In dealing with compressor failures as a result of system problems, as we are doing with this presentation, we should first identify the various general system categories to which most system related compressor failures can be traced. Each – of these general categories will then be discussed in terms of the damage they can cause and finally, a discussion only with solutions to each of these areas should help toward the identification of the system problem.
This dissuasion is not intended as a system design manual and you should be prepared, following this discussion, to look to other sources (such as the Trane Reciprocating Refrigeration Manual) to further develop these design techniques and skills.
Let us look at general system failure categories. Most compressor failures (with the exception of product defects) can be placed in the following general categories :
(Damage from hydrostatic pressure when a compressor attempts to compress a liquid–oil or a refrigerate or both)
(Excessive wear related problems caused,-by the lack of sufficient lubricating oil in essential areas)
(Extraneous materials resulting in excessive wear, mechanical motor damage and heat)
(Those electrical problems which can induce failure rather than those electrical problems caused by mechanical damage from (? 1-3 above))
Our, discussion will include some of the mechanical causes for electrical failures as well. Let’s take a closer look at each of these areas.
LIQUID SLUGGING DAMAGE
First let’s look at the mechanical damage that is associated with hydrostatic pressures from liquid slugging.
(Figure 2) The teardown of this Model M compressor revealed suction valve damage resulting from the attempt to compress Liquid refrigerant or oil, or both.
Since a liquid is virtually not compressible, the resulting hammering characteristically damages the suction valves of this machine. In this example, pieces of broken suction valve were wedged against the discharge valve and the side of the discharge gas passage.
|This is a severe case. More often, the suction valves remain intact but crack radically or chip when subjected to liquid slugging.|
(Figure 3) This- is the piston and rod assembly that was removed from the same compressor. The piston damage occurred when it contacted the broken valve parts.
When this type of damage Is found, the cylinders are generally damaged to the point where they must be repaired. Similarly, other compressors may show suction valve and spring damage when subjected to severe liquid slugging. The top of the piston will generally have marks caused by contact with spring or valve fragments.
Anytime valve or spring damage is observed, remove the motor and Inspect it and the motor barrel carefully. It Is possible that valve or spring fragments have lodged them- selves In the motor barrel or stator windings where they can cause future spot burns.
An improperly oversized expansion valve selection is one of the principal causes of liquid flood-back and the resultant slugging. While an oversized valve may perform well at full load, at part load it may lose control.
The reason is, at part load the valve attempts to control at its superheat setting but its oversized port passes more liquid than is needed. This overfeeds the evaporator, causing a rapid reduction in the superheat of the leaving gas. In response, the valve closes until normal superheat is re-established.
At this point, the valve again opens to pass another slug of liquid. This “hunting” condition will allow liquid to flood through the evaporator and into the suction line from where it can enter the compressor to cause damage.
It is important to note that some manufactured packaged products are intentionally designed with expansion valves rated for larger tonnage. In this case, the valve has been carefully selected and tested to insure that it meets specific design objectives. Do not confuse this type of valve selection with a “field” selected valve discussed above.
In many cases. field installed valves have been selected by inexperienced personnel. An experienced serviceman should always be suspicious of field selected and installed valves.
Reduced air flow across a direct expansion coil, resulting from dirty air filters, fan problems, air blockage, etc. can also result in liquid slugging.
The reduction in air flow reduces the load on the coil, resulting in coil frosting. Frost insulates the coil heat transfer surfaces which further reduces the load that the coil actually sees.
At this reduced coil load condition, the expansion valve is generally unable to control accurately. In a sense, it is oversized for the job it is attempting to do and will act in the same manner as described above for the improperly sized expansion valve. A water chiller will show the same symptoms when it is badly fouled or the water flow rate is low.
A Similar problem may be encountered when the airflow distribution across the face by an evaporator is not uniform. This air distribution causes unequal loading of the coil refrigerant circuits; resulting in an erratic suction temperature sensed by the expansion valve.
This may cause even a properly designed valve to shut, resulting in possible flood-back of refrigerant through the lightly loaded circuits.
Poor air distribution may be evidenced by spot frosting or the appearance of spot condensation on the coil.
Migration is the result of refrigerant condensation in the coldest part of the system. Refrigerant which circulates as a vapour is trapped as a liquid when it condenses in the coolest place. Usually this is the compressor or the evaporator when outdoor ambient temperatures are warm.
It causes no problem when it condenses in the condenser where it’s supposed to. Certain types of systems lend themselves to migration problems because of the close association with other adjacent systems.
