Motor wont start/work, what's wrong?

[Badasssapper67]

Sanborn 60 gal air compressor with a Century "5 hp spl" motor I don't know what 5 hp spl means but I bet it means it's not really 5hp.
I have replaced the start switch, made sure the unloader valve is working, replaced the start capacitor, taken the belt off and spun it by hand and it free wheels nicely but does make a grinding sound.
When I apply power (240) it makes that "Hum of death" It still spins freely but wont start.
What has me scratching my head is replacing the motor, do I need a real 5 hp motor to replace or do I really need a 2hp motor?

 

[rsanter]

Start capacitor?
Remove the belt, get the motor spinning the correct direction.
Apply power. If it runs fine then.....you have a bad capacitor

Bob

 

[MTW]

Your motor is a "special" SPL designed for compressor use. Typically they have about a 2HP run winding and a 5HP start winding. You can find them aftermarket with the same rating and frame sizes.

From what it sounds like, you may have a bad centrifugal start switch inside the motor. You said you replaced the start switch, but I think you meant the pressure switch. If the motor hums when applying power the pressure switch is getting power to the motor.

The motor has 2 windings, start and run. The run winding is always energized while their is power to the motor. The start winding is energized through the start capacitor and centrifigual start switch, only until the motor gets up to speed. At which time the centrifugal switch cuts off power to the capacitor and start winding.

You said that you replaced the capacitor already. That only leaves the centrifugal start switch or the winding itself remaining as the problem. You mentioned a grinding noise when turning by hand, this is indicative of a worn out centrifugal start switch.

I would remove the motor and open up the rear endbell to inspect the switch and centrifugal mechanism.

If you want to just go the replacement route, get another "SPL" with the same frame, RPM and shaft size. A normal "real" 2HP will lack the starting torque necessary, and a real 5HP will require a larger circuit and a new pulley for a larger shaft, it's larger size may not even fit on your tank baseplate.

MTW Ω

 

[Badasssapper67]

Awesome. Thanks for the great advice MTW. Yeah, I meant the pressure switch.