Humming means voltage is reaching the motor but it can't develop enough torque to turn. On recips this is very often caused by starting against head pressure — not a bad motor.
1
Starting under load (unloader not bleeding / check valve leaking)
medReciprocating
CheckDrain the tank to 0 psi and try to start. If it starts fine empty but not with pressure in the tank, the head isn't being unloaded.
FixRepair unloader / check valve per their flows.
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2
Failed start capacitor or centrifugal switch (single-phase)
medCheckLOTO, discharge capacitor safely, measure µF with a capacitance meter — replace if more than ~10% below rated or bulging/leaking. A stuck centrifugal switch prevents the start winding from engaging.
FixReplace capacitor; clean or replace centrifugal switch.
ToolsCapacitance meter · Insulated screwdriver
SafetyCapacitors can hold a lethal charge — discharge through a resistor before handling.
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3
Low voltage / undersized wire or extension cord
medCheckVoltage drops more than 10% when starting.
FixIncrease wire gauge, plug directly into a dedicated circuit.
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4
Seized pump
highReciprocating
CheckLOTO, remove belt guard. Try turning the flywheel by hand. If it won't move, the pump is seized (often from low oil).
FixRebuild or replace the pump.
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5
Lost a phase (3-phase)
highCheckMeasure all three line voltages and amps — one missing leg.
FixReplace blown fuse/failed breaker pole; check contactor.
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Safety
Lockout/tagout before opening enclosures. Verify zero energy. Bleed system pressure before opening any pneumatic connection.