Refrigerant migration is a concern particularly with duplex water chillers. Since system water is usually pumped through the inactive chiller circuits as well as the active circuit, the temperature and pressure difference between the condenser and chiller of the idle circuit is considerable. And since the idle compressor may remain that way for days at a time, it is possible for large amounts of refrigerant to migrate from the condenser to the chiller idle circuit through any small leak and condense.
Such leakage can take place through the liquid line solenoid valve or compressor valves which normally do not have an absolutely tight shutoff. This means that, in time, a large percentage of the total refrigerant charge will end up in the low side of the system.
Of course, when the idle circuit is finally put into service, large amounts of refrigerant will flood-back through the suction line and this will result in slugging and oil dilution.
The best solution to this problem is to install a Trane Timed Periodic Pump-out in parallel with the auxiliary pump down contacts of the compressor starter (1 M) and the non-recycle relay (CR1 ).
The control is adjusted to close at timed intervals (generally 30 minutes). If when the timer contacts close the contacts of the low pressure control are also closed, indicating the presence of refrigerant, a pump down cycle is started.
This timed pump down cycle keeps the low pressure side clear of excessive amounts of liquid refrigerate, preventing flood-back problems at compressor start-up. It is important to note that migration to the evaporator will not be prevented by the availability of an oil sump heater. The quantity of refrigerant involved will overwhelm the sump heater.
The flood-back of oil can be equally as damaging as the flood-back of liquid refrigerant in terms of liquid slugging only. A well designed piping system will promote a uniform movement of oil, preventing the accumulation of damaging oil slugs.
System piping is particularly critical, for instance with Systems that must Operate for extended periods of time at minimum load when the gas velocities required for oil movement may be marginal. If an improper piping design permits large amounts of oil to be trapped at minimum load, the oil may return as a slug when a compressor returns to higher capacity operation.
To avoid gas velocity problems associated with minimum capacity operation, it is absolutely necessary that accepted piping design and sizing practices be strictly adhered to.
If it is found that a piping design is allowing excessive amounts of oil to be trapped in the lines at minimum load, the problem may be solved temporarily by preventing the compressor from unloading to less than the part load rating where the velocities become marginal. However, when this is done, the compressor can be expected to cycle frequently. This may create a new problem of motor overheating and may require the addition of anti-cycle protection to prevent nuisance motor shutdown during this temporary period.
It must be understood that this is only a temporary solution to a piping problem. The piping design must be corrected at the earliest opportunity and the compressor return to full range of capacity control to properly corrected problem.
Among the common categories of compressor lubrication problems are oil dilution or loss and reduced oil, viscosity due to compressor overheating. Oil dilution, probably the most common lubrication problem is oil dilution.
Since oil has a strong affinity for refrigerant, it can be easily understood how oil can become excessively diluted by the refrigerant during prolonged shutdown, causing it to lose much of its lubricating qualities. And within certain normal temperature ranges, depending upon the oil type.
It is possible for an oil-refrigerant mixture to saturate; causing a separation of the two fluids. The more dense refrigerate-rich mixture seeks the bottom of the crankcase while the less dense oil-rich mixture seeks the top. 1n addition, any refrigerant which has migrated and condensed In the evaporator further dilutes the oil on start-up.
When a compressor is started with excess refrigerant in the crankcase, a refrigerant-rich mixture is drawn by the oil pump. Being an excellent solvent, the refrigerant washes oil from the bearings. In addition, heavily diluted oil foams excessively and may cause the oil pump to actually lose its ability to pump for a time after the crankcase pressure is reduced at start-up.
Add to this mixture, a secondary slug of migrated refrigerant from the evaporator and the stage for mechanical failure because of severe oil dilution refrigerant washing. The marginal lubricating qualities of foamy oil, coupled with little or no oil flow from the pump and the washing action of the liquid refrigerant from dilution, results in the scoring of bearing and cylinder surfaces.
The degree of scoring that takes place during anyone start-up depends upon the amount of refrigerant present and the time required to purge the refrigerant from the oil, the time required to re-prime the oil pump, and the extent of compressor loading while all this is occurring.
The bearing damage which results from excessive oil dilution is usually confined to the rod bearings that are closest to the oil pump. The remaining. bearings may not show damage e the refrigerant portion of the mixture may escape or through the closest rod bearings before it reaches the extreme end of the lubrication loop which will allow adequate lubrication in these sections.
