1. The following mechanically related conditions account for approximately 80% of compressor failures.

• 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.

• 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.

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.

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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.
 

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.

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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.

Slugging

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.

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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

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.

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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.

Loss of Compressor Oil

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.

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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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General or Uniform Stator Burn.

Improper or unbalanced voltage, poor motor cooling or poor compressor high load limiting.

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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

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.

One Half of a Part Wound Motor is Burnt

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.

Spot Burn

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.
 

Shorted Compressor Terminals

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.