Author Topic: Air brush compressor.  (Read 9262 times)

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Offline Ian.M

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Re: Air brush compressor.
« Reply #25 on: August 29, 2018, 07:33:39 pm »
Many welders wont do it because they don't want to be accessories to the theft of a leased cylinder, or they aren't certified to weld on pressure vessels, and the safety argument lets them reject the job easily.  However there's no need to weld on it. You can drill and tap the base collar and/or the valve guard collar without any adverse consequences to the tank integrity, and leaving the tank body 'stock' permits easy future replacement.   The only downside is the inverted 'Christmas tree' I described above that you'll need to build to replace the valve to get by with only one port, and the need to build a stout base attached to the valve guard collar to accommodate the height of the 'Christmas tree' and support the inverted tank, possibly by cradling its shoulders if you aren't happy with the valve guard collar total weld length and diameter.

Also, do strap it down so 'Christmas tree' failure cant lead to 'rocket mode'!  :scared:
« Last Edit: August 29, 2018, 07:40:15 pm by Ian.M »
 
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Offline sureshotTopic starter

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Re: Air brush compressor.
« Reply #26 on: August 29, 2018, 09:42:00 pm »
Rocket mode lol  :-DD that inverted way would work ok I think, as long as the inlet was further down the single tube insert to avoid pulsing on the output air. Since I've added PTC relay in the search words its opened up a lot more. There's a guy on YouTube that shorts the start winding briefly then removes it and the compressor runs. I know this is unsafe, but reading on electromagnet and other stuff throws up many ideas. I know a relay is that, but something differ that could incorporate a timer or microcontroller, Arduino although I've never used one in my life. It's all on that PTC disconnect, but replace that, but factor in some thermal overload for safety. But I'm starting to think something purpose built for control of that start winding is possible. Yes I agree machining can overcome any welding, cut threads etc.
 

Offline coppercone2

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Re: Air brush compressor.
« Reply #27 on: August 29, 2018, 09:44:30 pm »
http://www.cagi.org/pdfs/tap106.pdf

Painting
systems use compressed air to atomize and spray paint onto surfaces.  Trace amounts of oil and water can create fish-eye craters in the finished surface, and moisture can affect the paint’s surface tension and texture.  Small  industrial  paint  booths  normally  use  air  dried  to  40°F  dew  point,  while  large-scale  automotive  paint  operations call for at least -40°F dew point air


Generally  speaking,  air  used  for  critical  industrial  applications  is  required  to  have  a  moisture  content  below  -100°  F  dew  point,  less  than  1  ppm  carbon  dioxide,  and  less  than  0.003  ppm  hydrocarbon  vapor.    In  less  critical  areas, -40°F dew point air is sufficient, and for even less critical applications, +40°F dew point is acceptable.

Page 26 has recommendations for spray painting
http://teamtechnical.com/html/Parker%20Basics%20of%20Coalescing.pdf




https://static1.squarespace.com/static/5141e2dee4b00c2dc63d1b79/t/52d437f1e4b04774445785af/1389639665069/Ind+Binder+2012.pdf

Industrial  grade painting: (non automotive grade)

Two stage unit with patented "Inverse Flow ™" and "Depth Filtration" technology. The 1st stage is
composed of 4 layers of filtration material to include activated carbon for hydrocarbon removal to 0.03
ppm. The 2nd stage is a 0.01 micron "Grade 1" coalescer


DRAD-25-PAM
Production spray painting application was having problems with contamination causing
peeling and poor color finish reducing the quality of their finished products. Reading
Technologies, Inc. DRAD system removed all contamination from the compressed airline
resulting in a quality finished product free of defects. This is a desiccated unit that
requires the first state prefilter to be changed every 4- 6 months, and the 2nd stage
coalescer to be changed every 6-9 months.



Might not be as easy as you think to get good results. especially with a piston compressor. I think those are meant to attach to screw compressors which are pretty clean to begin with.

