5.2 Compressor Diagnostics, Mechanical Failures, and Electrical Faults

5.2 Compressor Diagnostics, Mechanical Failures, and Electrical Faults

Quick Summary: Diagnosing hermetic and semi-hermetic compressor failures requires mastering both motor winding resistance relationships and refrigeration circuit operating pressure dynamics. Technicians must accurately distinguish between electrical faults (open windings, shorted turns, ground faults) and mechanical damage (slugging, broken valves, scroll bypass) to perform root-cause remediation and prevent recurring motor failures.

Compressor Winding Diagnostics & Electrical Testing

Single-phase hermetic compressors utilize two distinct motor windings embedded inside the welded steel housing: the Run winding (main winding, constructed of heavy gauge wire with lower resistance) and the Start winding (auxiliary winding, constructed of thinner gauge wire with higher resistance).

Terminal Identification & Resistance Formula

Single-phase compressors feature three electrical terminals on the terminal block: Common (C), Run (R), and Start (S). The mathematical relationship between winding resistances is:

$$R_{SR} = R_{CR} + R_{CS}$$

• Start-to-Run (S-to-R): Highest resistance reading (sum of both windings in series).
• Common-to-Start (C-to-S): Intermediate resistance reading.
• Common-to-Run (C-to-R): Lowest resistance reading.

Diagnostic Example: If measuring terminals on an unknown compressor yield:

• Terminal X to Y: 4.2 ohms
• Terminal Y to Z: 1.4 ohms
• Terminal X to Z: 2.8 ohms

Since 4.2 ohms (X-to-Y) equals 1.4 + 2.8, Terminal Z must be Common. Terminal Y (lowest resistance to Z) is Run. Terminal X (higher resistance to Z) is Start.

Winding Fault Identification

1. Open Winding: An ohmmeter displays infinite resistance (O.L.) across C-to-R or C-to-S. This can be caused by a melted winding conductor or an open internal line break overload protector.
2. Shorted Winding (Turn-to-Turn): Resistance measured across a winding drops significantly below nameplate resistance values due to breakdown of wire enamel insulation. This causes high running amperage and rapid thermal tripping.
3. Shorted to Ground (Ground Fault): Resistance measured between any electrical terminal (C, R, or S) and bare unpainted copper line or grounded metal casing shows continuity or a reading under 100 megohms on an insulation resistance tester (megohmmeter). Any reading below 1.0 megohm on a standard multimeter confirms a catastrophic ground fault.

Internal Line Break Overload Protector

Single-phase residential compressors contain an internal thermal-overload disk wired in series with the Common terminal. If compressor temperature exceeds ~225°F or current draw exceeds set limits, the disk warps and opens the circuit.

• Diagnostic Pitfall: Measuring resistance across C-to-R and C-to-S while the compressor is hot will show an open circuit (O.L.) on Common, mimicking a burnt open winding. Technicians must cool the compressor shell with water or airflow and wait up to 2 hours before declaring the motor permanently failed.

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Electrical Start Components & Soft Starters

Single-phase PSC and CSCR (Capacitor-Start, Capacitor-Run) compressors require auxiliary electrical components to generate initial starting torque.

Potential Relays

Potential relays feature normally closed (N.C.) contacts wired in series with the start capacitor across terminals 1 and 2. The relay coil (terminals 2 and 5) is wired across the start winding to sense Back-Electromotive Force (Back-EMF).

• Operation: As the compressor motor accelerates to ~75% to 80% of full speed, the spinning rotor generates high voltage across the start winding (Back-EMF). This pick-up voltage energizes the relay coil, opening the N.C. contacts and dropping the start capacitor out of the circuit.
• Failure Modes:
  • Welded contacts: Start capacitor remains continuously in circuit, causing start winding overheating and capacitor explosion.
  • Open coil or stuck-open contacts: Start capacitor never engages, preventing compressor starting and causing LRA tripping.

Hard Start Kits & Bleed Resistors

A hard start kit combines a high-microfarad start capacitor (typically 88-324 uF) with a potential relay. Start capacitors must always feature a 15,000 to 18,000 ohm, 2-watt resistor soldered across their terminals to bleed off stored DC charge between cycles, preventing contact arcing across the relay.

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Mechanical Failure Modes & Refrigeration Symptoms

Mechanical compressor failures severely impair heat pump and air conditioning refrigeration cycles. Isolating mechanical damage from refrigerant charge problems requires analyzing operating pressures, compressor amp draw, and compression ratio.

Compression Ratio Calculation

Compression Ratio is defined as Absolute Discharge Pressure divided by Absolute Suction Pressure:

$$\text{Compression Ratio} = \frac{\text{Discharge Pressure (psig)} + 14.7}{\text{Suction Pressure (psig)} + 14.7}$$

Normal air conditioning compression ratios range between 2.5:1 and 4.0:1. Ratios exceeding 8:1 (e.g., low suction pressure due to restriction or high head pressure due to dirty condenser) create excessive discharge temperatures and thermal oil breakdown.

Mechanical Breakdown Categories

1. Internal Valve Leakage / Scroll Bypass: Broken or warped reed valves in reciprocating compressors or worn seals in scroll compressors allow high-pressure discharge gas to leak directly back into the low-pressure suction side.
   • Symptoms: High suction pressure, low discharge pressure, low compressor amp draw (motor doing less work), low compression ratio, and very low temperature split.
2. Liquid Slugging: Entry of incompressible liquid refrigerant or oil into the compressor cylinders during startup or operation. Slugging causes instantaneous destruction of connecting rods, crankshafts, reed valves, or scroll wraps.
3. Liquid Floodback: Continuous return of liquid refrigerant during running cycles caused by low airflow or overcharging. Liquid dilutes oil in the crankcase, causing bearing lockout, metallic scoring, and crankcase burnout.
4. Thermal Oil Carbonization: Operating with discharge line temperatures exceeding 225°F (measured 6 inches from compressor outlet) breaks down polyolester (POE) or mineral oil into carbon slurry and acids, seizing bearings.