This is a crankshaft that was subjected to a refrigerant washout. The key to this observation is the way the metal from the aluminium rods is literally smeared onto the cranks surfaces.
The crank throws do not show any discoloration from temperature because the water evaporated instantly and the evaporating refrigerant in the crankcase and lubrication passages absorbed most of the heat of friction.
With a rapid failure such as this, the crankshaft will not heat up.
A rod from the same machine shows a similar smearing of the aluminum of the bearing surfaces.
Again, the crank shows little or no discoloration from heat.
Scaring of the piston In both the ring land and skirt areas is not uncommon when excessive oil dilution and refrigerant slugging occur.
This failure mode results because if heavy dilution is occurring from a washout with migrated refrigerant, the upper cylinder walls and rings are being washed with refrigerant and are receiving virtually no lubrication.
The rings and cylinder warts begin scoring initially and this scoring results in galling of the piston skirts and ring lands and the eventual piston seizure and break-up within the liner.
The top of this piston was scarred when the loss of rod bearing material produced sufficient clearance for the piston to strike the discharge valve cage retaining bolt.
(Figure 9) & (Figure 10)
The pump-end bearing and the motor end bearing show similar smearing of the bearing material.
From this discussion, it becomes apparent that failures produced by excessive oil dilution are such that the metal of the opposing bearing surfaces attempt to weld; producing the smeared appearance with very little indication of excessive heat beyond the point of failure.
Migration of refrigerant within an idle refrigeration circuit is one of the principle causes of oil dilution. As discussed previously, refrigerant vapour migrates to the coolest part of the system where it eventually condenses. This will continue until the pressure-temperature relationship of the refrigerant is equalized throughout the system. This migration to the compressor is aided by the oil-refrigerant affinity discussed earlier.
Since the compressor is constructed of a large mass of cast iron, it is the last thing to cool shutdown and it is typically the last component of the refrigerant system to warm as the ambient temperature rises. Therefore, it is frequently the coolest part of the system after several hours of shutdown. In the compressor, the motor barrel is one place where the migrating refrigerant may collect and condense. In addition, since all commonly used refrigerant oils have an affinity for refrigerant, dilution takes place readily unless some preventive device is used to reduce oil/refrigerant affinity.
To counteract this occurrence, most compressors are now equipped with crankcase heaters. This preventive device functions to maintain an oil temperature 10-20?C above ambient to reduce its affinity for refrigerant, thereby discouraging the migration of vapour to the compressor.
It is not the function of the heater to prevent refrigerant dilution of oil resulting from migration to other parts of the system or to vaporize quantities of refrigerant already in dilution in the compressor. The migration problem can be solved by installing an interval timer to periodically pump down the system.
The loss of oil robs a compressor of bearing lubrication and cooling, resulting in the generation of an excessive amount of heat, bearing scoring and rod seizure.
Compare this crankshaft with the previous one, Figure 5. Note the color difference.
The dark color of the throws and counterweights is due to the heat of friction resulting from the loss of lubrication. Also notice the appearance of the bearing surfaces.
These surfaces show grooving instead of the smeared aluminium that is typical of refrigerant washout. Although the fine grooving can also the result of dirt in the system, the real key to the differences in these two crankshafts is the discoloration from heat and the longer time before failure.
There are several causes of compressor oil loss. Some of the common causes are short cycling, excessive oil foaming, leaking hydraulic un-loaders and long periods of minimum load operation coupled with a poor piping design.
During extended periods of short cycling, it is possible for the compressor to pump oil into the system at greater rate than it is being returned. This of course, results in a reduced oil level.
Short cycling may be caused by a low refrigerant charge which is causing the compressor to cycle on the low pressure control of, differential setting of the controlling thermostat set too narrow, minimum load conditions, etc. These conditions are all accompanied by a low mass flow of refrigerant which in turn, results in low gas velocities.
If the system is subjected to rapid load fluctuation, for which cause frequent stopping and starting, the cycling can be eliminated through the application of a hot gas bypass system.
Excessive foaming within the compressor crankcase is another cause of oil loss. When oil foams up within the crankcase it enters the suction passage where it is entrained by the refrigerant gas and is pumped out into the system foaming persists, it is possible that the oil level may severely.
A certain amount of foaming is expected when a compressor is started. However, once the excess refrigerant is boiled from the oil, the foaming will subside if system refrigerant flow control is proper and the proper oil recommended by the manufacturer is being used.
