Disable machine #1 when machine #2 is turned on (shared circuit)

TonyS · Oct 23, 2014

Not sure where this should be posted so I'll try here.

Due to having filled up a conduit, I plan to have a couple of infrequently used sanders share a 220v line with my air compressor. I would like the normal situation to be that the compressor is powered and allowed to start/stop as needed. If, however, I turn on one of the sanders, I would like power to the compressor to be interrupted; then when the sander is turned off, power to the compressor should be restored. IOW, instead of having to flip a DPDT or other control switch each time, I want the interrupt to occur automatically.

I have a little experience wiring up relays but haven't been able to figure out how it could be done that way. Does this require fancier control circuitry? If so, can it be bought for a reasonable price?

TIA,
Tony

Steve in SoCal · Oct 24, 2014

You could perhaps use a reversing contactor to switch the load, you would still need some form of control to signal the correct machine.

You do understand that more than three current carrying conductor in a raceway requires a derate of the ampacity of the conductors. If you are close to max on the wires rating you may want to consider a parallel conduit. In any case, a second raceway and wiring is likely to be more cost effective than switch gear.

Steve

wippin' boy · Oct 24, 2014

instead of having to flip a DPDT or other control switch each time

man !!
your work load must be unimaginable
you better just hire an assistant

how about a floormat (N.C. contact in line with compressor coil) switch that kills compressor when your standing in front of sander

I was gonna sketch a sarcastic circuit in paint but I find that windows 7 doesn't have it
damn
better just move on

sorry

didn't sleep very good

Red James · Oct 24, 2014

Since the sanders are not used frequently, just hook them all up and depend on the circuit breaker if too much amperage is used. Starting current for a sander only lasts a very short time, then load is likely not enough to trip your breaker. If your usage of the sander(s) increases, then spend money to move the sander or add a circuit . . . do not make complicated switching circuits in your building's wiring . . . you might move a machine someday and leave a strange circuit behind.

Chip Chester · Oct 24, 2014

In the RV world, there's a blue box that will manage air conditioner turn-ons so that you can use 30A shore power when you really need 50A. As one would expect, there is some wiring involved, and it's a two-device unit rather than three, but it's a start. It's probably a current-sensing coil around one AC's conductor that uses a relay to disable the other AC's conductor.

This looks handy:

Current sensor relay

My first thought was plugs on the devices and one receptacle.

You could wire pilot lights into the sanders, and a house light opto switch to the air compressor, and put 'em all in a dark box. No light in box means the air compressor can run. The time delays would cover you for when you're on the sander, but just between parts and it's temporarily off. Air compressor won't be bothered by waiting an extra 5 minutes to turn on, unless you're painting or sandblasting at the time.

Picking up on the switched floor mat idea above, have the floor mat switch turn on the lights in the box. No wiring on machine needed.

It's a logic problem. And, it's a logic problem.

Chip

Scottl · Oct 24, 2014

Not exactly what you are looking for but the 220V version can wirelessly control your compressor.

Dust Collection - Long Ranger III Remote Dust Collector Switch and Motor Starter

No doubt there are also 220V current monitor devices that can turn on a dust collector when a sander is turned on. You would then need to adapt it so it turned off instead of on.

Jraef · Oct 24, 2014

Something turns on your sander, right? Whatever that someting is (either a "Manual Motor Starter" or "Magnetic Motor Starter"), will have the ability to add what is called an "auxiliary contact" to it, a small control circuit contact that operates with the main power contacts. If you get a "NC" (Normally Closed) aux contact for that starter, you can simply wire that in series with your pressure switch for the compressor. That pressure switch will be either feeding the control coil circuit of a motor starter on the compressor, or it will be feeding power to the compressor directly if it is small. Just make sure that if the pressure switch is wired directly to the motor, the aux contact will not likely be rated for that motor power, they rarely are. So in that case, you will have to add a properly rated relay (contactor) at the compressor, and have that aux contact control the coil of that contactor, then run the compressor motor power through the main contacts of that contactor.

By the way, expecting a circuit breaker tripping to make this work is like using the idiot light on your car that says "Over Heat" to tell when it is time to add coolant. By the time it works, it might be too late...

TonyS · Oct 24, 2014

LOL! All sarcasm accepted in good humor. Yes, my workload is unimaginable, as I recently retired. Why not replace work with obsessive behavior, I asked myself. It is true that a simple on/off switch would be fine, just thought I'd see if there were an easy relay-based solution I was missing. My brain couldn't see how to do it without an additional control circuit. Since the solutions offered all involve added circuits--except letting the breaker trip--I don't feel so bad now.

The floormat switch idea is intriguing. The pilot light and auxiliary contact ideas require adding a circuit from each machine, which is beyond my obsession level for this project. As is the price of the RV blue box.

I use X10 remote stuff to control my DC and air cleaner. Cheap, standard.

Thank you everyone.

Tony Quiring · Oct 24, 2014

Out there...

I believe there are current sensing relays or switches used in single phase motors that do not have the governor type.

I think they are potential relays.

You may be able to get one and wire in in series with the controlling outlet...where the sander goes.

You then need to look at tge compressor.

If it has magnetic starting where the pressure control uses a relay for the control of the motor then you can add a relay here to control the compressor via the relay to interrupt the control from the pressure switch.

You also can add 120 vac coil relay to allow compressor yo be slaved from overhead lighting.

We have an outlet next to compressor wired to light switch.

Relay in compressor control box has standard plug in...plugs into light switch controlled outlet.

Lights out no 7.5 hp compressor firing off while eating dinner.

Only issue is turns on with lights if one forgets to turn off air...

Tony Quiring · Oct 24, 2014

If sanders are hard wired then just get control transformer and connect to motor side of switch to provide control voltage for compressor.

TonyS · Oct 24, 2014

I'm thinking it could be done with a cheap current sensor on one of the wires of the supply line that powers the sanders. The sensor output would control a relay on the supply line to the compressor. If necessary, the current sensor could control a small relay whose output would control the power relay. Rainy day project.

awander · Oct 24, 2014

Window 7 does too have paint......

wippin' boy said:
man !!
your work load must be unimaginable
you better just hire an assistant

how about a floormat (N.C. contact in line with compressor coil) switch that kills compressor when your standing in front of sander

I was gonna sketch a sarcastic circuit in paint but I find that windows 7 doesn't have it
damn
better just move on

sorry

didn't sleep very good

wippin' boy · Oct 28, 2014

found it..........

rwbaldwin · Oct 28, 2014

There are dust extraction units and even wet/dry shop vacuums that which switch on when the power tool plugged into them is switch on. Find one with a dead motor and use the circuit board to control a relay.

Chip Chester · Oct 28, 2014

For that matter, there's a Sears stand-alone product for shop vacs that does that, but in the 120V realm. Even has two controlling outlets, that govern the controlled outlet. Relays to get to 220, and you're golden.

Chip

TonyS · Oct 29, 2014

Ha ha, I should be lucky indeed to be worrying about anything in 40 years. When the house goes to the next Pilgrim, it will be with the shop subpanel feed disconnected at the house main panel and a big notice added to that stack the realtor makes you sign, "You want to use all that shop electrical, have it carefully inspected." But I do also maintain a list of 'Things the next Pilgrim should know'.

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Disable machine #1 when machine #2 is turned on (shared circuit)

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