AC Compressor Failure and Case Analysis

Figure 1: Copeland scroll compressor.

The refrigerator compressor is the energy core of the air-conditioning refrigeration (heat) system. In the refrigeration system, it mainly plays the role of compressing the refrigerant and promoting the continuous circulation of the refrigeration oil. In the air conditioning system, the compressor can be divided into three types: reciprocating, rotary and scroll. At present, the most commonly used are rotary and scroll refrigeration compressors.

The following introduces some common faults of air-conditioning compressors, combined with several actual case studies to let you quickly understand the faults of air-conditioning compressors!

1. Cause Analysis of AC Compressor Failure

1.1 Damaged Compressor Motor

Possible reasons: first, the wiring of the compressor terminal is incorrect and the motor is burned; second, the system refrigerant leaks.

While the high-pressure gas of the rotary compressor is discharged from the compressor, it is also responsible for taking away the heat generated by the electric motor. If the refrigerant in the system leaks, only a small amount of high-pressure gas will be discharged from the compressor, so that the heat generated by the compressor motor when it is powered on will always accumulate, and if things go on like this, it will cause the compressor motor to burn out.

Figure 2: Refrigeration compressor motor coil burned.

When the compressor is locked, the motor factor should be eliminated as much as possible first, so the insulation resistance of the motor and the windings of the main and auxiliary coils should be measured first to determine whether the motor is burned.

1.2 Compressor Capacitor Problem

Possible reasons: first, the capacitor is damaged (short circuit, open circuit); second, the capacitor specification does not match the compressor.

Note: This item is only applicable to single-phase compressors. Because a three-phase induction motor is used in a three-phase compressor, a rotating magnetic field is generated by feeding three-phase alternating current into the stator core, and a capacitor is not required.

1.3 Frequent Operation of Compressor Thermal Protection

Possible reasons: First, the thermal protector is abnormal. You can refer to the performance diagram and text description of this item in the specification provided by the compressor manufacturer.

Second, the wiring of the power line is unreasonable, and the compressor starts with low voltage. Or the wiring of the compressor terminal is not correct, or the inverter of the inverter air conditioner is running under a phase loss: check the current between the three phases to see if there is a short circuit or open circuit.

Third, the system starts up before the high and low pressure are balanced. Generally, the air conditioner is required to be turned on at least 3 minutes after it is turned off; it may also be that the capillary flow of the system is too small to balance the high and low pressure as soon as possible.

Figure 3: Hisense air conditioner in working.

Fourth, water hammer caused by liquid return, long-term shutdown start, low ambient temperature start, etc. In the state of long-term shutdown and low temperature, the refrigerant in the compressor dissolves in the refrigerating machine oil and raises the liquid level (the mixture of liquid refrigerant and lubricating oil). When starting the compressor, the liquid refrigerant in the closed shell evaporates from dissolved lubricating oils and a strong foaming phenomenon appears.

1.4 Water Intrusion in System

The refrigeration system has strict requirements on moisture, and it is generally stipulated that the moisture content in the refrigeration system is less than 0.2ml. If water invades the compressor, it will cause serious damage to the compressor as follows:

First: The mechanical parts of the compressor are rusty. When the rusting reaches a certain level, the clearance fit between the mechanical parts of the compressor will be reduced, and in severe cases, the compressor may be stuck.

Second, the motor coil coating, insulating materials, etc. are corroded, resulting in a short circuit of the motor.
Third, the deterioration of refrigerant and refrigeration oil.

In general, water intrusion may be caused by incomplete vacuuming or refrigerant leakage on the low-pressure side of the system.

1.5 Abnormal Wear of Compressor

Possible reasons: First, the clearance between the internal components of the compressor is small, which is generally a problem of the compressor itself.
Second, the problem of refrigeration oil, such as: poor oil return hole, insufficient oil quantity, oil deterioration, poor oil return, etc.

Refrigeration oil plays a role of lubrication inside the compressor, which can effectively prevent the wear of the mechanical parts of the pump body, and its oil seal function can maintain the pressure difference between high and low pressure, avoid high and low pressure gas mixing, and prevent the cooling capacity from decreasing.

In addition, due to the continuous circulation of the refrigerated oil, the heat generated between the friction surfaces can also be taken away in time. When the amount of refrigerated oil is insufficient, the mechanical parts inside the compressor will be abnormally worn out because they cannot be lubricated in time, which will eventually cause the compressor to be stuck.

Figure 4: The severely worn fixed and orbiting scrolls.

Main reasons for poor oil return in the system are as follows:
First, the length and drop of the indoor and outdoor connecting pipes exceed the specified value. Generally, the indoor and outdoor connecting pipes shall not exceed 15m, and the maximum drop is 5m.

Second, the capillary of the system is blocked or selected unreasonably and the liquid separation of the indoor heat exchanger is uneven, causing the refrigerant to return to the compressor without completely evaporating.
Since it is not completely evaporated and its flow rate is relatively slow, the refrigerated oil may be attached to the tube wall of the heat exchanger, resulting in poor liquid return, which also affects the improvement of system performance.

Third, the leakage of the refrigerant in the system took away the refrigerant oil, and the refrigerant oil was not replenished when re-adding the refrigerant, resulting in poor oil return in the system.

1.6 Foreign Matter Entering Compressor

Possible reasons: The impurities from the compressor itself and the air conditioner system.

2. AC Compressor Failure Case Analysis

The following is a combination of several actual cases to analyze the failure of the air conditioner compressor.

2.1 Insufficient Refrigerant

Figure 5: Air conditioner unit.

