Compressor Failure Modes, Symptoms and Corrections
A handy for analysis of a in. lure compressor, to aid in finding cause of failure take
correction action. Replacement new or remanufactured 6 compressors fail I four times
the rate of original compressors indicating replacement failures are caused by system
problems.
Liquid Slugging
Broken reeds rods, or crankshaft. Loose or broken discharge bolts. Blown gaskets.
Slugging is a a result of trying to compress liquid in the cylinders. Liquid may be either
refrigerant or oil or more likely a combination of both. Slugging is a a result primarily of
refrigerant migration into the oil on #he off cycle.
Correction:
1. Check pump down cycle operation
2. Is TXV sized and operating properly.
3. Suction line sized properly?
4. Check unloading
Liquid Washout
Worn Pistons and/or rings. Cylinders worn. Scored pins. Scored and/or broken rods.
Worn Bearings. Scored crankshaft.
This is a mutt of refrigerant washing oil off surfaces. Off cycle migration of saturated
refrigerant into crankcase. Compressor starts up resulting in a mass of foam which
when pumped washes bearing surfaces of oil film necessary far proper lubrication.
WASHOUT is a minor condition of SLUGGING.
Correction:
1. Check TXV bulb and super heat setting.
2. Is TXV oversized?
3. Check crankcase header, (On during off cycle.)
Liquid Dilution
Rotor Drag. Worn bearings. Scored and/or broken rods. Scored Crankshaft.
This is a result of liquid refrigerant returning to compressor during running cycle. Oil
becomes diluted and lubrication for oil pump and end bearing my be adequate, but as it
progresses down the crankshaft insufficient oil to lubricate the rods and main bearings
will occur.
Correction:
1. Check TAY bulb.
2. Clock superheat setting.
3. Check defrost cycle.
High Discharge Temperature
Discolored valve plate (Cannot rub off). overheated or burned valve reeds. Worn rings
and patent. Worn cylinders. Scored rods, bearings, and crankshaft. Spot bum in stator.
This is a a result of temperatures in the compressor head and cylinders becoming to
hot that the oil lam its ability to lubricate.
Correction:
1. High compression ration: check far law suction and high discharge pressures. Law
load and evaporator problems.
2. Check low pressure control setting
3. Check for dirty condenser, inoperative condenser fan and ambient temperature.
4. Check air flow across compressor.
Lack of Lubrication
Scored bearings. Broken rods. Scored crankshaft. Low oil in crankcase.
This is a a result of lock of enough oil in crankcase to properly lubricate the running
gear.
Correction:
1. Check oil failure witch.
2. Chock pipe sizing and also for oil traps
3. Inadequate defrost.
4. Law load.
5. Eliminate shad cycling.
Electrical
Many motors fail as a mutt of a mechanical or lubrication failure. Many foil due to
malfunctioning external electrical components.
General or Uniform Burn
Entire winding is uniformly overheated or burned.
Correction:
1. Check far low voltage.
2. Rapid cycling of compressor,
3. Loose terminal connection.
4. Unbalanced voltage.
Single Phase Burn
Two phases of a three phase motor am overheated or burned.
A result of not having current through the unburned phase and overloading the other
two phases.
Correction:
1. Check contacts in starter and contact slide mechanism for binding.
2. Terminal connections on compressor.
3. Unbalanced voltage
4. Blown fuses.
Half Winding Single Phase Burn
This shows as when one hall of the motor has a single phasing condition on a PART
WIND MOTOR with a two contactor system.
Correction:
1. Chad both contactors as we will be defective.
2. Chad timer far proper time delay.
Start Winding Burn
Only the start winding is burned in a single phase motor due to excessive current
flowing through the start winding.
Correction:
1. Check C,S, and R, wiring.
2. Starting capacitor and/or start relay.
3. Compressor overloaded.
Run Winding Burn
Only the run winding is burned in a single phase motor.
