Power source interlock question 3

[72mattski]

Good morning,

I have a question that I am sure I could figure out, but I also know I do not need to re-invent the wheel and there is A LOT of expertise on this site...so I thank you ahead of time for your help and hopefully constructive comments.

To the situation: I currently have two 480VAC power sources supplying via independent/separate but identical circuits powering two air compressors. I would like to install a third, standby air compressor but I can not install new 480 VAC infrastructure as my distribution panel is maxed out. The new air compreessor is identical to the existing two in power and capacity. Simply put, two power supplies, three loads.

I would like to install an interlock, mechanical or electrical, such that I avoid all the bad combinations, two compressors on one circuit, two circuits tied together, yada yada. What is your suggested interlock? If mechanical, do you have a suggested product? If electrical, can you provide the same and or a simple controls diagram?

Again, thank you all for your time.

Respectfully

 

[Skogsgurra]

I do not see any smart solution to this. Cannot reduce the Boolean equations. But the "dumb" solution is simple enough: If two of the companion compressors are running, the third isn't allowed to run. Like this:

A = /B + /C
B = /A + /C
C = /A + /B

In the first case: If any of B OR C is off, it is OK to run A. The other ones follow the same rule.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[VEBill]

Conceptually it's very simple.

Three transfer switches: each of the two primary circuits would have a transfer switch for primary or back-up, and then the back-up would have a switch to select one or the other of the two redirected feeds. The three transfer switches might be manual switches, or contactors (but they'd would then require a control panel, lower voltage power supply, etc.).

Or you could use suitable plugs and receptacles and unplug/plug them in manually. I'm not sure if suitable plugs and receptacles are available or practical at your boltage and power levels.

Note: 480VAC is way outside my area. So stand by for further responses from others with appropriate expertise.

 

[VEBill]

"boltage" -> voltage, obviously.

 

[IRstuff]

Seems to me that any such control circuitry would result in the equivalent of blowing the breaker(s). Have you done the cost/benefit analysis? The only way to "know" some of the conditions is to measure the current or voltage on the units, and then add circuitry that can drive breakers. Seems to be somewhat nontrivial, so is it worth it?

TTFN
faq731-376

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[DRWeig]

You can do VE1BLL's transfer switch scheme no problem, but you only need three transfer switches, one per compressor, that have a positive-off position. Smarts is just a small PLC. As IRstuff mentioned, you'll need some sensors too. Electrical interlock is fine, it's done regularly with 480V emergency panels and molded-case circuit breakers.

I don't have diagrams to share, but I've designed similar systems. IRstuff, again, is right in saying it is not trivial. If your compressors are 480V 10A, it will be cheap. If they are 480V 1000A, it will not be cheap. In either case, a design is needed and a bit of PLC programming.

Think $100 PLC and handful of $50-each adjustable-setpoint current switches. I would throw a motor protector relay on each compressor too if you don't already have them, upstream of the starter and disconnect. It'll have an output to use for sensing voltage for source availability, plus phase loss, brownout, whatever. There will be a few standard relays in the game too, $20 range each. Add the cost of your three transfer switches that will vary with current rating, and then figure out labor.

Best to you,

Goober Dave

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[djs]

As the third compressor is a backup, why not use a mechanically interlocked dual breaker at one of the existing compressors. One way (normal) the existing compressor has power. The other way (backup) the new compressor has power.

BTW 480 VAC panel boards are not that expensive.

 

[Skogsgurra]

I feel very lonly with my interlock equations. Don't you guys read Boolean equations any more?
Nothing was said about compressor power consumption. Nothing was said about contactors or not. This could be cheap and simple if the contactors are already in place or it can be somewhat costly if power is in the hundreds HP range and no contactors available.

There are simple electro-magnetic locks that can be used to block a switch if two compressors are already running - and allow the switch to be actuated if only one is running.

480 V is everyday stuff. Nothing special about it.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[IRstuff]

Your equations only covered one of the OP's questions, though; they don't distinguish that there could be only two units, but both are on the same circuit. That requires a supply-side logic circuit.

TTFN
faq731-376

Need help writing a question or understanding a reply? forum1529

 

[Skogsgurra]

They do, actually. Don't they?

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[waross]

Transfer switch one: connects feeder one to compressor one or transfer switch three.
Transfer switch two: connects feeder two to compressor two or transfer switch three.
Transfer switch three connects compressor three to either transfer switch one or transfer switch two.
My Boolean is quite rusty Gunnar. I haven't used it in a while.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[72mattski]

OUTSTANDING!!!

Thanks guys. You can make a pretty smart fellow like me feel not so smart as the solution truely is quite simple: Three sets of two contactors. Each set has a mechanical or electrical interlock to prevent both from being shut. Two (the primary two) are powered from the two sources. They will both supply one compressor each as well as the third set of contactors. The third set will supply the third comprssor.

Fortunately, they are small compressors so the overall cost is not insurmountable.

Again, thank you all. Your input has been very helpful.

 

[VEBill]

Many of the solutions described above require no interlocks as such.

The transfer switches (or contactors) shall be DPDT switches that inherently select either one or the other. No interlocks required. The avoidance of bad combinations is inherent and fool-proof.

The only decision left is how to prioritize in the event that both primary compressors fail.

 

[ScottyUK]

"The transfer switches (or contactors) shall be DPDT switches that inherently select either one or the other."

Unfortunately most contactors are normally open devices, and those with normally closed poles usually have limited switching capability. This means that most contactor-based solutions will require two NO contactors with an interlock. The technology is well-proven and well-understood.

The alternative might be to use a bar & shaft type contactor which has multiple NO and NC poles on one actuator. These available in very large ratings. Be careful that any rotating plant will tend to maintain a voltage for a short period after de-energisation, and it is entirely possible that a fast transfer using NO and NC poles on one contactor may result in an out-of-step closure with resultant high currents and high torque on shafts.

 

[waross]

Probably the cheapest solution is four breakers with a slider bar interlock.
One breaker for each normal compressor and two breakers for the standby.
The slider bar blocks all the breakers from being turned on, but has slots spaced such that the breakers may only be turned on in safe combinations. This slider bar system is used when 5 breakers must be used and interlocked for the isolation and bypass of a UPS.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[zif63]

Check SOCOMEC ATyS automatic transfer switches. I've used them for supplying two DC drives from two different sources.