Refrigeration compressor motor burnout detection and repair

Refrigeration compressor motor burnout detection and repair

The core component of the refrigeration system is the compressor. The faults of the motor refrigeration compressor (hereinafter referred to as the compressor) can be divided into motor faults and mechanical faults (including crankshaft, connecting rod, piston, valve plate, cylinder head gasket, etc.). Mechanical failure often causes the motor to run over load or even stall, which is one of the main reasons for motor damage.

　　The damage of the motor is mainly manifested as damage (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. After the winding is burnt, some phenomena or direct causes that caused the burn are covered, making it difficult to analyze and investigate the cause afterwards. However, the operation of the motor is inseparable from the 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 reasons for the unit burning are nothing more than the following six:

(1) Abnormal load and locked rotor; (2) Winding short circuit caused by metal chips; (3) Contactor problem; (4) Power supply lack of phase and abnormal voltage; (5) Insufficient cooling; (6) Vacuum with compressor . In fact, motor damage caused by multiple factors is more common.

　　 1. Abnormal load and locked rotor

　　The motor load includes the load required for compressed 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; and the frictional resistance caused by the failure of lubrication and the motor lock in extreme cases will greatly increase the motor load.

　　Lubrication failure and increased frictional resistance are the primary reasons for abnormal load. Liquid return to dilute the lubricating oil, overheating of lubricating oil, deterioration of lubricating oil, and lack of oil, etc. will destroy normal lubrication and cause lubrication failure. Liquid return dilutes the lubricating oil, affecting the formation of the normal oil film on the friction surface, and even scouring the original oil film, increasing friction and wear. Overheating of the compressor will cause the lubricating oil to become thinner or even coking at high temperature, which will affect the formation of normal oil film. The oil return of the system is not good, the compressor is short of oil, and it is naturally unable to maintain normal lubrication. The high-speed rotation of the crankshaft, the high-speed movement of connecting rod pistons, etc., the friction surface without oil film protection will quickly heat up, and the local high temperature will quickly evaporate or coke the lubricating oil, making it more difficult to lubricate the part, which can cause severe local wear within a few seconds.

　　 Lubrication failure, local wear, the crankshaft rotation requires more torque. Low-power compressors (such as refrigerators and household air-conditioning compressors) often have a stall (motor can’t rotate) after lubrication failure due to the small motor torque, and enter the "locked rotor-thermal protection-locked rotor" endless cycle, and the motor burns only Time issue. The high-power semi-hermetic compressor motor has a large torque, and local wear will not cause a locked rotor. The motor power will increase with the load within a certain range, which will cause more serious wear and even cause cylinder biting (piston stuck in the cylinder Inside), the connecting rod is broken and other serious damage.

　　 The current when locked rotor (locked rotor current) is about 4-8 times the normal operating current. At the moment the motor starts, the peak current can approach or reach the locked-rotor current. Since the heat generated by the resistance 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 locked, but it generally does not respond quickly and cannot prevent the winding temperature changes caused by frequent starts. Frequent starting and abnormal loads make the windings withstand 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 temperatures or using a low-temperature compressor for high temperatures will affect the load and heat dissipation of the motor, which is inappropriate and will shorten the service life of the electrode. After the winding insulation performance deteriorates, if there are other factors (such as metal chips forming a conductive loop, acid lubricating oil, etc.), it is easy to cause a short circuit and damage.

　　2, short circuit caused by metal shavings

　　The metal shavings in the windings are the main culprit for short circuit and low ground insulation. The normal vibration of the compressor during operation and the twisting of the windings due to electromagnetic force each time the compressor is started will promote the relative movement and friction between the metal shavings contained in the windings and the winding enameled wire. Sharp metal shavings will scratch the enameled wire insulation and cause a short circuit.