CAUSES OF COMPRESSOR OVERHEATING
Among the common causes for compressor overheating are a high compression ratio (low suction and high discharge pressures), low refrigerant charge and unloading the compressor below its design unloading limits:
Each of these conditions produce the same thing low refrigerant mass flow. Since the motor heat and heat of friction produced by the compressor are always present, any condition that reduces the flow of refrigerant gas below the design minimum requirement deprives the compressor of needed cooling, resulting an overheated condition.
When overheating is suspected, it is confirmed by checking both the oil and discharge temperatures. The oil temperature is taken on the outside surface of the oil sump at the discharge end of the compressor. Similarly, the discharge temperature is 1aken on the surface of the discharge line approximately six pipe diameters from the compressor.
These temperatures are to be taken on smooth, clean surfaces which are free of paint, corrosion, etc. The thermometer or thermistor is to be attached tightly to the surface and insulated to obtain the best possible reading:
The readings obtained will not be precise due to heat conduction losses through the metal and this is an important consideration when using the temperature guidelines given below.
Since the oil viscosity becomes marginal at temperatures between 90 and 95?C, any oil temperature reading within this- range is approaching the point where oil films can break down, resulting In metal-to-metal contact and eventual mechanical failure.
The discharge temperature, on the other hand, should not exceed 135?C. If the discharge temperature is 135?C, the temperature in the area of the compressor valves is approaching a temperature that can be in furious to the oil.
This temperature range should not be thought of as a strict dividing line between good and bad. The oil breakdown process. extends over a wide temperature range arid in the ranges mentioned, this process is in an area of accelerated breakdown. This is why this range of temperatures is critical but still subject to many other variables.
A high compression ratio is generally attributed to con- denser problems, evaporator problems, improper system control, or a combination of all three problems. The solution to this problem is to check the cleanliness of the evaporator and condenser, the condenser and evaporator air or water flow rate and the entering and leaving water or air temperatures. In addition, operation and control of the system must be monitored closely to identify any other operating mode which could contribute to low refrigerant mass flow.
A low refrigerant charge, on the other hand, is characterized by the appearance of flash gas in the sight glass at the expansion valve, low suction pressure and highly super heated suction gas. Of course, adding refrigerant to the system is the solution to this problem. Obviously, be doing this the source of the leaking refrigerant should be determined.
Finally, the unloading of a compressor below the minimum capacity specified by the manufacturer for certain ranges of suction and discharge temperature difference, can cause a refrigerant mass flow that is less than that required for adequate compressor cooling. The: solution is to limit the unloading of the compressor to that specified by the manufacturer for the existing system design conditions.
Some of the contaminants most frequently found in refrigeration systems are moisture, copper oxide, dirt, flux, and flaked or powdered aluminium. While the filter-drier will usually trap these materials, this is not always the case.
Moisture and water in a system can lead to further contamination due to formation of rust, corrosion, refrigerate decomposition, oil slugging or general deterioration. By coupling each of these failure components with the resultant damage which can occur with their presence, further areas of failure become obvious.
Excessive heat from friction, copper plating, and unnecessary wear of precision bearing surfaces all could be connected to this contaminant. In addition, ice formed (usually only in R-12 systems) in the expansion valve will restrict refrigerant flow or stop it completely.
One way to detect moisture in a system is with the ~J~-~, liquid Line indicators: sufficient time should be allow the indicator to reach a point of equilibrium and to indicate the proper color. It should be located ahead of the expansion valve and any driers (if present).
In an installation that has been properly installed and dehydrated there is virtually no way that moisture should be an initial prob1em in any reciprocating system. If a leak is present in a chiller so that water enters a refrigeration system, the resultant refrigerant leak would be apparent long before the moisture should be a problem.
The main cause or source of moisture contamination is from air introduced into the system during piping installation of any of the refrigerant lines. An other source or moisture introduced to the system is improperly handled refrigerant oils used as service replacement to the compressor. This can be remedied by closely following recommended oil handling techniques.
Without proper evacuation procedures, sufficient water can remain in refrigerant lines to induce corrosion and accelerate formation of other forms of contamination described earlier. Moisture is usually detected by use of a dry-eye moisture indicator located in the liquid line.
A common field procedure used to evacuate moisture and air from a system is the triple evacuation or ?blotting” method. Alternately, evacuating to a 1 mm Hg absolute pressure and then breaking the vacuum with dry nitrogen or refrigerant will normally minimise the time required to properly dehydrate a system.
