Interlocking Two Air Compressor's .

[Davebones]

I was informed we have two ( 2 ) used air compressor's coming from another plant we own that is being shut down . I'm told these each need a 480V , 300 amp feed . My boss is telling me he doesn't want to run two 300 amp circuits as he says only A or B compressor will run at the same time . I'm told there is a 400 amp fusible safety switch for each compressor . My question is how could I interlock these so that only one compressor can run at a time ? Thought about having two contactors with a switch for either A or B compressor just not sure if this is the best way to do this ...

 

[SceneryDriver]

Feed both compressors from the same circuit, and interlock their controls so only one contactor can pull in at a time. You will need some logic to accomplish this; either a manual selector switch or a smart relay / small PLC. Do you want to run one unit all the time unless there's a problem, or alternate their use to even the wear and tear on the units?




SceneryDriver

 

[Davebones]

We would alternate between the two . This is for plant air so we would just switch between the two every week .

 

[bobsherwood]

interlocking two air compressors

interlocking two air compressors

I would install an alternating relay.

 

[growler]

I was informed we have two ( 2 ) used air compressor's coming from another plant we own that is being shut down . I'm told these each need a 480V , 300 amp feed . My boss is telling me he doesn't want to run two 300 amp circuits as he says only A or B compressor will run at the same time . I'm told there is a 400 amp fusible safety switch for each compressor . My question is how could I interlock these so that only one compressor can run at a time ? Thought about having two contactors with a switch for either A or B compressor just not sure if this is the best way to do this ...

It's very common to have air compressors set up like this. One will run while the other cools off and when maintenance is needed you lock it out with a selector switch.

The reason I say this is beacause they may have evrything you need at the plant that being closed down. Either you or your boss should give them a call.

 

[kwired]

I would install an alternating relay.

How simple that may be will depend on other factors.

If these are equipped with VFD's and are essentially "always on", you may just manually switch them periodically or even put them on a timer to transfer them periodically.

Often any equipment connected to an alternating relay is set up to have a limit that kicks in the second unit should the first one fail or not keep up with demands, but you can't really do that in a situation where there is only supply capacity for one unit, though you probably can arrange an automatic transfer if it is detected that one unit has failed.

 

[Jraef]

What you are creating is referred to as a "duplex controller", it's very common. If you don't already have the motor starters for these, you can buy a Duplex Panel from most of the major control suppliers that will accomplish this for you. It will have an "alternating relay" in it that will automatically cycle from one to the other each time the pressure switch calls for a compressor, that way the wear and tear on the compressors is evened out between them. Most Duplex Panels will have an option for calling the second compressor if the first one is not enough, but per your boss, you would just not hook that up.

If you already have your own motor starters, you can accomplish the same thing, but it will take some control circuit skills to pull off. We don't know how adept you are st that, but this is simple enough that we could guide you through it if you provide all of the details on what you have now.

 

[FionaZuppa]

I was informed we have two ( 2 ) used air compressor's coming from another plant we own that is being shut down . I'm told these each need a 480V , 300 amp feed . My boss is telling me he doesn't want to run two 300 amp circuits as he says only A or B compressor will run at the same time . I'm told there is a 400 amp fusible safety switch for each compressor . My question is how could I interlock these so that only one compressor can run at a time ? Thought about having two contactors with a switch for either A or B compressor just not sure if this is the best way to do this ...

can you use a Siemens DTNF365. put a locking hole on either side of the handle OFF position so that you retain disco functionality but cant power up the other compressor unless you remove the lock.

 

[Smart $]

You didn't mention that the installation had to be automatically switched, so just run one circuit sized for one to both safety switches. Switch them manually. If someone switches on both, the supply OCPD should trip when both run simultaneously. Mechanical or electrical interlocking is not required.

 

[petersonra]

You didn't mention that the installation had to be automatically switched, so just run one circuit sized for one to both safety switches. Switch them manually. If someone switches on both, the supply OCPD should trip when both run simultaneously. Mechanical or electrical interlocking is not required.

I think by code it is though. otherwise you would need a bigger feeder circuit.

 

[Smart $]

I think by code it is though. otherwise you would need a bigger feeder circuit.

Well, please let us know what Code section that is...

 

[petersonra]

take a look at the feeder requirements for motor circuits. there is an exception for motors that are interlocked so they can't run simultaneously.

 

[FionaZuppa]

a 3pole dbl throw switch will not work ??

 

[petersonra]

a 3pole dbl throw switch will not work ??

kind of pricey though for the size motor.

 

[Smart $]

take a look at the feeder requirements for motor circuits. there is an exception for motors that are interlocked so they can't run simultaneously.

Section number, please.

 

[Smart $]

Section number, please.

Nevermind... it's 430.24 Exception No. 3.

FWIW, 430.24 is about conductor ampacity for several motors or a motor(s) and other loads. The text goes on to say "Conductors supplying several motors..." So let's just stop right there. If only one disconnect at a time is closed, the conductors are supplying ONE motor only. Granted there will be a violation if anyone closes both disconnects. But that is not supposed to happen. I'm all for signage that states something to the effect of closing only one disconnect at any time.

 

[kwired]

Nevermind... it's 430.24 Exception No. 3.

FWIW, 430.24 is about conductor ampacity for several motors or a motor(s) and other loads. The text goes on to say "Conductors supplying several motors..." So let's just stop right there. If only one disconnect at a time is closed, the conductors are supplying ONE motor only. Granted there will be a violation if anyone closes both disconnects. But that is not supposed to happen. I'm all for signage that states something to the effect of closing only one disconnect at any time.

Or interlocking controls that won't allow both to run simultaneously.

 

[Smart $]

Or interlocking controls that won't allow both to run simultaneously.

I'm not against it and easily done if you have two motor starters. OP didn't mention having any motor starters but compressors requiring a 480V 3Ø 300A feed should. :happyyes:

...and it would only require one circuit and disconnect.

 

[kwired]

I'm not against it and easily done if you have two motor starters. OP didn't mention having any motor starters but compressors requiring a 480V 3Ø 300A feed should. :happyyes:

...and it would only require one disconnect.

You would think there is some control method besides just a manual motor switch or simple line voltage pressure switch on that large of a unit

 

[FionaZuppa]

so what was the final solution to this problem?