However, on the bright side, powder coating can tolerate alot more crap then paint can IIRC
« Last Edit: August 29, 2018, 09:59:32 pm by coppercone2 »
 
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Offline Ian.M

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Re: Air brush compressor.
« Reply #28 on: August 29, 2018, 11:27:12 pm »
Rocket mode lol  :-DD that inverted way would work ok I think, as long as the inlet was further down the single tube insert to avoid pulsing on the output air. Since I've added PTC relay in the search words its opened up a lot more. There's a guy on YouTube that shorts the start winding briefly then removes it and the compressor runs. I know this is unsafe, but reading on electromagnet and other stuff throws up many ideas. I know a relay is that, but something differ that could incorporate a timer or microcontroller, Arduino although I've never used one in my life. It's all on that PTC disconnect, but replace that, but factor in some thermal overload for safety. But I'm starting to think something purpose built for control of that start winding is possible. Yes I agree machining can overcome any welding, cut threads etc.
I said to build the 'Christmas tree' with a coaxial 'stinger' tube to draw the output air from near the (new) top of the tank to avoid much of the oil mist and pressure pulsations.

All the PTC thermistor does is gradually disconnect the start winding when it gets hot a second or two after power is appied.   All you need to do to start it is to power the start winding via a momentary switch that you hold down at main power switch-on and release a second later, though a RC snubber across the winding would be a good idea to damp the inductive spike when its released.  If its a higher HP one with an external start capacitor, you'd need to put the cap in series with the momentary switch.

Add an isolated hall current sensor for main winding current, two isolated mains relays (ideally solid state ones) for master power and start winding control and some way of tripping a MCB if the controller detects a fault (e.g. a kettle element and another solid state relay), and a temperature sensor aluminum foil taped to the housing, and a hacked wallwart to power the Arduino, and you can fairly easily do an Arduino based controller.   Add a 5V analog output pressure sensor and a LCD and keypad shield and a solonoid controlled unloader valve and blow-down valves on the main tank drain and oil trap and you can get as fancy as you like with features that would normally be found on a high-end commercial compressor.
« Last Edit: August 29, 2018, 11:29:48 pm by Ian.M »
 
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Offline sureshotTopic starter

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Re: Air brush compressor.
« Reply #29 on: August 30, 2018, 12:39:59 am »
I've problems visualising that level of complexity. I'm not sure I could pull it off, I'd have to do a crash course in microcontrollers. Not to mention software programming, of which I've done neither. At the moment I'm still thinking it through, being in an apartment means I've got to keep noise levels down. I'm tempted to buy an ex dental compressor unit, cheers for the tip on that who ever mentioned that. If I can't find a reasonably simple solution to avoid start tripping being a problem, I've got two alternatives. First is a low noise oil free piston compressor coupled with a  large receiver, not ruled out diaphragm compressors either. The low cost airbrush compressor mounted on a 50 litre air tank. Or splash out on a second user dental type compressor. The former would serve well I think, I know you can buy them with 3 or 4 litre receivers on them, but that's just to small in volume for two people using at the same time. The hobby compressor on a large air receiver is straight forward and quick, also reasonably cheap. The later might be an impulse buy. But I'm still thinking it over on how to overcome the frequent stop start issues of hematically sealed compressors. Paint contamination is of little concern, I can't see it being a problem. Years ago I worked in the holiday bungalow / timber out buildings, firing thousands of nails an hour daily. I know how quick that air can be consumed, and that was a massive screw compressor. The fridge compressor has a low volume output, but it has silence and excellent service life on it's side. The hobby compressors are all China stuff with cheap an cheerful everything, with bearings that normally bite the dust first. It's only that quite and long service life that has still got my interest in the fridge type hematically sealed compressors.
 

Offline Ian.M

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Re: Air brush compressor.
« Reply #30 on: August 30, 2018, 02:29:02 am »
Well, you *can* do the 'programming' with your soldering iron by building and old-skool analog logic + sequencing circuit using OPAMPs, and/or comparators, diodes, transistors and 555 timers with a liberal sprinkling of pots/presets and passives, and, just maybe back in 1970 that would have been the smart way to do it except RCA had introduced CD4000 series logic a couple of years earlier so the smart designer would have dumped as much of the analog hairball as possible and done it in logic, only keeping comparators and pots or presets for over-current, at-speed (by motor current), over-temperature and pressure, and transistors for relay driving.

