaking, one of the long-standing problems in power plants.
In thermal power plants, the starting cage (also known as the outer cage) of the high-voltage double squirrel-cage induction motor’s starting cage (also known as the outer cage) is broken or even broken, thus damaging the stationary coil of the motor, which is still the most common fault until now. From the production practice, we realize that the initial stage of desoldering or fracture is the phenomenon of fire at start-up, and the lamination of the semi-open rotor core on the side of the desoldering or fractured end melts and gradually expands, eventually leading to fracture or desoldering. The copper bar is partially thrown out, scratching the static iron core and coil insulation (or even breaking a small strand), causing serious damage to the motor’s static coil and possibly causing a larger accident. In thermal power plants, steel balls and coal condense together to produce a large static moment during shutdown, and feed pumps start under load due to lax outlet doors, and induced draft fans start in reverse due to lax baffles. Therefore, these motors have to overcome a large resistance torque when starting.
There are structural problems in the starting cage of the domestic medium-sized and above high-voltage double squirrel-cage induction motors. Generally: ① the short-circuit end ring is supported on all the outer cage copper bars, and the distance from the rotor core is large, and the inner circumference of the end ring is not concentric with the rotor core; ② the holes through which the short-circuit end ring passes through the copper bars are mostly straight-through holes ③The gap between the rotor copper bar and the wire slot is often less than 05mm, and the copper bar vibrates greatly during operation.
①Copper bars are connected by surfacing welding on the outer circumference of the short-circuit end ring. The motor of the powder discharger in Fengzhen Power Plant is a high-voltage double squirrel cage motor. The copper bars of the starting cage are all welded to the outer circumference of the short-circuit end ring. The quality of surfacing welding is poor, and de-soldering or breakage often occurs, resulting in damage to the stator coil. ②The form of the short-circuit end hole: the hole form of the short-circuit end ring of the domestic high-voltage double squirrel-cage motor currently used in the production field, generally has the following four forms: straight hole type, semi-open straight hole type, fish eye hole type, deep sink hole type type, especially the most through-hole type. The new short-circuit end ring replaced on the production site usually adopts two forms: fish-eye hole type and deep sink hole type. When the length of copper conductor is suitable, the space for filling solder is not large, and silver solder is not used much, and the soldering quality is high. Easy to guarantee. ③ Welding, desoldering and breaking of copper bar and short-circuit ring: The failure cases of de-soldering and fracture of the starting cage copper bar encountered in all the more than one hundred high-voltage motors in contact are basically the short-circuit end ring. The eyelets are straight-through eyelets. The conductor passes through the outer side of the short-circuit ring, and the copper conductor ends are also partially melted, and the welding quality is generally good. The copper conductor penetrates about half of the end ring. Because the temperature of the electrode and the solder is too high and the welding time is too long, part of the solder flows out and accumulates through the gap between the outer surface of the copper conductor and the hole of the end ring, and the copper conductor is prone to breakage. ④Easy to find the solder joints of welding quality: For high-voltage motors that often spark during startup or operation, generally speaking, the starting cage copper conductors are desoldered or broken, and it is easy to find the copper conductors that are desoldered or broken. It is very important for the high-voltage double squirrel cage motor in the first and second overhaul after the new installation and into operation to comprehensively check the copper conductors of the starting cage. During the re-soldering process, attention should be paid to replacing all the starting cage conductors. It should be cross-welded symmetrically, and should not be welded in sequence from one direction, so as to avoid deviation of the short-circuit end ring. In addition, when repair welding is performed between the inner side of the short-circuit end ring and the copper strip, the welding place should be prevented from being spherical.
3.3 Analysis of broken cage of rotor
① Many of the motors of the main auxiliary machines of the power plant have broken cage bars. However, most of the motors with broken cages are those with heavier starting load, longer starting time and frequent starting, such as coal mills and blowers. 2. The motor of the induced draft fan; 2. The newly put into operation of the motor generally does not break the cage immediately, and it will take several months or years to operate before the cage breaks; 3. At present, the commonly used cage bars are rectangular or trapezoidal in cross-section. Deep-slot rotors and circular double-cage rotors have broken cages, and the broken cages of double-cage rotors are generally limited to the outer cage bars; ④ The connection structure of the motor cage bars and short-circuit rings with broken cages is also various. , Motors of a manufacturer and a series are sometimes different; there are suspended structures in which the short-circuit ring is only supported by the end of the cage bar, and there are also structures in which the short-circuit ring is directly embedded on the weight of the rotor core. For rotors with broken cages, the length of the cage bars extending from the iron core to the short-circuit ring (extension end) varies. Generally, the extension end of the outer cage bars of a double-cage rotor is about 50mm~60mm long; The length of the extension end is about 20mm~30mm; ⑤ Most of the parts where the cage bar fracture occurs are outside the connection between the extension end and the short circuit (the cage bar welding end). In the past, when the motor of Fengzhen Power Plant was overhauled, two halves of the old cage bar were used for splicing, but due to the poor quality of the splicing, the splicing interface cracked in the subsequent operation, and the fracture appeared to move out of the groove. Some cage bars originally have local defects such as pores, sand holes, and skins, and fractures will also occur in the grooves; ⑥ There is no significant deformation when the cage bars are broken, and there is no necking when the plastic material is pulled off, and the fractures are well matched. Tight, is a fatigue fracture. There is also a lot of welding at the welding place between the cage bar and the short-circuit ring, which is related to the quality of the welding. However, like the broken nature of the cage bar, the source of the external force for the damage of the two is the same; ⑦ For motors with broken cages, the cage bars are in The rotor slots are relatively loose, and the old cage bars that have been repaired and replaced have grooves oriented by the protruding part of the silicon steel sheet of the iron core groove wall, which means that the cage bars are movable in the grooves; ⑧ The broken cage bars are not For a long time, sparks can be seen from the stator air outlet and the air gap of the stator and rotor during the starting process. The starting time of the motor with many broken cage bars is obviously prolonged, and there is obvious noise. When the fracture is concentrated in a certain part of the circumference, the vibration of the motor will be intensified, sometimes resulting in damage to the motor bearing and sweeping.
The main manifestations are: motor bearing damage, mechanical jamming, power switch phase loss, cable lead connector burnout and phase loss, cooler water leakage, air cooler air inlet and air outlet blocked by dust accumulation and other reasons for motor burnout.
After the above analysis of the faults and their nature of the high-voltage motor, as well as the elaboration of the measures taken at the scene, the safe and stable operation of the high-voltage motor has been effectively guaranteed, and the reliability of the power supply has been improved. However, due to poor manufacturing and maintenance processes, coupled with the influence of water leakage, steam leakage, moisture, improper operation management and other factors during operation, various abnormal operation phenomena and more serious failures will occur. Therefore, only by strengthening the strict control of the maintenance quality of high-voltage motors and strengthening the all-round operation management of the motor, so that the motor can reach a healthy operation state, can the safe, stable and economical operation of the power plant be guaranteed.
Post time:
Jun-28-2022