It is important to remember that removal of standing water in a system will require a lengthy evacuation process. At 50 mm Hg absolute pressure, water boils at approximately 38?C. It is obviously impractical to heat the entire system to over 38?C. The only practical way to effectively dehydrate a system contaminated with standing water is with a vacuum pump capable of producing a blank off pressure of less than 1 mm Hg absolute.
Most deep vacuum pumps will go far below this blank off pressure and this ability will assure satisfactory dehydration. The time factor for water removal is often ignored; it takes time to boil off water trapped in a system using only the heat from room ambient temperatures. Remember that water is changing state and that a latent load is present in the water.
Another important consideration is vacuum pump size. A pump which is too large may reduce the pressure fast enough to actually freeze the water making it almost impossible to remove. The possibility also exists that damage can result to portions of coil or the piping if freezing takes place.
Dirt and air contamination foreign matter such as dirt, solder flux, or chemicals can, along with air, result in chemical in balances which can cause oil molecules to break up, this situation combined with the heat from higher system discharge temperatures and friction, temperatures, can result in the formation of acids, sludge, or combination of the two.
Coupled with the increasing friction which this whole process can produce, the whole system begins a process of self-destruction. In addition to the basic problems induced, more complex chemical reactions result in the formation of oxides and the elements needed for copper plating.
In addition to moisture, dirt and air from poor installation practice can result in severe problems after the system is put into operation. Dirt can be prevented from entering the system by insuring that only clean tubing is being used on field constructed system. piping. Proper evacuation and purging of the system with oil pumped dry nitrogen while making brazed piping joints will prevent air induced problems.
~A~~rG,eOf .dirt and air (along with moisture in the air) in a ~rse-tvlce operation may occur while adding oil to the compressor. Since oil has strong oxidizing characteristics and can quickly entrain air and water; care must be taken when transferring oils to the compressor. Oil should always be stored so that air is displaced by an inert gas above the oil level
Oxides can appear in the form of:
1. Red iron oxide (Fe2 03)
2. Black iron oxide (Fe2 04)
3. Red copper oxide (CU2 0)
4. Black copper oxide (Cu 0)’
The contaminant is an oxide which was wiped from the tubing walls or from within the compressor by the refrigerant and carried to the coil sump where it was deposited on the surface of the strainers.
In fact, the lower strainer became so thoroughly plugged that the pressure drop across the strainer screen caused it to collapse.
Of course, a plugged strainer robs the compressor main and rod bearings of lubrication, causing scoring. The nature of the scoring is similar to that caused by oil loss, Figure 11, or overheated oil, Figure 15. In addition, many times it is possible to see fragments of the oxidized material imbedded in the softer bearing surfaces.
The extent of the damage depends upon the oil flow reduction. The bearings at the extreme end of the oil passage will be affected first. Since the cylinder walls are splash lubricated, the piston skirts should show no particular wear.
The formation of oxides on the interior walls of refrigerant piping takes place when the heat required to sweat high temperature silver solder joints is applied to the pipe in the presence of air. Oxidation is prevented by displacing the air within the pipe with an inert gas, such as oil pumped dry nitrogen, before heat is applied.
A nitrogen-rich atmosphere within a pipe is obtained by placing a piece of tape over the open pipe end opposite the nitrogen connection. A small hole is pierced in the tape and the flow of nitrogen is adjusted until escaping gas can be felt at the hole.
If oxides should be discovered in the oil, they may be cleared from the system by installing a suction line strainer or filter- drier to trap the material before it can enter the compressor. The oil is then changed. as necessary, until it remains clean.
The parts where copper plating is most frequently found are close tolerance parts that operate at the higher temperatures, such as the compressor valves, bearing journals and the oil pump. The source of the plated copper is the system piping.
No attempt should be made by the serviceman to determine, by colour, the exact contaminant. This must be done by chemical analysis. In addition, due to the combination of contaminants, the basic black colour of the resultant product which you will see will give few clues as to the origin.
The crank bearing surface on the left shows copper plating. The reason this particular bearing surface was plated is the rod was probably a little tighter than the others, causing it to operate at a higher temperature.
When copper plates out on a crank journal bearing, it is generally flaked-off and rolled into balls by the rod, causing severe scoring of aluminium.
In addition, flaked-off copper will affect the life of open compressor crankshaft seals by reducing their sealing efficiency when the copper flakes become imbedded in the seal carbon face.
