Refrigeration compressor motor burnout detection and maintenance methods, quickly collect it!

Refrigeration compressor motor burnout detection and maintenance methods, quickly collect it! What are the methods of detection and maintenance of refrigeration compressor motor burnout? Do your friends know? Let's get to know more with the coldest bacteria today! The core component of the refrigeration system is the compressor. The failure of the motor refrigeration compressor (hereinafter referred to as the compressor) can be divided into motor failure and mechanical failure (including crankshaft, connecting rod, piston, valve plate, cylinder head gasket, etc.). Mechanical failures often overload the motor and even block it, which is one of the main causes of motor damage. The damage of the motor is mainly manifested as the destruction (short circuit) and open circuit of the stator winding insulation layer. After the stator winding is damaged, it is difficult to be found in time, which may eventually cause the winding to burn out. After the winding is burned, some phenomena or direct causes that caused the burnout are masked, making post-mortem analysis and cause investigation more difficult. However, the operation of the motor is inseparable from normal power input, reasonable motor load, good heat dissipation and protection of the winding enameled wire insulation. Starting from these aspects, it is not difficult to find that the cause of the unit burned out is as follows: (1) Abnormal load and locked rotor; (2) Short circuit of winding caused by metal shavings; (3) Contactor problem; (4) Absence of power supply phase and voltage; (5) Insufficient cooling; (6) Vacuuming with compressor . In fact, motor damage caused by multiple factors is more common. 1 Abnormal load and stall The motor load includes the load required to compress the gas and the load required to overcome mechanical friction. If the pressure ratio is too large or the pressure difference is too large, the compression process will be more difficult. The increased frictional resistance caused by lubrication failure and the motor stall in extreme cases will greatly increase the motor load. Lubrication failure and increased frictional resistance are the primary causes of abnormal loads. Dilution of the lubricating oil back to the liquid, overheating of the lubricating oil, coking and deterioration of the lubricating oil, and lack of oil will destroy the normal lubrication and cause lubrication failure. Dilute the lubricating oil back to the liquid, which affects the formation of the normal oil film on the friction surface, and even wash away the original oil film, increasing friction and wear. Overheating of the compressor will cause the lubricating oil to dilute or even coke at high temperature, affecting the formation of normal oil film. The oil return of the system is not good, the compressor lacks oil, and naturally cannot maintain normal lubrication. The crankshaft rotates at a high speed and the connecting rod and piston move at a high speed. The friction surface without oil film protection will quickly heat up. Local high temperature makes the lubricating oil evaporate or scorch quickly, making lubrication of this part more difficult. It can cause serious local wear within a few seconds. Lubrication failure, local wear, and greater torque are required to rotate the crankshaft. Low-power compressors (such as refrigerators and household air-conditioning compressors) often have blocked torque (the motor cannot rotate) after lubrication failure due to the low torque of the motor, and enter the "locked-thermal protection-locked-rotation" dead cycle. The motor burns only Time issue. The high-power semi-hermetic compressor motor has a large torque, and local wear will not cause stalling. The motor power will increase with load within a certain range, which will cause more serious wear and even cause the cylinder to bite (the piston is stuck in the cylinder) Inside), severe damage such as broken connecting rod. The current at stall (stall current) is about 4-8 times the normal operating current. The moment the motor starts, the peak value of the current can approach or reach the locked rotor current. Since the heat dissipation of the resistor is proportional to the square of the current, the current during start-up and stall will cause the winding to heat up rapidly. Thermal protection can protect the electrode when the rotor is blocked, but generally there will be no quick response, and it cannot prevent the winding temperature change caused by frequent startup and the like. Frequent start-ups and abnormal loads make the windings subject to high temperature tests, which will reduce the insulation performance of the enameled wire. In addition, the load required for compressed gas will increase as the compression ratio increases and the pressure difference increases. Therefore, using a high-temperature compressor for low temperature or a low-temperature compressor for high temperature will affect the motor load and heat dissipation, which is not suitable and will shorten the service life of the electrode. After the insulation performance of the winding has deteriorated, if there are other factors (such as metal chips forming a conductive circuit, acid lubricant, etc.), it is easy to cause short circuit and damage. 