It looks like a capacitor start, capacitor run motor. Some of these type of motors have limited numbers of starts per hour or require some minimum time between starts so not shutting off the motor when at pressure may be due to this to ensure it does not overheat. The other possibility is that the motor will struggle to start if it is trying to work against existing pressure in the tank on restart. I have a compressor that bleeds off air until the motor is up to speed to allow it to start in a reasonably short time.

Martin C

Whether it's worth 'doing' depends to a large extent on what you are going to be using it for and whether, once done, it will be the machine 'that you want it to be'…

What capacity is it, and what tasks do you hope it to perform with it?

..it looks like quite a small receiver for that type of pump, but it's hard to tell from a picture…

Edited By DiogenesII on 04/09/2023 06:54:11

Edited By DiogenesII on 04/09/2023 07:05:14

this looks like a commercial machine that hardley ever switches off….old style……

not sure about the electric side of things but the big fitting on the bottom of the copper pipe looks like a non return valve…….

a standard on/off pressure switch, the hiss u hear when it shuts the motor down is unloading the pressure from the comp down to the non return valve…..

that way the motor will start without pressure in the system……

personally I'd try a new type switch…..less tha £20…..

just have to get a couple of new fittings so that u can use the switch and the saftey valve from one tank fitting / port….

the small pipe are the unloader valves…..won't be needed with the new switch…….

nice old machine either way…..

Just a guess but I notice the heads have an air intake at the front, and connect to a shared outlet manifold at the back, which then connects via a big pipe to what could be a non-return valve on the receiver. In addition both heads have thin pipes leading to the top: I suggest these move a valve that directs air back to atmosphere rather than compressing it into the tank. The higher the pressure in the tank, the more air is escapes back out the inlet, greatly reducing the load on the motor.

Not an efficient set-up, but it allows the motor to run continuously which is good for reliability because frequent restarts are bad for single-phase motors. I think the compressor is either designed for continuous operation or it predates electrical motor control. Another system uses a clutch to disconnect the pump from a continuously running motor.

I wouldn't attempt to modernise it: if it works leave it be.

My main concern is the receiver. Air tanks tend to collect water, which causes them to rust inside on the bottom, maybe cutting a groove into the metal along a water line. A tank that's perfect on the outside can be badly damaged inside. Poor maintenance or long-term storage with water inside are widow-makers.

Dave

Small continuous run compressors fitted with unloader valves frequently use relatively small motors that don't have sufficient power to start against a pressurised tank. Motor is fine running up to pressure but doesn't have the oomph to start it under load.

To convert it to  motor start – stop operation you will need a modern pressure controlled starter switch with an outlet vent in the off position.

Easy way to set up is to add an auxiliary tank / manifold to the outlet side connected to the tank via a non return valve and vented via the pressure switch. When the tank reaches the set pressure the motor turns off and the manifold vents down to atmospheric pressure so there is no back pressure when the motor restarts. Auxiliary tank / manifold doesn't have to be very large. From memory the outlet collection manifold on my Atlas Copco KE1 16 cfm rated Vee twin was around 3" diameter and a bit over 1 ft long. Plenty big enough. Your compressor looks smaller so maybe your manifold is large enough as is.

Swanky way is to add an electrically controlled valve between the tank and the unloader valves in place of the existing pressure controlled valve. Use a time delay relay to open the valve for a few seconds when the pressure controlled motor switch kicks in.

I have a PDF of the KE1 manual with the basic details on how continuous operation and starter pressure switch unloading is set up on commercial machines. PM me if you'd like a copy.

I simply vented the manifold and the KE1 worked fine. Far as I know it's still going strong 3 owners and 20 years down the line. I'd have heard if it broke!

Clive

Edited By Clive Foster on 04/09/2023 10:09:45

By the look of it it could be 50+ years old. Before you modify it I would do a hydraulic test ! IF it proves sound then consider modification. The motor may not be large enough in a stop/start set up and it would be a good idea to use a Dol starter to protect the motor (correctly rated ). Noel..

If it were my compressor before I messed about with pressure switches etc I would get an air filter put on the left hand cylinder, as it is at the moment it is sucking in all the c**p in the workshop air. The right hand air filter doesn’t look too special either.

If it's any help in sorting out what you need to do I used to have an almost identical compressor and it was badged as Sherry's of London. Mine had a Danfoss pressure switch that had a small pipe to the compressor to unload the compressor for starting and if memory serves a non return valve between the compressor and tank to allow the compressor to unload. Long gone as I needed something with greater capacity, but it was nice and quiet and would run a small spraygun. Search for Sherry's compressor may turn up some information, there again you may just get a cocktail recipe!

Just tried a google search for Sherry's air compressor and there are a few results, there's one for sale that comes up for a vintage and salvage company identical to mine and similar to yours,the one in red faded as mine was to a fetching pink, that shows the Danfoss switch. Yours may just have had the pressure switch and associated plumbing removed .

Edited By Dave Wootton on 07/09/2023 12:18:55

Trying to use a pressure washer as a hydraulic source will almost certainly result in a receiver failure ! Many pressure washers run to 1500psi and some MUCH higher ! 1.5 X working pressure eg 150 = 225 psi Noel.

Only if the tester is so dumb that they do not Tee-in a bypass, so the pressure at the receiver is carefully adjusted. I did write the word ‘carefully’ in my reply, but I do know that many don’t take notice of what is actually written.🙂

I expect a diesel fuel injector ‘pop-tester’ could be used to raise the pressure – but with such a small volume per stroke and hand operated it would take a while.🙂

Doubtless some pressure washers will attenuate to a quite low pressure delivery – but, yes, any dim-wit supplying full pressure directly from a pressure washer (100Bar upwards) could end up with a scrap receiver.

Unfortunately the term ‘engineer’ (in the forum title) does not run true to the term when used to describe a proper engineer.