Dear Members:

I kindly ask for your help in a project...while I have Mechanical Engineering background (retired), I am not very proficient with electrical-electronic issues.

I have a 1/3 HP single phase motor that will draw 7-8 amps at start-up...This motor will be running a High Pressure compressor that has a 5 amp 3000 psi pressure switch.

I want the Pressure Switch to stop the motor but I don't want it to turn it back on when the pressure is vented...I want to use an ON-OFF-MOMENTARILY ON toggle switch to start it at the Momentarily ON position and then let the pressure switch turn it off when the pressure reaches 3000 psi.

In other words: I don't want the PS controlling both: The ON and OFF cycles, I want it just for the OFF part of the cycle upon reaching 3000 psi and I want to use the toggle's Momentarily ON position for starting the motor once again.

This is a precautionary feature I am trying to implement for preventing unexpected motor start ups (accidents)

I believe that I will need a relay ???? but I don't know how to wire it and I don't know the specs I need for the relay either. Any help will be greatly appreciated.

Best Regards,

AZR

 

What you need is a relay with its coil in parallel with the motor AC supply, same voltage, on the relay use a N.C. contact in series with the N.C. pressure sw.
Across the relay N.C. contact, wire a N.O. P.B. switch to reset the circuit when the pressure drops.

Max.

 

I erred, make that a N.O. contact on the relay and the P.B. will be also required to initiate the first start/run also.
Max.

 

I think this is what you asked for:

 

Thank you so much for your help and prompt response Max and #12...I have another question that I just thought:
Since the Pressure Switch is only 5 Amps and the motor supposedly draws 7-8 amps (Start up), how do I deal with this issue? Should the motor be started from the relay ?
Is there any particular relay (Brand and Model) that you would recommend?
Thanks,
AZR

 

I think this is what you asked for:

Your mom P.B. is shown N.C. I believe it should be shown N.O. as also the P.S.?
Max.

Since the Pressure Switch is only 5 Amps and the motor supposedly draws 7-8 amps (Start up), how do I deal with this issue? Should the motor be started from the relay ?
Is there any particular relay (Brand and Model) that you would recommend?
Thanks,
AZR

The fact the switch will already be closed when starting the motor it will not see the start up current on the contact closure, especially with the circuit shown, I think an Ice cube relay shown should do it, P&B, Omron, Idec are a source of relays.
Max.

 

I could re-design that circuit to have the pressure switch only control the relay, but that is not safe. Relays get stuck. Besides, nobody uses parts with their normal run current rated for the start surge current. You probably already have a Pressure Switch that is rated to carry the start surge. You can look for a Pressure Switch that is rated for 10 amps of normal run current, or you can look for pressure switches that have their Maximum Surge Current listed in their specifications.

"Safe" is terribly important here. If this fails, you would have a serious explosion.

There are lots of relays rated for 10 amps at whatever voltage you are using.

 

Your mom P.B. is shown N.C. I believe it should be shown N.O. as also the P.S.?
Max.

That's what I had in my drawing inventory, so I labeled them to remove any doubts.

 

I would likely do it using a relay as Max posted available from McMaster Carr Supply Switch Two Circuits On or Off (DPDT)—8 Pins Part Number 7170K21. Part Number 7122K19 is the needed relay socket. The drawing below reflects a 3PDT relay so the pin numbers for the DPDT will be different. The below circuit shows Run and Stopped lamps which are not really necessary but sometimes a nice to have. I assumed 120 VAC mains power so for other mains voltages the relay should be changed accordingly. The start button is just a normally open push button switch, any of dozens would work. I also added a normally closed E-Stop push button switch which is really optional but would serve to shut down a running compressor if you wanted to shut it down before tank operating pressure was reached.



Any of the circuits or circuit info posted would work just fine, just a matter of what you want or need to meet your requirement.

Ron

 

This was my original concept.
Max..

 

Wow!...Thank you all very much, this is a very friendly & helping Forum!
I ordered the parts and will try all three diagrams for the purpose of learning how these things work.

