BOSAN

Air Compressor Motor Overload or High Current: Troubleshooting Steps

Do not treat a motor-overload alarm by repeatedly resetting the overload relay. Check three-phase voltage and current, discharge pressure, inlet and separator restrictions, air-end rotation, lubrication, contactors and motor insulation to identify the electrical or mechanical load.

· BOSAN

Air Compressor Motor Overload or High Current: Troubleshooting Steps

Overload protection prevents the motor and cables from continuing to heat during overcurrent, phase loss or mechanical seizure. Resetting without diagnosis can turn a correctable filter restriction or bad connection into motor, drive or air-end damage.

Typical Symptoms

  • The unit trips immediately during starting, trips after loading, or operates for a period before the thermal overload opens.
  • The controller reports main-motor overload, overcurrent, stall or excessive motor temperature.
  • Phase currents are unbalanced, the motor hums or overheats, or contactors and cables become hot.
  • Discharge pressure or separator differential pressure is high, or the air end produces a heavy rubbing sound.

Diagnostic Sequence

  1. Record whether the event occurs during starting, loading, full load or deceleration, together with pressure, frequency, ambient temperature and current.
  2. A licensed electrician should compare the motor nameplate, overload setting and controller parameters, then measure three-phase voltage and current. Check phase loss, imbalance and voltage drop across contactor poles.
  3. Confirm that discharge pressure, unload pressure and system back pressure do not exceed the model limits. Compare the pressure transducer with a calibrated gauge.
  4. Check differential pressure across the inlet filter, inlet valve, oil filter and air/oil separator. Restriction can increase air-end load or cause incorrect control.
  5. After shutdown and depressurization, follow the manual to verify that the air end, coupling or belt drive rotates freely. Inspect bearings, belt tension, alignment and foreign-object interference.
  6. Verify lubricant type, level, condition and circulation. Insufficient lubrication, unsuitable viscosity or cold starting can increase torque demand.
  7. On VSD units, read the drive fault history and verify motor data, acceleration/deceleration time, minimum speed and cooling. Insulation and winding tests may be required.

Corrective Actions

  • Correct phase loss, voltage imbalance, poor contacts and incorrect overload settings, but never raise a setting beyond the motor and package limits.
  • Restore correct pressure settings; replace filters that have reached differential-pressure or service-life limits; repair an inlet valve that cannot fully open and a control circuit that cannot unload correctly.
  • Repair coupling, belt, bearing or air-end binding. Confirm free rotation and correct lubrication before energizing the unit.
  • After one controlled reset, monitor all phase currents unloaded and loaded. If the alarm repeats, stop resetting and perform professional diagnosis.

Stop the Compressor and Call a Qualified Technician When

  • The motor, contactor or cable produces smoke, a burnt smell, abnormal heat or insulation-test failure.
  • The air end does not rotate freely, metallic impact is heard, or current rises rapidly.
  • The same overload returns after reset or phase current remains seriously unbalanced.

Prevention

  • Trend full-load phase current, discharge pressure and separator differential pressure.
  • Service inlet, oil and separator filters according to actual environmental conditions.
  • Periodically torque power terminals and perform thermographic and motor-insulation checks.
  • Use the specified heating and warm-up procedure in cold environments to avoid starting with high-viscosity lubricant.

Important Note

The overload relay, variable-speed drive and motor thermistor may report separate faults. Identify the protection source first and use the package wiring diagram and drive manual for diagnosis.