Compressor fails to start — branching by hum, sustained LRA, or instant trip

Question

Accepted Answer

Branch 1 — 'Hum': power on and current flows but rotor doesn't spin. Check: Check: ① Place a hand on the shell — strong 60/50 Hz hum, no rotation. ② Clamp meter shows ~5–6× RLA sustained, the LRA level (e.g., RLA 5A → LRA 25–30A). ③ Protector trip time: single-phase 5–10 s, three-phase usually within 5 s. Candidates: a) single-phase — failed start capacitor/relay/PTC (PSC: Permanent Split Capacitor), b) any type — bearing stuck (liquid refrigerant condensed during off-time locks the rotor), c) system pressures not equalized (restart against high differential, low start torque), d) three-phase — phase loss. What to do: Steps: ① Single-phase — measure start capacitor capacitance (LCR meter, within ±10%). Nameplate 100 μF reading 70 μF → replace. Test PTC resistance (within ±20% of nominal at room temp). Remove the start relay and bench-test contacts. ② Any type — stop ≥5 min and retry (pressure equalization). If it then starts, root cause is c. ③ Still humming + LRA — bearing stuck. Tap the compressor shell gently with a soft mallet to dislodge condensed liquid (experienced techs only). If still no start, replace the compressor. ④ Three-phase — measure R-S, S-T, T-R voltages. If one phase reads 0V or differs, phase loss → check upstream breaker/contactor. ⑤ Add anti-short-cycle timer (if missing) to enforce ≥5 min between stop and restart. Branch 2 — instant trip (breaker or solid-state relay drops out immediately). Check: Check: ① Breaker trips almost immediately (<1 s) on attempt. ② Just before the trip, clamp meter shows >10× RLA or overflows (not the usual 5–6× LRA). ③ Megger (500V) — winding insulation <0.5 MΩ → insulation failure (turn-to-turn or to ground) is nearly confirmed. ④ External — soot/rust at compressor terminal area, damaged wire insulation. ⑤ Internal terminal block — feed-through insulation failure possible (winding terminal contacts the shell or external). What to do: Action: instant trip + insulation <0.5 MΩ → decide to replace the compressor and run the burnout clean-up procedure (see compressor-motor-overheat case). ① Winding shorts are not field-repairable. ② Treat acid byproducts — recover → replace compressor → blow nitrogen through liquid and suction lines → install fresh filter-drier + Acid Burnout Filter Drier → evacuate → recharge. ③ Audit upstream breaker and overload — verify protection is healthy before installing the new compressor (an external short can damage the new one too). ④ One week later, retest oil acidity and megger insulation — additional treatment if residual acid or low insulation.

Compressor fails to start — branching by hum, sustained LRA, or instant trip

Likely causes

Branch 1 — 'Hum': power on and current flows but rotor doesn't spin

Branch 2 — instant trip (breaker or solid-state relay drops out immediately)

Related cases