Determining The Cause Of Death
If you answered COSS (Compressor Operation and Service Seminar), held at the Copeland Corp. factory here and at other locations throughout the year, you’d be correct. The three-day course has been offered by Copeland since 1986, and is now part of a broader selection of HVACR training programs available through the Emerson Climate Technologies family of companies.
The first two days cover refrigeration basics, plus design and application details of specific Copeland brands, all of which are necessary to fully appreciate the troubleshooting and teardown portion. On the third day, Scott Lanzer and Steve Brown, both technical specialists, delve into details that can turn an ordinary technician into something of a troubleshooting specialist.
Lanzer said that for three-phase Copelamatic® and Copelaweld® compressors, it can be assumed that they have higher starting torques and higher running efficiencies compared to single-phase compressors. The nomenclature will tell their voltage, phase, and Hertz.
Critical ChoicesMost U.S. three-phase units have wye-wound motors. All three legs are joined at the middle of the wye, Lanzer explained. This type of winding is very beneficial, but it also means contractors and technicians need to use care when making certain repair choices.
Single-phase burnouts are not the compressor’s fault, he said. Some may be prevented in cases where the contractor or technician has some degree of control.
For example, “If you have a choice between fused disconnects and circuit breakers, choose circuit breakers,” Lanzer said. Why? “If a fuse goes, it would just take out one leg and could single phase the motor. The compressor could burn out.”
Contactor selection is also critical. “Whenever you buy a new compressor, you should buy a new contactor,” Lanzer pointed out. Contactors have a limited life and should be routinely inspected, he explained.
If you don’t replace the contactor when replacing the compressor, the old contactor could stick, “and burn out that new compressor.”
Contactor replacement is mandatory on four-, six-, and eight-cylinder compressors because they have pilot-duty circuits and rely on the contactor to open.
When troubleshooting, most servicers will examine the solid-state sensor(s). First off, “Know what kind of sensor you are measuring,” said Lanzer. Copeland has used Texas Instruments (TI) and Robertshaw. (Today’s Copeland compressors are all TI, but contractors may still come across Robertshaw sensors on older models, the company pointed out.) To know which sensors they are dealing with, contractors should use an ohmmeter.
Read from the common terminal to each of the three sensors, Lanzer said; the old Robertshaw will show 75 to 125 ohms; the TI will show 500 to 20,000 ohms.
Caution: Use an ohmmeter with a maximum of 9 VAC for checking, Lanzer added. The sensors are sensitive, easily damaged, and no attempt should be made to check continuity through them with anything other than an ohmmeter. Don’t use a megohmmeter.
Before a servicer condemns the solid-state module, he should check for module power. Then Lanzer advised the following:
This delay is built into the module, he explained. If the contractor doesn’t allow for this delay, he could still get a reading that would seem to indicate that the module is at fault.
Detective WorkTrainer Steve Brown went into more detail on why compressors fail, and how to become a service detective. System problems need to be identified and corrected to prevent repeat compressor failures.
Most of the time, a compressor fails due to system problems, he said. “The compressor, being the heart of the system, will be affected by what goes on in the system. Just because a compressor fails, doesn’t mean that it was the compressor’s fault,” he said. “You need to be a bit of a detective. It leaves a trail.”
Previous Copeland failure analyses found that approximately 50% of motor failures are actually the result of mechanical problems.
“Check your superheat,” Brown advised. “Check your subcooling.”
Contractors probably hate hearing this, but about 37% of the Scroll compressors returned to Copeland have nothing wrong with them, Brown said.
In order to perform their detective work, he recommended that contractors conduct compressor analyses. An in-warranty welded compressor should not be opened in the field because this would void the warranty.
An out-of-warranty welded compressor may be dissected using a handheld grinder to grind through the girth weld, Brown said. Once the inspection is completed, the contractor can return the internal components with the compressor so the company can conduct its own analysis. Contractors can request that Copeland conduct an inspection analysis by going through a PrimeSource wholesaler. (These wholesalers can be located by visiting www.copeland-corp.com.)
Why Compressors FailThe teardown at the Sidney facility focused on semi-hermetic compressors. To tear down a Copeland semi-hermetic, contractors need to first make sure there is no pressure remaining in the high or low side of the compressor, the company advised. Then remove the head and valve plate.
Look for scoring in the cylinders, Brown said. Also look for broken reeds. From here, depending on what is found, the contractor may choose to remove the front bearing cover and/or the bottom plate.
To continue their detective work, contractors and techs need to know the signs and symptoms of the five main causes of compressor failures:
1. Refrigerant floodback;
2. Flooded starts;
4. Overheating; and
5. Loss of oil.
Floodback occurs when liquid refrigerant returns to the compressor during the running cycle, Brown explained. This may or may not be a problem with the expansion valve, so don’t assume! There can be multiple system causes.
