The faults of electric compressor (hereinafter referred to as compressor) can be divided into motor faults and mechanical faults (including crankshaft, connecting rod, piston, valve plate, cylinder gasket, etc.). ). Mechanical failure often makes the motor overload or even stall, which is one of the main reasons for motor damage.
The damage of the motor is mainly manifested in the damage (short circuit) and open circuit of the stator winding insulation layer. It is difficult to find the damaged stator winding in time, which may eventually lead to winding burning. After the winding is burned, some phenomena or direct causes leading to the burning are covered up, which makes it difficult to analyze and investigate the causes afterwards. However, the operation of the motor can not be separated from normal power input, reasonable motor load, good heat dissipation and the protection of enameled wire insulation layer of the winding.
Starting from these aspects, it is not difficult to find that the causes of winding burning loss are nothing more than the following six:
(1) The load is abnormal and the rotation is blocked;
(2) Short circuit of winding caused by metal chips;
(3) contactor problem;
(4) The power supply is out of phase and the voltage is abnormal;
(5) insufficient cooling;
(6) vacuumizing with a compressor.
In fact, motor damage caused by many factors is more common.
Second, the abnormal load and locked rotor
Motor load includes the load needed to compress gas and the load needed to overcome mechanical friction.
Excessive pressure ratio or pressure difference will make the compression process more difficult; However, the friction resistance caused by lubrication failure increases, and in extreme cases, the motor stall will greatly increase the motor load. Lubrication failure and friction resistance increase are the main reasons for abnormal load. Dilution of lubricating oil by returning liquid, overheating of lubricating oil, coking and deterioration of lubricating oil and lack of oil will destroy normal lubrication and lead to lubrication failure. Dilution of lubricating oil by reflux affects the formation of normal oil film on friction surface, and even washes away the original oil film, increasing friction and wear. Overheating of compressor will lead to dilution or even coking of lubricating oil at high temperature, which will affect the formation of normal oil film. The oil return of the system is not good, and the compressor is short of oil, so it is naturally impossible to maintain normal lubrication. When the crankshaft rotates at high speed and the connecting rod and piston move at high speed, the friction surface without oil film protection will heat up rapidly, and the local high temperature will make the lubricating oil evaporate or coke quickly, making it more difficult to lubricate this part, and it will cause local serious wear in a few seconds. Lubrication failure and local wear require more torque for crankshaft rotation.
When the lubrication of low-power compressors (such as refrigerators and household air-conditioning compressors) fails, the phenomenon of locked rotation often occurs (the motor cannot rotate), and it enters the infinite cycle of "locked rotation-thermal protection-locked rotation". It's only a matter of time before the engine burns out. However, the motor torque of high-power semi-closed compressor is very large, and local wear will not cause locking rotation. The power of the motor will increase with the load in a certain range, which will cause more serious wear and even cause serious damage such as cylinder seizure (piston stuck in the cylinder) and connecting rod fracture. The current in locked rotor (locked rotor current) is about 4-8 times that in normal operation. At the moment when the motor is started, the peak current can approach or reach the locked-rotor current. Because the heat release of the resistor is proportional to the square of the current, the current during starting and locking will make the winding heat up rapidly. Thermal protection can protect the electrode when the rotor is locked, but it generally has no quick response and cannot prevent the winding temperature change caused by frequent starting. Frequent starting and abnormal load make the winding unable to withstand the high temperature test, which will reduce the insulation performance of enameled wire. In addition, the load required to compress gas will also increase with the increase of compression ratio and pressure difference.
Therefore, it is not suitable to use high-temperature compressor at low temperature and low-temperature compressor at high temperature, which will affect the load and heat dissipation of the motor and shorten the service life of the electrode. If there are other factors (such as metal chips forming conductive circuit, acid lubricating oil, etc.) after the winding insulation performance becomes worse, it is easy to cause short circuit and damage. ).
Third, the winding short circuit caused by metal chips.
