Remote Compressor Start/Stop

[wbrian63]

A couple of members have asked me about the remote compressor control I've got in my shop.

Here are the details of the setup. It revolves around a DP (Definite Purpose) contactor I got from Grainger.

http://www.grainger.com/Grainger/items/2UTN5

It's rated for up to 5hp at 230v.

Think of it as a big magnetic switch. Apply voltage to the coil and the magnet energizes and closes the contacts in the switch, passing voltage thru from the line to the load.

The coil on this contactor is rated for 120v, so all you need to do is get 120v to the coil when you want the compressor to run.

Rather than wiring a direct circuit, I used a remote-control Insteon appliance module I got from Smarthome.com:

http://www.smarthome.com/2456S3/App...Plug-in-Appliance-Control-Module-3-pin/p.aspx

This device is nothing more than a small DP contactor that can be triggered by other Insteon devices. I also bought a Insteon relay switch (don't want a dimmer in this situation):

http://www.smarthome.com/2476S/Swit...ontrol-On-Off-Switch-Non-Dimming-White/p.aspx

I put the DP contactor in an enclosure like this one:

http://www.homedepot.com/h_d1/N-5yc...splay?langId=-1&storeId=10051&catalogId=10053

Ran 220v into the box to one side of the contactor (there are 4 spade lugs on each side, 2 for each pole of the switch). From the other end of the contactor, two wires out to the pressure switch on the compressor.

Added a cord to the box with a 3-pole plug on the end. Hot to one side of the coil, and neutral to the other side of the coil. (I put the hot on the coil connector in the back of the box - there are 2 spade lugs for each coil connection, and the bare lug is hot when 120v is present).

Plug the cord into the appliance module. Plug the appliance module into a wall socket.

Install the switch in a box that has hot and neutral wires (must have neutral). Cap the line lead from the switch.

Press and hold the "on" side of the toggle until the switch beeps. Within 4 minutes, press the "learn" button on the appliance module to link the two devices.

Insteon uses RF and neutral line X-10-ish protocols so it's very reliable.

Of course, you don't have to use the Insteon controls, that adds $80 to the cost, you just have to have a switched 120v line going to the contactor coil.

If the compressor is like mine (Quincy QV41C60VC), it has an unloader valve that only engages when the pressure switch reaches full value. That can be a problem if you turn the compressor off before it reaches full pressure and try to turn it on before the head pressure has a chance to bleed off.

I solved this by t'ing in a electric valve at the check valve on the top of the tank. The valve is normally open (requires voltage to close the valve).

This is completely optional. If you don't cycle the compressor on and off in mid-cycle, or if you wait a few minutes from off to on below full pressure you should be fine.

I started out with this valve:

http://www.grainger.com/Grainger/items/2G499

I already had this valve from another project.

It's perfect because it's a 1/8" valve, and the check valve has a 1/8" port.

However, the coil is making a hell of a racket when engaged, and the max operating pressure is 150psi, while my compressor cut-out is 175psi. I don't know if this is because the valve is old or damaged or if it's PO'd about the extra 25psi.

I've just ordered this - part # 4738K712 - rated for 175psi operating pressure:

http://www.mcmaster.com/ctlg/DisplC...track=true&ScreenWidth=1280&McMMainWidth=1066

The valve is wired to the free pair of lugs for the 120v coil on the DP contactor. When the contactor is triggered, the valve closes. When the contactor is released, the valve opens and vents the compressor head pressure.

I also have another one of these valves on the drain for the compressor, but the coil is a 220v model - I wired it up to the terminals on the pressure switch. When I cut the power to the compressor, the tank dumps completely. I'm frequently away from the shop for days and sometimes weeks at a time - I prefer to completely drain the tank every time I leave.

Total cost:

Contactor: 24.33
Enclosure: 14.77
Insteon Switch: 45.99
Insteon Appliance Module: 34.99
175psi Unloader valve 91.89

You could get all this for as little as $39.10, or blow the whole wad for $211.97 (plus freight and applicable sales tax).