Its worth noting that the board full of chips approach has been obsolete even for hobbyists since the mid 90's as Microchip introduced the first electrically erasable and reprogrammable MCU, the PIC16C84 family back in 1993 and it was possible to bit-bang its programming protocol over a PC serial port with an interface built of very few cheap discrete components.

However if you haven't got the programming skills yet, an air compressor is *NOT* a good place to start, so just maybe you should build it really really old-skool, with relay logic and time delay relays so it does what you expect when you expect, or simply bite the bullet and get a small off-the shelf compressor rated for enough continuous CFM for your needs.

A big external tank plumbed as an 'air-pig' before the regulator gives you a lot longer before the compressor cuts back in, and lets you run high CFM tools for a few minutes far in excess of hat your compressor can supply continuously, but the down-side is the extended runtime required to get it up to pressure in the first place which will strain smaller compressors not rated for continuous duty.  TANSTAAFL applies.
« Last Edit: August 30, 2018, 02:34:35 am by Ian.M »
 
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Offline sureshotTopic starter

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Re: Air brush compressor.
« Reply #31 on: August 30, 2018, 10:36:01 am »
I would go simple relay / transistors timers etc or Auduino and take that idea up, only because its now in real time. The old analogue is really interesting to watch ( Dave's videos) but trying to do up to date technology. I've no idea how long it would take me to learn programing and the Auduino system. In the past with projects I've gone weeks thinking it through for a solution,  it usually comes eventually. Some times its right under my nose and i didn't even realise it lol. This is a hobby I'd like to get into sooner rather than later, i have a bit more time to kick ideas on these hematically sealed compressors about. What i don't get is the number of YouTube videos on diy airbrush compressor builds with fridge compressors, but no one seems to have hit the load trip out problem of frequent stop starts. Unless they just live with it, or wait for the unit to cool recover, and it starts back up. VFD looked the most possible way forward, but finding single phase units that put out single and not 3 phase is a problem. Sharing the work load between two compressors might be an option, as each has time to cool and recover switching alternatively. Coupled with a receiver that also had enough capacity to avoid frequent stop starts as well. The low pressure of airbrush working pressure is a plus in terms of less work load. And feed rate in no where near that of work shop air tools, that has to make duty cycle some what easier on hardware. Thank you for the ideas and suggestions.
 

Offline sureshotTopic starter

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Re: Air brush compressor.
« Reply #32 on: August 30, 2018, 11:31:19 am »
I might have hit on a good idea, as long as the air receiver is large enough to allow off times of the compressor..
With regard to cooling, using a fan underneath the compressor to cool the oil anyway, that has to be a good thing. So when the compressor stops, use a relay and fan to speed up cooling of the PTC. As when the compressor stops the PTC is still hot, start winding not avaliable for use. Force cool the PTC during the compressor off period, by the time the demand for air is needed, the start winding is avaliable for start up of the compressor. Careful choice of an air receiver allowing enough compressor off time to not overload any windings. From what I've read and seen, its that PTC being hot that won't allow early restart. As when it trys with a hot PTC that trips the overload protection. The only barrier is the PTC relay casing which would slow any PTC cooling. Maybe modify the PTC relay case to allow fast cooling when the compressor turns off. Something like a small 40mm axial fan, pushing air over the PTC whilst the compressor is in the off state. Might be an idea..
« Last Edit: August 30, 2018, 11:33:16 am by sureshot »
 

Offline Ian.M

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Re: Air brush compressor.
« Reply #33 on: August 30, 2018, 12:58:32 pm »
I don't think the cooling and stop/start problem is anywhere near as bad as you think as long as you've either got something better than the PTC thermistor starter or have a time delay relay in series with it set for a few seconds delay so it can cool back down while the motor is running. With an unloader valve + noise suppressor on its exhaust so you don't make all the neighbourhood dogs howl with your inadvertent ultrasonic whistle, + another timer relay controlling main power to the motor set to allow the motor to run on about five minutes after the unloader opens, at most demand levels it wont short cycle, and if it does, the PTC and the start winding are already pretty cool as it was over five minutes since they were last used, and the motor and compressor head are fairly cool as they've been pumping ambient temperature air through at no pressure differential for the last five minutes.