Metal chips in the winding are the chief culprit of short circuit and low grounding insulation value.
The normal vibration of the compressor during operation and the torsion of the winding under the action of electromagnetic force every time it is started will promote the relative movement and friction between the metal chips mixed between the windings and the enameled wire of the winding. Metal chips with sharp edges and corners will scratch the insulation layer of enameled wire and cause short circuit. The sources of metal scraps include copper pipe scraps left during construction, welding slag, internal wear of compressor and metal scraps dropped when parts are damaged (such as valve plate rupture). For fully enclosed compressors (including fully enclosed scroll compressors), these metal chips or fragments will fall on the windings. For semi-closed compressor, some particles will flow in the system with gas and lubricating oil, and finally gather in the winding due to magnetism; However, some metal chips (such as bearing wear and motor rotor and stator wear (hole sweeping)) will directly fall on the winding. It is only a matter of time before the short circuit occurs after the metal chips accumulate in the winding. Special attention should be paid to two-stage compressors. In a two-stage compressor, the return air and normal oil return directly enter the first stage (low-pressure stage) cylinder, and then enter the cooling winding of the motor cavity through the medium-pressure pipe after compression, and then enter the second stage (high-pressure stage cylinder) like a common single-stage compressor.
The return air contains lubricating oil, which makes the compression process on thin ice. If there is any backflow liquid, the valve plate of the first stage cylinder will easily break. The broken valve plate can enter the winding after passing through the medium voltage tube. Therefore, the two-stage compressor is more prone to motor short circuit caused by metal chips than the single-stage compressor.
Unfortunately, things tend to come together, and the faulty compressor often smells the burning of lubricating oil when starting the analysis. When the metal surface is seriously worn, the temperature is very high, while the lubricating oil is in 175? Coking begins above c. If there is too much water in the system (the vacuum pumping is not ideal, the water content of lubricating oil and refrigerant is high, and air enters after the negative pressure oil return pipe breaks). ), the lubricating oil may be acidic. Acid lubricating oil will corrode copper tube and winding insulation. On the one hand, it will cause copper plating. On the other hand, this acidic lubricating oil containing copper atoms has poor insulation performance, which provides conditions for winding short circuit.
Fourth, the contactor problem
Contactor is one of the important components in motor control circuit. Unreasonable choice will destroy the best compressor.
It is extremely important to select the contactor correctly according to the load. Contactors must be able to meet harsh conditions, such as fast cycle, continuous overload and low voltage. They must have a large enough area to dissipate the heat generated by load current, and the contact materials must be selected to prevent welding under high current conditions such as starting or blocking. For safety and reliability, the compressor contactor should disconnect the three-phase circuit at the same time. Gulun Company does not recommend the method of disconnecting the two-phase circuit. In the United States, the contactors approved by Gulen Company must meet the following four requirements:
Contactors must meet the working and testing standards specified in ARI Standard 780-78 "Special Contactor Standard".
The manufacturer must ensure that the contactor can be closed at 80% of the lowest nameplate voltage at room temperature.
When using a single contactor, the rated current of the contactor must be greater than the rated current (RLA) on the motor nameplate. At the same time, the contactor must be able to withstand the locked-rotor current of the motor. Whether there are other loads downstream of contactor, such as motor fan, etc. , also must be considered.
When two contactors are used, the locked-rotor rating of the branch winding of each contactor must be equal to or greater than the locked-rotor rating of the compressor half winding.
The rated current of contactor should not be lower than the rated current on the nameplate of compressor.
Contactors with small specifications or poor quality cannot withstand the large current impact when the compressor is started, locked and at low voltage, and it is easy for single-phase or multi-phase contacts to shake, weld or even fall off, resulting in motor damage. Contactors with vibrating contacts often start and stop the motor. The motor starts frequently, and the starting current is huge, and heating will aggravate the insulation aging of the winding. At each start, the magnetic moment will make the motor windings move slightly and rub against each other. If there are other factors (such as metal chips, lubricating oil with poor insulation, etc. ), easy to cause short circuit between windings. The thermal protection system is not designed to prevent such damage. In addition, the shaking contactor coil is also prone to failure. If the contact coil is damaged, it is easy to have a single-phase state.