2 Short circuit caused by metal shavings The metal shavings in the winding are the culprits of short circuit and low ground insulation value. The normal vibration of the compressor during operation and the winding twisted by the electromagnetic force at each start-up will promote the relative movement and friction between the metal shavings interposed between the windings and the winding enameled wire. Sharp metal chips can scratch the insulation of the enameled wire and cause a short circuit. The sources of metal shavings include copper pipe shavings left during construction, welding slag, metal shavings that fall off during compressor internal wear and parts damage (such as broken valve discs). For hermetic compressors (including hermetic scroll compressors), these metal chips or debris will fall on the windings. For semi-hermetic compressors, some particles will flow in the system with gas and lubricating oil, and finally gather in the windings due to magnetism; and some metal chips (such as those caused by bearing wear and motor rotor and stator wear (bore sweeping)) It falls directly on the winding. It is only a matter of time before a short circuit occurs after metal chips have accumulated in the winding. Special attention should be paid to the two-stage compressor. In the two-stage compressor, the return air and normal return oil directly enter the first-stage (low-pressure stage) cylinder, after compression, enter the motor cavity cooling winding through the medium-pressure tube, and then enter the second stage, just like the ordinary single-stage compressor. (High-pressure cylinder). The return air contains lubricating oil, which has made the compression process like thin ice. If there is liquid return, the valve plate of the first stage cylinder is easily broken. The broken valve piece can enter the winding after passing through the medium-pressure pipe. Therefore, two-stage compressors are more prone to motor short circuit caused by metal chips than single-stage compressors. Unfortunately, things often get together. The problematic compressor often smells of the scorched smell of lubricating oil when it is turned on. When the metal surface is severely worn, the temperature is very high, and the lubricating oil starts to scorch when it is above 175oC. If there is more water in the system (the vacuum is not ideal, the lubricating oil and refrigerant have a large water content, and the air enters after the negative pressure return air pipe ruptures), the lubricating oil may appear acidic. Acid lubricating oil will corrode the copper tube and the winding insulation layer. On the one hand, it will cause copper plating. On the other hand, the acidic lubricating oil containing copper atoms has poor insulation performance and provides conditions for winding short circuit. 3 Contactor problem The contactor is one of the important components in the motor control loop. Unreasonable selection can destroy the best compressor. It is extremely important to select the contactor correctly according to the load. The contactor must be able to meet demanding conditions such as fast cycling, continuous overload and low voltage. They must have a large enough area to dissipate the heat generated by the load current, and the selection of contact materials must prevent welding under high current conditions such as starting or blocking. The rated current of the contactor cannot be lower than the rated current on the compressor nameplate. Contactors with small specifications or inferior quality cannot withstand compressor start-up, stalling and high-current impact at low voltage, and are prone to single-phase or multi-phase contact jitter, welding or even shedding, causing motor damage. Contactors with jittering contacts frequently start and stop the motor. The motor starts frequently, and the huge starting current and heat will aggravate the aging of the winding insulation layer. At each start, the magnetic torque causes the motor windings to move slightly and rub against each other. If there are other factors (such as metal shavings, lubricating oil with poor insulation, etc.), it is easy to cause short circuit between windings. The thermal protection system is not designed to prevent such damage. In addition, jittery contactor coils are prone to failure. If the contact coil is damaged, it is easy to appear single-phase state. If the selection of the contactor is too small, the contacts cannot withstand the arc and the high temperature due to frequent start-stop cycles or unstable control circuit voltage, which may be welded or detached from the contact holder. The welded contacts will produce a permanent single-phase state, making the overload protector continuously cycle on and off. It should be particularly emphasized that after the contactor contacts are welded, all controls that rely on the contactor to disconnect the compressor power circuit (such as high and low pressure control, oil pressure control, defrost control, etc.) will all fail, and the compressor is in unprotected status. Therefore, when the motor burns out, checking the contactor is an essential process. The contactor is an important cause of motor damage that is often forgotten. 