# Compressor bogs down



## kcabpilot (3 mo ago)

Generic 30 gal Kobalt belt driven 3 hp 2 cylinder V-type pump 120VAC

Compressor starts and runs normal until it gets to around 40-50 psi then it slowly starts to bog down to where it's barely turning. I can hear the belt begin to squeak so it seems the motor is still trying to turn the pump. This goes on for 10 or 20 seconds then it slowly begins to gain speed until it is running normal again and will continue to do so all the way up to max pressure where it automatically shuts off.

So I think it's a pump problem but can't figure out why it does it around 50 psi but then has no problem going up to 125 psi after getting over that "hump"??? I've been using it like this for months, it never pops the breaker.

EDIT: Correction - motor is 1.5 hp


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## stevon (Mar 5, 2019)

kcabpilot,

Sounds like the check valve might be partially clogged. I would replace it and the air intake filter. If you had a side by side two cylinder head I would suspect the head gasket leaking in between the two cylinders but that is not for your case with the V-twin setup. What are the amps being drawn when the compressor bogs down? a cheap ammeter could tell you a lot. Does the electric motor overheat? pictures might help.









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Stephen


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## stevon (Mar 5, 2019)

kcabpilot, 

There is no such thing as a home use "3 HP 120 volt" AC motor. To produce an actual 3 HP requires at least 30 amps or more at 120 volt. Also the plug and outlet would have to be rated at 40 amps or more and wire would have be 8 gauge or at least 10 gauge but even that would overheat. A lot of sellers market a tool with "develops peak 3 HP" with a 120 volt power cord but the most you can get from that cord and the outlet is 15 amps for an hour or less which works out to an actual 1.5 HP output. You can pull 3 HP from a 120 volt circuit for a second or two until the house breaker or the motor thermal overload trips. So the marketing statement and labeling statements of "develops 3 HP" is not illegal to state or mark on the product. If you read the markings on the motor the FLA amps or full load amps tells you what actual HP is produced by that motor under full load. So if your motor states FLA= 15 amps @120 volt you really have a 1.5 HP motor at best. This is where the ammeter comes in handy and tells the real story

Stephen


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## kcabpilot (3 mo ago)

Stephen you are correct I mistyped. It is a 1.5 hp motor. I don't have a clamp-on meter and normally would not buy a cheap one from HF but for 15 bucks it might be worth it although it's plugged into a 20 amp circuit and doesn't trip the breaker, nor does it trip the breaker on the motor so I'm guessing it's not going over 15 amps. The motor never stops but it comes close to it for 5 or 10 seconds. When it does I can hear it trying to slip on the belt.


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## stevon (Mar 5, 2019)

kcabpilot,

I have seen funny things happen with an old belt that was age hardened and has polished sides. Is your belt in good condition and properly tensioned? A clamp on ammeter could tell you under load how much power or HP you are drawing under load and especially when the motor is bogging down. The amps will reach FLA rating of the motor if there is unusual resistance or you have reached the top air pressure that the compressor was designed for. Does the pump turn or "roll" by hand easily with no air in the tank? Did you take the check valve out and test it? I would just change it out myself. It is a maintenance part like an air filter or the oil that needs to be periodically changed. Check valves are relatively cheap as they often fail and thousands of them are made each year to meet the demand. When they leak the motor struggles against the air tank's back pressure to the pump. A bad check valve sign usually is the unloader valve is constantly venting or "hissing" instead of a short release after the electric motor is shut off by the pressure regulator. Your current load should go something like this: Power on, 0 rpm, LRA start at 50 amps then turning, 10 amps @ 0 psi pumping air gradually increasing to FLA 15 amps @ 135-150 psi at the top air pressure.
I would start with basic maintenance, replace the belt, check valve, oil and with an ammeter observe operation to point to problems such as the start centrifugal switch, bad capacitors, motor windings, etc.

Stephen


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