Air brush compressor.  - Page 1
Forum section: electronics / projects

author: sureshot
author_rank: Regular Contributor
timestamp: August 28, 2018, 01:16:21 pm
content: I'm looking at putting together an airbrush compressor. I'd prefer the silent fridge type unit, but they cost a lot. I'm familiar with gases and gas laws, but wanted to run a fridge type compressor at better than 50% duty cycle which there designed for. Does anyone know if forced air cooling from below would allow longer running times ? I know it would cool the oil in the unit, but unsure about motor winding temperatures. As i understand it, start up is the highest current draw. So running shouldn't over heat the windings. Do you think if the oil is kept cool, constant run use can be achieved ?I've looked at on and off demand, and the motor is not suited to frequent stop start cycles where it would overheat.Thanks for reading, any thoughts ideas appreciated.

author: Miyuki
author_rank: Frequent Contributor
timestamp: August 28, 2018, 01:25:47 pm
content: They are gas cooledYou can open it and blow air to windings and also measure temperature for protection (some people do it this way)As it is AC induction motor it draw almost no power without load and can draw like 10 times more when stall, so you can measure power at your pressure to seeAlso I recommend to use big air tank, then it can run in long cycles and let it cool down with forced air cooling

author: Ian.M
author_rank: Super Contributor
timestamp: August 28, 2018, 01:40:45 pm
content: A refrigeration compressor is designed to run with the gas return to its intake not much above 4 deg C and at fractional PSI. Running it with 15 PSI air at 25 deg C to its intake is already overloading it and increasing its thermal stress so you'll have to drastically reduce the duty cycle to avoid overheating.Getting a BLDC compressor (without a controller) and building a VFD controller so the pressure can be regulated by varying the speed would be the best option if you want to avoid the need for a large tank, but that's not going to be cheap.Also, a refrigeration compressor will give you a lot of trouble with oil mist carryover, + the oil in a salvaged one will be unsuitable for reliable running open loop as its hygroscopic.

author: bob225
author_rank: Frequent Contributor
timestamp: August 28, 2018, 02:02:21 pm
content: What is your budget ? the small air brush compressors are fairly quite as it is, The silent ones can run a tad hot but that's the trade off, I always run a filter with whatever air source I am using anwaywhat country are you from ? as its pointless posting any links.In the uk there is everythingairbrush -https://www.everythingairbrush.com/compressors.htmlI have had great service from them over the years with parts for my DeVilbiss Dagr and some inks when I was playing with my Iwata brush

author: DaJMasta
author_rank: Super Contributor
timestamp: August 28, 2018, 02:34:02 pm
content: I know it's not in the spirit of building your own with a project, but second hand good compressors can be had for cheap, sometimes. When I was looking around and was in the market, I found someone selling a Badger brand compressor for like $80 because it had just been sitting around for a bit. New start capacitor and an oil change and it's been going great for years since.I also think tank size is going to be important if you are designing your own. An airbrush doesn't actually have very high demands in terms of volume unless you're cranking up the pressure to use it as an air duster, so as long as you keep your tank small enough to not start overheating on the initial fill, the pressure maintenance in the tank should be a much lower duty cycle. As mentioned above, I don't think external forced air cooling will have a big effect. The housing does get warm in use, but at least on mine it doesn't seem to be relied on for cooling in the initial design, no fins or obvious heat transfer point mean it's probably not well thermally coupled to the hot components inside.

author: amyk
author_rank: Super Contributor
timestamp: August 28, 2018, 06:39:06 pm
content: Older refridgeration compressors did have cooling fins.That said, the assembly inside is usually mounted on a few springs, so there is certainly not much thermal contact.

author: sureshot
author_rank: Regular Contributor
timestamp: August 28, 2018, 07:26:43 pm
content: Thank you for your replys. Years ago i opened a fridge compressor carefully cutting above the hematic seal with an angle grinder. So yes I'm familiar with the anatomy of these units. I did a test on start up and measured just over 5 Amps on the unit i tested, from memory that drop a lot lower once running. Only climbing a little under load, years ago i tried using one. Back then i found 15 minutes was about the same temperature as if it was run in a fridge, its intended location. I was hoping the filter trap would catch any would be fine oil mist, not a lot i don't think. But some oil in an open loop is possible. I've seen airbrush compressors that use these fridge motors, manufacture called Bambi i think. There rated at 50% duty cycle of a maximum of 30 minutes, they don't use forced air cooling. I did look in some detail years ago at frequent stop starts, but its just not possible with the split phase configuration. I was hoping to run a unit for continuous use with forced air cooling, i know its only the outer shell and oil that would get cooled. As you've said above the windings and compressor arrangment is mounted on sturdy springs. I'm not sure what the vfd control for another motor type compressor is. My idea was to find a cheap normal work shop compressor, remove the original motor and drive. Then fabricate the fridge compressor in its place. Connect up the electrical control gear, but some how alter the stop start time, if it could be run with forced air cooling. Opening up the fridge compressor for cooling is not really viable as it would quickly rust badly due to its construction. Thanks again for the thoughts on this.