It would be a good idea to stick a manual reset KLIXON or similar high temperature thermal cutout to the compressor housing and put it in series with incoming power.  Adding a muffin fan to cool the compressor is probably only nessercery if you are going to run the compressor flat out for extended periods.  If you do add one, the easiest option would be a mains one controlled by a strap-on tank thermostat on the compressor housing, so you can tweak it not to run when demand is light.

----------------------------------

As for Arduino, get a UNO, a pair of good sized breadboards (or one large multi-panel one) and either a pack of jumper wires with Dupont pin ends, or a 40 pin turned pin wirewrap socket you can cut up to make adapters for the Arduino female headers as their contacts are a touch too wide to grip 24AWG solid wire properly, stick or bold down the Uno to a base right next to the breadboard  and dive right in, with the help of one of the many Arduino tutorials and the Arduino official forum, reading switches and sensors, flashing LEDs, mutiplexing 7 segment displays, and controlling low voltage relays etc.  Getting started is as simple as downloading the Arduino IDE (which includes the compiler etc.), installing it, plugging a USB cable between your Arduino Uno and your PC for power and data, making sure the Uno is selected as the board type you are using, opening Examples: 01.Basics: Blink and clicking the Upload button (right facing arrow on round button) to build the example code and send it to the board to start its LED blinking under control of this program:
Code: [Select]
// the setup function runs once when you press reset or power the board
void setup() {
  // initialize digital pin LED_BUILTIN as an output.
  pinMode(LED_BUILTIN, OUTPUT);
}

// the loop function runs over and over again forever
void loop() {
  digitalWrite(LED_BUILTIN, HIGH);   // turn the LED on (HIGH is the voltage level)
  delay(1000);                       // wait for a second
  digitalWrite(LED_BUILTIN, LOW);    // turn the LED off by making the voltage LOW
  delay(1000);                       // wait for a second
}

If you find that figuring out what's going on with an Arduino program is tough, learning to write C programs*, to run on your PC, from a book that isn't excessively PC/Windows centric can help, as just about any development environment on a PC offers much much better debugging than an Arduino does, with the ability to step though programs line by line and peek at and even modify the contents of your program's variables.   If you tell the development environment to create a new console application project (i.e you aren't trying to write any GUI stuff, just a program that does pure text input and output in a system command line (console) window), the learning curve is pretty shallow and a 'getting started' program can be as simple as:
Code: [Select]
    /* Hello World program */
#include <stdio.h>
#include <stdlib.h>

int main(int argc, char *argv[])
{
    printf("Hello World!\n");
    return 0;
}
which just displays the text Hello World! in the console.  C on your PC wont have any Arduino specific stuff (like the commands that actually do stuff with pins of the Arduino board, or any of the Arduino libraies for talking to common stuff you might want to control) but the basics of executing statements sequentially,making decisions, looping, doing maths and handling text as sequences of ASCII characters are the same.  N.B. typically one doesn't use printf() to display text on an Arduino, as it has a family of simplified print() and println() functions for that purpose  Arduino print() and println() are like table knives - they each do one thing. printf() is more of a swiss army knife, with a lot of blades that can do many things, but figuring out which one to use and how to get it open without cutting yourself can be more challenging! 

* The Arduino language is  C++ 'under the hood' using the AVR build of the GNU GCC compiler with a thin wrapper that supposedly simplifies it for novices, but as C++ is very close to being a superset ANSI C, ANSI C is much simpler to learn and the C subset of C++ is enough to handle anything a novice is likely to want to do in the main file of an Arduino sketch.
« Last Edit: August 30, 2018, 01:12:36 pm by Ian.M »
 
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Offline james_s

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Re: Air brush compressor.
« Reply #34 on: August 30, 2018, 02:55:20 pm »
Why is this getting so incredibly complicated? Like I said, my friend actually built one of these and it has been in service for more than a decade without problems, I can get some pictures if you want. It consists of a compressor from his old refrigerator, a couple of propane torch (blowlamp) disposable tanks, an electromechanical pressure switch with built in dump valve and a regulator, that's it. No electronics, no microcontroller, just the original thermal protector that came with the compressor. He's used the heck out of it all this time and it just works.
 
