Branch breaker trips or motor overload kicks during start or run.
1
Locked rotor / seized airend or pump
criticalCheckLockout/tagout, then bar over by hand. Should rotate freely.
FixIf seized, airend rebuild or pump replacement required. Do not attempt to free with power.
SafetyLOTO before any rotational check. Stored air pressure must be bled to 0 PSI.
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2
Single-phasing (3Φ)
highCheckMeasure all three legs at motor under load; check phase loss relay.
FixRestore lost phase; check fuses, contactor tips, terminations.
ToolsMultimeter · Clamp meter
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3
Low supply voltage
CheckVoltage at motor terminals under load — must be within ±10% of nameplate.
FixCorrect supply; check wire size and length (voltage drop). Use voltage-drop calculator.
ToolsMultimeter · Clamp meter
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4
Voltage imbalance >2%
Rotary screw
CheckMeasure L1-L2, L2-L3, L1-L3. Imbalance = (max deviation / avg) × 100.
FixHave utility/electrician balance phases. 1% imbalance → ~6–10% current imbalance.
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5
Wrong overload setting
CheckOL set to 115–125% of motor FLA on nameplate.
FixReset dial / select correct heater element.
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6
Discharge pressure above setpoint
CheckVerify pressure switch / load setpoint; verify unloader function.
FixAdjust setpoint; repair unloader.
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7
Motor winding short / ground fault
criticalCheckMegger windings to ground (>1 MΩ minimum cold, >100 MΩ on a new motor).
FixRewind or replace motor.
ToolsMegohmmeter (500V or 1000V)
SafetyDischarge windings to ground after meggering.
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8
Starting too frequently (recip)
Reciprocating
CheckCount starts/hour. Recip motors generally allow 6–10 starts/hr max.
FixAdd storage receiver; widen pressure differential.
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9
Inrush during start (recip without unloader)
Reciprocating
CheckVerify centrifugal unloader or pilot unloader releases head pressure on shutdown.
FixRepair pilot/unloader valve so the next start is unloaded.
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10
Capacitor failed (single-phase)
Reciprocating
CheckTest start/run cap with capacitance meter. Must be within ±10% of rating.
FixReplace cap with same μF and ≥ original VAC rating.
ToolsCapacitance meter
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Safety
Lockout/tagout before opening enclosures. Verify zero energy. Bleed system pressure before opening any pneumatic connection.