The only change I will make is to try a single PB instead of 2 where suggested, I will like using one PB for everything as I am Too old and more prone to accidents; the SPDT Toggle On/Off Mon ON switch
seems to be the answer to my concerns...Very Simple Operation: Turn the PB to the middle ON or NC position and it will be ready, move the toggle momentarily to the right and the motor/compressor will start and then it will spring back to the ON (NC) position. If I have an emergency turn the PB all the way it to the left for power OFF (NO) position. Does this sound reasonable?

The ON lamp circuit is a nice feature, I will be using this circuit for an Hour Meter...It is very important to keep track of compression times for appropriate rebuilding the compressor's pistons, general maintenance and lubrication.

Once again, thank you all for your time & courtesies to my requests.

Best regards.

AZR

 

I could re-design that circuit to have the pressure switch only control the relay, but that is not safe. Relays get stuck. Besides, nobody uses parts with their normal run current rated for the start surge current. You probably already have a Pressure Switch that is rated to carry the start surge. You can look for a Pressure Switch that is rated for 10 amps of normal run current, or you can look for pressure switches that have their Maximum Surge Current listed in their specifications.

"Safe" is terribly important here. If this fails, you would have a serious explosion.

There are lots of relays rated for 10 amps at whatever voltage you are using.

Serious question: is a relay more likely to "get stuck" (i assume you refer to the contacts welding shut?) than a pressure switch? I can't say I've ever seen a pressure switch stuck, but I have seen liquid level switches stuck, so without having knowledge of the internals of both types of switch i can't predict if one is less suceptible to getting stuck than the other, but until i read your post my gut swag would have been that they have equal chance, as well as both having the same chance as a relay. Now I'm totally uncertain. Please enlighten me.

 

On the bright side any good well made compressor has a purely mechanical pop off valve so if the pressure switch does fail and the compressor continues to run the pop off valve will release tank pressure.

Ron

 

@Reloadron
One problem here might be in the definition of, "compressor". Some people think a compressor is the entire assembled machine with motor, tank, controls, and safety devices. I don't. I chronically work with just the motor/pump assembly. I believe the TS is in the same situation. Most of my machines will pop an over-current safety in the motor windings before the pressure gets to 500 PSI, but most tanks sold to the public are not rated that high. Even a 1/4 HP refrigerator compressor can turn a Harbor Freight tank or a re-purposed water heater tank into a bomb. It might require an hour or more, bit it WILL blow if there is no Pressure Switch or pop-off installed. I'm sure a few Shade Tree Mechanics have collected a Darwin Award for this.

The simple answer to @strantor is, Pressure Switches are designed with full expectation that welding is a life threatening failure, relays aren't. I suppose Pressure Switches do weld, though I've never seen that happen, and that's why pop-offs and over-current devices are also used. Anybody with good sense will have more than one safety in a lethal quality machine. I hope the TS has good sense!

 

I agree, many do tend to lump the entire unit as one and call it compressor. This is also why it is unwise to lump a bunch of parts together and roll your own unless one knows what they are actually doing. Bad things can happen.

Ron

 

The OP mentions it is a high pressure unit rated for 3000lb/sq" and intends using it at that pressure.
If the whole thing is one off the shelf unit I can see it being made with the build in safety when used at those levels.
Max.

 

The OP mentions it is a high pressure unit rated for 3000lb/sq" and intends using it at that pressure.
If the whole thing is one off the shelf unit I can see it being made with the build in safety when used at those levels.
Max.

Oh, that's correct. A 3,000 PSI unit is not quite your everyday off the shelf unit. Only units I worked with like that were large N2 compressors, they did have mechanical high pressure relief valves but weren't exactly everyday stuff.

Ron

 

The compressor will have an adjustable 600-5000 psi PS, and of curse, a burst disk for either 3000 or 4500 psi depending of the material of the tanks being filled....This is an experimental 3 stage very small compressor (scaled down to run with a 1/3 - 1/4 hp motor) I made intended for filling breathing air tanks, so there are no holding air tanks per-se as part of the set up; external SCUBA tanks are being filled to 3000 psi and CARBON FIBER tanks like the ones used by the Fire Department can be filled to 4500.

The objective is to experiment with durability and efficiency of new oilless/breathing safe materials and components for the internals of the compressor...I have thought for years about this and now that I have been retired for a while with nothing else better to do at home, I decided to work and have some fun in my shop.

Regards....

AFZ