Compressor motor hums but won't start

Symptoms

• Motor buzzes or hums when switched on
• No shaft rotation or air movement
• Humming stops as soon as you switch it off
• May trip the breaker if left humming too long

Likely causes

• CommonWeak or failed start capacitorThis capacitor gives the motor its initial torque to start turning; without it working, the motor just vibrates.
• CommonMotor trying to start under tank pressureIf the pressure switch's unloader valve isn't releasing pressure from the pump head, the motor can't overcome it at startup.
• PossibleSeized or very stiff pump/motor bearingRust or lack of lubrication over time in storage can stiffen the mechanism.
• PossibleLow voltage reaching the motorA long or thin extension cord can starve the motor of the voltage it needs to start.
• Less commonFailed motor windingsMore likely on older units and needs professional diagnosis.

What to do

1. Switch the compressor off immediately if it's humming without starting; don't let it continue, as it can overheat.
2. Drain the tank fully through the petcock so the motor will start unloaded next attempt.
3. Check that you're plugged directly into a wall outlet, not a long or thin extension cord.
4. Try turning the compressor's pulley or fan by hand (unplugged) to see if it spins freely; heavy resistance suggests a seized bearing or pump.
5. If it spins freely by hand but still just hums when powered with an empty tank, the start capacitor is a likely culprit.
6. Do not repeatedly try starting a humming motor, since each attempt without success builds heat in the windings.
7. Have a technician test and replace the start capacitor or inspect the motor windings if the issue continues.

Tools and parts

• None for basic checks; multimeter for capacitor testing

When to call a pro

If the tank is empty and the pump turns freely by hand but the motor still just hums, have a technician test the start capacitor and motor windings.

Safety