Auto Transfer Switch Question

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mkgrady

Senior Member
Location
Massachusetts
I have a customer that has one feed to a manual transfer switch. The switch feeds two 75 HP compressors but just one at a time. One compressor is the normal and is run with a VFD. The other compressor is the back-up and has just a line voltage starter. Each is a packaged unit with all kinds of internal controls

The customer wants to make the transfer switch automatic. When the pressure drops in the compressed air system, they want the back-up compressor to come on-line and have the normal compressor go off.

Now here is the problem, two suppliers say their suppliera are not interested in making a custom auto transfer switch. Custom because the normal auto-transfer switch switches between sources not loads. Also, the normal auto switching takes place upon a loss or normal power. In my case I want the switch to transfer upon loss of pressure.

Although the components involved would be large because of the motor sizes, the control sequence seems pretty simple. It occurred to me that if the parts are available I could make the auto transfer switch with some very large relays. The voltage is 208-3 phase. The largest total compressor package amps is 228 and it has a 250 amp contactor for the motor. Does somebody make a three pole double throw 250 amp relay? Anybody have any advice on this?
 

crossman

Senior Member
Location
Southeast Texas
Hmmm.... good question!

If you can't find a 250 amp 3PDT contactor, could you use two sufficiently large contactors in place of the transfer switch and interlock them through the control circuit so that only the appropriate one is energized?

or... do away with the transfer mechanism altogether and feed the VFD and the starter all the time. Make the motors turn on and off when you want them to by wiring the appropriate logic into the control circuits of the VFD and the motor starter. Interlock the controls so that both motors cannot run at the same time. This may not be the best option if the Motors and controls are "package units" with integrated controls as you mention.

We need to consider the code though. Without the manual transfer switch, would it be appropriate to have a feeder which is big enough for only one motor to serve two motors even if they are electrically interlocked and only one of them can be on at any given time?
 

mkgrady

Senior Member
Location
Massachusetts
crossman said:
Hmmm.... good question!

If you can't find a 250 amp 3PDT contactor, could you use two sufficiently large contactors in place of the transfer switch and interlock them through the control circuit so that only the appropriate one is energized?

or... do away with the transfer mechanism altogether and feed the VFD and the starter all the time. Make the motors turn on and off when you want them to by wiring the appropriate logic into the control circuits of the VFD and the motor starter. Interlock the controls so that both motors cannot run at the same time. This may not be the best option if the Motors and controls are "package units" with integrated controls as you mention.

We need to consider the code though. Without the manual transfer switch, would it be appropriate to have a feeder which is big enough for only one motor to serve two motors even if they are electrically interlocked and only one of them can be on at any given time?

Seem to be all good thoughts. Thanks. I don't know what code would be violated.
 

crossman

Senior Member
Location
Southeast Texas
I think the code issue may be the possibility of both motors running at the same time which would overload the feeder. (I am assuming the existing feeder is sized for only one of the motors.) Even though the control circuit should prevent this, I'm not sure if you could convince the AHJ of this. You may have to have a seperate feeder for each motor. And now I am assuming that there is an AHJ and an inspection involved. From your situation, I am thinking that this probably isn't going to be inspected, right? Electrically and safety wise, I see no problem with it.
 

QES

Senior Member
Location
California
You can buy ATS with input control. For example you can wire up an ats to an plc and the plc can remotely switch load. For your case the control will be a pressure switch logic.
 

mkgrady

Senior Member
Location
Massachusetts
crossman said:
I think the code issue may be the possibility of both motors running at the same time which would overload the feeder. (I am assuming the existing feeder is sized for only one of the motors.) Even though the control circuit should prevent this, I'm not sure if you could convince the AHJ of this. You may have to have a seperate feeder for each motor. And now I am assuming that there is an AHJ and an inspection involved. From your situation, I am thinking that this probably isn't going to be inspected, right? Electrically and safety wise, I see no problem with it.

The work will be inspected but since both motors will be interlocked so they can't run at same time, I don't know why it would be a code problem. You're right, the feeder is sized to run only one motor. Running a seperate feeder would be cost prohibitive.
 

mkgrady

Senior Member
Location
Massachusetts
QES said:
You can buy ATS with input control. For example you can wire up an ats to an plc and the plc can remotely switch load. For your case the control will be a pressure switch logic.

I've never done a PLC so I don't know how, but I could wire a control scheme that would work.

