Automatic Alternating 120v Switch???

[WOTDesigns]

just got a new toy that will require an additional compressor. I only have 110 in my shop and the power is already stained.

I'm trying to figure out how to prevent both compressors from at the same time. seems simple- relay that only allows one side to be open at a time then clicks OFF and when one of the two fires it blocks power to the other, then OFF then the second can fire.

the plan is to have one for my general air and mist system, and one for my shiny new pneumatic spindle. the ideal would be to wire 220 and get a big compressor but thats not going to be cheap or realistic as this is a rental im planning on leaving in the next year or two. Plus its a duplex with a single drop split to both so amps are at a PREMIUM to say the least.

all I have found are these but they seem to need a second switch to initiate the cycle.

TWO COMPRESSORS, TWO SEPARATE AIR CIRCUITS its what I'm after, but I cant have them fire at the same time.

lost. not great with electronics. HELP please.

[Al_The_Man]

Depends if you want it automatic or manual, but the easiest way I would think is to have a power relay for each with each having a N.C. aux contact.
The aux contact would be wired in the circuit of the opposite contactor or relay coil.
IOW, an electrical interlock.
The other similar method is to pick up a reversing contactor off ebay, these have mechanical interlock as well as electrical.
Using it this way it would obviously supply one of two motors rather than reverse one.
Al.

[WOTDesigns]

Depends if you want it automatic or manual, but the easiest way I would think is to have a power relay for each with each having a N.C. aux contact.
The aux contact would be wired in the circuit of the opposite contactor or relay coil.
IOW, an electrical interlock.
The other similar method is to pick up a reversing contactor off ebay, these have mechanical interlock as well as electrical.
Using it this way it would obviously supply one of two motors rather than reverse one.
Al.

I'm afraid I cant quite picture it. wouldn't that require me to toggle them on?

I'm wanting it completely automatic. they will be fighting each other to "fill" from a single outlet. both air circuits will be running simultaneously (one for the spindle, and one for mist coolant). I can't be involved in the switching process- I just need them to play nice and share ab outlet. otherwise I have to plug one into the breaker the cnc its on and I REALLY don't want to do that.

[Al_The_Man]

Well if automatic, something has to switch them at an appropriate time, what do you have in mind for this??
What would be the sequence required?
I understood you wanted one or the other running in order to avoid electrical O/L, what is this simultaneously running?
Al.

[WOTDesigns]

I keep going over it in my head then my head hurts. I'm thinking I might understand what you suggested though:

wire a 120v relay with 120v switching N/C to the AC in on each one and wire the switch to the opposite compressor on the back side of the relay built into the compressor? that way when compressor 1 kicks on it cuts power to compressor 2. when compressor 1 fills it represses the relay on compressor 2 and when compressor 2 kicks on it kills power to compressor 1.

right?

[TXFred]

I'm confused about why two compressors taking turns is going to be advantageous over a single compressor running full time. Is this just a duty cycle problem, where a single compressor would melt down if left running full time?

How many cfm do you need total?

I know nothing about the electronics side of this. I'm just curious about your goals.

[Al_The_Man]

It may not be quite that simple, but if using say one pressure switch off of one compressor, if it has a NO and a NC both with the same common, this would work if they are fed off of the same supply?
The master N.O. would feed to the normal N.C. pressure switch of the slave.
You would make one compressor the master, the other the slave, IOW the master would take precedence.
In this instance the one you make the master, probably would be the one with the most demand.
This method would require no other components.
For any other options it may be wiser to use something like a Siemens smart relay, this way you will have more options when it comes to the required logic.
Al.

[WOTDesigns]

I need 6 cfm for the spindle (which is pushing the top end of what can be done on these small compressors) and ??? cfm for my kool mist. Just to run my spindle I will have to get a larger compressor (mine runs 4.5 cfm) and likely have to lower the PSI (thus slowing the spindle) to be able to keep up the air supply on the spindle alone.

if I had 220 I would have to spend $600+ on a 5hp to keep up with both and have a 50% duty cycle. No way do I trust these compressors to run 1.5 hr strait very many times.

this just seems like the only way to do it in my situation :-/

swung by orvac on the way home and grabbed a few surplus 120v coil 10a @ 120vac relays for $10 ea. also picked up some anderson type connectors for the switching wires so I can move the compressors to drain.

if this works I will be VERY grateful! can't see how it wouldn't though.

[WOTDesigns]

does this crappy hand drawing make any sense?

[Al_The_Man]

It would be clearer drawn in ladder logic, but I think you are going to get conflict if you do not make one compressor the master, the other the slave using simple logic.
With the drawing you have to indicate which NC is tied to which relay?
Check the compressors for existing relay and/or the type of pressure switch and contact configuration, if both N.O. and N.C and if they use the same common or are double pole?
Al.

[WOTDesigns]

gonna go cut for a while then I'll draw it in color.

it should work fine (and easy actually). the simple explanation is that I am going to add a NC relay on the AC line to each compressor as a KILL SWITCH (KS). compressor 1 (C1) motor line is wired to trip KS on C2 and vise versa. So if C1 is running its sending power to KS on C2 so C2 has no power until C1 shuts off. Even if they both drop to 90psi at almost the same time whichever one gets there first gets the power and the other will kick on as soon as the first is done.

[WOTDesigns]

I hope this helps anyone understand that is thinking of doing something like this.

First one shows two compressors plugged in normally and one is running.

The second shows adding two NC relays to the AC line in to the compressor and using the power to the motor to OPEN the relay on the other compressor so it can not turn on.