Compressor Failure Reasons

There are a variety of reasons why a compressors usually fails and I felt it would be a good idea to list and comment on the most common mechanical and electrical reasons for failure.

Electrical Reasons. 
Start winding Burn

This is one of the principle reasons why a lot of compressors fail and can be caused by a technician not wiring up the compressor properly. Other reasons why this failure occurs include:
- Wrongly sized or bad capacitor
- Loose or badly corroded terminals
- Compressor short-cycling
- Motor overloaded due to pressures not equalised
- Low voltage
- Compressor not turning
Run Winding Burn Out
A run winding burn out often appears as a general burnout due to the start winding getting burnt out also. The reasons associated with start winding burn out are very similar to a run winding failure..
Spot Burn

Spot Burn is usually caused by a surge in electrical current or voltage to the compressor. This is usually out of the technicians hands due to an electrical supply problem. A spike in voltage or current can result in arcing through the winding points of weaker insulation. A spot burn is a localised burn which can be within a winding, between windings or from winding to ground.

Mechanical Reasons

Liquid Floodback / Flooding
Liquid Floodback is when liquid refrigerant returns back to the compressor while running.  As we already know liquid cannot be compressed. The liquid refrigerant floodback will also dilute the compressor’s oil which causes pistons, cylinders and ring wear.

To prevent floodbacks happening an accumulator can be placed between the evaporator and the compressor which will catch any liquid refrigeration before accessing the compressor acting like a reservoir for any liquid.
Liquid Slugging
Liquid slugging is a condition which occurs when liquid is allowed to enter one or more of the compressor cylinders. Slugging can occur when moisture has entered the sealed system of from floodback problems mentioned above.
Because liquid is practically non-compressible, the compressor seals may be compromised when the pistons attempt to compress the liquid. Liquid slugging can lead to permanent valve damage and reduced compressor efficiency.

Low Compressor Oil
Compressors have moving parts and need oil to avoid seizure and overheating. The parts that compress the refrigerant vapour have to be lubricated and some oil always leaves the compressor with the refrigerant.
There is a certain amount of oil pumped out of the compressor with each stroke of the piston (usually 3% of discharged volume). The same amount of oil must return to oil-sump if the compressor is to be sufficiently lubricated. In a properly designed refrigeration system the oil travels along with refrigerant, completing the full circle.
The following can lead to lack of lubricant in the crankcase:

- Gas velocity too small to pick-up the oil:
- Due to lack of refrigerant in the system
- Due to oversized suction line
- Compressor short cycling
- Blockage in the system

High Discharge Temperatures
Occasionally discharge temperatures in a compressor can become so high oil can lose its viscosity for proper lubrication. This results are similar to having low oil where the parts that need lubrication start to wear out and become damaged. Evidence that this problem has occurred can include:
- Discoloured valve plate which can’t be rubbed clean
- Burned discharge valve reeds
- Burned and worn pistons, rings and cylinders
- Stator spot burn from metal debris.

In general, the biggest reasons for high discharge temperature are:
- High condensing pressures
- High system superheat
- Low suction pressures
- High compression ratios

credit: http://www.e-refrigeration.com