author: coppercone2
author_rank: Super Contributor
timestamp: August 28, 2018, 07:34:03 pm
content: The main thing I think would be to ensure the output is sufficiently filtered from oil. Compressor oil getting into your paint is likely going to fuck it up, given how crazy people are about preparation and cleanliness with painting.I don't know how to quantify air purity, you can go by commercial standards of 'clean dry air' maybe. Don't know what sensors you use to measure the vapors found in a compressor would be.

author: Ian.M
author_rank: Super Contributor
timestamp: August 28, 2018, 08:02:07 pm
content: It will probably rust badly anyway unless you either have consistently low RH or have a large capacity air drier on its intake.If you want to go down the tankless variable speed route, 12V and 24V compressors from marine or RV fridges & freezers typically have low voltage BLDC motors that can be speed controlled by a low voltage three phase VFD drive. As they can be used with eutectic holding plates, they support continuous operation to chill down the holding plate while power is available. Use a RC ESC to drive it that supports tuning to limit the max RPM to 3500, and implement the pressure control system with an Arduino and a 10 bar pressure transducer.Seehttps://www.mmair.com/wp-content/uploads/2016/09/BD35.BD50.Danfoss.pdffor the manual of a typical one (with OEM controller).Its fairly common for the control board to get water or corrosion damaged in a marine installation, so you should be able to pick up just the compressor without breaking the bank if you call around marinas with workshops, boat-breakers and second hand chandlers.

author: DaJMasta
author_rank: Super Contributor
timestamp: August 28, 2018, 09:37:30 pm
content: Filtering the output air shouldn't be a big problem, you can get an inexpensive regulator/oil trap combo that just threads on the output of the tank and has done a good job with mine. You could use a second stage trap or one with a filter inline with it for extra removal as well.

author: coppercone2
author_rank: Super Contributor
timestamp: August 28, 2018, 09:49:30 pm
content: Quote from: DaJMasta on August 28, 2018, 09:37:30 pmAre you sure that will stop oil vapor, not just mist? Trace contamination might be enough to effect paint. Your not even supposed to let a work piece sit in air for more then like some short time period after sanding.

author: sureshot
author_rank: Regular Contributor
timestamp: August 28, 2018, 10:11:50 pm
content: Thanks again for the replys and the link there Ian. Yes I've come across Danfoss before, but not that type of motor. I will definitely have a good read up on this, VFD sounds an interesting alternative. I'm well versed in clean air as a diver. That has a few filtration stages. I did think for a split second on cylinder use as an air source. But realised quickly that airbrush artist's have some air on continuously, only adding paint where they want it to go. I would empty a 15 or 18 litre 232 or 300 bar in no time. And the initial outlay for valve regulator control gear is expensive. It was a nice thought while it lasted lol, complete silence...If there was a "magic" way to alter that heat generated in that start winding, some how...Stop starts would be possible, and problem solved. Any AC experts know if there is the slightest chance to take the heat of that start winding ?

author: DaJMasta
author_rank: Super Contributor
timestamp: August 28, 2018, 10:14:55 pm
content: I haven't done any significant work on the subject, but my compressor made for airbrushing seems to be fine with it. Cheap filter on the intake, compressor that dumps into the tank, then a drain vent in the bottom of the tank, then the output comes up from the tank and into a combined regulator/oil trap, which has done just fine in all of my painting. Even if there's some vapor content in what comes out of the pump, it cools down in the tank and then any remainder is taken out in the moisture trap on the regulator (has a basic airstone like filter.)

