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Unit 6 — Refrigeration System Components
Section 1 — Compressors

1.4 — Common Compressor Failures

Compressor failures result in system downtime, expensive repairs, and potential product losses. Understanding common failure modes, their causes, and remedies enables technicians to prevent problems through proper installation and maintenance, and to diagnose issues when they occur.

Motor Windings Flood Back Flood Start Lubrication Mechanical Migration 313A / 313D

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1.4.1 — Motor Winding Failures

Motor winding failures are among the most common causes of hermetic compressor failures. They occur when insulation between windings breaks down, causing short circuits or grounds.

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Causes

  • Overheating due to inadequate cooling (low refrigerant charge, restricted airflow)
  • Voltage abnormalities (overvoltage, undervoltage, voltage imbalance)
  • Excessive cycling causing thermal stress
  • Contamination (moisture, acid, debris in system)
  • Lightning or power surge damage
  • Locked rotor conditions
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Symptoms

  • Compressor trips on overload or internal protection
  • Windings show low resistance (short) or infinite resistance (open)
  • Windings show resistance to ground (grounded)
  • Burned or discolored oil
  • Acid present in system
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Remedies & Prevention

  • Verify proper voltage at all times (within ±10% of rated)
  • Check phase voltage balance (within 2%)
  • Ensure adequate refrigerant charge for motor cooling
  • Install surge protection devices
  • Minimize short cycling through proper controls
  • Maintain system cleanliness and dryness

1.4.2 — Contactor and Relay Failures

Contactors and relays that control compressor operation can fail from welded contacts, coil burnout, or mechanical wear.

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Causes

  • Excessive cycling
  • Voltage abnormalities
  • Arc damage from inadequately sized contacts
  • Dirt and contamination
  • Vibration
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Prevention

  • Use properly rated contactors
  • Maintain clean electrical enclosures
  • Verify proper voltage
  • Replace contactors at recommended intervals

1.4.3 — Flood Back (Liquid Flooding)

Flood back occurs when liquid refrigerant returns to the compressor during operation. Liquid refrigerant cannot be compressed and causes immediate damage to compressor components.

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Causes

  • Expansion valve overfeeding (stuck open, oversized, improper superheat setting)
  • Loss of evaporator load (fan failure, blocked coils, sudden load reduction)
  • Excessive refrigerant charge
  • Oil logging in evaporator returning suddenly
  • Defrost cycle returning liquid to an operating compressor
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Symptoms

  • Low or zero suction superheat
  • Frosted compressor crankcase
  • Low suction pressure with normal or high discharge pressure
  • Knocking or banging sounds from compressor
  • Broken valves, pistons, or connecting rods (post-failure)
  • Diluted oil (refrigerant–oil mixture)
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Remedies & Prevention

  • Maintain proper suction superheat (10–20°F / 5.5–11°C)
  • Size expansion valves properly for the application
  • Install suction accumulators on heat pumps and systems prone to flooding
  • Ensure proper evaporator loading
  • Use crankcase heaters to prevent liquid refrigerant accumulation during off cycles
  • Install suction line filter-driers to protect the compressor from liquid slugs

1.4.4 — Flood Start (Liquid Slugging)

Flood start occurs when liquid refrigerant accumulated in the compressor crankcase during the off cycle causes damage at startup. During off periods, refrigerant migrates to the coldest point in the system — often the compressor crankcase.

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Causes

  • Refrigerant migration during off cycle
  • Crankcase temperature lower than other system components
  • Long off cycles in cold ambient conditions
  • No crankcase heater, or heater failure
  • Excessive refrigerant charge
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Symptoms

  • High-pitched squeal or hammering at startup
  • Oil foaming visible in sight glass
  • Low oil pressure after startup
  • Compressor trips on overload shortly after starting
  • Broken valves, rods, and pistons (post-failure)
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Remedies & Prevention

  • Install and verify operation of crankcase heaters
  • Keep crankcase heater energized during all off cycles
  • Install pump-down solenoid to remove refrigerant from the low side before shutdown
  • Use hard shut-off expansion valves or solenoid valves to prevent refrigerant bleed-through
  • Allow crankcase heater warm-up time (2–4 hours minimum after power restoration)
  • Install non-bleed thermostatic expansion valves

1.4.5 — Lubrication Failures

Inadequate lubrication causes excessive wear, overheating, and eventual mechanical failure of compressor components.