　　The sources of metal shavings include copper pipe shavings left during construction, welding slag, metal shavings dropped when the compressor is worn out and parts are damaged (such as broken valve plates). For hermetic compressors (including hermetic scroll compressors), these metal chips or particles will fall on the windings. For semi-hermetic compressors, some particles will flow in the system with gas and lubricating oil, and finally collect in the windings due to magnetism; and some metal chips (such as bearing wear and motor rotor and stator wear (sweeping)) will Land directly on the winding. It is only a matter of time before a short circuit occurs after metal shavings have accumulated in the winding.

　　 It is the two-stage compressor that needs special attention. In a two-stage compressor, the return air and normal return oil directly enter the first-stage (low-pressure stage) cylinder, and then enter the motor cavity cooling winding through the intermediate pressure tube after compression, and then enter the second stage like an ordinary single-stage compressor (High pressure cylinder). The lubricating oil in the return air has made the compression process like walking on 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 tube. Therefore, two-stage compressors are more prone to motor short circuits caused by metal chips than single-stage compressors.

　　Unfortunate things are often put together. When the compressor with a problem is turned on and analyzed, it is often the burnt smell of lubricating oil. The temperature is very high when the metal surface is severely worn, and the lubricating oil starts to coke when it is above 175oC. If there is a lot of water in the system (the vacuum is not ideal, the water content of the lubricating oil and refrigerant is large, the air enters after the negative pressure return pipe is broken, etc.), the lubricating oil may appear acidic. Acidic 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, this acidic lubricating oil containing copper atoms has poor insulation performance, which provides conditions for winding short-circuit.

　　3, contactor problem

　　The contactor is one of the important parts in the motor control circuit. Unreasonable selection can destroy the best compressor. It is extremely important to correctly select the contactor according to the load.

　　The contactor must be able to meet harsh conditions such as rapid cycling, continuous overload and low voltage. They must have a large enough area to dissipate the heat generated by the load current, and the contact material must be selected to prevent welding under high current conditions such as starting or stalling. For safety and reliability, the compressor contactor must disconnect the three-phase circuit at the same time. It is not recommended to disconnect the two-phase circuit.

　　The contactor must meet the following four items:

　　1) The contactor must meet the working and testing criteria specified in the ARI Standard 780-78 "Special Contactor Standard".

　　2) The manufacturer must ensure that the contactor can be closed at room temperature and at 80% of the z*lower nameplate voltage.

　　3) When a single contactor is used, the rated current of the contactor must be greater than the current rating (RLA) of the motor nameplate. At the same time, the contactor must be able to withstand the locked-rotor current of the motor. If there are other loads downstream of the contactor, such as motor fans, etc., they must also be considered.

　　4) When using two contactors, the locked rotor rating of each contactor must be equal to or greater than the locked rotor rating of the compressor half winding.

　　The rated current of the contactor cannot be lower than the rated current on the nameplate of the compressor. Contactors with small specifications or inferior quality cannot withstand compressor start-up, locked-rotor and large current impacts at low voltages, and are prone to single-phase or multi-phase contact vibration, welding or even falling off, causing damage to the motor.

　　 Contactors with trembling contacts frequently start and stop the motor. Frequent starting of the motor, huge starting current and heat, will aggravate the aging of the winding insulation. 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, poorly insulated lubricating oil, etc.), it is easy to cause a short circuit between the windings. The thermal protection system is not designed to prevent such damage. In addition, the trembling contactor coil is prone to failure. If the contact coil is damaged, it is easy to have a single-phase state.

　　 If the contactor type selection is too small, the contact can not withstand the arc and the high temperature generated by frequent on-off cycles or unstable control circuit voltage, and may weld or fall off from the contact holder. The welded contacts will produce a permanent single-phase state, so that the overload protector will 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 will be unprotected status.

　　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 unbalance between the three phases cannot exceed 5%. High-power motors must be powered independently to prevent low voltage 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 there will be a large load current. The motor windings will overheat very quickly, and the compressor will be thermally protected under normal circumstances. When the motor windings are cooled to the set temperature, the contactor will be closed, but the compressor will not start up, a locked rotor will appear, and a "locked rotor-thermal protection-locked rotor" endless loop will enter.

