another compressor question

[Frances IoMParticipant @francesiom58905]

having created a small enclosure with some small amount of sound abatement I had intended to use a contactor to remotely control my small compressor (12L Impax cheapie from Screwfix) – however the instruction book states very clearly that on/off switching must be done via the compressor mounted switch – why?

[Frances IoMParticipant @francesiom58905]

remote switching of compressor

[steveteeParticipant @stevetee]

Because switching off at the switch vents pressure from the line between the pump and the reservoir, so that when the motor restarts it is against no load. If the motor is switched off externally then pressure is left in the pipe so the motor could be starting against 80 psi or more.

[Frances IoMParticipant @francesiom58905]

thanks – that makes sense – but raises the query how is that pressure released and could that be activated remotely? – I did however build the enclosure so that the switch is readily accessible but just wanted to avoid the walk to the compressor when I realised I needed to use my air blow gun

[Gordon WParticipant @gordonw]

I usually leave my cheap compressor switched on at the mains, but if switched off at mains and later switched on either the tank is still up to pressure and it does not switch on, or it has lost pressure and the switch on the machine turns it on. Which is what stevetee is saying really.

[Les Jones 1Participant @lesjones1]

The on/off switch on the compressor must also be an unloader valve. You could add an NORMALLY OPEN solenoid valve to bleed of the air pressure between the compressor output and the connection to the reservoir when the power is removed. There needs do be a non return valve on the inlet to the reservoir for this to work. I think there must be an air pipe connected to the on/off switch for the original system to work. A diagram of the piping which shows the location of the pressure switch and any valves on the compressor would be helpful. It is possible the pressure switch could also be part of the on/off switch.

Les.

[Frances IoMParticipant @francesiom58905]

I can see a pressure detector connected to the tank with two leads attached leading towards the motor but as yet have not removed the cover – since the manual states that when the reservoir pressure switch operates the air in the pump cylinder,.. supply tube and pressure switch is discharged – thus if I monitor the current to the compressor and only operate the remote switch when motor is not running then it should stop with a full reservoir but in a fit state to restart when mains is again connected – feeding the live supply wire thru a small toroid with a small secondary winding the induced current being detected by a small bit of electronics might work.
Or am I missing something ?

[MM57Participant @mm57]

Good question as I plan to do the same (remote switch on cheapy screwfix compressor) and saw the same "warning" in the manual.

I really can't see the problem in using a remote switch anytime that the compressor isn't running? Whether it's completely discharged, at some intermediate pressure due to some leakage/use since last powered off or at full pressure?

I have noted no loss of pressure at all from full over at least a week of non-use. I suppose I shouldn't really leave it like that though…

[Les Jones 1Participant @lesjones1]

The pressure is only released in the pipework between the compressor and the reservoir as there is a non return valve where the air enters the reservoir..

Les.

[Clive FosterParticipant @clivefoster55965]

Assuming you have the common push button on-off switch with subsidary pressure operated contacts the simplest way is to leave the compressor mounted switch on all the time and put your remote contactor in the feed to the compressor switch. Worked fine for a goodly number of years on my old compressor which, like yours, lived in a hutch outside the workshop for noise abatement reasons. Another advantage was that the reomote contactor made the system NVR so the compressor wouldn't automatically restart if power was lost. The standard switch and pressure control dooby had a simple mechanically locked pushbuttons.

Clive

[Frances IoMParticipant @francesiom58905]

the switch is mechanically locking on/off (tho with an illuminating power indicator light) rather similar to much domestic equipment – don’t think it has any additional mechanical connections – seeing the pressure sensor has two mains insulated leads suggests to me some form of electrical operation to operate a pressure discharge valve

[Phil WhitleyParticipant @philwhitley94135]

I have the same problem with my small compressor, which is a Clark with a 3hp 2860 rpm motor. If you use a contactor to control the feed to the compressor it will work fine a long as the contactor stays on till the compressor is full and switched off by the pressure switch, which also dumps the pressure between compressor and tank. If the contactor switches the power back on without dumping the pressure, it will blow the 13A fuse in the plug. This always happens when I am going round the workshop last thing turning everything off, and the compressor is running, and I turn it off at the wall instead of using the red pull up/push down switch on the pressure switch. Turn it of on the compressor, and it dumps the pressure in the feed pipe regardless of whether the tank is full or not, so when you restart, no problems. To put it simply you cannot control this type of compressor with a contactor on the feed. Why not put a long control rod from the switch to the outside of the soundproof cabinet, or use a cycle brake cable to operate the switch. There is a way round this, but it depends on using the switch on the compressor to control the relay in such a way that when you switch off the contactor, the compressor continues to run until the tank is full, the back pressure is dumped, and then the pressure switch interrupts the feed to the contactor, and the compressor will not restart, even if the pressure drops. when you next turn the contactor on the compressor will restart when it needs to, or instantly if the pressure has dropped overnight. Having thought about that it is getting so complex that it would probably be easier to remote mount the pressure switch and extend the air pipe to it. I will look at mine tomorrow, and may come back with a solution.

Phil

[Clive FosterParticipant @clivefoster55965]

Phil has a valid point but I never found it a problem after the first time. A bit of discipline and feel for how long it takes to pump the tank up were sufficient for me. Could put an input pressure guage on the regulator / drier, or dual gauge ones aren't that much more than singles. Pressure switches can be got to wire in line with the start button or you could probably do something similar via the switch on the compressor itself. Maybe simplest to wire an indicator light across the motor so you don't try to stop it whilst its running but that doesn't help trying to restart against a full pressure head if you do have to stop willy nilly.

Clive.

[Author]

Posts