Cheap Compressor

[rtbcomp]

The silent ones are fridge motors, they have an external capacitor and a start relay, so there will be a start wiring on the motor.

The red one (It's made by Fini of Italy) does unload like a conventional compressor.

 

[eil]

Theres a start winding on both. but when you see a start cap its high starting torque as you probably know, when you get a min could you post a pic of the relay/starting device on the fridge ones.

 

[rtbcomp]

Could it be a start capacitor?

.......... I'm not sure of its duty cycle though........

I could limit the duty cycle using a couple of methods.

1) constrict the outlet to increase the time the tank takes to empty. I may have to fit a bigger tank to avoid short cycling.

2) fit a solenoid operated valve on the output controlled by a 24 hour tappet timer, 15 mins, 15 mins off during the time I need the compressor.

 

[rtbcomp]

Here are some pics of fridge motor type compressors

Note that the dump valve in the pressure switch isn't used.

 

[eil]

I see yes, the only reason i can see for that is to keep it quiet hence the shhh, interesting to note that there is no manufacturer name stamped on the pot.

 

[rtbcomp]

They were used to power air brushes in a tanning salon

The one that was completely dead had a burnt-out cutout. I've replace it but it's still having difficulty starting up.

There was no sign of any oil in it. Note the sight-glass on the side, I reckon this was retro-fitted.

The manufacturer's plate on the case says

MGF Italy
Model Baby 6/30
0.25 hp
8 bar
30 lt/Min (about 1cfm)
6 lt tank

Manual here http://www.obeeliks.com/Manuals/INSTRUCTION%20MANUAL%20MGF.pdf
Made in 2005

 

[HasBean]

Sorry, off topic I know but is there a reasonable website or book or whatever that explains how compressors work (prefferably for the hard of thinking, i.e. me)

Paul

 

[rtbcomp]

Sorry, off topic I know but is there a reasonable website or book or whatever that explains how compressors work (prefferably for the hard of thinking, i.e. me)

Paul

To be honest I'd thought of doing a thread on here on that very subject!

 

[rtbcomp]

.......The one that was completely dead had a burnt-out cutout. I've replace it but it's still having difficulty starting up.

I've some 10/40 in it and it's started running after about 1/2 hour of trying!

 

[eil]

doggy start cap maybe?

 

[rtbcomp]

doggy start cap maybe?

No, these motors have pathetic starting torque and putting the wrong oil (motor oil 10/40) instead of the proper stuff makes it dificult for the motor to start.

What happens is the motor tries to start, but it can't because the oil is too thick so the overload cutout trips after a few seconds. In this time the current in the motor warms the oil a bit. After the cutout has cooled down it has another go.

This cycle is repeated until the motor manages a few revs, this stirs the oil up and warms it further. After a while the oil is thin enough for the motor to start.

Once up and running it will stop & start OK.

Have you seen this thread http://forums.lr4x4.com/index.php?showtopic=9710

 

[eil]

pop it in the oven first for a warm up

 

[rtbcomp]

Some motors start straight away, freezer motors are pretty good. What I thought of doing was devising a way of circulating the oil between several motors, either by installing them in a common sump or pumped circulation.

That way the freezer motors would help the smaller motors along.

IMHO I would bother with a motor of less than 100 watts

 

[eil]

The trouble I encounter with starting is mostly start caps open circuit,and mainly they have electro-magnetic solenoid relay, but you do have some that are ptc and as soon as they warm up they need at least three mins to cool off with no power so as to allow power through to your start winding.

 

[eil]

This might interest you
http://www.ra.danfoss.com/Technical...250250gh_r134a_12-24vdc_09-2008_ed100b302.pdf

 

[rtbcomp]

This is the first time I've encountered a start cap. on a fridge motor, as all the non-cap motors (8 in all) act the same way I'm sure it's not the cap.

 

[rtbcomp]

This shows the most common configurations.

 

[eil]

They are on the high starting torque/ high back press ones those are the ones you should keep an eye out for instead of the ptc ones for what you are using them for, have a look around that danfoss side about the twin comps they are piped together give`s you some ideas. i have a supco capacitor tester its been a verry usefull tool, even though you get them on multimeters nowadays.
If the comp hasent been started in a while I nonmally take the relay off the pins and give it a damn good shake then test contininuity, they have a habit of sticking after lay offs.

 

[rtbcomp]

I've stopped goinbg for larder fridges, I had a LEC that wouldn't pull the skin off a rice pudding. I prefer fridge/freezers - I can use the carcase as a cupboard. It's suprising how many show up on Freecycle with broken pipes at the back and all the gas has escaped

 

[rtbcomp]

Just tried to start it again - wouldn't have it and the cutout started glowing, this is obviously something to do with why it packed up in the first place. I'll either fit another fridge motor to it or dismantle it for spares.