Correction:
1. Chock relay
2. Chock run capacitors
Primary Single Phase Burn
This will show as only one phase burned. Other two will be O.K. A result of losing we
phase in the primary of a ฦ to Y or Y to ฦ transformer.
Correction:
1. Check transformer far proper voltage incoming and outgoing.
CORRECTIVE
MEASURE
TROUBLE
POSSIBLE CAUSE
High condensing
pressure.
Air or non-condensable gas in
system.
Purge air from
condenser.
Insufficient water or air flowing
through condenser.
Increase quantity of
water or air.
Evaporative condenser clogged or
limed.
Clean condenser
water tubes.
Too much liquid in receiver,
Draw off liquid into
condenser tubes submerged in liquid service cylinder.
refrigerant.
Low condensing
pressure.
High suction pressure.
Low suction pressure.
Too much water or air flowing
through condenser.
Reduce quantity of
water or air.
Condensing water too cold.
Reduce quantity of
water.
Liquid refrigerant flooding back from
evaporator.
Check expansion
device adjustment,
examine fastening of
thermal expansion
valve bulb(s).
Leaky compressor discharge
valve(s).
Remove heads,
examine valves.
Replace any found
defective.
Overfeeding of expansion device.
Regulate expansion
valve, check bulb
attachment and
superheat adjustment.
Leaky suction or discharge valves.
Remove head,
examine valves and
replace if worn.
Malfunction of compressor capacity
control system.
Check capacity control
system.
Excess load.
Reduce load to
normal.
Restricted liquid line, or suction,
strainer screens.
Pump down, remove
restriction, examine
and clean screens.
Insufficient refrigerant in system.
Check for refrigerant
shortage.
Too much oil in system.
Remove oil.
Improper adjustment of expansion
valve(s) or liquid control devise(s).
Adjust device(s) for
proper superheat approximately 10°F.
Expansion valve power element dead Replace expansion
or weak.
valve or power
element.
Compressor will not
run.
Electric power cut off
Check power supply.
Fuses blown.
Test fuses and renew
if necessary.
Overload devices tripped.
Check overload
devices and find
cause of overload.
Low voltage.
Check voltage (should
be within 10% of
nameplate rating).
Trouble in starting witch or control
circuit.
Close switch manually
to test power supply. If
OK check control
circuit including
temperature and
pressure controls and
capacity control
device.
Seized compressor.
Repair or rebuild
compressor.
Compressor runs
Shortage of refrigerant.
continuously with
insufficient reduction of Individual cylinders not loading.
load temperatures.
Repair leak and
recharge system.
Check and correct
pumping ability of
individual cylinders replace suction and/or
discharge valves and
parts as needed.
Check and correct
capacity control
system.
Incorrect control switch settings.
Compressor short
Presence of air or foul gas.
cycles or stops on high Insufficient water or air flowing
pressure cutout.
through condenser, clogged
condenser.
Reset control switches
or replace.
Purge condenser.
Check -to, or air Flow.
Check for scaled or
fouled tubes in water
cooled condenser. In
evaporative type,
check for fouled
surfaces and
insufficient air or spray
water.
In air cooled type,
check for fouled
surfaces, or lack I air
flow.
Compressor stops on
Oil Failure Switch
Plugged Oil Strainer.
Excessive liquid repair in sump????
Clean oil strainer.
Pump down observe
level.
See symptoms chart.
IDENTIFYING COMPRESSOR FAILURES
Most compressors fail due to system malfunctions, which must be corrected to prevent repeat
failures. After a compressor fails, field examination of the failed compressor often will reveal
symptoms of system problems. Proper corrections will help eliminate future failures.
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REFRIGERANT FLOODBACK
This is a result of liquid refrigerant returning to the compressor during the running cycle. The
oil is diluted with refrigerant to the point it cannot properly lubricate the load bearing surfaces.
Air Cooled Compressors
Worn pistons and cylinders
No evidence of overheating
The liquid washed the oil off the pistons and cylinders during the suction stroke causing them
to wear during the compression stroke.
