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    The main cause of the burning of refrigeration compressor windings

    The main cause of the burning of refrigeration compressor windings The damage of the motor coil is mainly manifested as the destruction (short circuit) and open circuit of the stator winding insulation. 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. 　　 Starting from these aspects, it is not difficult to find that the reasons for the winding burnout are as follows: 　　 (1) Abnormal load and locked rotor; 　　 (2) Short circuit of winding caused by metal shavings; 　　 (3) Contactor problem; 　　 (4) The lack of power phase and abnormal voltage; 　　 (5) Insufficient cooling; 　　 (6) Use the compressor to evacuate. In fact, motor damage caused by multiple factors is more common. Factor 1: Abnormal load and stall 　　 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, excessive heating of the lubricating oil, coking and deterioration of the lubricating oil, and lack of oil will destroy 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 become thinner or even scorch 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. 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 during stall (stall current) is approximately 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, the use of high-temperature compressors for low temperatures or low-temperature compressors for high temperatures will affect the motor load and heat dissipation, which is not suitable and will shorten the service life of the electrodes. 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. 　　 Factor 2: Winding short circuit caused by metal shavings 　　 The metal shavings in the winding are the culprits of short circuit and low ground insulation value. 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. 　　 In the two-stage compressor, 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. 　　 Factor 3: Winding short circuit caused by metal shavings 　　It is extremely important to select the contactor according to the load. When using a single contactor, the rated current of the contactor must be greater than the motor nameplate current rating (RLA). At the same time, the contactor must be able to withstand the motor stall current. If there are other loads downstream of the contactor, such as motor fans, it must also be considered. 　　When two contactors are used, the sub-winding stall rating of each contactor must be equal to or greater than the compressor half-winding stall rating. 　　 Contactors with small specifications or inferior quality can not withstand the large current impact of compressor starting, stalling and low voltage, and are prone to single-phase or multi-phase contact jitter, welding or even falling off, causing motor damage. 　　 If the size 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. 　　 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. 　　 Factor 4: Power supply phase loss 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 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: nominal 380V three-phase power supply, the voltage measured at the compressor terminal is 380V, 366V, 400V The average value of the three-phase voltage is 382V, and the maximum deviation is 20V, so the percentage of voltage unbalance 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, a 5.2% unbalanced voltage may cause a 50% current unbalance. 　　 The National Electrical Manufacturers Association Motor and Generator Standards publication states that the percentage of phase winding temperature rise caused by unbalanced voltage is approximately twice the square of the voltage unbalance percentage point. In the previous example, the number of voltage unbalance points was 5.2, and the percentage increase in winding temperature was 54%. The result is that one phase winding is overheated while the other two windings are at normal temperature. 　　 Factor 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 refrigeration equipment often finds that a large amount of refrigerant leaks only when the cooling effect is poor. 　　 Frequent protection will occur after the motor is overheated. Some users do not check the cause in depth, or even short circuit the thermal protector. After a short time, the motor will burn out. Compressors have a range of safe operating conditions. The main consideration for safe operating conditions is the load and cooling of the compressor and motor. 　　 Factor 6: Use the compressor to evacuate 　　 There are also some on-site construction workers in the refrigeration industry who have retained the past habit of using a compressor to evacuate, air plays the role of an insulating medium. After the vacuum is evacuated in a closed container, the discharge phenomenon between the electrodes in it is easy to occur. 　　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 it is strictly forbidden to energize the compressor when the system and the compressor are in a vacuum state (no refrigerant is added after the vacuum has been completed). 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; heat dissipation Insufficient 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, is the fundamental method to avoid damage to the compressor motor

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