4 Power failure and abnormal voltage Abnormal voltage and phase loss can easily destroy any motor. The power supply voltage variation range cannot exceed ±10% of the rated voltage. The voltage imbalance between the three phases cannot exceed 5%. High-power motors must be powered independently to prevent low voltages when other high-power equipment on the same line starts and runs. The motor power cord must be able to carry the rated current of the motor. If the compressor is running when a phase loss occurs, it will continue to run but will have a large load current. The motor winding will quickly overheat, and the compressor will be thermally protected under normal conditions. When the motor winding is cooled to the set temperature, the contactor will be closed, but the compressor will not start up, there will be locked rotation, and enter the "locked-thermal protection-locked rotation" dead cycle. The difference in the windings of modern motors is very small, and the difference in phase current when the three-phase power supply is balanced can be ignored. In an ideal state, the phase voltage is always equal, as long as a protector is connected to any phase to prevent damage caused by overcurrent. It is actually difficult to ensure the balance of the phase voltage. The method of calculating the percentage of voltage unbalance is the ratio of the maximum deviation between the phase voltage and the average value of the three-phase voltage and the average value of the three-phase voltage. For example, for a nominal 380V three-phase power supply, the voltages measured at the compressor terminals are 380V and 366V, respectively , 400V. You can calculate the average value of three-phase voltage 382V, the maximum deviation is 20V, so the percentage of voltage imbalance is 5.2%. As a result of voltage imbalance, the load current imbalance during normal operation is 4-10 times the percentage point of voltage imbalance. In the previous example, 5.2% unbalanced voltage may cause 50% current unbalance. The National Electrical Manufacturers Association (NEMA) standard publication for motors and generators states that the percentage of phase winding temperature rise caused by unbalanced voltage is approximately twice the square of the voltage unbalance percentage. In the previous example, the number of voltage unbalance points is 5.2, and the percentage increase in winding temperature is 54%. The result is that the one-phase winding is overheated and the other two windings have normal temperatures. A survey completed by U.L. (Underwriters Laboratories, USA) showed that 43% of power companies allow 3% of the voltage imbalance, and another 30% of the power companies allow 5% of the voltage unbalance. 5 Insufficient cooling Compressors with higher power are generally return air cooled. The lower the evaporation temperature, the smaller the system mass flow. When the evaporation temperature is very low (exceeding the manufacturer's regulations), the flow is not enough to cool the motor, and the motor will run at a higher temperature. Air-cooled compressors (generally no more than 10HP) have little dependence on the return air, but they have clear requirements on the ambient temperature and cooling air volume of the compressor. A large amount of refrigerant leakage will also reduce the system mass flow, and the cooling of the motor will also be affected. Some unattended cold storage, etc., often find that a large amount of refrigerant leaks only when the cooling effect is poor. After the motor is overheated, frequent protection will occur. Some users do not check the cause in depth or even short-circuit the thermal protector, which is a very bad thing. Before long, the motor will burn out. Compressors have a range of safe operating conditions. The main considerations for safe operating conditions are the load and cooling of the compressor and motor. Due to the different prices of compressors in different temperature zones, it has been common to use compressors in the domestic refrigeration industry in the past. With the growth of professional knowledge and the improvement of economic conditions, the situation has improved significantly. 6 Use the compressor to evacuate The open refrigeration compressor has been forgotten by people, but there are still some on-site construction workers in the refrigeration industry who have retained the past habit of using the compressor to vacuum. This is very dangerous. Air acts as an insulating medium. After evacuating the airtight container, the discharge phenomenon between the electrodes inside easily occurs. Therefore, with the deepening of the vacuum in the compressor casing, the insulation medium is lost between the exposed terminals in the casing or the windings with slight damage to the insulation layer. Once energized, the motor may be short-circuited and burned in an instant. If the case leaks, it may cause electric shock. Therefore, it is forbidden to use the compressor to evacuate, and when the system and the compressor are in a vacuum state (the refrigerant has not been added after the vacuum has been drawn), it is strictly prohibited to energize the compressor. 7 Summary After the motor burned out, the phenomenon of winding damage was masked, which caused certain difficulties in fault analysis. However, the root cause of compressor motor damage will not disappear. Abnormal load caused by poor lubrication or failure or even blocked rotation, insufficient heat dissipation will shorten the life of the winding; the inclusion of metal shavings in the winding provides a benefit for the short circuit; the welding of the contactor will make the protection of the compressor impossible to perform; If the power supply on which the motor runs is abnormal, any motor will be destroyed fundamentally; using the compressor to evacuate may cause discharge of the internal terminal. Unfortunately, the above-mentioned unfavorable factors can also cause each other: abnormal loads and large currents during stalling may cause contactor welding; single contact arc drawing or even welding will cause phase imbalance or single phase; phase imbalance will cause Heat dissipation problem; insufficient heat dissipation will cause wear; wear will produce metal chips... Therefore, the correct installation and use of the compressor, as well as reasonable daily maintenance, can prevent the emergence of adverse factors, and is the fundamental method to avoid damage to the compressor motor.

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