Anybody know of a manufacturer that makes an off the shelf ATS in this size range?
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
I have designed automatic throw-over schemes using mechanically interlocked contactors (think reversing starter). These contactors are intended for switching motor loads and prevent both loads from coming on at the same time. You can create your own voltage sensing and timing logic. You definitely do not need all of the fancy (i.e. test and gen start) controls of an ATS.
 

charlie tuna

Senior Member
Location
Florida
many large building control air compressors are set up like this--with the feeder sized to handle only one compressor, so the code issue shouldn't be a problem. automatic transfer switches are very expensive and i'm sure the manufacturers don't want anything to do with "custom design controls" -- they are very independent! go with your vfd acting as the prime control motor starter and just program it's logic to start the secondary compressor if the vfd shuts down. i would add a second hard wired interlock in the secondary starter's "start" circuit to prevent the possibility of someone "fiddlin" with the vfd program which might allow for it calling for both compressors. somehow, we have had too many incidents with unqualified people getting into vfd programs and changing limits and even equipment safeguards. never forgot a press that had a maximum speed limit of 1500 rpm. the owner got to messing around with the vfd -- set it back to factory settings and then when he re-programmed it, he mistakenly added an extra zero----15000!!!!!!!!
 

mkgrady

Senior Member
Location
Massachusetts
jim dungar said:
I have designed automatic throw-over schemes using mechanically interlocked contactors (think reversing starter). These contactors are intended for switching motor loads and prevent both loads from coming on at the same time. You can create your own voltage sensing and timing logic. You definitely do not need all of the fancy (i.e. test and gen start) controls of an ATS.

So I guess I should consider buying a reversing starter and wire it up with the controls I'm looking for.
 

mkgrady

Senior Member
Location
Massachusetts
charlie tuna said:
go with your vfd acting as the prime control motor starter and just program it's logic to start the secondary compressor if the vfd shuts down. i would add a second hard wired interlock in the secondary starter's "start" circuit to prevent the possibility of someone "fiddlin" with the vfd program which might allow for it calling for both compressors.QUOTE]

I prefer to stay out of the packaged compressors controls. I don't want any warranty issues. I want to just feed the main unit or the back-up.
 

mkgrady

Senior Member
Location
Massachusetts
jrannis said:
I dont why you couldnt use a three pole double throw ASCO switch and some simple control wiring? Whats the problem? It can only feed one compressor at a time.

Sounds good to me. I'll go look up an ASCO switch. I don't have experience with relays of this size so I didn't know where to start.
 

wireguru

Senior Member
look into the Fuji Odyssey Series 3N Contactors. a 285amp rated one will run you about $490.

They have N.O. and N.C. auxillary contacts, you can run the coil circuit on contactor A through the NC auxillary contact on contactor B, and vise versa to interlock them so they both cant operate at the same time. The N.O. auxillary contacts could be used for pilot lights to show you which contactors are engaged.

use all 120vac control and the 2 contactors, pressure switch, hoffman box, and misc stuff and you should be out of there for around $1300 in parts.
 

mkgrady

Senior Member
Location
Massachusetts
wireguru said:
look into the Fuji Odyssey Series 3N Contactors. a 285amp rated one will run you about $490.

They have N.O. and N.C. auxillary contacts, you can run the coil circuit on contactor A through the NC auxillary contact on contactor B, and vise versa to interlock them so they both cant operate at the same time. The N.O. auxillary contacts could be used for pilot lights to show you which contactors are engaged.

use all 120vac control and the 2 contactors, pressure switch, hoffman box, and misc stuff and you should be out of there for around $1300 in parts.

Thanks. This kind of validates what I was originally thinking. Having a relay manufactures name really helps and having your suggested control scheme saves me alot of time as I don't do alot of this.
 

wireguru

Senior Member
Heres the fuju part number: 3NC5F0122 "Fuji IEC 285A contactor, non-reversing, 3-pole, 169 mm frame width, SUPERMAGNET(tm) coil accepts 110-120 VAC or DC, includes 2 N.O. and 2 N.C. aux contacts "
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
Regardless which manufacturer you chose for your contactors, I would still recommend adding their mechanical interlock mechanism if simultaneous operation must be prevented.
 

mkgrady

Senior Member
Location
Massachusetts
jim dungar said:
Regardless which manufacturer you chose for your contactors, I would still recommend adding their mechanical interlock mechanism if simultaneous operation must be prevented.

Is a mechanical interlock something the relay manufacturers offer as a standard part? Would you expect I could order two relays and one mechanical interlock?

It does seem like a good idea. Even though I can't imagine how both relays could be energized at once, if it does happen the feeder breaker (600 amp)may not open and the 300 mcm CU THHN feeder could draw too much and cook the wires.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
mkgrady said:
Is a mechanical interlock something the relay manufacturers offer as a standard part? Would you expect I could order two relays and one mechanical interlock?

Yes the contactor manufacturers offer them.

Relays are normally rated 20A or less. Contactors are normally rated 30A or more. Starters are contactors paired with overload relays.
 
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