author: Ian.M
author_rank: Super Contributor
timestamp: August 28, 2018, 10:41:54 pm
content: Quote from: sureshot on August 28, 2018, 10:11:50 pmThe start winding phase shift is typically provided by its resistance being dominant, while the main winding's inductance is dominant so the current lags near 90 deg. Therefore some heat buildup in the start winding from each start is unavoidable. However, many compressors use a PTC thermistor to cut the current into the start winding to near zero after the compressor has started. This is basically a timed start and cant be short-cycled because the thermistor and the start winding have to cool down before it can restart.To improve the situation, if you add an unloader valve on the output, the compressor can run on, (on a timer) at virtually zero head, so minimal motor current and high flow, which will cool it down. Also it wont have to start against any back pressure which will reduce the starting current and prevent stalling. If you also add an electronic soft-starter it will operate the start winding for the minimum time required to get up to speed, reducing winding heat buildup and eliminating the PTC thermistor and its need to cool down. You'll need something to sequence it all, so the motor runs up to speed before the unloader valve closes, and once the pressure setpoint is reached the unloader operates but the motor is allowed to run on for up to N minutes to cool down and avoid short cycling by closing the unloader if the pressure drops below the setpoint while the motor is still running.

author: james_s
author_rank: Super Contributor
timestamp: August 29, 2018, 12:34:35 am
content: My friend built a quiet air compressor using the hermetic compressor from his old refrigerator. He built it about 10 years ago and it is still going strong, so while there are various possible issues, his experience proves that it is indeed possible. If you let one sit too long they can rust and seize up but so long as you use it semi regularly it will be ok. The nice thing is that you can get fridge compressors for little or nothing so if it does fail you can get another one. The one special part he used is an unloader valve that releases the head pressure so the compressor isn't starting under load, they usually lack sufficient torque to do that.His friend's dad is a retired dentist and had a small compressor which was commercially built using a refrigerator type hermetic design except the top of the dome was held on by a band clamp rather than welded and it had a threaded bung for a dip stick to check the oil.

author: sureshot
author_rank: Regular Contributor
timestamp: August 29, 2018, 01:36:33 am
content: Some interesting replys, so thank you for that. I think if I can get a decent sized air receiver that will limit the start stop cycles and less run time between fills. 20 to 25 psi seems normal for spraying the majority of media, only one it two odd ball paints might need less or a bit more pressure. I think a really large air receiver would help. Maybe to compressors for efficiency, bold plans. The PTC thing I didn't realise, so thanks for that Ian. See if I can find a pair of matched compressors, and a large receiver. After seeing some YouTube stuff, I've been thinking air conditioning compressors. They must endure a hard work load.

author: DaJMasta
author_rank: Super Contributor
timestamp: August 29, 2018, 03:12:56 am
content: If it helps any, my compressor uses a 5L tank and pressurizes to about 110psi, then relies on the regulator to feed the brush. I paint between like 18 and 25psi, and 50-80psi is where I use it for dusting stuff off. The initial pressurization is 30-45s and even when using it to dust parts the duty cycle is no more than 50%... probably closer to 30%. You could also consider looking at the design for dental air equipment (they were mentioned before), but the construction is essentially the same as an airbrush setup, just with larger airflow requirements. The way they generally get around this is to use multiple compressors on the same tank, partly because compressors of this type above about 1/2hp don't seem to exist, so it's not uncommon to see a big tank with 3 or 4 1/2hp compressors on it with a little additional equipment to make sure the air is extremely dry (doesn't matter as much for airbrushing).

author: Ian.M
author_rank: Super Contributor
timestamp: August 29, 2018, 04:09:46 am
content: Hmm, my suggestion in reply #8 to use a RC ESC may be questionable, depending on the make of BLDC compressor chosen. Apparently the Danfoss controller incorporates a boost converter, and runs the 3 phase H bridge driving the motor at a significantly higher DC bus voltage. Most RC ESCs wont output enough voltage to drive a Danfoss BD35 BLDC compressor at anywhere near rated speed. High end ones for 12S packs may be suitable. This Youtuber is getting results using an eBike controller.If you trawl through the Danfoss spec. sheets you can determine that for a BD35F, the absolute minimum speed is 1850 RPM, recommended minimum speed is 2000 RPM, max operating speed is 3500 RPM, winding resistance is 2.2 ohms, displacement is 2 cc, and the max power consumption is 78.5W input to controller. *IF* you can find one with a working or repairable OEM controller, speed can be controlled with a simple variable resistor.