The metering device may lose control, but this does not mean it is a metering device problem. What happens if the evaporator coil freezes over, if an evaporator fan fails, questioned the instructors — the liquid will not be properly evaporated and liquid returns to the compressor.
Also, depending on the style of compressor, different failure patterns will be found. A refrigerant-cooled compressor is one that requires the return gas to flow across the motor before entering the compression chamber, the instructors explained. Here, the liquid refrigerant would mix with the oil in the crankcase and would result primarily as low-end wear, such as worn bearings and worn rods.
An air-cooled compressor is one where the return gas does not flow across the motor; it enters into the side of the compressor, very near the compression chamber. Here the liquid would enter the compression chamber and more likely result in a liquid slug (compression of liquid in the cylinder area).
Can you have floodback in a welded compressor? It’s possible, said Brown. All welded compressors are refrigerant-cooled, so contractors should look for damage in the running gear, bearings, rods, and crankshaft areas.
Liquid refrigerant displaces oil as it travels through the crankshaft. Also, liquid refrigerant is a solvent and washes the oil film from adjoining surfaces; this, in turn, causes wear. Scoring will be evident.
While the compressor is operating, the contractor should check superheat at the compressor. The amount of desired superheat varies according to the application. If floodback has been a problem, a minimum of 20 degrees F superheat would be desirable, according to the company.
Flooded starts usually are the result of off-cycle migration. In order for flooded starts to occur, the compressor needs refrigerant and oil in contact with each other while the compressor is off. The longer the compressor is off, the greater the chance of migration, Brown said.
During a flooded start, refrigerant explodes from the oil and washes the oil from the lubricated surfaces, bearings, journals, etc.
Defrost cycles and increased capacity control can help control off-cycle migration. However, “If you have a compressor sitting off for long periods of time, you may have some migration,” Brown said.
Slugging is different from flooded starts, Brown explained. When slugging occurs in a semi-hermetic compressor, liquid is between the top of the piston and bottom of the compressor. Therefore, during a teardown a contractor would see different types of internal damage. It can be caused by floodback (in air-cooled systems) or migration (in liquid-cooled systems).
Overheating is a common cause of failure, Brown said, and is often overlooked. Overheating can be caused by numerous problems. Possible causes include low suction pressure, high return gas temperature, high head pressure, etc. Signs of overheating include discolored valve plates and reeds, possible cylinder and ring wear, and under extreme conditions, could result in lower-end damage.
Loss of oil occurs when oil logs out in the system and fails to return to the compressor. This will typically result in running gear failure such as bearing, rod, and crankshaft wear. Possible causes include compressor short cycling, inadequate defrost cycles, improper piping, etc.
The TeardownNext, this COSS class broke out into groups and opened six randomly selected, field-returned, semi-hermetic compressors in the company’s teardown lab. Here is what they found.
Compressor 1 was two years old and refrigerant-cooled. There was no discoloring of the valve plate. There was light wear on the crankshaft. The internal line break protector showed evidence of tripping, but the motor was good. No other evidence of mechanical or electrical problems was found. This could have been a low-voltage problem.
Compressor 2 had a valve plate that was discolored from high temperature. Parts were broken and scoring was evident. The overload showed evidence of tripping. This was not a slug, said Brown; the damage was caused by heat. The overheating caused the oil to break down, which resulted in the scoring and the broken parts. The system probably would have shown high superheat.
Compressor 3 was a remanufactured compressor with which nothing was found wrong. This compressor checked out OK both mechanically and electrically. In fact, it probably never ran. Could it have been a starting component or voltage problem?
Compressor 4 had pistons that were worn on the loaded side only. “This could be the result of liquid floodback washing away the oil film,” Brown said. This was an air-cooled compressor. Did the contractor calculate superheat? Probably not, Brown said.
Compressor 5 (a two-year-old compressor) showed evidence of some overload tripping. However, the stator, rotor, valve plate, reeds, bearings, and oil were all good. The only evidence here is the overload trips. There may have been a miswiring issue or a starting component issue.
Compressor 6 had an obvious motor burn. However, that is not the primary cause of failure. The valve plate was severely discolored. There were signs of severe heat resulting in thin oil and piston ring wear. The students on this teardown team found the center and rear bearings seized to the crankshaft. This failure was the result of an overheat condition, leading to oil breakdown, which wore the mechanical components, causing the motor failure.
What’s inside that failed compressor? There’s only one way to find out.
For more information on Copeland’s training programs, contact the Educational Services Group at 937-498-3617 or visit www.copelandcorp.com/education.
Publication date: 01/27/2003