# Motor won't spin when pressures drops below the pressure switch setting.



## misterand (Apr 21, 2017)

Hello,
I was given an old AJAX SRW-3 compressor as a gift from a friend. The compressor was in used 17 years ago, but since then, it has been stored in a warehouse until last week. I had to buy a new pressure switch because the old one was leaking. 
Now, it is back and almost usable but for a few drawbacks. It starts and fills the tank to about 110psi, motor stop, I drain the tank to about 50psi, where the pressure switch connects the motor again. Motor tries to start but it does not. It hums though. 
I supposed that the start capacitor (it only has one capacitor on top of the frame), was bad, but if I remove the belt, the motor does not have a problem starting as many times as I apply energy to it. Not a single failure.
I supposed that the torque on start was too high because there was some defect of the release valve (I have no idea if that is the right name, I am referring to the thin metal pipe that goes from the top of the cylinder block to the pressure switch. At the pressure switch side, it connects to a special fitting that shows a small needle inside. Such needle, as I understand release pressure from the cylinder block when the motor starts, so the load presented to the motor is not as big as it would without releasing the pressure. 
So, I removed that thin pipe leaving an opening where air escape, the tank won't get filled but the load is at a minimum. Even in that condition, the motor fails to start. (it does hums and draws a lot of current, tripping the breaker after a few seconds)
The start capacitor was measured and its value matches the value on the label (580-708 MFD). 
When the motor starts I can hear the internal switch clicking. (at the beginning and it accelerates and when it stops and starts decreasing speed)

In case it could be helpful, this URL shows a picture of a very similar compressor. Mine has the cylinder block in a 90 degrees vertical position, this one has it tilted. Other than that I don't see any difference.
URL: https://www.propertyroom.com/l/ajax-120-psi-air-compressor/10702035

After this long story, What do you think? 
What can be the faulty component? 

Thank you very much,
Andres


Disclaimer: I have no experience at all with compressors or AC motors. I apologise in advanced for the lack of knowledge. I hope that the tests made some sense for you to be able to peer into the problem. If that's not the case, please ask me anything and I will try to clarify the best I can.


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## misterand (Apr 21, 2017)

Correction:

I was wrong. When I wrote my post I was under the impression that the motor always turned on and spin if the tank w
as empty and it was the first time that I turned the motor on. 
It is not true. In reality, the problem is simpler to explain: The motor does not spin if attached to the compressor when you turn it on. Period.

It does hums but unless I help him by moving the pulley by hand it does not spin. Without load, I mean completely free, it always spins when turned on.
I also noticed that when detached from the motor the big pulley on the compressor spins pretty easily by hand. A 3HP motor should not had a problem moving that load.

Is this a problem of wrong capacitor values?
The motor is label 130/215 VAC but I don't find any wiring instructions. Does that mean that I can plug it into 220 or 115 using the same wiring it has now?
Is the motor defective? When it starts is able to fill the tank to 100-120psi without heating up too much tough.

Thank you very much


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## misterand (Apr 21, 2017)

Solved. 
Opened the motor, played a little with the centrifugal switch and put it back on. Now it works, starts and stops realiably again.
I still need to figure out how to set the pressure switch to start the motor once that the pressure drops to 85-95 psi...At the moment is turns off at 120psi but it won't start again until it drops below 60psi.


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