author: james_s
author_rank: Super Contributor
timestamp: August 29, 2018, 05:13:26 am
content: You want to run the highest pressure that is safe for whatever tank you're using and then drop the pressure with a regulator on the output. This will allow a smaller tank to hold a lot more air and give you a consistent pressure.Most small air conditioners use rotary compressors, I used one of those for several years as a vacuum pump for pumping down AC systems prior to charging. They have a compact can that fits tightly around the stator so it's easier to cool one externally but they are also noisier. Scroll compressors used in better quality central AC systems are nearly immune to seizing due to small amounts of rust but I've never seen one smaller than about 1.5HP (roughtly 1HP per ton of refrigeration capacity) and 240V.Don't over-think it, an ordinary reciprocating compressor from a domestic fridge works fine. Add an unloader valve and an electromechanical or electronic starting relay in place of the PTC if it uses one and it'll be fine.

author: sureshot
author_rank: Regular Contributor
timestamp: August 29, 2018, 10:38:43 am
content: Thank you for your replys and the video there Ian. Yes very interesting. I've worked out a decent 24L receiver would set me back around £60 I've got to host compressors from our old fridge and freezer, but not sure if to use those yet. I'd like to find a bigger retail type compressor out of a comercial chiller display unit. The control valve and regulator is £25 from a reputable seller. Then just hoses clips and hardware fittings. The cheapest Bambi purpose ready to go compressor is way over £300 I'd like that, but i also like £300 lol. Its far more cost effective to put one together from parts. I don't have the use of a decent work shop, so no welding or grinding indoors. I can get someone to do small welds if needed though. I will check out that video Ian, thanks for that. All your replys have been really helpful, putting real time use into perspective for me. Much appreciated

author: sureshot
author_rank: Regular Contributor
timestamp: August 29, 2018, 11:18:12 am
content: Had a quick look about on ebay and Amazon, most Variable frequency drives look like single phase to 3 phase operation. Video below shows a guy using a VFD to power what looks like a general standard fridge compressor. I know little about VFD's other than they vary the frequency and control AC motors etc. I'm wandering how the compressors split phase start system would be with a lower Hz input on start up. It looks like, if it worked would be a great addition to an airbrush compressor build. But i need a single phase drive output, there not to expensive, and power ratings look doable as well. Do you think it would power a standard fridge compressor straight off the bat ? Worth a try right.

author: sureshot
author_rank: Regular Contributor
timestamp: August 29, 2018, 11:39:41 am
content: Forgot to post these two, unsure on the red motor and fridge compressor motor differences.https://youtu.be/GEWAbttNPmIhttps://youtu.be/NuZsyDMP9osAny thoughts on these appreciated.

author: coppercone2
author_rank: Super Contributor
timestamp: August 29, 2018, 12:43:57 pm
content: Have you ever done any side by side tests of paint made by a oil filtered compressor and a oilless one? Like scratch test after curing for the same time period?I am really curious. But I already have a quiet air compressor so there is no point in me doing this fun project. Maybe if I got into model making one day. Those are like 40dB. I am not sure what kind of delicate work I would need to be doing to benefit from the reduction from 60 to 40dB.

author: Ian.M
author_rank: Super Contributor
timestamp: August 29, 2018, 12:51:56 pm
content: An abandoned 13 Kg propane bottle mounted upside down may be a cheap option for a tank of that size. Don't use a butane bottle as their max working pressure is much lower. A propane bottle should have been hydrotested to be safe with a max working pressure of 360 PSI @ 70 deg C, so unless its in really sh!tty condition has plenty of margin for use as a 150 PSI max air tank.Bore-scope it when you remove the valve to eliminate really sh!tty ones with deceptive exteriors. Deodorise with neat bleach, rinse thoroughly then passivate with a dilute phosphoric acid soak, rinse again, drain and dry in a warm place opening down, preferably with an airline through the opening for circulation.You'll have to braze up an adaptor, probably using the body of the existing valve to let you connect two T fittings coupled TT on their side with one rotated enough so fittings on their legs don't mechanically interfere with each other, with a concentric pipe straight through both T's to take the air out from near the top of the tank. You'll need a solid plug reducer at the bottom of the bottom one. Use the top T's leg for air in and the bottom leg for another T for safety valve on the top and drain cock or auto-drain valve on the bottom.Inspect regularly or replace every five years with a tank that was within its hydrotest interval when acquired.

