View Full Version : Old Prestcold converted to air compreesor

Iain 1959
29-10-2016, 09:52 PM
Hi I bought an old Prestcold that had been converted years ago to an air compressor when I went to see it I saw it run for 20 minutes or so nice and quite,anyway took it home and when I tried it it would not run see this video

29-10-2016, 10:47 PM
Is it controlled by a pressure switch?

If you keep restarting it like that it won't last long.

30-10-2016, 08:09 AM
Hi Ian.
Welcome to the forum.
I am thinking much along the same lines as Brian.
The comp sounds as if it is pushing against a blockage?
Which is why the pressure switch is relevant.
Is the pressure switch wired up correctly.
IE. dumping surplus pressure once desired air pressure is reached?

Just a thought did you purchase all the air lines etc to the air receiver?
Is there a non return valve in the air discharge line?
It sounds as if you are back pressurising the discharge and the comp will stall and get progressively worse as is shown!

If it ran before disconnection then you have reconnected something differently.
Is this comp 240v or 3 phase?

Iain 1959
30-10-2016, 09:20 AM
Thanks for your prompt replies.

I bought it complete with a converted Calor gas bottle as a receiver, it has no pressure switch the previous owner just turned it off at the mains when the pressure was what he wanted. I have completely taken it all apart, the readings for the windings are 3.4 8.5 & 10.7 ohms the 2 capacitors look ok but there wiring is too bad for reuse. I took the end off the motor and it turns fine also it has a sump full of clean oil.At some time in its life it was water cooled it had a water tank attached directly to the cylinder head. I will put up some photos but at the moment the photos on my phone cant be uploaded wrong format.


30-10-2016, 11:58 AM
No pressure switch fitted? Make sure you have made a will as odds are you are going to have a fatal accident if you operate this contraption. Ever heard of Health and SAFETY!

Iain 1959
30-10-2016, 01:13 PM
Thank you for your concern but I really need help in other ways to see if I can get this timepiece running again on my shopping list is http://www.ebay.co.uk/itm/142046075025?_trksid=p2055119.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AIT

Iain 1959
30-10-2016, 01:29 PM
I have now got these pictures up


The Viking
30-10-2016, 02:17 PM
Hi Iain,

If you unbolt the valves from the compressor you can test run it without any chance of blockages in the lines affecting it, se the green arrows in the picture for the bolts to remove. When you refit them make sure you use a good quality gasket sealant on the discharge side (less critical on the air intake side).
At least this way you know if there is an issue with the compressor itself or something in the lines.


The Viking
30-10-2016, 02:41 PM
Sorry Iain,

I just have to stress H&S...
Your Calorgas bottle is likely to have a test pressure of 10-15 Bar, that compressor of yours is capable of providing 40+ Bar!
To run it safely there need to be a high pressure cut out switch AND a high pressure relief valve. As the pressure builds up the switch should cut the compressor out, should the switch fail the valve will then dump the air.
Depending on the state of your Calorgas bottle and its marked test pressure the switch might be set at 2 Bar and the valve opening at 2.5 Bar.

To run your setup without safety devices is like checking the level of your petrol tank with a lit match...


Iain 1959
30-10-2016, 05:52 PM
Ok everyone as the pictures show it is just a bare unit at the moment I need advice on the the readings of the windings I need to know what the 3 terminals on the motor are I need to know what capacitors to use and a circuit diagram to give this a modern circuit for safety, in the video I think it was the klixon making it drop out??

30-10-2016, 07:52 PM
Hi Ian.
I have sent you a little booklet which has the wiring setup for the prestcold A10.
Hopefully it may help as it also explains how to test the motor windings?
Sadly the norm is a 10% difference between each phase winding!
Your 3.8 winding reading may be the issue!

30-10-2016, 08:55 PM
hi to identify your 3 winding pins
run to common = 3.4 ohm ( low reading)
common to start =8.5 ohms ( medium reading)
start to run =10.7 ohms (high reading)

your run and common windings should add up to or same as start windings
e.g 3.4 +8.5 =11.9 ohms and start winding is 10.7 ohms
therefore the readings are just about ok , i would change the klixon and also the run capacitor because if the run capacitor is faulty then the amps would go up and cause excessive heat to break the klixon

Iain 1959
30-10-2016, 10:09 PM
Thanks for that do I need to megger the windings ? Is there a way to test the klixon, what value capacitors should I use?

31-10-2016, 06:12 AM
Did you receive the booklet I emailed you?
If you did have a look at pages 7 & 9.
The Info may Help?
Good helpful advice Ghaz!

31-10-2016, 11:00 AM
Hi the capacitors depend on the horse power the motor is.if tell us what model or horse power then I can find out the capacitor sizes

Iain 1959
31-10-2016, 01:05 PM
Thanks got it

Iain 1959
31-10-2016, 02:04 PM
Label reads as follows
Model 100C62/SN
Part No. 17/5 916
220/240V 50C.P.S.
HP1 Phase1 4.9 Amps

31-10-2016, 06:44 PM
Hi going by the motor ohms reading the motor seems like its 0.5hp
which has 60uf start capacitor and 5uf run capacitor

your label info is 1hp
which has 220uf start capacitor and 15uf run capacitor

Iain 1959
31-10-2016, 07:28 PM
Thanks i'll take the old ally can ones off and see if any marking servive

Iain 1959
31-10-2016, 07:51 PM
Surprise both are the same


100uf 275v

Made by TOC marked Prestcold part no 15/2/293

Iain 1959
05-11-2016, 05:07 PM
I seem to have come to a stop in my last post I stated that both capacitors where the same & 100uf made by Prestcold, other replies have stated that I need 60uf & 5uf and 220uf & 15uf am I right in understanding that my 2 100uf go against the rules for start & run capacitors. What I really need to do do is to get 2 capacitors and the most basic wiring diagram to test run this compressor before I spend any more money ie if the windings need replacement this would not be economical.

monkey spanners
05-11-2016, 05:32 PM
Two 100uF capacitors wired in parallel make a 200uF capacitor (this is the start capacitor and is only in circuit for a fraction of a second as the compressor starts) There should also be a run capacitor, as with the start capacitor its capacitance value is matched to the motor size and can be a single capacitor or a number of them wired in parallel to get the correct value.

Iain 1959
05-11-2016, 06:50 PM
Hi no just these 2 if you look at the video they where mounted under the frame the whole thing was mounted look under the red light.

06-11-2016, 09:00 AM
What is on picture 4? Start relay?

06-11-2016, 10:49 AM
Or Maybe a Klixon (Thermal cut-out ?)

Cheers Grizzly

06-11-2016, 03:33 PM
Picture 4 looks like the old style prestcold relay, if you undo the two side screws and remove the cover you will find the coil and contact assembly. Was common to find the contacts either too pitted to make contact or arced together, just used to clean contacts with a small flat file, but if the coil is u/s that won't work. As you have 100 in the model i am fairly certain it is a 1 HP as prestcolds later models were S50-1/2hp, S75-3/4hp, S100-1hp,S200-2hp etc, etc.

06-11-2016, 03:46 PM
Hi your run capacitor should rated for 400v