Description of the problem: The 4 sets of 5 hp cabinet air conditioners newly installed in a certain factory issued exhaust pipe high temperature protection alarms from time to time. The user thought it was a machine problem and asked the manufacturer to assist in handling it.

On-site check: After arriving at the site, the manufacturer found that the pipes of 4 cabinet AC conditioners were extended about 8-10 meters. After asking whether there was leak detection and adding refrigerant during installation, the maintenance personnel replied that no leaks were found during the leak detection, but no additional refrigerant was added.

Cause analysis: When the connecting pipe is extended, but the refrigerant is not added, it is equivalent to the lack of fluorine in the system in different degrees. The lack of fluorine in the system will lead to insufficient oil return, poor lubrication of the scroll disk, the rise of central temperature, and high exhaust temperature. When the temperature reaches the protection value, the machine displays E4 protection.

On-site handling method: Quantitatively add refrigerant on site: 50g×8=400g. The exhaust temperature returned to normal status, and after being observed for a period of time, the high temperature protection of the exhaust pipe did not appear again.

2.2 Large Vertical Drop between Indoor and Outdoor Units

Description of the problem: A primary school installed more than 10 sets of 5-horsepower cabinet AC conditioners, and two of them often had abnormal protection.

On-site check: After door-to-door inspection, it was found that the drop between the inner and outer machines of the two sets was about 8 meters, and the connecting pipe did not have an oil return bend.

Reason analysis: Since the external machine is 8 meters higher than the internal machine, the lubricating oil has to overcome a large gravity in the circulation to perform oil return. The bigger the drop between the inner and outer units, the more difficult it is to perform oil return, and the lack of oil will cause frequent protection or failure of the compressor.

Figure 6: Installation location diagram of Air conditioner unit oil return bend/oil storage bend.

Handling method: For machines with a drop of more than 5 meters, oil return bends must be installed at every interval of no more than 5 meters. The oil return bends can be set in a "U" shape.

2.3 The Pipe is bent and Indoor Unit Makes Noise

Description of the problem: A 3-horsepower cabinet AC conditioner is installed in the office of a certain company, and it has been reported that there is a whistling sound within a certain period of time when it is turned on.

On-site inspection:
a: Observe the starting up process of the AC conditioner. The compressor starts whistling after starting for 1 minute. The sound becomes louder and lasts for about 2 minutes, and then the sound becomes smaller, but the whistling sound can still be heard.

b: After investigation, it was found that the connecting pipe at the position of the suction pipe was bent.

Figure 7: Bent connecting pipe.

2.4 The Voltage is Unstable

Description of the problem: The newly installed 3-horsepower cabinet AC conditioner has a loud noise after running for 3 minutes, and the fault still exists after replacing the compressor.

On-site check: After the compressor was started for 2 minutes, the voltage dropped from 198V to 173V. At this time, the compressor began to make a "humming" sound, and then stopped running.

Cause analysis and handing method: The compressor cannot be started normally due to the low power supply voltage. It is recommended that the user install a voltage stabilizer to deal with it.

2.5 Leakage of Refrigerant

Description of the problem: A 5-horsepower cabinet AC conditioner in a restaurant has been used for 3 years and its compressor has been changed 3 times, but it began to not cool a few days ago.

On-site check: On-site inspection found that the compressor does not start, and the resistance of terminals U, V, and W is about 60 ohms, which is normal. Start the compressor forcefully, and a buzzing sound is heard. It is judged that the compressor is stuck.

Cause analysis: There is a lot of oil dirt on the valve of the outdoor unit. After removing the nut of the small connecting pipe, it was found that the bell mouth of the small tube was cracked. The analysis was that the nut was fixed with too much force during the second maintenance, which caused the bell mouth to crack, resulting in the leakage of the system refrigerant.

On-site handing method: Discharge refrigerant, pour out the compressor oil and observe it. The color of the oil is very dark, as shown in the picture below:

Figure 8: Black and normal machine oil.

Since the color of compressor oil is close to black, it means that the system is seriously dirty. At the same time, because the liquid storage tank is difficult to clean, the compressor and liquid storage tank should be replaced simultaneously. The system should be flushed with nitrogen, the bell mouth should be re-expanded, and refrigerant should be added quantitatively. Then the AC conditioner works normally.

2.6 Compressor Burned Out

Description of the problem: When the 1 air duct unit in a building is operating in cooling at an ambient temperature of about 15°C, one compressor burns out, and the return air pipes of the other units freeze with thick ice and the evaporators of the indoor units also freeze.

On-site check: After scrutiny, the compressor was burned due to the liquid hammer phenomenon.

Cause analysis: If the outdoor ambient temperature is not within the specified range, long-term use of the compressor will cause the refrigerant liquid to not completely evaporate in the evaporator, and the liquid will enter the compressor to form a liquid hammer.

Under normal cooling conditions, when the temperature of the indoor coil is lower than -2°C for 2 minutes, the anti-freezing protection of the indoor coil is turned on, and when it is higher than 6°C or the compressor stops for 7 minutes, the air conditioner will automatically return to start.

On site, the evaporator of the indoor unit was frozen but the unit did not start the protection. After careful inspection, it was found that the indoor coil temperature sensor was installed in an improper position, and was not placed on the copper elbow that is most prone to frost, causing the unit to fail to activate the normal protection function.

Handing method: Move the indoor coil temperature sensor to the elbow that is prone to frost, so that it can accurately detect the evaporation temperature of the evaporator, and avoid severe frost or ice.

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