In this illustration, the oil pump at the right is heavily plated with copper. To perform properly, a gear pump such as this must be built to very close tolerance. Consequently, a deposit of metal on the pump parts can close the fine clearances, resulting in dragging and binding of the pump gear and eventual shaft breakage.
Once this occurs, there is a high probability that additional main and rod bearing damage will result even though the compressor shuts down on oil failure within a short time.
|Typically, additional damage occurs when operators, or service personnel reset the OPC (oil pressure control) to restart the machine without realizing that there has been a complete loss of forced lubrication within the compressor.|
Copper plating takes place in two steps. First, copper is dissolved in the by-products of an oil-refrigerant reaction. The quantity of copper dissolved is established by the nature of the oil, the temperature and the presence of impurities. As the second step, the dissolved copper is deposited on metal parts in a subsequent electrochemical reaction.
The common denominator in both the dissolving and depositing of the copper is high temperature. A second contributor to copper plating is the use of an improper oil. Certain oils react with refrigerants more readily than others under high temperature conditions to produce by products that solves copper. Finally, the presence if air, moisture and ~..Jfu contaminants all hasten the depositing of the copper:,.;WI!l
To prevent repeat failure due to copper plating, investigate and correct the cause or causes of high operating temperatures, use only the oils recommended by the compressor manufacturer and double or triple evacuate the system to assure the removal of air and moisture.
The selection of compressor oils by the manufacturer is a far more detailed procedure than most servicemen may be aware of. Oils are selected in accordance with their analytical components to meet objectives of proper lubricity in certain temperature ranges with chemical stability.
Other properties such as the inline point which affect seal and ?0? ring swelling, and those which limit the oil/refrigerant stratification at lower system temperature operating ranges must also be considered.
Only those oils tested and recommended by t1’1e manufacturer or their exact replacement may be used with the full knowledge that they are satisfactory for long term, .trouble free use. Other oils may also be used successfully, but it is not practical for any manufacturer to list all of the available oils to determine their suitability for extended use.
The first reaction of a servicemen when first viewing a motor burnout in a hermetic compressor is to assume that the motor or some component of the electrical system is at fault. Although it is sometimes difficult to prove and is usually not the case. The majority of motor burnouts that resulted from damage to the compressor and motor from system related causes, such as the areas previously discussed.
All attempts should be made to determine the mode of failure before assuming that the motor was bad. If the motor protection, circuit and control in functioning properly it is extremely difficult for a catastrophic failure to occur purely from electrical means.
The problems in this section are discussed to include mechanical and specific motor and electrical problems. Some of the common electrical-related compressor problems stem from the loss of power in one of the three phases (single phasing), low voltage, shorted motor windings, over- heating, dragging rotor and electrical control problems.
When a motor fails, it is strongly recommended that the windings be cleaned for inspection. Their appearance will generally lead to the apparent cause of the failure. Before proceeding, the stator windings of a three-phase, four pole motor will be examined.
The identification of the windings of each of the three phases is important when diagnosing a motor problem. Motors are available with 3, 6, 9 and 12 leads. However, regardless of the number of leads, the general appearance of the stator winding of these motors is the same.
This illustrates the stator coil arrangement of a three-phase, four-pole motor. For this purpose; the four coils, or poles, of each of the three phases are the same color for easy identification.
Note that the coils of the three phases appear in the color series yellow, which represents a phase silver, which represents a phase; and copper, which represents a phase and that this series is repeated at each 90 degree interval.
The phase windings of a typical motor are also easily identified because each winding is separated from the others by an insulating barrier.
A complete motor burnout is most apt to occur when the motor is i n the stalled position. At the moment the motor is energized. the electrical and physical stresses on the windings are the greatest. If at this time voltage is low or the compressor is mechanically locked up, the motor will burn unless the overloads are tripped within a very short time.
When a motor burns in the stalled position, the soot and other by-products of the burn are confined to the suction side of the system. This may help with your diagnosis.
With the absence of proper motor protection, another cause for burnout is inadequate motor cooling due to reduced flow or no flow of suction gas. Since this type of burn takes place when the compressor is operating, the by-products of the burn are frequently carried to the discharge side of the system.
When all phases of the motor are burned, check the compressor to see if it is free. If the mechanical parts are seized, it can be assumed that the cause of the motor burnout and compressor failure is mechanical. When this is the case, further investigation is required to determine the source of the mechanical failure.
If, on the other hand. the compressor is free and appears to be in reasonable operating condition, the source of the problem could be electrical. The electrical analysis should start with a voltage and phase balance check. The voltage should be within plus or minus 10 percent of the nameplate rating and the phase imbalance should not exceed two percent.