author: sureshot
author_rank: Regular Contributor
timestamp: August 29, 2018, 07:16:12 pm
content: Yes I did look at the 4 cylinder oiless compressor, it has a 4 litre receiver, on YouTube a video on it doesn't look to bad for noise. Price is £ 158 here in the UK, which is quite good. I've never done a side by side comparison of oiless verses oil compressor. I'm certain the tank and trap will remove any oil vapour. Most of the intended painting was going to be artist canvas or similar surfaces. I'm trying to get my Son interested also. He's got no interest in electronics sadly. Yes I can get hold of large propane cylinders that are empty, the problem there is finding someone to fill it with water before welding the feet and input output ports into it. Many welders are sceptical of welding old gas cylinders for obvious reasons, I know filled with water it's safe. As a retired diver, out of test cylinders look like an attractive option. Large twin set would do really well. They'd hold that pressure all day long, the walls are very thick indeed. But again it's the welding, welders see a cylinder and think oxygen, but it's not, it's compressed air or nitrox in it's previous use. EBay have a few sellers of air receivers but there expensive. There are also sellers of compressor / airconditoning motors I've found, but there not that cheap. A new bit of info above about the relay and PTC I was unaware of the PTC part of that, i knew it involved a relay, just asumed they used a centrifugal switch to disconnect the start winding and any capacitor. I'm wandering if there's a way to configure that PTC thermistor for frequent stop starts with out over heating any windings, but I doubt it...

author: Ian.M
author_rank: Super Contributor
timestamp: August 29, 2018, 07:33:39 pm
content: 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'!

author: sureshot
author_rank: Regular Contributor
timestamp: August 29, 2018, 09:42:00 pm
content: Rocket mode lolthat 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.

author: coppercone2
author_rank: Super Contributor
timestamp: August 29, 2018, 09:44:30 pm
content: http://www.cagi.org/pdfs/tap106.pdfPaintingsystems 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 airGenerally 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 paintinghttp://teamtechnical.com/html/Parker%20Basics%20of%20Coalescing.pdfhttps://static1.squarespace.com/static/5141e2dee4b00c2dc63d1b79/t/52d437f1e4b04774445785af/1389639665069/Ind+Binder+2012.pdfIndustrial grade painting: (non automotive grade)Two stage unit with patented "Inverse Flow ™" and "Depth Filtration" technology. The 1st stage iscomposed of 4 layers of filtration material to include activated carbon for hydrocarbon removal to 0.03ppm. The 2nd stage is a 0.01 micron "Grade 1" coalescerDRAD-25-PAMProduction spray painting application was having problems with contamination causingpeeling and poor color finish reducing the quality of their finished products. ReadingTechnologies, Inc. DRAD system removed all contamination from the compressed airlineresulting in a quality finished product free of defects. This is a desiccated unit thatrequires the first state prefilter to be changed every 4- 6 months, and the 2nd stagecoalescer 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

author: Ian.M
author_rank: Super Contributor
timestamp: August 29, 2018, 11:27:12 pm
content: Quote from: sureshot on August 29, 2018, 09:42:00 pmI 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.

author: sureshot
author_rank: Regular Contributor
timestamp: August 30, 2018, 12:39:59 am
content: 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.

author: Ian.M
author_rank: Super Contributor
timestamp: August 30, 2018, 02:29:02 am
content: 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.TANSTAAFLapplies.

author: sureshot
author_rank: Regular Contributor
timestamp: August 30, 2018, 10:36:01 am
content: 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.

author: sureshot
author_rank: Regular Contributor
timestamp: August 30, 2018, 11:31:19 am
content: 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..

author: Ian.M
author_rank: Super Contributor
timestamp: August 30, 2018, 12:58:32 pm
content: 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:// the setup function runs once when you press reset or power the boardvoid setup() {// initialize digital pin LED_BUILTIN as an output.pinMode(LED_BUILTIN, OUTPUT);}// the loop function runs over and over again forevervoid loop() {digitalWrite(LED_BUILTIN, HIGH); // turn the LED on (HIGH is the voltage level)delay(1000); // wait for a seconddigitalWrite(LED_BUILTIN, LOW); // turn the LED off by making the voltage LOWdelay(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:/* 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 textHello 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.

author: james_s
author_rank: Super Contributor
timestamp: August 30, 2018, 02:55:20 pm
content: 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.

author: amyk
author_rank: Super Contributor
timestamp: August 30, 2018, 05:04:54 pm
content: Quote from: james_s on August 30, 2018, 02:55:20 pmSome 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.

author: SeanB
author_rank: Super Contributor
timestamp: August 30, 2018, 06:41:46 pm
content: 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.

author: sureshot
author_rank: Regular Contributor
timestamp: August 30, 2018, 07:04:44 pm
content: 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.

author: Ian.M
author_rank: Super Contributor
timestamp: August 30, 2018, 07:55:02 pm
content: 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#c3Then 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.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.