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Offline amyk

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Re: Air brush compressor.
« Reply #35 on: August 30, 2018, 05:04:54 pm »
Why is this getting so incredibly complicated? Like I said, my friend actually built one of these and it has been in service for more than a decade without problems, I can get some pictures if you want. It consists of a compressor from his old refrigerator, a couple of propane torch (blowlamp) disposable tanks, an electromechanical pressure switch with built in dump valve and a regulator, that's it. No electronics, no microcontroller, just the original thermal protector that came with the compressor. He's used the heck out of it all this time and it just works.
Some people just can't resist to show off their "epik Arduino skillz" ::)

We are on an electronics forum after all... but I agree, sometimes the best solution is to not use electronics.
 
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Offline SeanB

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Re: Air brush compressor.
« Reply #36 on: August 30, 2018, 06:41:46 pm »
You can use a regular refrigerator non PTC starter/overload, provided it is rated for the motor current. Most refrigeration compressors have the starter mounted on the side, and there are 2 types, PTC and voltage relay, easy to tell as the voltage relay has a direction arrow on it, and must be mounted with the arrow up. PTC have a disc in the case, and are position independent. However you do get external start relays that simply connect to the 3 terminals on the compressor and are mounted outside the steel cage, but those are typically only on larger commercial units.

If you can find a scrapped out 6000BTU or 9000BTU unit, use the rotary compressor and associated 50uF capacitor in there. Those will run just as well, plus they also are larger, so the oil bath in the base cools the motor a lot better.  Turn it upside down and drain off the oil, it will more than likely be POE synthetic oil, which will turn to acid when exposed to moisture, and refill it with regular SAE30 engine oil, which will last well enough in there. These will survive rapid start stop cycles provided you use an unloader valve, and just filter the inlet air ( take a regular spin on oil filter and make an adaptor for the M20 thread to get to the copper diameter) and use a receiver tank for it. Fan cool it and the tank, and put a coalescing oil filter on it to get most of the oil mist out, and you get reasonable air out of it. The compressor will fill the 4l tank in under a minute.
 
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Offline sureshotTopic starter

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Re: Air brush compressor.
« Reply #37 on: August 30, 2018, 07:04:44 pm »
Thanks for the replys and ideas, my Son does code at school, I think he could help me out along the way for any Audino projects. It is to complex for this project, I'm  certain there are simpler options. I've found a compressor, it's a bit steep at £130 delivered, and regulator and control valve gear at £25 which isn't terrible. That just leaves the air receiver, I've seen examples around £60 or so for 25 to 50 litres units. I might be worried about nothing over the frequent stop starts, but messing about with our old fridge compressor some years ago.. 15 minutes on and 5 off and it would not restart, just overloaded and clicked off. From memory a few more minutes was needed. I think it hinges on how quick the air is used up. The idea of a different starter is an in interesting idea though. I just thought if that PTC was cooled quicker the start winding would be available sooner, and less likely to overload and trip out. The compressor I'm looking at is 1/2 horse power 370 watts, it's a bit bigger than your normal fridge compressor. I think it will move more air faster, so with the price of a receiver it's quite a bit under the retail price of a similar semi professional compressor.
« Last Edit: August 30, 2018, 07:07:25 pm by sureshot »
 

Offline Ian.M

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Re: Air brush compressor.
« Reply #38 on: August 30, 2018, 07:55:02 pm »
Admittedly its years since I tripped a refrigeration compressor out myself, but that doesn't sound unreasonable for cooldown time.  However if you cut power to the PTC a few seconds after starting, it should be restart capable 10 minutes later even if the compressor has been running in-between so either extend the run on timer to 10 minutes or add another time delay relay that stretches the run on time to 10 minutes from the last start, or fit an electronic starter.   