　　The difference in the windings of modern motors is very small, and the difference in phase currents when the power supply is balanced in three phases can be ignored. In an ideal state, the phase voltages are always the same, as long as a protector is connected to any phase to prevent damage caused by overcurrent. In fact, it is difficult to ensure the balance of the phase voltages.

The voltage unbalance percentage calculation method is the ratio of the maximum deviation of the phase voltage and the average value of the three-phase voltage to the average value of the three-phase voltage. For example, for a nominal 380V three-phase power supply, the voltage measured at the compressor terminal is 380V and 366V respectively ,400V. The average value of the three-phase voltage is 382V, and the maximum deviation of z* is 20V, so the voltage unbalance percentage is 5.2%.

　　As a result of voltage unbalance, the unbalance of load current during normal operation is 4-10 times the percentage of voltage unbalance. In the previous example, 5.2% unbalanced voltage may cause 50% of current unbalance.

　　The temperature rise of the phase winding caused by the unbalanced voltage is approximately twice the square of the voltage unbalanced percentage point. 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 one phase winding is overheated while the other two windings are at normal temperature.

　　 A completed survey shows that 43% of power companies allow 3% voltage imbalance, and another 30% of power companies allow 5% voltage imbalance.

　　 5. Insufficient cooling

　　Compressors with higher power are generally return air cooling type. The lower the evaporation temperature, the lower the system mass flow tends to be. When the evaporating 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 return air, but have clear requirements on the compressor's ambient temperature and cooling air volume.

　　 A large amount of refrigerant leakage will also reduce the mass flow of the system, and the cooling of the motor will also be affected. Some unattended cold storage, etc. often have to wait until the cooling effect is very poor before discovering a large amount of refrigerant leakage.

　　 There will be frequent protections after the motor is overheated. Some users do not check the reason thoroughly, and even short-circuit the thermal protector. That is a very bad thing. After a while, the motor will burn out.

　　 compressors have a safe operating range. The main considerations for safe working conditions are the load and cooling of the compressor and motor. Due to the different prices of compressors in different temperature zones, it was more common for the domestic refrigeration industry to use compressors beyond the scope in the past. With the growth of professional knowledge and the improvement of economic conditions, the situation has improved significantly.

　　 6. Vacuum with compressor

　　 Open-type refrigeration compressors have been forgotten, but there are still some on-site construction workers in the refrigeration industry who have retained the past habit of using compressors to vacuum. This is very dangerous.

　　Air plays the role of insulating medium. After the airtight container is evacuated, the discharge phenomenon between the electrodes inside can easily occur. Therefore, as the vacuum in the compressor housing deepens, the insulating medium between the exposed terminals in the housing or between the windings with small damage to the insulating layer will lose the insulating medium. Once energized, the motor may be short-circuited and burned in an instant. If the shell leaks, it may also cause electric shock.

　　 Therefore, it is forbidden to use the compressor to vacuum, and when the system and the compressor are in a vacuum state (the vacuum is completed and the refrigerant is not added), it is strictly forbidden to energize the compressor.

to sum up

　　 After the motor burned out, the phenomenon of winding damage was concealed, which caused certain difficulties in fault analysis. However, the root cause of damage to the compressor motor will not disappear. Abnormal load or even locked rotor caused by poor lubrication or failure, insufficient heat dissipation, will shorten the life of the winding; the inclusion of metal shavings in the winding will provide a profit for the short circuit; the welding of the contactor will make the compressor protection impossible; Abnormality of the power supply that the motor relies on will fundamentally destroy any motor; vacuuming with the compressor may cause the internal terminal to discharge.

Unfortunately, the above-mentioned unfavorable factors can also cause each other: abnormal load and large current during locked rotor may cause contactor welding; single contact arc 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 appearance of unfavorable factors, which is the fundamental method to avoid damage to the compressor motor.