author: sureshot
author_rank: Regular Contributor
timestamp: August 30, 2018, 11:02:43 pm
content: 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.

author: Lomax
author_rank: Frequent Contributor
timestamp: August 30, 2018, 11:46:24 pm
content: 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 gethot). Indeedyou sometimes see them listed including such enclosures.Other modelshave thepump and motor enclosed in an oil filled container, much like the fridge variety. I've got aJunAir 300myself, 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!

author: BradC
author_rank: Super Contributor
timestamp: August 31, 2018, 12:00:24 am
content: Quote from: sureshot on August 30, 2018, 07:04:44 pmIf 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.

author: salbayeng
author_rank: Frequent Contributor
timestamp: August 31, 2018, 05:39:31 am
content: 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.

author: sureshot
author_rank: Regular Contributor
timestamp: August 31, 2018, 10:16:42 am
content: 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.

author: sureshot
author_rank: Regular Contributor
timestamp: August 31, 2018, 10:55:42 am
content: 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.

author: Ian.M
author_rank: Super Contributor
timestamp: August 31, 2018, 12:07:39 pm
content: I think they've missed a few there. Seehttp://people.ucalgary.ca/~aknigh/electrical_machines/other/split_phase.htmlThere 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.

author: sureshot
author_rank: Regular Contributor
timestamp: August 31, 2018, 12:17:32 pm
content: 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.

author: sureshot
author_rank: Regular Contributor
timestamp: August 31, 2018, 12:30:45 pm
content: 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.

author: BradC
author_rank: Super Contributor
timestamp: August 31, 2018, 12:38:29 pm
content: 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.

author: sureshot
author_rank: Regular Contributor
timestamp: August 31, 2018, 12:52:22 pm
content: 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.

author: Ian.M
author_rank: Super Contributor
timestamp: August 31, 2018, 01:16:44 pm
content: *AND* a burst disk for belt&braces protection.Re: your reply #46 compressor photos,The underslung cylindrical metal can indicates the presence of a starting cap*, and the presence of a control box means its most likely to have a current relay or potential relay, as a PTC starter can be integrated on the three pin motor connector.For the second one, if its actually got the same model number the plastic box may simply indicate cost reduction and/or the advance of housing technology, or it may conceal an OEM customised electronic starter and overload board. If its a different model number it may be an integrated 'brain-box' for the whole system its from. You'd have to open it or find a wiring diagram to tell the difference.* Can't be anything else as the ones in the background clearly show there's no plumbing to it.

author: sureshot
author_rank: Regular Contributor
timestamp: August 31, 2018, 06:58:21 pm
content: Burst disk for protection, no idea what your reffering to. I know any system must have the requisite safety hardware incoperated in it. Yes "The brain box" that was what I thought when I saw that larger grey box. I didn't want a compressor I couldn't get up and running. That one is a bit steep at £160 delivered. The best new unit that would fit our requirements is about £380 new. But that's only got a small receiver. Anything a fare margin under that figure is worth a go. Dam that grey box... The slightly less 1hp at £125 also has a similar grey box near it in the picture. The seller is a refrigeration hardware dealer. I don't think I can ask, what is the grey box. Mainly because of the intended non standard use it would be purposed for.

author: Ian.M
author_rank: Super Contributor
timestamp: August 31, 2018, 07:39:34 pm
content: 'burst disk' aka 'rupture disk' = non-reclosing pressure relief devicehttps://en.wikipedia.org/wiki/Rupture_discBecause of their extreme simplicity, if mounted as specified, short of major debris in the system large enough and flat or round enough to physically block the port, there's no way for them to fail closed,

author: sureshot
author_rank: Regular Contributor
timestamp: August 31, 2018, 10:29:49 pm
content: Ah ok, never heard of those before, thanks for the info on that Ian. Well I was bold enough to ask the seller of the fridge compressor, what is the grey box for ? As he's selling to refrigeration customers I doubt I will get a reply. I did explain briefly what my non standard use was, I wasn't asking for advise. Really just enquiring what the grey box is for. I thought some Google terms like " refrigeration control systems" and similar might have thrown something up, but no nothing really. If I can't find data on the grey box, I won't proceed with that type of unit. Expect all large units of 1hp to 1.5hp all have them. It's only the chance that the host equipment is needed for that type of compressor to function properly, I really don't know.