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Causes

  • Low oil level (inadequate charge, leaks, poor oil return from system)
  • Oil viscosity problems (wrong oil type, excessive refrigerant dilution, overheating)
  • Oil pump failure or restriction
  • Clogged oil passages or filters
  • Oil foaming from refrigerant boilout
  • Wrong oil type for the refrigerant
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Symptoms

  • Low oil pressure (below manufacturer specifications)
  • Low or no oil visible in sight glass
  • High operating temperatures
  • Metallic particles in oil
  • Unusual noise (bearing rumble, knocking)
  • Seized compressor (post-failure)
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Remedies & Prevention

  • Monitor oil level and add the correct oil type as needed
  • Verify proper oil pressure (typically 25–60 psid above crankcase pressure)
  • Install oil pressure safety controls
  • Ensure proper piping design for oil return from the system
  • Use the correct oil type for the refrigerant in use
  • Use crankcase heaters to prevent refrigerant dilution of the oil charge
  • Perform regular oil analysis to detect contamination trends before failure

1.4.6 — Mechanical Failures

Mechanical failures involve physical breakdown of compressor components from wear, stress, manufacturing defects, or external causes such as liquid slugging and contamination.

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Valve Failures

Suction and discharge valves are subject to high-cycle fatigue and can break, warp, or develop internal leaks.

Causes: Liquid slugging, foreign material, fatigue, overheating, excessive pressure differential.

Prevention: Maintain proper superheat, install suction strainers, prevent liquid slugging, avoid excessive compression ratios.

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Bearing Failures

Bearings support all rotating components and fail when lubrication is inadequate or loads are excessive.

Causes: Lack of lubrication, contaminated oil, misalignment, excessive loads, liquid slugging.

Prevention: Maintain proper oil level and quality, avoid liquid slugging, ensure proper mounting and alignment.

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Piston & Cylinder Wear

Excessive wear between pistons and cylinders reduces volumetric efficiency and eventually causes failure.

Causes: Lack of lubrication, contaminated oil, excessive operating temperatures, foreign material.

Prevention: Maintain proper lubrication, install suction strainers, prevent overheating.

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Shaft Seal Failures

Shaft seals in open-drive compressors prevent refrigerant leakage where the drive shaft exits the compressor housing.

Causes: Wear, age, misalignment, lack of lubrication, contamination, excessive system pressure.

Prevention: Maintain proper oil level, verify shaft alignment, replace seals at manufacturer-recommended intervals.

1.4.7 — Off-Cycle Migration

Off-cycle migration occurs when refrigerant moves to the compressor crankcase during system off periods. This dilutes the oil, reducing its lubricating properties and causing foam-out at startup.

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Causes

  • Temperature differential (compressor colder than other system areas)
  • No crankcase heater, or heater malfunction
  • Expansion valve leakage (bleed-type or worn seat)
  • Long off cycles
  • Excessive refrigerant charge
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Symptoms

  • Oil level appears high (refrigerant dissolved in oil)
  • Foaming oil at startup
  • Low oil pressure after startup
  • Frosted crankcase on startup
  • Recurring lubrication-related failures
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Remedies & Prevention

  • Install and maintain crankcase heaters (sized per manufacturer recommendations)
  • Keep crankcase heater energized continuously — not just during off cycles
  • Use pump-down control sequence to evacuate refrigerant from the evaporator before shutdown
  • Install a liquid line solenoid valve to enable pump-down operation
  • Use non-bleed thermostatic expansion valves
  • Allow adequate warm-up time after power restoration before starting the compressor
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