You could take your target refrigeration compressor, measure the DC resistance of its start winding and start it into a small tank or a long airline with a pressure relief valve set to 150PSI then cut power as soon as the valve pops and start measuring the DC resistance immediately and plot it against time as it cools, so you can calculate the average winding temperature using the resistance vs temperature calculator here: http://hyperphysics.phy-astr.gsu.edu/hbase/electric/restmp.html#c3

Then run it continuously into the relief valve again till its nice and toasty, cut off and measure the start winding resistance to get the max temperature then, if its got an external KLIXON or bimetallic strip in the three pin connector for overload protection, immediately trip it by reapplying power without releasing the pressure, and measure the winding resistance and plot as before to see what the worst case temperature is and how long it takes to cool down again. If its got internal protection, its either going to be in the main winding feed, or the common.  Main winding protection, treat as external, but if you find the common has opened, plot the resistance of the main + start windings until the protection resets then measure both of them separately for at least as long as it took to reset, so you can extrapolate the start winding resistance back to when it tripped, then carry on with just the start winding resistance as before.

For bonus points, attach a thermocouple to the casing and monitor running temperature and the casing temperature as it cools and plot that on the same chart as the calculated winding temperature.

That should characterise it well enough to be able to design a reliable control system for it, without needing much equipment up-front.  :-DMM

N.B. the temperatures calculated from the resistance are an average for the whole winding and don't allow for hotspots, so even if you actually know the rated insulation temperature, *DON'T* use the data you will be collecting to try to push the limits past whatever the OEM protection acted at.  In fact the sensible thing to do would be baby it a bit and use the numbers you've got to set conservative cycling limits and also instrument the casing and air outlet temperatures.

I wont comment on the £130 for a compressor and I cant legally encourage you to skip-rat one from a dead fridge or freezer unless you have the appropriate equipment and permits for refrigerant recovery and disposal.  ;)  >:D
« Last Edit: August 30, 2018, 08:13:59 pm by Ian.M »
 
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Offline sureshotTopic starter

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Re: Air brush compressor.
« Reply #39 on: August 30, 2018, 11:02:43 pm »
Very interesting stuff there, never thought it through enough to test winding resistances. No I wouldn't abuse the insulation class rating, and would opt for running a margin under it's maximum. The disconnect of the PTC after run up, would that not throw that winding back into circuit ? Only ask as the high resistance of the PTC is keeping the start winding out of circuit. But having said that maybe removing power from the PTC has no affect on the start winding once it's running. I will give these ideas a go, so thanks for the tips and ideas. No I don't have means to get the gas removed from a closed loop, a standard fridge compressor is around £40 But as a one off the bigger unit is ok. I've been looking at scroll compressors, but I'm not getting in to those. I doubt there suited we'll to no refrigerant use. If I can achieve a system where two can spray at a reasonable duty cycle that will do fine. I did look at the cheap end of airbrush compressors, but a 3 or 4 litre receiver is pointless, the unit will work itself to death. Would be nice to have use of a few more hardware resources, but I don't.
 

Offline Lomax

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Re: Air brush compressor.
« Reply #40 on: August 30, 2018, 11:46:24 pm »
My $0.02: Dentist compressors are great for airbrushing and can sometimes be found for cheap on eBay; they're always oil-less designs, and many have built in driers and/or automatic water drain. Often quite small. The reason you never hear them running when in the chair is because they put them inside acoustically insulated boxes (in another room) - a plywood box with a mineral wool lining will do, with an anti-vibration platform for the compressor, and forced air ventilation (this one's important, these things get hot). Indeed you sometimes see them listed including such enclosures. Other models have the pump and motor enclosed in an oil filled container, much like the fridge variety. I've got a JunAir 300 myself, with a tiny 4 litre tank and auto drain. Think I paid £80 on eBay for it. Works beautifully for airbrushing, stapling, nailing, blowing, inflating (steady!), and also powers my EFD vacuum pick-up tool and dispenser. I also like playing with pneumatics for fun, and have a selection of regulators, rams, and solenoid valves to tinker with. It doesn't produce enough cfms to power sanders, drills and that sort of stuff, except for the briefest moment - but then neither would a fridge one!
 
