# An odd issue to a new compressor owner



## KenMathisHD

Hi guys,

I just recently bought my first air compressor, it's a 10 gallon 2.5Hp 125psi Electric Oil Lubed Compressor by Central Pneumatic. I've had the darn thing for maybe a bit more than a week, and it has come up with this weird little issue. At first startup it works fine, get's up to 125psi no problem and cuts off. Everything is fine until it reaches the cut-in point (about 80-90psi), it kicks on and runs good for about a second, then it starts sounding like it's struggling for about 3-5 seconds, and it cuts off. The pressure moves up about 1-2psi through this, and it won't kick on by itself again. Sometimes it throws the reset switch but not always. If I drain the whole tank and turn it off and on again it fills up fine, which sounds like the unloader valve from what I'm reading. The part I'm not sure about is this: when it starts struggling, if I put my hand over the intake almost immediately and cover it completely, it picks up strong and fills up the tank like nothing happened until it goes to cut-in on its own again. I have the warranty on it so I could go and get it replaced, but the nearest Harbor Freight is about 30-40 miles from me and I'd rather not sit in traffic for an hour just to get a replacement if I can pull it apart and fix it relatively easily. I don't see a warranty void if sticker removed/broken/whatever thing on it, so I don't mind pulling it apart if I can fix it.

Thanks!
~Ken


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## Osviur

Hi,

It seems to be the starter capacitor giving a weak starting torque, once at full RPM the motor has enough torque to overcome its load.

Regards.


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## KenMathisHD

Ah ok, so when I cover the intake does it create enough of a vacuum to help the motor overcome the initial torque loss? Or is it because the compressor is trying to compress a small, contained amount of air instead of pulling in air to do so? Or how does that work?


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## KenMathisHD

Additionally, is there a way I can test for the faulty capacitor to confirm before either replacing the cap or exchanging the compressor?


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## Osviur

Hi,

The compressor's work in to compress a fixed air amount in each stroke, this represents a mechanical load or resistance to rotate freely. This resistance is transmitted though the compressor shaft to the motor, which must have enough torque to overcome this resistance.

In the small compressors (up to 5 HP) the induction motor 1 phase is normally used. This motor needs a capacitor and an auxiliary winding to define the rotation direction and the initial torque to start. As the rotation speed increases the motor develops its own torque in the main or working winding. At a determined RPM (80-90% full speed)a centrifugal switch disconnects the auxiliary circuit (cap and start coil) and the working winding assumes the total load). This starting process, takes a few seconds (2-3), taking a very high current. If for any reason the starting cycle can not be completed, the overcurrent protection circuit (thermomagnetic or fuse) will cut energy, stopping the motor to avoid its destruction. There are too induction motors that maintain the cap connected all the time. 

In the case of a failing capacitor the starting torque will not be enough to overcome the compressor resistance and the starting cycle will not be completed. 

Closing the air inlet, there are not air to compress so there is not work to do, apart to overcome the friction losses which are a little fraction compared with the compressing work. In this way is easier to start the motor.

To define if the capacitor is the responsible or not, the simpler way is to replace it. Unless you have a multimeter with a capacitor measure capability .

I hope this note help you. Regards.


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## KenMathisHD

I wish the 20 or so sites I’ve been to could explain it as well as this. I’ll crack it open and see if I can get a model number on the capacitor, am I right to assume that these are typically too big to be soldered to something and probably have their own connector, or are they more likely to be a part of something that itself would have to be replaced?


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## motormonkey

> ...Everything is fine until it reaches the cut-in point (about 80-90psi), it kicks on and runs good for about a second, then it starts sounding like it's struggling for about 3-5 seconds, and it cuts off. The pressure moves up about 1-2psi through this, and it won't kick on by itself again. ...



This a bit vague to me. When it kicks back on, does it indeed come up to full speed, or does it just make a couple of turns and then start struggling? I ask because this could be a symptom of the unloader not functioning.