author: james_s
author_rank: Super Contributor
timestamp: September 01, 2018, 07:15:12 am
content: Quote from: sureshot on August 30, 2018, 07:04:44 pmFor £130 I would expect you could get a pretty decent ready to use quiet air compressor rather than having to build anything yourself. You should be able to get an old refrigerator compressor for peanuts, is there a scrapyard nearby? Maybe try asking at a HVAC shop or appliance repair place.You don't want a PTC starter, you need either a current relay or an electronic starter, I have a circuit somewhere that works, it's just a triac and a couple resistors and capacitors. With a proper starter the cycles won't hurt it, you can cycle it on and off all day long without any trouble. You do need a pressure switch that has a load dump to release the head pressure though.I'd have to ask but I think my friend spent about £30 building his, most of that was for the pressure switch and regulator. The other bits were all salvaged, including some steel tubing he welded together for the frame.

author: sureshot
author_rank: Regular Contributor
timestamp: September 01, 2018, 10:28:24 am
content: Thanks for that James, well I've got two appliances, fridge and a freezer. Both are getting replaced. I've got no means of safely removing the refrigerant, i know some would just cut them out, but I'm a bit more responsible than that. I'm very interested in the alternative starting device, current relay and or triac circuit. That sound like it won't trip out with moderate time interval. Other scrap parts, not really. Here in the UK years ago you could take stuff from the tip or old scrap yard, but there so hot on health and saftey these days that many sites won't entertain removing scrapped stuff. Its like the air receiver, i don't have equipment to weld it myself. I can find any number of old empty gas cylinders, even large out of test dive cylinders. No welder i know would touch one with a barge pole. So only option is to find a dead work shop compressor, the control gear pressure switch dump valve is easy and cheap to source. Where that ridiculous here in the UK that the seller i doubt would reply to my grey box question, mainly through fear i joe public me that is, blowing myself up. He'd see his answered question as a liability risk, which is just ridiculous.

author: sureshot
author_rank: Regular Contributor
timestamp: September 01, 2018, 12:11:17 pm
content: Won't drag the thread on further, just wanted to say thank you for all the help and advice, much appreciated. I think I've found the grey box purpose, it looks like the control electrical gear. For some strange reason i started looking at 3 phase compressors and VFD's.... Back down to earth now lol. The images are 3 phase units up to 4.5hp. I'm sure now i can fashion something together. Thanks again all.

author: james_s
author_rank: Super Contributor
timestamp: September 01, 2018, 05:51:36 pm
content: Quote from: sureshot on September 01, 2018, 10:28:24 amMy friend who built his compressor is in the UK so he had access to the same resources you have.What is the refrigerant? In some UK/Europe units they use hydrocarbons like propane or butane, which obviously you can just vent. Others frequently use R134a which due to the way the laws are written you're not supposed to vent it from a refrigeration system, however it's perfectly legal to use a can of it as airbrush propellant or to power one of those air horns where it is obviously vented, so unless it is R12 (CFC) or R22 (HCFC) I would not worry about it. Surely you could also take the thing to a recycler or refrigeration shop and have the refrigerant removed, they usually charge little or nothing because the recovered refrigerant has value.There should be no need to weld the cylinders, I'm not personally comfortable welding pressure vessels. I think my friend drilled out the original valve and tapped the hole to fit whatever standard fitting he used.

author: sureshot
author_rank: Regular Contributor
timestamp: September 01, 2018, 06:59:14 pm
content: I might look at getting the refrigerant removed, as I do have two compressors going free soon. I've not looked at the back to see what the refrigerant is yet, there quite old so might be lucky on a non toxic harmful variant. As for air receivers I'm looking at single cylindrical types, these heave no feet and generally no more than a single main opening of quite a large diameter. Old gas cylinders would still need a bit of prep work before being suitable for use, same with an old diving cylinder. Scrap yards round here won't let you take anything because of HSE regs.Just another question..I don't want to take up to much forum space. Those DOL motor starters of single phase, would one of those be suitable as a motor starter for a compressor ? And if they are, what happens when the compressor reaches pressure and blows down. Does the motor starter just default to it's off status as the pressure switch has shut off power to the motor ? I really don't know. Or does the motor starter status stay engaged and on until the pressure switch kicks back in. I'm not sure.

