Air Compressor Pump Issue

[MikefromMI]

My compressor will start and run fine till about 40ish pounds. Then the pump starts putting so much of a load on the motor it almost stops it. Then it releases (about 5sec later) the load and spins up to normal again. And continually cycles like that until the pressure switch shuts it off. I've never tore into a pump. Looking for advice from someone who knows what's going on. I can't see spending too much on a rebuild when there are so many compressors offered used with spent motors. I have a video I can send if someone would like to see it. Thx - Happy Thanksgiving!

 

[PILOON]

I'd suspect the pressure switch and or the unloader.
Not the pump itself.
Actually the unloader is probably the prime suspect.

The unloader's job is to relieve pressure so that the motor has no load on starts and somewhere there is also a check valve to retain the already pressure in the tank.

Also, what type compressor?
Big tank with oil based compresser head?
or small oil less direct drive (no V belt)

To divert, many motors sport start and run capacitators, Could be the run.*

Then other designs have start windings in which case there is a mechanical 'switch' inside the rear housing that kicks out once the motor has started spinning.
Crud can be a cause as welded contacts also.

* two 'cans' external to the motor would confirm this style'

 

[ray66v]

Ditto on suspecting the unloader valve.

It's usually the last thing in the line from the pump, and attached to the tank. It's where the air normally comes out, when it stops running at the end of the cycle.

If it is that type, you can remove it, and make up a bypass with fittings to see if it runs okay. Then, find a replacement.

It may also be part of the pump.

 

[goeduck]

Agree ^^

 

[Industrial Toys]

I don't see how. But as always, I could be wrong. The unloader relieves pressure, not put a restriction on your pump. If it was defective, as in stays open, the tank would build no pressure as air is going out the unloader. Also, what does the pressure switch have to do with it? It's struggling at 40psi. Pressure switch would mean not runing at all or not shutting off at set pressure.

Did you check running current of motor? Oil in crankcase?

 

[Fallon]

I don't see how. But as always, I could be wrong. The unloader relieves pressure, not put a restriction on your pump. If it was defective, as in stays open, the tank would build no pressure as air is going out the unloader. Also, what does the pressure switch have to do with it? It's struggling at 40psi. Pressure switch would mean not runing at all or not shutting off at set pressure.

Did you check running current of motor? Oil in crankcase?

I concur. The unloaded kicks in when the motor stops. It's the hiss of air you hear being released. The OP said it pumped fine until 40psi. That's way after any startup backpressure issues. The unloader not working would cause those issues as soon as the motor started.

If the motor is tripping out it's due to excessive load. Bad pressure gauge & the actual pressures are higher. Bad bearings in the pump or possibly motor. Plumbing restriction between the pressure gauge & pump causing excessive backpressure before the gauge. Something like that.

 

[PILOON]

LOL, once on a DIY compressor setup I did not have an unloader but had a check valve.
My system 'D' was to leave a fitting loose between the check valve and the compressor.
Worked just great.
Kind of hissed a tad but did the trick.

 

[MikefromMI]

Darn, I never returned back up here. Ready for this..... The **** belt was loose. I know everyone responding assumed that. I pulled the **** motor hard. No good. I put a baby bottle jack up there so I could keep the pulley tight while I tightened the motor bolts. The speed cycling was do to the belt heating up and grabbing. Then she would spin up till the belt cooled. At that point there was compression built up so she wouldn't spin up again. And... I shut it off cause I didn't want to burn up what I thought was a motor issue.... I'm embarrassed to even finish this post. Just hooking up the next dufuss.

 

[GLyford]

Been there, done that.

I also read somewhere that changing the compressor oil can also help a reluctant to restart issue, and for me it did.

 

[PILOON]

Well, Mike, we appreciate the follow up.
So many posters start a topic to never provide a conclusion.