Hermetic Compressor
The pump at the heart of the refrigeration circuit, common types, and how to evaluate one before condemning it.
Safety
What it is
The compressor raises the pressure of refrigerant vapor so it will condense at a temperature above the outdoor air, and it circulates refrigerant through the system. In a hermetic compressor the motor and pump share a sealed shell cooled by returning suction gas.
| Type | Where used | Notes |
|---|---|---|
| Reciprocating | Older residential and light commercial | Pistons and valves, tolerant of a wide range of conditions |
| Scroll | Most current residential equipment | Two scrolls, quieter and more efficient, sensitive to reverse rotation on three phase |
| Rotary | Small systems, ductless units | Compact, low tolerance for liquid return |
| Variable speed inverter | Modern high efficiency systems | Driven by a board, capacity modulates with load |
Why it matters
A compressor is the most expensive component in the system, and most compressor failures are caused by something else. Liquid floodback, chronic overheating from low airflow or a dirty condenser, acid from a burnout, or repeated hard starting all kill compressors. Replacing one without finding the cause usually means replacing another.
Diagnose the system, not just the part
How it works
Compressor: Low pressure vapor is compressed into hot, high pressure discharge gas.
Suction vapor enters the shell, cools the motor windings, then is compressed and discharged. Oil circulates with the refrigerant and returns through properly sized and sloped suction lines. Internal overload protection opens on excessive current or shell temperature and resets after cooling, which is why a unit may restart on its own after a trip.
How to diagnose it
Measure voltage at the compressor terminals while it attempts to start
Sag below the manufacturer minimum points to supply, wiring or contactor problems
Clamp run current and compare to rated load amps
Locked rotor current with no start means a starting or mechanical problem
Measure winding resistance common to start and common to run with power off
Open windings or a short to the shell confirm electrical failure
Compare suction and discharge pressures after a run
A compressor that cannot build a pressure difference has internal valve or scroll damage
Check the run capacitor and contactor before condemning
Both are far more common failures and present the same way
Inspect oil and check for acid if a burnout is suspected
Determines the cleanup procedure required on replacement
High voltage and high pressure
Tools involved
- Multimeter and megohmmeter where appropriate
- Clamp ammeter with inrush
- Digital manifold
- Recovery machine and cylinder
- Acid test kit
Symptoms
Practice & Mastery
- Hermetic Compressor Troubleshooting Scenarios2
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