If the contactor selection is too small, the contact can't withstand the arc and high temperature caused by frequent start-stop cycles or unstable control circuit voltage, and may be welded to death or fall off the contact frame. The welding contact will produce a permanent single-phase state, so that the overload protector can be continuously and circularly turned on and off. It is particularly important to emphasize that all controls (such as high and low pressure control, oil pressure control, defrosting control, etc.) after contactor contact welding. Depending on the contactor to disconnect the compressor power supply circuit will all fail and the compressor will be in an unprotected state.
Therefore, when the motor burns out, it is necessary to check the contactor. Contactor is an important cause of motor damage, which is often forgotten.
Five, lack of phase and abnormal voltage of power supply
Abnormal voltage and lack of phase can easily damage any motor.
The variation range of power supply voltage cannot exceed 10% of rated voltage. The voltage imbalance between the three phases cannot exceed 5%. High-power motors must supply power independently to prevent other high-power equipment on the same line from running at low voltage. The motor power cord must be able to carry the rated current of the motor. If the compressor is running when the phase loss occurs, it will continue to run, but there will be a large load current. The motor winding will overheat quickly, and the compressor will be protected by heat under normal circumstances. When the motor winding is cooled to the set temperature, the contactor will be closed, but the compressor will not start, resulting in locked rotation and entering the infinite cycle of "locked rotation-thermal protection-locked rotation". The difference of modern motor windings is very small, and the difference of phase current when the power supply is three-phase balanced can be ignored. Ideally, the phase voltages are always equal, so as long as a protector is connected to any phase, the damage caused by overcurrent can be prevented. In fact, it is difficult to ensure the balance of phase voltage. The calculation method of voltage unbalance percentage is the ratio of the maximum deviation of phase voltage and three-phase voltage average value to three-phase voltage average value. For example, for a three-phase power supply with a nominal voltage of 380V, the voltages measured at the compressor terminals are 380V, 366V and 400V, respectively. It can be calculated that the three-phase average voltage is 382V and the maximum deviation is 20V, so the percentage of voltage imbalance is 5.2%. Due to voltage imbalance, the imbalance of load current is 4- 10 times of the percentage of voltage imbalance during normal operation. In the previous example, an unbalanced voltage of 5.2% may lead to an unbalanced current of 50%. NEMA motor and generator standard publications point out that the percentage of phase winding temperature rise caused by unbalanced voltage is about twice the square of the percentage of unbalanced voltage. In the above example, the number of voltage unbalance points is 5.2, and the percentage of winding temperature increase is 54%. The result is that one winding is overheated and the other two windings are normal. A survey by U.L. (American Underwriters Laboratory) shows that 43% of power companies allow 3% voltage imbalance, and another 30% allow 5% voltage imbalance.
Six, insufficient cooling
High-power compressors are usually of the return air cooling type. The lower the evaporation temperature, the smaller the mass flow of the system.
When the evaporation temperature is very low (exceeding the manufacturer's regulations), the flow rate is not enough to cool the motor, and the motor will run at a higher temperature. Air-cooled compressor (generally less than 10HP) has little dependence on return air, but it has clear requirements for compressor ambient temperature and cooling air volume. A large amount of refrigerant leakage will also reduce the mass flow of the system and affect the cooling of the motor. Some unattended cold storage, often have to wait until the refrigeration effect is very poor to find a large number of refrigerant leakage. After the motor is overheated, there will be frequent protection. Some users don't check the reason deeply, or even short-circuit the thermal protector, which is a very bad thing. Before long, the motor will burn out. The compressor has a safe working range. Safe working conditions mainly consider the load and cooling of compressor and motor. Due to the different prices of compressors in different temperature zones, compressors were used in the domestic refrigeration industry in the past.