Refrigerant Cooled Compressors
Center and rear bearings worn or seized
Dragging rotor, shorted stator
Progressively scored crankshaft
Worn or broken rods
The liquid dilutes the oil in the crankcase and the refrigerant rich oil will be pumped to the
rods and the bearings through the crankshaft. As the refrigerant boils off, there will not be
enough oil for sufficient lubrication at the bearings farthest from the oil pump. The center and
rear bearings may seize or may wear enough to allow the rotor to drop and drag on the stator
causing it to short.
Correction:
(1) Maintain proper evaporator and compressor superheat.
(2) Correct abnormally low load conditions.
(3) Install accumulators to stop uncontrolled liquid return.
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FLOODED STARTS
Worn or scored rods or bearings
Rods broken from seizure
Erratic wear pattern of crankshaft
This is the result of refrigerant vapor migrating to the crankcase oil during the off cycle. When
the compressor starts, the diluted oil cannot properly lubricate the crankshaft load-bearing
surface causing an erratic wear or seizure pattern.
Correction:
(1) Locate compressor in warm ambient or install continuous pump down.
(2) Check crankcase heater operation.
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SLUGGING
Broken reeds, rods, or crankshaft
Loose or broken backer bolts
Blown head gaskets
This is the result of trying to compress liquid refrigerant and/or oil, in the cylinders. Slugging is
an extreme floodback in air-cooled compressors and a severe flooded start on refrigerant
cooled compressors.
Correction:
(1) Maintain proper evaporator and compressor superheat.
(2) Correct abnormally low load conditions.
(3) Install accumulators to stop uncontrolled liquid return.
(4) Locate compressor in warm ambient or install continuous pump down.
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HIGH DISCHARGE TEMPERATURE
Discolored valve plate
Burned valve reeds
Worn pistons, rings and cylinders
Stator spot burn from metal debris
This is the result of temperatures in the compressor head and cylinders becoming so hot that
the oil loses its ability to lubricate properly. This causes rings, pistons and cylinders to wear
resulting in blow by, leaking valves, and metal debris in the oil.
Correction:
(1) Correct abnormally low load conditions.
(2) Correct high discharge pressure conditions.
(3) Insulate suction lines.
(4) Provide proper compressor cooling.
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LOSS OF OIL
All rods and bearings worn or scored
Crankshaft uniformly scored
Rods broken from seizure
Little or nor oil in crankcase
This is a result of insufficient oil in the crankcase to properly lubricate the load bearing
surfaces. When there is not enough refrigerant mass flow in the system to return oil to the
compressor as fast at it is pumped out, there will be a uniform wearing or scoring of all load
bearing surfaces.
Correction:
(1) Check oil failure control operation if applicable.
(2) Check system refrigerant charge
(3) Correct abnormally low load conditions or short cycling.
(4) Check for incorrect pipe sizes and/or oil traps.
(5) Check for inadequate defrosts.
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ELECTRICAL
While most motors fail as a result of mechanical failures, some are true electrical failures.
GENERAL OR UNIFORM BURN
All windings uniformly overheated or burned.
Correction:
(1) Check for proper voltage.
(2) Check for unbalanced voltage.
(3) Check for inadequate motor cooling.
SINGLE PHASE BURN
Two phases of a three phase motor overheated or burned as a result of the contactor opening
only one of its contacts.
Correction:
(1) Replace contactors with properly sized contactors.
(2) Check for proper motor protection.
HALF WINDING SINGLE PHASE BURN
In a two contactor application, two phases of one half of a three phase motor overheated or
burned as a result of one
contactor opening only one contact.
Correction:
(1) Replace contactors with properly sized contactors.
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HALF WINDING BURN
Half of all phases on a two contactor three phase motor are overheated or burned due to only
one contactor opening.
Correction:
(1) Replace contactors with properly sized contactors.
(2) Check for feed back circuit holding one contactor closed.
PRIMARY SINGLE PHASE BURN
Only one phase of a three phase motor is overheated or burned as the result of opening one
phase on the primary side a delta to wye transformer.