Another source of the problem may be low suction gas flow or no flow. Check the condition of the contactor. If the contacts are welded, it is possible that the compressor pumped down the system and then failed to stop. Continued operation with no gas flow over the motor caused it heat and eventually burnout.
Compressor short cycle operation to meet the small needs of a weekend load, for example, can also cause motor over- heating. The repeat starts with the corresponding in-rush current coupled with the reduced flow of suction gas over the motor during the brief operating cycles results in motor heating that can ultimately cause a burnout.
The mechanical evidence of the lack of compressor cooling is the scoring of the piston skirts with no apparent rod or main bearing damage. Since the suction gas cools other working parts of the compressor as well as the motor, a reduced gas flow or loss of flow causes the pistons and cylinder to heat. And since the thermal expansion rate of aluminium is greater than that of the cylinders, the pistons bind within the cylinders. causing the scoring,
When a motor fails, regardless of cause, always check the condition of the contactor. The high current that accompanies a burnout will frequently damage or weld the contacts. If the system is subject to extended periods of minimum load operation or load fluctuations that cause it to cycle frequently, a recycle timer should be installed to limit the starts to four or five an hour.
The recycle timer contacts (TR1) are wired in series with the chilled water or conditioned air thermostat (TC1) and in parallel with the non-recycling relay (CR1 ) and the auxiliary contacts of the compressor motor contact (1 M).
This arrangement provides both a time delay between successive starts plus a timed pump down cycle. The timer is controlled by a control relay (CR3). A compressor start-up energizes CR3 which, in turn, opens its contacts.
This de-energises the timer clutch, resetting the timer to zero. After the controlling thermostat (TC1) is satisfied and the pump down cycle is completed, control relay CR3 is de-energised, closing its contacts.
This energizes the timer clutch which opens contacts TR1 and holds them open for the duration of the time-out period. After the time-out period has elapsed, contacts TR1 close. If at this time TC1 is again calling for cooling, the system is restarted. Or, if TC1 is open however the low pressure control (LPC) is closed, indicating a build-up of low side pressure, a pump down cycle is started.
Each time the compressor is started CR3 is energized, reset- ting the timer to zero. When the compressor is stopped, it cannot be restarted until the timer again times out.
Metal fragments resulting from a mechanical failure can become lodged in the motor windings. Here they can act like cutting tools, causing damage to the motor insulation.
The stator illustrated was subjected to such dam- age. In this example a piece of valve became lodged between the rotor and stator where it cut through both the winding and slot insulation, causing several windings to burn.
A spot burn, such as this, may take place weeks or months after a repaired compressor has been returned to service.
|The particles lie in the motor until the~ ~re work~-d in~ a position to cause damage. Consequently, It’s always good practice to remove and inspect the motor and barrel after a failure that involves broken parts.|
A spot burn can also be caused by relative movement between the individual turns of a coil. When a motor is started, the coil ends flex slightly, causing the wire turns to rub against one another.
In time, this may cause a break down of the insulation, resulting in a turn-to-turn short.
The heat from this short will burn away the insulation of adjacent wire turns, resulting in an eventual phase-to-phase or phase to-ground short. Notice that the short originated where the end portion of the coil enters the motor slot.
A pressure point between the turns may have existed here or a metal fragment may have been imbedded between the turns which accelerated the wearing away of the insulation.
It is absolutely necessary that all such material be removed before any motor replacement is attempted.
When a compressor valve fails, it is possible for a piece of valve or valve spring to be forced into the suction side of the compressor where it can become lodged in the motor windings. Here it can cause a short between the motor turns, resulting in a spot burn.
A spot burn may also be caused by stresses on the motor. If examination of a spot burned motor reveals no evidence of metallic particles, either imbedded in the windings or in the jC- ~~.r~ barrel, it may be suspected that the insulation breakage resulted from normal stress.
Single phasing and causes the loss of power in one of the phases causes a three-phase motor to single phase. This causes the remaining two phases to draw excessive current. If the overloads do not take the motor off the line quickly, these two phases will burn.
This is the appearance of a motor that was subjected to single phasing. Note that the windings of two phases are burned while the four poles of the coil remaining unburned.
~ca1tiiguration is two windings burned-one unburned, two windings burned-one unburned, etc.
When this is observed, the motor has undoubtedly been single phased.
