LIQUID SLUGGING
Slugging is 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 primarily the result of off cycle refrigerant migration on refrigerant-cooled compressors and floodback on air-cooled compressors.Symptoms:
• Broken reeds, rods, or crankshaft
• Loose or broken discharge belts
• Blown gaskets
Correction:
1.Maintain proper compressor and evaporator superheat.
2.Prevent uncontrolled liquid return (particularly oil) with accumulators.
3.Locate compressors in warm ambient or install pump down cycle.
4.Correct abnormal low load conditions.
LIQUID WASHOUT
This is a result of refrigerant washing oil off wearing surfaces or off cycle migration of saturated refrigerant into the crankcase. Compressor starts up resulting in a mass of foam which, when pumped, washes bearing surfaces clear of the oil film necessary for proper lubrication. Severe migration results in slugging.Symptoms:
• Worn rods and bearings
• Worn pistons and cylinders on lower end
• Worn crankshaft and oil pump
• Scored cover bearing and crankshaft
Correction:
1.Locate compressor in warm ambient or install pump down cycle.
2.Check crankcase heater operation.
LIQUID DILUTION
This is a result of refrigerant returning to the compressor during running cycle. Oil becomes diluted and lubrication for oil pump and end bearing may be adequate, but as it progresses down the crankshaft, insufficient oil to lubricate the rods and main bearings will occur. This may allow the rotor to drag on the stator and short out the stator.Symptoms:
• Rotor drag/shorted stator
• Worn bearings
• Scored and/or broken rods
• Scored crankshaft
• Worn oil pump
Correction:
1.Maintain proper compressor and evaporator superheat.
2.Prevent uncontrolled liquid return with accumulator if necessary.
3.Correct abnormal low load conditions.
4.Check defrost cycle.
5.Check for oversized TXV.
HIGH DISCHARGE TEMPERATURE
This is a result of temperatures in the compressor head and cylinders becoming so hot that the oil loses its ability to lubricate.Symptoms:
• Discolored valve plate (cannot rub off)
• Overheated or burned valve reeds
• Worn rings and pistons
• Worn cylinders
• Scored rods, bearings, and crankshaft
• Spot burn in stator
Correction:
1.High compression ratio: check for low suction and high discharge pressures; low load and evaporator problems.
2.Check low-pressure control setting.
3.Check for dirty condenser, inoperative condenser fan, and ambient temperature.
4.Check airflow across compressor.
LACK OF OIL
This is a result of a lack of enough oil in the crankcase to properly lubricate the running gear.Symptoms:
• Scored bearings
• Broken rods
• Scored crankshaft
• Low oil in crankcase
Correction:
1.Check oil failure switch operation.
2.Check pipe sizing and also for oil traps.
3.Check for inadequate defrost.
4.Correct abnormal low load conditions.
5.Eliminate short cycling.
ELECTRICAL
Many motors fail as a result of a mechanical or lubrication failure. Many fail due to malfunctioning external electrical components.UNIFORM BURN
Symptom:• Entire winding is uniformly overheated or burned.
Correction:
1.Check for low voltage.
2.Check for rapid cycling of compressor.
3.Check for inadequate motor cooling.
4.Check for unbalanced voltage.
SINGLE PHASE BURN
This is a result of not having current through the unburned phase and overloading the other two phases.Symptom:
• Two phases of a three phase motor are overheated or burned.
Correction:
1.Replace the contactor.
2.Check terminal connections on compressor.
3.Check for balanced voltage.
4.Check for blown fuses.
HALF WINDING SINGLE PHASE BURN
Symptom:• One half of the motor has a single phasing condition on a PART WIND MOTOR with a two contactor system.
Correction:
1.Check both contactors as one may be defective.
2.Check timer for proper time delay.
START WINDING BURN
Symptom:• 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.Check starting capacitor and/or start relay.
3.Check for compressor overloading.
RUN WINDING BURN
Symptom:• Only the run winding is burned in a single phase motor.
Correction:
1.Check relay.
2.Check run capacitors.
PRIMARY SINGLE PHASE BURN
Symptom:• Only one phase is burned; the other two will be okay.
Correction:
1.Check transformer for proper voltage incoming and outgoing.
Reprinted with permission from CMP Corp.’s Technical Reference Manual. CMP is a manufacturer of compressor replacement parts headquartered in Oklahoma City. For more information, visit www.cmpcorp.com.
Publication date:08/04/2008