Correction:
(1) Check primary side of a delta to wye main power transformer.
START WINDING BURN
The start winding only of a single phase motor is uniformly overheated or burned.
Correction:
(1) Check for proper wiring of compressor.
(2) Check starting capacitor and/or starting relay.
(3) Correct compressor overloading.
SPOT BURN
A localized burn within a winding, between windings, or from windings to ground. If not the
result of mechanical problems, check for spikes or surges of high current flow.
SHORTED TERMINALS
A break down of the insulation between terminals and compressor body generally the result of
over torquing terminals.
Reciprocating Compressor Failure Modes
1. The following mechanically related conditions account for approximately 80%
of compressor failures.
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Flood Back: Liquid refrigerant returning to the compressor during the
running cycle.
Flooded Start: Crank case oil diluted with liquid refrigerant due to off cycle
vapour migration.
Liquid Slugging: Liquid refrigerant or excess amounts of oil entering the
cylinders during the running cycle.
Excessive Discharge Temperatures: Higher than design superheated
discharge gas temperatures.
Compressor Oil Loss: Quantity of oil returning from the system is less than
that leaving the compressor.
2. The following electrically related conditions account for approximately 10% of
compressor failures.
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Stator burn: Improper or unbalanced voltage and poor motor cooling.
Single Phase Burn: Loss of one phase on a three phase system.
Half Part Wind Burn: Loss of supply to one winding set on a two winding
motor(Part wound).
Loose Connections: Loose electrical joins causing a voltage drop.
Starting Components: Damaged starting capacitors and relays.
Inpure Power Supply: Spikes or surges of current flow.
Shorted Power Terminals: Over torquing power terminals.
Mechanical Failures
Flood Back
Liquid return during the running cycle. More commonly known as refrigerant
flood back. The expansion valve plays a part in all the conditions here
whether directly or indirectly this is obviously because the refrigerant can
only really come this route.
Read More!
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A flood back would cause any of these wear patterns or any combination.
Firstly, air cooled compressors, where the gas goes directly into the cylinder
head suction manifold, Liquid washes oil off cylinders and pistons during the
suction stroke causing cool and dry wear during the discharge stroke
resulting in:
1. Worn pistons.
2. Worn cylinders and rings.
3. Metal debris falling into the oil.
Then with refrigerant cooled compressors where the gas first travels over
the motor before rising to the suction manifold. Liquid cannot rise to the
suction manifold and instead enters the crankcase to dilute the oil. This
refrigerant rich oil is then pumped through the crankshaft evaporating and
washing as it goes along reslting in:
1. Conrod/crankshaft wear which worsens furthest from the oil pump
therefore:
2. Centre and rear bearings worn or seized.
3. Conrods possibly broken.
4. Motor end bearing wear is greatest causing the rotor to drop and drag on
the stator shorting the windings.
For both air cooled and refrigerant cooled compressors any wearing will be
without signs of heating due to the cooling effect given by the vaporising
refrigerant. There will therefore be no discoloration or carbonisation of the
metal parts or oil. The white bearing metal would normally be smeared with
a lumpy appearance on opposing surfaces.
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Flooded Start
Crank case oil diluted with liquid refrigerant which has migrated from other
parts of the system especially from saturated areas. The migration is usually
by vapour during the off cycle.
Read More!
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A flooded start would cause the following damage or any combination. It must
be stressed that there will be no recognised wear pattern and this in itself is the
signature of a flooded start.
1 Worn pistons and rings.
2 Worn or scored connecting rods or bearings.
3 Connecting rods broken from seizure.
4 Erratic wear pattern on the crankshaft.
Any wearing will be without signs of heating due to the cooling action given
by the vaporising refrigerant. There will therefore be no discoloration or
carbonisation of the metal parts or oil. Lumps of white metal bearing would
be smeared on opposing surfaces.