I welcome questions and suggestions.

Regards

 

[BigE]

Could you explain the pneumatic circuit? I'm trying to understand where that valve is dumping head pressure vs. tank pressure vs. open air, etc.

 

[wbrian63]

Could you explain the pneumatic circuit? I'm trying to understand where that valve is dumping head pressure vs. tank pressure vs. open air, etc.

As I understand the concept, dual-stage compressors utilize 2 or more cylinders to boost output pressure. The first stage compresses ambient air and outputs it into the intake for the second stage, which compresses the air further.

The compressed air exiting the second stage travels to the storage tank, passing thru a manifold at the tank inlet. This manifold contains a one-way (check) valve, and 3 ports. This check valve prevents the tank from emptying back into the second stage when the compressor isn't running.

Attached to the second port on the manifold is a safety pressure release valve. This valve opens to vent excess pressure, should the compressor fail to shut off at cutoff pressure.

Attached to the third port is a copper tube that travels down to a valve inside the pressure switch box. The valve is not physically part of the pressure switch, it's just under the pressure switch cover.

Both of these connections are "upstream" of the check valve (or to look at it another way, they're between the second stage outlet and the check valve).

When the pressure switch senses that the desired cutoff pressure has been obtained, a mechanical "trip" occurs in the switch, which breaks the circuit feeding electricity to the motor. This trip also depresses a pin which actuates a one-way valve on the end of the previously mentioned copper tube.

When the valve is opened, all of the pressure between the check valve on the tank and the compressor is released.

This is called the unloader valve, and the work it does is referred to as "unloading the compressor."

The reason for this unloading event is to allow the motor to spin the compressor when next activated by the pressure switch, without having to overcome the high pressure that would be present in the second stage.

Attached to the pressure switch is a lever that swings to two positions - 'Auto' and 'Off'. When manually moved to 'Off', a lever actuates the same pin the pressure switch depresses, unloading the compressor.

This is a VERY simple drawing of the previous layout description:

OK - so that's how it worked on my pre-modified compressor.

In my scenario, the pressure switch is left perpetually in the Auto position. The DP contactor sends electricity to the motor when energized, and breaks the electrical connection when released.

If the signal to the DP contactor is removed after the compressor has reached full pressure (in which situation the unloader valve will have already done its job), no issue exists.

If the signal to the DP contactor is removed at any time before the compressor reaches full operating pressure, the unloader valve will not operate and pressure will remain in the second stage between the compressor outlet and the check valve. If the compressor is restarted while this condition exists, the load on the motor will be excessive.

To solve this problem, I removed the safety pressure release valve from the manifold at the tank inlet and replaced it with a street-ell (male threads on one end, female on the other). To that ell, I attached a 4" pipe ****** to bring the pipe up above the compressor/motor framework. On the end of the 4" ******, I added a tee. One port in the tee gets the safety pressure release valve I previously removed. The other hole gets a normally open electrically actuated valve.

Normally open means that the valve is open when no voltage is present, and closed when voltage is applied.

The same 120v signal that causes the DP contactor to "make" and send electricity to the pressure switch, and from there on to the compressor motor also energizes the previously mentioned valve, causing it to close. If the signal voltage is interrupted at any point, the valve opens and unloads the compressor. If the compressor has already reached operating pressure and the mechanical unloader valve has opened, then when he electric valve opens, nothing happens, because there's no pressure in the circuit to vent.

This is truly a "belt and suspenders" implementation. It is conceivable that I might come into the shop and need air very briefly. I might turn the compressor on, wait until sufficient pressure was achieved, do what I needed to do and shut the compressor off. The only likely scenario for this type of behavior would likely involve me leaving shortly thereafter, and the compressor would remain "loaded" when I switched off the DP contactor remotely.

That being said, the second stage will eventually vent off through normal leakage, so leaving for even 2 or 3 minutes, returning and restarting the compressor would probably cause no problems.

