fridge compressor to air compressor wiring problems

[capoman]

hello
iv extract my old fridge compressor and and i want to turn it as an  air compressor 
but iv found dificulties to turn it on directly by mountin a switch
i cant determin wich wire i will use to turn it on

[hermit]

First, it will make a poor air compressor and will probably fail quickly when run open to air.  Was there no schematic with the fridge?  Can you find one online by using your model number?

[james_s]

They work fine as light duty air compressors, my friend has been using one for close to 10 years now. You do need an unloader valve between the compressor and tank though as they will not start against a high head pressure, and some of them are prone to seizing if not run for an extended period.

Most refrigerators have a PSC motor and there are 3 terminals on the motor. One is common which goes to neutral, one is live which goes through the thermal protector shown in the picture there to the motor, and the other connects through a run capacitor to live.

[floobydust]

Welcome to the forum 
Can you give us a make/model for the old fridge? It will help figuring out the wiring by looking at the old appliance's schematic.

[PaulAm]

In the last 10 years or so, most refrigerator compressors are run by an inverter.  Before that they used a start relay and capacitor.  Get the schematic and you should be able to figure out what you need.

Some of the oil used is highly hygroscopic and it may be useful to drain it out and replace it with a similar quantity of normal vacuum or compressor oil.

[shakalnokturn]

When a start relay is used it's the (over) current of the stalled motor drawn on the run winding that triggers the relay to switch the start winding in.
Some models use a PTC thermistor instead of a relay on the start winding. Looking at the photos I'm guessing it is the case here.
A PTC will not allow the motor to start after a short stop, the PTC needs time to cool and return to a low enough resistance.

[Electro Detective]

I hope OP is wearing some personal protection, tight glasses/faceshield and gloves etc

Air compressors and their fittings going -BANG!- don't care if you have family and friends that don't need the bad news 

[capoman]

the fridge model its  REX KBI 300 
and the compressore  model its a ZEM r600a
there is some pdfs on google of that ompressore
showing the plugs .... but i dindt understand fridge schematics ....
i dont know if i shoild use a capacitore as showen in some videos  of converting fridge compressore to air compressor....

thank you all for your answers

[Electro Detective]

@ capoman   there are many Youtube videos about DIY air compressors using fridge compressors

Good luck and play safe mate 

[Ian.M]

Most small hermetic single phase refrigerator compressors use a start winding that creates the necessary phase shift by its high resistance relative to its inductance, without a capacitor, and due to the thin wire used for the start winding it will burn out if continuously powered, so will typically have a high current PTC thermistor in series with the winding to provide a crude time delay mechanism to cut (most of) the power to it after start.   TThis prevents rapid re-starting as the PTC thermistor must cool  first, but it stays hot as long as the motor is running.  There's also normally a Klixon or similar over-temperature trip in series with the common to both windings, to provide some protection if for any reason the motor fails to start.  Both the PTC and the Klixon are usually integrated in the connector that plugs onto the motor pins.

If it didn't have a start or run cap don't add one!

If you need rapid restart capability, the PTC needs to be able to cool down while the motor is running.  Adding a time delay relay to cut start winding power is one approach, either in series with the PTC or replacing it. 

[capoman]

[Ian.M]

Confirmed.  You can see the PTC thermistor (disk on edge between spring contacts at the top) and the disk face on,(bottom left) is probably the Klixon thermal trip.  It looks like the start winding connector contact is the left top one, and the main winding contact is the right top one.  The start winding contact wont go anywhere except the PTC, but the main winding contact goes to the PTC and one or more external connections.   I'd also bet on the klixon contacts being in series with the bottom motor winding contact which should be the common.

We had a thread about repurposing refridgeration compressors last year.  Rather than re-hashing it, I'll point you to the comments I made in it: https://www.eevblog.com/forum/projects/air-brush-compressor/

[capoman]

thank you
but i still dont know how to hook up the wires to make it work perfectly and saafe ?

[Ian.M]

To make the compressor run, you need to power the main winding, with the start winding + its series PTC in parallel.  If you link to the wiring diagram you found we can attempt to explain it, but there's no substitute for hands-on  continuity and winding resistance checking to confirm your wire/terminal identification.

Identify the two wires that go to the Klixon in series with the compressor's common pin, and that go to the main winding pin + to the PTC in the plug.   Apply mains across them and it should start and run.

Once you've got it to run, making it work as a usable air compressor is *LOT* more complex - see the thread I linked, but if you cant even get it to run, you are S.O.L. and cant proceed further. 

[soldar]

Always provide a pressure relief valve if you want to prevent explosions.

[Ian.M]

Yes.  In the old topic I also recommended adding a burst disk for a last resort final safety backstop in case of both pressure switch failure and safety valve clogging.

[floobydust]

the fridge model its  REX KBI 300 
and the compressore  model its a ZEM r600a

I still don't have the compressor model number. ZEM is the manufacturer, r600a is the refrigerant.
Fridge compressors need a capacitor (start or run) and so far it is missing from your pictures.

[Ian.M]

Fridge compressors need a capacitor (start or run) and so far it is missing from your pictures.

That may be true of the 120V 60Hz compressors on the large fridges common in North America, but in the 220V-240V 50Hz rest of the world, small and medium domestic refrigerator and freezer compressors traditionally have used a thin wire start winding so its resistance dominates and provides the required phase difference relative to the inductive main winding, without a capacitor.

See "Split Phase Motor" here: http://people.ucalgary.ca/~aknigh/electrical_machines/other/split_phase.html

[soldar]

Googling for compressore zem gives dozens of hits including some youtube videos with explanations. You will find more info in Italian than in English.

[floobydust]

Fridge compressors need a capacitor (start or run) and so far it is missing from your pictures.

That may be true of the 120V 60Hz compressors on the large fridges common in North America, but in the 220V-240V 50Hz rest of the world, small and medium domestic refrigerator and freezer compressors traditionally have used a thin wire start winding so its resistance dominates and provides the required phase difference relative to the inductive main winding, without a capacitor.

See "Split Phase Motor" here: http://people.ucalgary.ca/~aknigh/electrical_machines/other/split_phase.html

I think this is what you are proposing, no start/run capacitor at all.

[Ian.M]

Yes.  That diagram has a split phase motor with the PTC starter in series with the start winding and the Klixon thermal trip in series with the common.  You can see the PTC and the Klixon integrated in the push-on three pin motor connector in the photo in the O.P's reply #10, so its 99%+ certain that's what he's got.

[capoman]

Modell : ZEM  HQM 12AA

[capoman]

this its the board where the cables are connected

what if i try to start it normally without the ptc   just with the 3 pins   Comun Run Start

[floobydust]

It's hard to figure out the wiring block. I can't see where the black wire goes, or the brown from the Klixon. That terminal blade seems top have no wire.
My guess is to short black wire to brown wire (a to b), and leave the extra blue wire unconnected. Yellow-green is protective earth-ground.
HQM 12 AA 2260050592 uses mineral oil 210cc, 12A start 1A run.

[capoman]

iv tried to follow wires  to make this simple diagram
iv tried also to test the terminals 
i hope i did the right setting on the multimeter  :p