Two common courses of action are taken to avoid migration and they are 1.
heating the oil during the off-cycle or 2. Setting a pressure switch to run the
compressor intermittently with the liquid line closed to maintain a safe low
crankcase pressure, this is called a pump down and the principle here is that
the lower the crankcase pressure the less refrigerant found in solution. Often
a combination is applied where the compressor at the off-cycle will continue
running until the crankcase pressure has dropped to a predetermined
pressure, after which, the compressor is locked out only to recycle on load
demand while then a crankcase heater acts to keep refrigerant from settling
in the oil. Locking the compressor out protects it against short cycle damage
should there be an unexpected cause of this such as leaky liquid line
solenoid valves. Whichever methods are used it is extremely important that
a compressor is not started under flooded conditions. However, neither of
these methods work to protect the compressor if there has been a power
interrupt.
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Slugging
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Liquid refrigerant or excess amounts of oil entering the cylinders during the
running cycle is commonly called liquid slug. This is most often the result of
flood back on air cooled compressors or flooded starts with refrigerant
cooled compressors.
Read More
A liquid slug would cause the following damage or any combination.
The liquid slug can be either liquid refrigerant or oil.
With air cooled compressors slugging will take place during extreme flood
backs.
With refrigerant cooled compressors slugging is the result of a severe
flooded start.
1 Broken discharge or suction valve reeds, connecting rods or crankshaft.
2 Loosened, thread stripped, or broken discharge valve backer bolts.
3 Blown valve plate and head gaskets with the loss of charge.
Maintaining correct superheat is important here. Also look out for low loads,
cool compressor ambients and migration control.
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High Superheated Discharge Temperatures
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Discharge gas temperatures or superheated discharge temperatures which
are higher than designed for. This is high discharge gas superheat which is
the result of high suction gas superheat and/or high compression ratios. The
high compression ratios can be a result of abnormally high discharge
pressures, abnormally low suction pressures or a combination.
Read More!
High discharge temperatures would cause the following damage or any
combination.
Cylinder and head temperatures become so hot that the oil loses the
required viscosity for proper lubrication. Resulting ring wear causes
discharge gases to blow past the rings and pressurise the crankcase
preventing oil return from the system. Metal debris dropping to the
crankcase will eventually cause stator spot burn when arriving between the
rotor and stator.
1 Discoloured valve plate which can’t be rubbed clean.
2 Burned discharge valve reeds.
3 Burned and worn pistons, rings and cylinders.
4 Stator spot burn from metal debris.
Look out for a high compression ratio i.e. low suction and high discharge
conditions. Check the low and high pressure control settings. On low temp
systems check for proper liquid injection or head cooling air flow. Also
insulate the suction lines especially those that pass through warm zones. To
reduce discharge superheat it may be necessary to reduce suction
superheat. Check for or install discharge thermisters or Klixons.
An example of a not so obvious fault here is a refrigerant cooled compressor
which has been overcharged with oil. Refrigerant cooled compressors have
higher suction and discharge superheats after cooling the motor especially
with low temperature applications. The higher amperage resulting from the
extra power required to churn the high level oil adds to total motor heat. A
shortage of oil will also cause higher amps due to increased friction.
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Loss of Compressor Oil
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Quantity of oil returning from the system is less than that leaving the
compressor. Since the very parts that compress the refrigerant vapour have
to be lubricated an amount of oil always leaves the compressor with the
refrigerant. We find conditions where oil leaving the compressor can
increase also where oil returning is decreased.
Read More!
A loss of crankcase oil would cause the following damage or any combination.
The most common causes of poor oil return is too low a mass flow in the suction
line to sweep the oil back or improper design of suction line risers.
1 All rods and bearings worn or scored.
2 Crankshaft uniformly scored and heat discoloured.
3 Rods broken from seizure.
4 Look for little or no oil in the crankcase and much discolouring.
Any wearing will be of scoring in character which is very different to the
wearing caused by liquid washout. There will be much evidence of
overheating i.e. staining of metal parts and carbonisation of the oil. Look for
a resulting dirty oil strainer
Check for the following:If applicable check the oil protection.
