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compressor

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Unread postAuthor: POLAND_SPUD » Wed Apr 29, 2009 3:16 pm

I had the same discussion with jimmy sometime ago... so I am used to this :wink:

look closely - at the circled area in the pic - the piston (as I am talking about a typical fridge compressor not a scroll pump) is exposed on both sides.. but the pressure acting on the other side of it is not at atmospheric pressure (as if it was in case of a tyre pump or a shock pump) but equal to the inlet pressure...

if inlet pressure is 50 psi then to get 100 psi I need to do the same amount of work as when inlet pressure is atmospheric but I want to get 50 psi at the air output



to make this simpler to understand think how shrader valves work... their crack pressure is AFAIK close to 60 psi.... if you connect a chuck to one and provide air at 61 psi the valve would open...

but would it open if the pressure inside the tyre was 200 psi ?? no... how much more pressure would you need to do exactly the same thing which requires exatly the same amount of work ( that is overcoming the force of the spring inside it)?

well you would have to supply air at higher pressure.. at (200psi + 61 psi) 261 psi... just to do exactly the same job - that is overcome the force of the spring that's equal to 60 psi


now before you think that this proves your theory.... :wink: (acctually after writing this I thought -WTF!! is he right ?!? one would have to use more pressure and the result would be the same ?!? - LOL I had moments of doubt)
....mind you that the spring force in this example is an equivalent of load put on the motor


ok another example... here potential energy of water is used to represent pressure

there is a bucket with water standing on the ground and you have two ideantical pumps that can pump water 10 meters high... so you put the pump in the first bucket and pump water from it to a bucket that you placed 10 meters higher... now you put another pump in the second bucket in order to pump water 10 meters higher... is there more load put on the motor of the second pump than the first one ? no height difference/pressure differential is the same and there is exactly the same load put on both pumps

I really don't know how to explain it better... but it's simple...

I hope this helped
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Unread postAuthor: daberno123 » Wed Apr 29, 2009 5:43 pm

Interesting discussion guys but since I have both an A/C compressor and a dehumidifier compressor (practically the same as one out of a refrigerator). I can do some timed tests of filling the same volume to a certain pressure. I could also test how high they can pump in a certain amount of time. I have a small 1200 psi gauge I can use. That's really the only way to get a concrete answer on this.

I know for a fact that the A/C compressor can fill roughly 3 cubic inches to ~800 psi in under a minute (pretty sure, although I wasn't counting). I have not tested the dehumidifier one yet, since I just got it.

However the dehumidifier compressor seems to have a higher flow than the A/C compressor just by feeling the air coming out of the outputs. This may be a bit skewed since the A/C compressor has a slightly larger output tube.

I could hook them up to my shop compressor (100 psi) too and test how this affects fill times and total pressure achieved.

The earliest I could do these tests are this weekend, unless I can get to the hardware store earlier to get some stuff to get the dehumidifier running.
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Unread postAuthor: Technician1002 » Wed Apr 29, 2009 6:48 pm

daberno123 wrote:Interesting discussion guys but since I have both an A/C compressor and a dehumidifier compressor (practically the same as one out of a refrigerator). I can do some timed tests of filling the same volume to a certain pressure. I could also test how high they can pump in a certain amount of time. I have a small 1200 psi gauge I can use. That's really the only way to get a concrete answer on this.

I know for a fact that the A/C compressor can fill roughly 3 cubic inches to ~800 psi in under a minute (pretty sure, although I wasn't counting). I have not tested the dehumidifier one yet, since I just got it.

However the dehumidifier compressor seems to have a higher flow than the A/C compressor just by feeling the air coming out of the outputs. This may be a bit skewed since the A/C compressor has a slightly larger output tube.

I could hook them up to my shop compressor (100 psi) too and test how this affects fill times and total pressure achieved.

The earliest I could do these tests are this weekend, unless I can get to the hardware store earlier to get some stuff to get the dehumidifier running.


Excellent test. Do you have a AC clamp on Ammeter? The power draw would be good to know. Also pay attention if the motor current goes up enough and the start relay picks up again. If you miss this and it runs with the start relay engaged due to high run current, it can quickly burn out the motor. Watch for this to save the compressor.
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Unread postAuthor: daberno123 » Wed Apr 29, 2009 8:44 pm

Technician1002 wrote: Excellent test. Do you have a AC clamp on Ammeter? The power draw would be good to know. Also pay attention if the motor current goes up enough and the start relay picks up again. If you miss this and it runs with the start relay engaged due to high run current, it can quickly burn out the motor. Watch for this to save the compressor.


Could you elaborate on how I would tell if the start relay picks up again? Is there a good chance that this is happening? I don't want to kill my compressor.
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Unread postAuthor: POLAND_SPUD » Thu Apr 30, 2009 1:14 am

first of all, it will make a slighly different sound but I think that you should just measure motor current...

hmm it seems like an interesting test
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Unread postAuthor: Technician1002 » Thu Apr 30, 2009 1:48 pm

daberno123 wrote:
Technician1002 wrote: Excellent test. Do you have a AC clamp on Ammeter? The power draw would be good to know. Also pay attention if the motor current goes up enough and the start relay picks up again. If you miss this and it runs with the start relay engaged due to high run current, it can quickly burn out the motor. Watch for this to save the compressor.


Could you elaborate on how I would tell if the start relay picks up again? Is there a good chance that this is happening? I don't want to kill my compressor.


An easy way is to use a 25 foot 16 gauge extension cord. Plug in a incandescent desk lamp and the compressor. When the compressor starts the voltage will sag a little and you will notice the light dim while it starts. If suddenly dims again, it probably hit the starter again.

I didn't mention that MOST but not all starter relays have an overload thermal protector. If the compressor doesn't start (Stalled) it will over heat and click off to try again in a minute or two. These work well on a cold compressor. An overloaded and already over hot compressor may still burn out if it kicks back and the compressor overheats before the protection operates.

A short off on cycle with AC pressure still high in hot weather causes many AC units to die in hot weather. Pay attention to the compressor if it is overly hot. Hot is normal. Really hot is not.

The starter relay simply uses the motor run current to operate. When the compressor is stalled (startup zero RPM) the run current is very high, which pulls in the relay and applies power to the start winding also. After it picks up speed, the run current drops off to about 1/5 the value and the start winding is shut off. It does make a click on the mechanical ones. Some solid state ones are silent.

If the compressor is severely overloaded the increased motor run current may pick up the start circuit again. With a mechanical relay, a click is heard. On a solid state relay the hum suddenly changes a little and the lights may dim. If the overload is working, it may click off completely a second or two later.
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