I just like being 100% sure that I'm not going to accidentally harm my brand new "pretty blue thing."

I hope this answers your question without being too simplistic.

Regards

 

[larry4406]

Good write up.

Me - I leave the tank charged while not used so it is always ready. The compressor is by the shop door. When I leave, I close the ball valve on the tank discharge so any minor line leaks don't cause the compressor to run, turn the control to "off", and walk away.

 

[wbrian63]

That's one option.

Sadly, on occasion, I may be gone from the shop for weeks at a time.

I have another valve attached to the tank drain that is closed when power is applied to the pressure switch. When power is removed, the tank dumps.

This way, I never have to remember to drain the tank.

Trade-off is that the compressor has to recharge every time I come to the shop...

 

[jake26]

Very nice writeup. I am not ready yet but I am gonna tag this one!

 

[torqueman2002]

Very nice writeup. I am not ready yet but I am gonna tag this one!

+1

 

[tig]

I was planning on using my home control system to control power to my compressor.

See here:

http://www.garagejournal.com/forum/showthread.php?p=1409479#post1409479

This thread, particularly the part about the unloader valve, is giving me pause.

My two-stage compressor (see thread above) only has one cylinder. Does it also have an unloader valve, and should I also be concerned about undue wear if I power the compressor off mid-cycle occasionally?

 

[wbrian63]

I don't know how the compressor can be two stage with only one cylinder. Typical design of a two-stage compressor is first cylinder intakes outside air and compresses it. The compressed air is fed to the second cylinder which further compresses the air. Hence the two-stage designation.

Regardless, it's hard to see if there's an unloader valve. On my Quincy, it's part of the pressure switch. You know if you have one if you hear a hissing sound after the motor cuts off at full pressure.

I would be surprised if that small a compressor has a unloader valve.

The only time it matters is if you attempt to restart the compressor within a few minutes of shutting it off before the unit has reached cut-out pressure.

The unloader valves that are part of the pressure switch are only triggered when the unit reaches cut-out pressure. If the compressor is turned off before it reaches that pressure, the pressure remains in the cylinders and makes it hard to restart. However, after a few minutes, the pressure will dissipate via normal leakage internal to the compressor.

Larger compressors have unloader valves that are mechanical and connected to the crank of the compressor. They're valves that are closed with centrifugal force when the compressor is running, and open when the compressor stops. They don't care whether the compressor has reached cut-out pressure or not.

I wouldn't worry about it.

 

[tig]

I have successfully finished the project of making my compressor power be remotely controlled by my home automation system. Details in this thread:

A photo of the pump-start relays in place:

 

[cdenton]

Well, I am all for doing this... My main motivation is that if just left on the lines will bleed down over time causing the compressor to cycle unnecessarily. To combat this in my current compressor I have a manual valve that I rotate when I am going to use the compressor. This works swimmingly well as long as I remember to close it when I leave the shop. In my new shop I am planning to do the same thing. I will place a ¾ 120v actuated solenoid valve to the output of the compressor and wire it in to a light switch near the door. I should be able to do this for $100 or so. This offers everything in your solution with the addition of the compressor not bleeding down and with the subtraction of the auto drain. This also simplifies needing to vent the accumulated pressure at the high side for restart.
Another potential mod would be to put a IR presence detector to auto switch the valve.
Let me know what you think.
CD

 

[tig]

@cdenton - In my case, I don't think I have any leakage. Time will tell.

The down-side, I think, of only shutting off the air supply, and not the power, is you still run the risk of a disaster if a seal or valve goes wrong on the compressor side of your solenoid valve. FWIW, the Hunter PSR-52 pump start relay was ~$50...

 

[cdenton]

That is a good price on the motor starter. It has been my experience that over time at the very least you will have some leaks at rotational joints etc for reels etc. I suspect a combination of our solutions would yield a bulletproof solution. You would just need a relay for the 110 to 24vac signal to open both at once.
CD