When the pressure switch kicks off at high pressure, it opens an unloader valve which releases the air pressure on the outlet of the compressor. A check valve downstream from the unloader prevents air from the tank from coming back down the line. With no high pressure on the outlet of the compressor, the compressor motor doesn't have to start up with such a large load on it. If there is still high pressure on the outlet of the compressor when it starts up, the motor may not have enough torque to get up to operational speed.


It's easy enough to tell if the unloader is working. Listen to the compressor when it kicks off. You should hear a momentary hiss of air as the unloader releases the air on the compressor outlet.


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## KenMathisHD

It runs at full speed for about a second, then starts struggling/bogging for 3-5 seconds (motor runs at an audibly reduced speed, the whole thing rocks back and forth while it does though i figure it’s because it’s running so slowly), then it shuts off. I’ll fill it up when I get home and see if I can hear it hiss when it reaches cutoff. It doesn’t reach cutoff when it kicks in with pressure in the tank (that’s when it will struggle/bog after kicking on) unless I put my finger over the intake. It’s kicked in before and didn’t have issues filling it up with pressure in the tank for the first couple days I had it, so I’m thinking the motor has enough torque to overcome the pressure, but something is interfering with keeping it going.


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## KenMathisHD

Sorry, runs at what sounds like full speed for about a second.


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## StevenHill

On the tank of it is there a check valve? Sometimes the non return valve on the tank passes back air into the pump, this means the unit is trying to start against the pressure what’s in its tank and it cuts it out due to pulling to many amps


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## KenMathisHD

I think so? I know there’s some sort of release valve on the very top of the motor, there’s a valve at the pressure gauge that has a little ring attached to it that looks like something you’d put keys on (I think it’s an emergency release valve in case the pressure gets too high), and then there’s this kinda bulbous three way fitting that connects to a hose on the motor, the tank, and the box that has the on/off switch for the motor. It looks bulbous enough to be a check valve.


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## StevenHill

Yes the second picture with the ring on it is the safety valve in case the pressure gets too high. The 3rd picture is the check valve that comes from the unit into the tank and out to the pressure switch, once the machine stops And is up to pressure crack the pipe going to the pump. If there’s still pressure your check valve is passing back, if this is the problem which it sounds like you can open it up and give it a clean there might be a bit of debris stoping it from sealing


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## StevenHill

Obviously if your going to open up the check valve depressurise the full system


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## KenMathisHD

But cracking it can still be done while the system is pressurized yea? I figure it kind of has to be but just want to make sure I don't accidentally blow myself or my compressor up :tango_face_devil:

If it's dirty, any recommendations on what to clean it with?


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## motormonkey

It is a bad idea to loosen any fitting on a pressurized vessel... even a little.


If the check valve into the tank were leaking, air from the tank would be released into the open unloader valve. Tank pressure would drop and you'd hear it.


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## KenMathisHD

Would the pressure drop a little or would it eventually empty itself? The tank pressure stays constant, doesn't drop unless I use it


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## motormonkey

If the check valve is leaking, pressure from within the tank would back feed to the compressor and the unloader. The unloader should only be open when the pressure switch is in high pressure cutoff. If the check valve was leaking, then, yes, the pressure would leak through the open unloader... until the compressor kicked on again at low pressure cut-on. If the pressure is not dropping, then the check valve is likely okay.



You never did say if you heard the unloader working when the compressor kicks off at high pressure.


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## KenMathisHD

motormonkey said:


> You never did say if you heard the unloader working when the compressor kicks off at high pressure.


Sorry about that, yea you can hear the unloader when it reaches cut-off pressure, a quick hiss of air.


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## stevon

KenMathisHD,

On my Sanborn compressor and turning off the power (before pressure switch) I have to manually push the release pin on the unloader release valve to unload the head.

With air in the tank I would push the pin on the unloader release valve to see:

1. verify check valve is working and doesn't leak
2. check the function of the release valve
3. and after top psi is reached pressure switch trips the release valve

all free tests without parts

Stephen


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