The situation just described is a typical single phasing burnout. However, in a single phasing condition, it is possible for one phase to heat faster that the other causing only one phase to burn. When one phase is burned, check the windings of the remaining two phases. If one shows heat damage, it can be assumed that single phasing was the cause for the burnout.
A dragging rotor is another cause for motor electrical problems. Since the rotor-to-stator spacing is extremely close, bearing wear can cause the rotor to dip sufficiently to drag on the stator.
The stator illustrated shows signs of grooving caused by a dragging rotor. The rotor dragged the laminations causing a failure of the slot insulation, resulting in a phase-to-ground short.
Note that two or three windings are burned, not the entire motor. The rotor that was removed from this motor showed similar grooving.
Bearing wear that is severe enough to cause rotor drag must be investigated. Some of the possibilities are oil dilution or oil that is contaminated by dirt or other abrasives.
Bearing surfaces that show smearing, similar to Figure 10, indicates an oil dilution problem. See “Oil Dilution?, page 4. Cloudy oil which contains suspended particles coupled with a dirty oil strainer screen, on the other hand, signals the presence of dirt, moisture or other contaminating materials.
After the replacement compressor is started, it is wise to periodically check the color and .clarity of the oil. If it becomes discolored by suspended material shortly after startup, change the oil, as necessary, until it remains clean. In some cases it may be desirable to install a suction filter to trap contaminates prior to entering the compressor. Obviously, if the oil continues to indicate a continual build up of suspended material after installation of a suction filter, there is a good possibility that the particles are the result of another mechanical failure taking place.
Up to this point, we have assumed that by careful analysis the serviceman should be able to trace back through failed compressor parts and determine the real cause for failure. This may not always be the case.
In the real world, time, shop space, and the freedom to explore causes for system failures are not usually ideal. In addition, some compressor scrambles beyond the point of determining which mode of failure came first. Even with extensive cleaning of damaged parts, the baked-on varnish, carbon, and sludge may be so extensive that you wonder how the compressor lasted as long as it did.
In these situations you must simply explore all probable system causes based on a conclusion that more than one factor was involved in the actual failure. The conditions that lead to a catastrophic compressor failure have probably come through a long and involved path. Combining all of the ideas from the earlier discussion in this presentation, start now in an attempt to avoid a repeat of the same sequence of events that caused the first failure.
While performing the necessary clean up operations that are described in the next step, use what time you have to check the electrical system for such things as inadequate wire sizes, burned motor contacts, and loose terminals. Check to see that the motor protection circuit is not bypassed and that pressure safety controls are working and wired properly. Is the expansion valve sensing bulb installed properly and is the valve in good working order ?
Even these simple and obvious items can be overlooked in the haste of installation and start up. Finally, take extreme care to be sure that the system is cleaned and put back on line properly. When it is restarted you will be able to monitor the pressures and temperatures that may give you your final answers on the original failure.
After a compressor motor failure or a failure resulting from the presence of Contaminants within the system, the life of the repaired or replacement compressor is dependent upon the care exercised in system cleanup.
To reduce the amount of contaminants to be removed from the system, inspect the suction and discharge piping adjacent to the compressor. If either contain soot or other by- products of a motor failure, clean the line with a refrigerant parts cleaner before the compressor is installed.
Note: Cleaners that contain chlorine should not be used. When installing or rebuilding a compressor in a system which has experienced an electrical failure, install a suction line filter-drier of an appropriate size.
After the leak test has been completed and the test pressure released, replace the liquid line drier, and evacuate to 1 mm Hg absolute pressure. Break the vacuum with oil pumped dry nitrogen and again establish the 1 mm absolute pressure. Allow the system to remain under vacuum for 12 hours. If the vacuum reading remains unchanged, the system is leak and moisture free and ready to receive its charge of refrigerant.
Charge the system and perform the following checks before the system is put into continuous service.
1. First, inspect all wiring connections for tightness. Properly tightened connections are very important since loose wire connections will cause voltage drops which can be instrumental as a primary cause for various electrical failure.
2. Check the condition of aH contactors. If the contacts are
1n poor condition, replace them. Specific information is available from manufacturers on how to determine if the contacts are past their service life.
3. The voltage at the compressor contactor should be checked to make sure it is within plus or minus 10 percent of the compressor nameplate rating.
4. The voltage balance between the phases should be tested. Voltage imbalance, in percent, is found by deter- mining the sum of the three voltage differences from the average, dividing this figure by twice the average and then multiplying the result by 100.