1. System refrigerant charge or lack of.
2. Correct abnormally low load conditions or short cycling.
3. Check for oversized suction pipes or lack of oil traps.
4. Check for inadequate defrosts otherwise known as oil harvests.
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Electrical Failures
General or Uniform Stator Burn.
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Improper or unbalanced voltage, poor motor cooling or poor compressor
high load limiting.
Read More!
Check for the following most common causes:
Low running voltage due to over loading of the main circuit or loose
connections anywhere from the main transformer to the compressor. V loss
or IR drop: ( V loss=I×R ) where R would be line resistance plus the added
resistance of any poor connections.
1. Unbalanced voltage due to unbalanced phase usage.
2. Over loading of the motor i.e. motor not sized for the suction density or
pressure, usually the complete condensing unit would then be undersized
resulting in higher discharge pressures too. check for maximum operating
pressure controls.
3. Rapid cycling of the compressor meaning there is insufficient time for heat
generated by the inrush current to dissipate.
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Single Phase Burn
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Loss of one phase on a three phase system.
This condition is known as single phasing. Loss of voltage on one phase
results in the loss of motor torque by the loss of that winding. The rotor then
slips i.e. slows down effectively loosing back emf to the remaining windings
which then draw more current tending to burn those windings unless
protected. If the torque demand on the motor is high then it will stall and
should have tripped the breaker easily.
Check for the following causes:
1. Contact damage or sticky contactor sliding mechanism.
2. Voltage imbalance.
3. Improper electrical connections along that phase right back to the
transformer.
4. Blown fuse.
5. Proper motor protection.
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One Half of a Part Wound Motor is Burnt
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Loss of supply to one winding set on a two winding motor (Part wound).
Squirrel cage motors will draw approximately 6 times their running current
on start up due to the initial absence of inductive reactance . If the building
supply cables are undersized then a voltage drop throughout that building
will occur every time the compressor starts due to this IR line voltage drop.
There are numerous ways to reduce this effect. Star Delta starting will limit
the peak starting current to approximately 2 times running current but the
starting torque is limited to 1/3 of the norm. However, part wound motors
will have a starting torque proportional to the relative size that the first half
winding. Unless the compressor is much unloaded, the loss of power to one
half of the total winding or one of the tandem windings will result in over
working and thus overheating of the remaining half winding.
Check for the following causes:
1. Faulty control circuit.
2. No interlocks.
3. Blown contactor.
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Loose Connections
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Loose electrical joins cause a drop in the supply voltage reaching the motor
windings meaning extra current need be drawn to compensate in the
attempt to maintain motor speed. This added I2R heat production in the
motor windings will cause motor overheating.
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Start Winding Burn
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Damaged starting capacitors and relays on single phase motors.
These components usually blow as a result of power problems or
compressor short cycling. By my experience loss of refrigerant charge and
the subsequent LP switch induced compressor short cycling is the biggest
cause of failure to these components. When these components do blow the
compressor will often burn the run winding. However, the run winding will
also burn if the compressor is overloaded causing the starting relay to
repeatedly call the start winding into circuit whilst the compressor is already
running. Surprisingly common is the miss wiring of compressor motors often
resulting in instantaneous nuisance motor damage by burning the start
winding.
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Spot Burn
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Caused by spikes or surges of current flow or more likely system or
compressor debris damage.
Rapid changes in a buildings electrical load can be resistive or inductive.
Especially with rapid changes in high inductive loads the result is normally a
spike in the voltage allowing arcing to occur through winding points of
weaker insulation. Damage to the compressor is usually in the form of a spot
burn. A spot burn is a localised burn which can be within a winding, between
windings or from winding to ground. Remember also that a localised spot
burn can be caused by metal debris finding its way between the rotor and
stator.
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Shorted Compressor Terminals
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Over torquing power terminals on the compressor can damage the fuseglass
otherwise known as fusite insulation resulting in shorts occurring to the
compressor motor body.