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Offline BradC

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Re: Air brush compressor.
« Reply #41 on: August 31, 2018, 12:00:24 am »
messing about with our old fridge compressor some years ago.. 15 minutes on and 5 off and it would not restart, just overloaded and clicked off.

If that was still in the fridge at the time then that is expected behaviour.

It can take quite a while for pressures to equalise during an off cycle, and a fridge compressor won't start into a head. A fridge with r134a can be pushing up to a couple of hundred psi into the condenser with the evaporator sitting at 10-15 psi. The cap tube is long and tiny and when the compressor stops it can take quite some minutes for those pressures to equalize down to a level the compressor can start into. So you hit the compressor, it sits locked up until the windings heat up and it trips its overload. By the next time the overload has cooled down the pressures will have equalized and it'll be off an running where the superheated return gas will help get the temperature back under control.
 
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Offline salbayeng

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Re: Air brush compressor.
« Reply #42 on: August 31, 2018, 05:39:31 am »
Maybe 40 yrs ago as a boy I used a discarded R12 compressor (mineral oil) , it worked perfectly fine for years with just occasional use.

The cooling inside these is achieved mostly through the oil, if you look at the bottom of the motor shaft, it is conical, with the pointy bit dipping in the oil reservoir, as the motor spins , centrifugal force tries to fling the oil outward, but because the shaft is conical the oil moves up as well as out, so basically flinging a fine stream of oil droplets against the windings, and the shell, and drip back down to the sump. In refrigeration applications the discharge pressure can be around 200psi, whereas for air brushing / painting you might only use 50psi-100psi. As motor current is directly proportional to pressure, you only draw half or less of rated current, and winding loss goes as I2R so only 1/4 of the heat in the windings. (the iron loss will be unchanged, as its frequency dependant, or could be lower with a BD35 running at low speed) ,  and you still have air flowing through the compressor so you are getting gas cooling,  so in summary, the hobby use should be more benign in terms of heating.

About 20years back I made a compressor with an old style automotive 2 cylinder York compressor and a 1/2HP motor, the motor spins continually, and the pressure switch just operates the magnetic clutch, it also makes a great vacuum pump. It has an oil seperator too.

At BHPR we used a bunch of the BD35's in some airborne environmental controllers, as Ian says above they have a boost inverter and make more than 100v across the windings (I guess the windings are similar to a 110VAC compressor) we have a spare BD35 we use for flushing the plumbing with R141, and it cops a bit of abuse, but still runs fine (we swapped out the hydroscopic oil though). The BD35's don't like starting with high back pressure, and we had to plumb in a bypoass capillary to equalise pressure for hot restarts, and fiddle with our electronic controller.

Another option for pressure regulation with refrig compressors is to throttle the suction side, with some kind of unloading valve, I played with this , and yes it is possible, but I couldn't find a suitable valve to hack up to make it work well.
 
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Offline sureshotTopic starter

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Re: Air brush compressor.
« Reply #43 on: August 31, 2018, 10:16:42 am »
Thank you for your replys, great info, I've gained a lot of ideas knowledge in this thread. I've still been looking at start stop options. I thought there was one type of start relay with PTC, so far I've found three variations on starting these motors. So I'm not 100% sure what type of start system the one I'm going to get uses. Thanks for the heads up on the dental compressors, i found a few that would be close to budget. I've only actually built one compressor years ago now, that used a volvo air brake compressor i found in a tool machine sellers shop. Powered by a lawn mower engine, i had the frame housing and receiver made in a local fabrication shop to my drawings. It was for nail gun use mainly. But back on the single phase hematically sealed units, i was unawre of the different starting options. Seems there is no single industry standard on the electrical side of these units. From memory the unit i tried some years ago, i don't remember a start, or start run capacitor. But from looking about for a unit plenty have capacitors bolted to the side of them.
 

Offline sureshotTopic starter

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Re: Air brush compressor.
« Reply #44 on: August 31, 2018, 10:55:42 am »
These are the configurations on wiring i found. Be interesting to know if the overload is in the windings or not. Looking at these diagrams i think maybe not.
 

