Air compressor cycling

[StewPidass0]

Been lurking on the boards for a long time & finally registered to see if I could get some ideas on this issue.

Moved about 2 years ago & thought I didn't need my 60gal Sanborn single-stage air compressor...got a great offer on it & decided to sell so I didn't have to move it to the new house...only to regret that as since then it's been fun even trying to fill tires with my little 6gal pancake compressor (& forget about any actual tools).

Anyway, I bought a used 60gal 2-stage air compressor from a business that was closing for literally nothing (they threw it in with some lathe tooling I was buying)...was their backup compressor & looked like new (Kobalt brand but looks the same as Sanborn, Dewalt, etc - these all seem to be the smae re-branded model with CH pumps). Fired it up at their shop & let it build to 45/50psi or so & figured it was good to go. Get home, hook up permanantly to 240v 30a circuit (even though it only needs 20a) & it gets to 75/80psi & the motor cycles. I figured it was the pressure switch as that's a pretty cheap/common fix but same issue. Verified 240v at switch & motor. Verified pulling around 15a at start-up but then once pressure starts getting higher went all the way to about 29a before cycling (pic below is during the cycling it shoots up to almost 40a before cycling down to zero). The capacitors checked out, but I happened to have exact replacements for both so I replaced them anyway while I was in there. Verified the check valve is working properly. Tried bypassing the pressure swtich (just in case the new pressure switch was faulty) by just hardwiring the motor to line & same issue. Hoooked up my little pancake to it & filled to 120psi & it held for days without dropping at all, so nothing appears to be leaking.

In doing some research everything I could find was pointing to valves or gaskets, but before I crack it open to look at those (as it seems to be building pressure fine) was going to ask here for thoughts...not sure why the amperage is getting so high, unless maybe the 3.7hp motor just cant chooch enough & needs replaced (maybe with a 5hp motor)?

Not sure where to go from here...any insight is welcome, thanks all!

 

[The Cobbler]

sounds like you've ruled everything but the motor out, it sure points to a bad motor

 

[redmondjp]

sounds like you've ruled everything but the motor out, it sure points to a bad motor

Not necessarily; notice that this motor has a built-in thermal overload on it. This could be kicking in (and should be at the amperages quoted) in order to protect the motor. Now, what is causing this? It could be overloaded externally (pump taking more power to turn than it should), or the motor could be partially-shorted, resulting in it drawing excessive current. I would remove the belt and monitor the no-load running current of the motor. This should be significantly less than it's full-load current of 17.2 amps. Perhaps 4-5 amps, just taking a guess. If it's higher than this, say 10-12 amps at no load, I would concur that the motor is the culprit.

 

[Jswain]

Did you measure the voltage at the motor while under load?

I would do that, then disconnect the belt like mentioned and see what the no load amps are while running.

Spin both pulleys by hand to make sure everything is spinning freely while it's disconnected.

 

[StewPidass0]

I didn't measure voltage while under load, I'll go do that from empty to when it starts resetting.

I just pulled off the belt & motor pulls 1.45-1.53a...let it run for a while to see if that would build any heat/etc but never went higher.

I can spin the flywheel on the pump by hand there's no real resistance...I can feel when it's compressing but no binding (going that slow it's not building enough pressure to make the pressure switch kick on...I do hear puffs coming & being bled off so it's doing its thing).

 

[Lwel9226]

Did the PO change the motor pulley to a larger size??? That would put a strain on the motor.....

LynnW

 

[StewPidass0]

Did you measure the voltage at the motor while under load?

I would do that, then disconnect the belt like mentioned and see what the no load amps are while running.

Spin both pulleys by hand to make sure everything is spinning freely while it's disconnected.

You know how when something doesn't work like it should you start ruling out what it can't be? It can't be the wiring, it can't be this, it can't be that. Well, I had tested amperage on one leg only & didn't bother to test the other leg...turns out I had 120v going to the leg that I was testing & nothing on the other, so the motor was only getting half of the angry pixes that it wanted.

Cord tested good, plug tested good, outlet...hmmm...one hot leg wire had snapped right at the screw terminal. Pumped right up to 175psi & shut off like it should.

Thanks everyone for the help!

 

[Jswain]

I love when testing fixes the problem vs parts cannon! Good score, by the looks of it you have yourself a brand new compressor.