author: james_s
author_rank: Super Contributor
timestamp: September 01, 2018, 10:31:29 pm
content: He used a pair of cylinders like this:https://www.walmart.com/ip/Torch-Kit-Pencil-Flame-Propane-BERNZOMATIC-UL100/295966342I think they are just strapped to the frame he built, I'd have to check though.For the motor starter the old style used a current relay which was wired in series with the main winding. When power is applied the locked rotor current is high enough to pull in the start relay which connects power to the starting winding. Once the rotor starts turning the current falls and the start relay drops out.These are the old style magnetic starters.https://www.amazon.com/Compressor-Relay-Starter-Refrigerator-Black/dp/B00W8UCPS0

author: sureshot
author_rank: Regular Contributor
timestamp: September 02, 2018, 02:41:20 am
content: Thanks for the links James, there's a guy on YouTube that used similar cylinders with brass fittings and nylon air line, compression fittings I think. The relay link I will try and find UK sales for it, thanks for that.I'm only going to do this once...A bucket list thing if I'm around long enough. Some years ago I got interested in airbrushing from a library book lol, before computers where around. Some years later I pulled a fridge compressor and fired it up, with a plan to build a unit for airbrushing, it never happened.This time I was hoping to get my Son interested, he's into the Marvel stuff and super hero's etc, thought he might want to try airbrushing some stuff. As I'm only going to get one chance at this, and after looking across the web, I've kind of got taken with 3 phase. I know it cost a fare wack more, but I wanted a unit my Son could still use in years to come. So at the moment VFD's look the best option for this, the only slight off putting thing is the winning you get from the frequency carrier, I think it's that. But hoping the better quality ones are not so noisy. 3 phase fridge compressor, VFD control, usual pressure switch and regulator for output air. Coupled up with a 25 or 50 litre air receiver. I did briefly look at 100 litre air receiver and quickly worked out that's just daft. The VFD would just offer better control of the compressor. 3 phase as these look like much higher volume of air output. The idea being some quite times between compressor filling the receiver, although there fairly quite anyway. As it's something I want to last I might as well throw a few bob at it.

author: sureshot
author_rank: Regular Contributor
timestamp: September 02, 2018, 06:47:08 pm
content: This really is a final question before I throw all sensibility out the window, spend to much, and get horse whipped by the missesSo I've been doing a bit more searching on a switching system to start up the smaller compressors. After seeing a couple of videos of starting with out a PTC relay, the start winding was briefly shorted. I think I might have found a suitable timer relay to achieve this, but that would look something like the pressure switch voltage low at 12 Volts. I'm not sure in the scheme of things if that's a viable option. I think it might be, as the pressure switch is mechanical I believe. Just the contacts breaking and making the compressors circuit.There is an image of the timer relay I'm considering, so my question is, do you think it's usable / suitable to switch the start run windings in and out of circuit ?Thank you for reading, thoughts on this last question appreciated.Ps. Just a correction, I think the pressure switch would need to break the mains to the compressor timer relay, and the mains side of the 12 Volts supplying the timer relay board. Think that the right way.

author: Epatsellis
author_rank: Newbie
timestamp: September 03, 2018, 04:39:18 am
content: When I airbrushed a lot, I found very quickly that the best air supply was a 5 ft tall co2 bottle. Quiet, cheap to refill and as long as you had good ventilation, far better than any compressor I used, including compressors from Badger, California Air Peoducts and several refrigeration compressor based ones.

author: coppercone2
author_rank: Super Contributor
timestamp: September 03, 2018, 04:42:30 am
content: Quote from: Epatsellis on September 03, 2018, 04:39:18 amYou have the benefit of a really clean gas here if you get it from a welding supplier.

author: sureshot
author_rank: Regular Contributor
timestamp: September 03, 2018, 10:40:12 am
content: We have an industrial gas supplier / welding company up the road here, its called SIP Analytical. I'm not comfortable using C02, my place is quite small, i also have cardiac issues, don't want it affecting blood gases. I've been up to SIP Analytical in the past to look into compressed air control gear, it is doable but outlay a bit pricey. But the biggest factor would be how quick I'd get through 3480 litres of air 232 bar 15 litre cylinder and 4500 litres of air, same size cylinder at 300 bar. Then the hassle of taking it to my diving club house for refill. But i did look into it. What I've noted is airbrush artists have a flow of air continuously with the trigger on the brush, only pulling it back for paint delivery. They don't seem to cut the air during painting. This uses the avaliable air up much faster. I'm not sure on a 3 phase compressor build yet, from what I've seen the VFD to drive one could be whiny powering the motor windings. So i might go with single phase still, but try to get the higher end unit of about 1hp if i can find one. I did find a thread in another electronics forum where another guy had the same start stall problems with a PTC relay, he also used the air up quickly before the PTC had chance to cool off. He's solution seemed to be a timer relay, and he done away with the PTC relay.