Offline Ian.M

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Re: Air brush compressor.
« Reply #45 on: August 31, 2018, 12:07:39 pm »
I think they've missed a few there.  See http://people.ucalgary.ca/~aknigh/electrical_machines/other/split_phase.html

There is also such a thing as a shaded pole induction motor. They get by with one winding and split pole pieces with a copper or aluminum band on one part, to provide a lagging flux at that part of the pole, but they have very low starting torque and aren't terribly efficient, so you'll *never* see one in a compressor.  They are invariably low fractional HP and are popular in small fans, crappy small bench grinders and any other application that has negligible load before its spun up to above half speed.

A split phase motor with PTC thermistor starter and overload/over-temperature cutout by bimetallic strip heated by the motor current, both integrated into the female three pin connector is about as cheap and nasty as you can go for small refrigeration compressors.   Its vastly improved by adding a time delay relay to open the PTC + start winding circuit a few seconds after motor power on so the PTC can cool down while the motor is running.  Alternatively replace the PTC with a fairly sensitive current relay as shown above but with plain wire instead of the capacitor.  Don't try to use a potential relay as the lack of a cap confuses them as the S to C potential doesn't shift as a split phase motor comes up to speed.

To size the current relay, measure the full load running current of the motor (with the compressor at max pressure differential for the application - i.e. fully cooled down and on 'boost' mode if still in the fridge or freezer, or popping off a pressure release valve at a bit bore than the max tank pressure you want on the output if you've already cut the lines and want a more accurate max current), and also if possible measure the current during the starting period (which requires a scope + isolated current probe of some sort) so you can see what current differential you have to work with.  In general 20% above the measured max full load current would do the job, but higher may be appropriate to reduce start winding heating. 

You can always put a neon indicator (with resistor) across the PTC or the current relay contacts, which will light once the PTC or current relay cuts the starter circuit, and check the start on timing that way.

Due to the refrigerant gas producing really really nasty acids if you pass an arc through it,  and the oil mist, inside the can of *ANY* type of  hermetic refrigeration compressor is not a good place for a thermo-mechanical self-resetting overload trip, especially one that may need replacing.  OTOH there is no better place to measure the temperature than buried under part of the main (run) winding.   I think on all except the largest which may have a thermistor buried in the winding + an external electronic protection circuit, the overload is likely to be entirely external, and will be some sort of delayed (thermal) self-resetting over-current trip.
« Last Edit: August 31, 2018, 12:17:25 pm by Ian.M »
 
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Offline sureshotTopic starter

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Re: Air brush compressor.
« Reply #46 on: August 31, 2018, 12:17:32 pm »
Seems like a lot of variables, I'm not phased by that though, no pun intended. Yes i would flush it out and replace with suitable polyester compressor oil. Below is a unit I'm looking at, i see an electrical control box on the compressor itself, but the other image is the same unit. I've no idea what the blown plastic molded case is for. Any ideas appreciated.  Yes i know there's no single solution, depending on the unit i end up with dictates the testing i need to do.
 

Offline sureshotTopic starter

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Re: Air brush compressor.
« Reply #47 on: August 31, 2018, 12:30:45 pm »
I understand the split phase principle, and AC phase angle to start these things, and windings are offset from each other. Seems that there's many variations on the one principle. I'm curious as to what the light grey box is above, i see the control box on the compressor itself, but unsure what the other box is. Would that grey box be a party stopper on getting it running ? That unit is rated at 1.5 hp single phase, something like that married up with a 50 litre receiver, its cheaper to buy a garage compressor for the receiver and control gear than it is to buy a separate receiver or extra hardware.
 

Offline BradC

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Re: Air brush compressor.
« Reply #48 on: August 31, 2018, 12:38:29 pm »
I saw it mentioned elsewhere on the thread, but I want to reinforce it. Please put an adequately sized pressure relief valve on the tank.

Those things can generate well upwards of 300psi into a good head and a ruptured tank is not pretty.
 

Offline sureshotTopic starter

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Re: Air brush compressor.
« Reply #49 on: August 31, 2018, 12:52:22 pm »
Yes i will put a pressure relief valve in the air receiver, that goes with out saying. I'd never contemplate using any compressor with out one.
 


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