ARCO rpc sizing

[lws09]

I am currently looking for a larger RPC to power a couple of my not so though out auction purchases. I have a 20HP monarch and a Ingersoll 2545 two stage 10HP air compressor that I currently cannot run. I came across an Arco RPCNC40-230WS but after talking to the grumpy guy who answered the phone at Arco I have my doubts that this will be enough. According to Arco this unit should have had multiple units all wired together. Is this actually the case? Will this unit struggle to start this air compressor? Or will it be more than adequate?

Thanks in advance!

 

[lws09]

Or would this one be better suited to my needs? Anyone have pros or cons to either?

 

[JST]

The second one leaves you guessing a bit, since the plate you show is the idler motor data plate. That leaves you to figure out what it ought to start and run when used as an RPC.

The ARCO gives you specifically what HP they expect it to be able to start, and what the total connected load HP should be, maximum, as an RPC. That is the data you need, assuming it is correct.

If that data is correct, it should be able to run both, since you would have 30HP connected, and they allow 40.

Compressors can be hard to start. But that ought to start the compressor with no issues by itself (and the compressor should have an unloader). If the lathe (20 HP) is running, and is not under max load, it will add some "effective idler capacity" to the system, and again per the numbers, there should be no issues. Plus, the remaining stated capacity is double the compressor power (40 HP total, less 20 HP for the lathe).

I use an ARCO unit. Mine is larger capacity than the biggest single load, and I never have two 3 phase machines running at once. So I have never come particularly close to maxing it out, and cannot speak to that from experience. It has been bulletproof and reliable so far, and I got it used from another shop.

My inclination would be to go with the ARCO, given the data available. They state an actual RPC output capacity that should do what you need.

As for what the ARCO "should have been" per your call to them, I have no clue. It sounds as if there may have been some mis-communication.

But, unless there are obvious connections unused that would go to these mysterious multiple units, I'd be dubious. I don't see any such connections, and the contactors etc are all wired up as if what you see is what you get.

The control box is about what I would expect for a motor like that, certainly not more capacitors than the motor would need.* The motor and control box both carry the same rating tag, there is no mention of anything like "when used with auxiliary unit Model XXX", and the motor physical size is credible for the rating.

I have heard one or two people say that ARCO rates things optimistically, but I have no input to that, since I don't strain the limits of mine. Per your information, and the dataplate, you have a 30% cushion over the total connected HP you plan to use, as well as for your least advantageous starting situation.

* ARCO has used a "dual cage" motor design, that draws less start current, and needs less start capacitance, than typical general purpose motors. So the relatively few capacitors visible makes sense. RPC control boxes for general purpose motors usually have more.

 

[lws09]

The second one leaves you guessing a bit, since the plate you show is the idler motor data plate. That leaves you to figure out what it ought to start and run when used as an RPC.

The ARCO gives you specifically what HP they expect it to be able to start, and what the total connected load HP should be, maximum, as an RPC. That is the data you need, assuming it is correct.

If that data is correct, it should be able to run both, since you would have 30HP connected, and they allow 40.

Compressors can be hard to start. But that ought to start the compressor with no issues by itself (and the compressor should have an unloader). If the lathe (20 HP) is running, and is not under max load, it will add some "effective idler capacity" to the system, and again per the numbers, there should be no issues. Plus, the remaining stated capacity is double the compressor power (40 HP total, less 20 HP for the lathe).

I use an ARCO unit. Mine is larger capacity than the biggest single load, and I never have two 3 phase machines running at once. So I have never come particularly close to maxing it out, and cannot speak to that from experience. It has been bulletproof and reliable so far, and I got it used from another shop.

My inclination would be to go with the ARCO, given the data available. They state an actual RPC output capacity that should do what you need.

As for what the ARCO "should have been" per your call to them, I have no clue. It sounds as if there may have been some mis-communication.

But, unless there are obvious connections unused that would go to these mysterious multiple units, I'd be dubious. I don't see any such connections, and the contactors etc are all wired up as if what you see is what you get.

The control box is about what I would expect for a motor like that, certainly not more capacitors than the motor would need.* The motor and control box both carry the same rating tag, there is no mention of anything like "when used with auxiliary unit Model XXX", and the motor physical size is credible for the rating.

I have heard one or two people say that ARCO rates things optimistically, but I have no input to that, since I don't strain the limits of mine. Per your information, and the dataplate, you have a 30% cushion over the total connected HP you plan to use, as well as for your least advantageous starting situation.

* ARCO has used a "dual cage" motor design, that draws less start current, and needs less start capacitance, than typical general purpose motors. So the relatively few capacitors visible makes sense. RPC control boxes for general purpose motors usually have more.

Thanks for your input!

 

[lws09]

I did some google searching and came up empty handed. would anyone have spec sheets or manuals for this particular Arco rpc? What is a fair price?

 

[lws09]

This is the reply I got after emailing Arco to ask about the capacity of this unit.

I spoke with you yesterday about this unit. It is only (1) of (4) pcs. that would have been linked electrically in parallel with each to start and operate a 40HP motor load. If you only have ¼th of the phase converter, then the maximum load capability would be a 10HP load only for start-up and operation. This unit will not handle the 20HP engine lathe.


this seems crazy to me just due to the shear size of this particular model.

I'm hoping someone that has some experience with one of these will chime in.

 

[JST]

That does seem odd. Odd that the data plate shows nothing about that, and implies that the thing has those ratings as-is.

I guess that falls under the "if that is correct" heading... It was not.

You would think that there would be a central control panel for the lot, if the setup was to use 4 idlers. Since the panel you show has the on-off switch, it appears to be that panel, only apparently it is not?

At least the seller was clear about how it was set up. It does confirm that the ARCO "grumpy guy" was talking about that unit.

I could find ZERO data on-line. And the website for ARCO units was fouled up, so the RPC section just went back to the "home" page for "Kirby-Risk", who apparently own ARCO now.

If it worked, it might have provided better information.

 

[lws09]

That does seem odd. Odd that the data plate shows nothing about that, and implies that the thing has those ratings as-is.

I guess that falls under the "if that is correct" heading... It was not.

You would think that there would be a central control panel for the lot, if the setup was to use 4 idlers. Since the panel you show has the on-off switch, it appears to be that panel, only apparently it is not?

At least the seller was clear about how it was set up. It does confirm that the ARCO "grumpy guy" was talking about that unit.

I could find ZERO data on-line. And the website for ARCO units was fouled up, so the RPC section just went back to the "home" page for "Kirby-Risk", who apparently own ARCO now.

If it worked, it might have provided better information.

I'm assuming that if he is correct that each idler may have had its own control panel.

I also had zero luck finding any info online.

I'm still hopeful there is someone on here that has had experience with one of these that will either confirm or deny what he is telling me.

I have not had very much luck finding a used rpc big enough to do what I want to do, and I'm not sure I want to have to buy a new one.

 

[JST]

Other idlers may have been started as needed, although they would not require starter panels with start capacitors, just a switch to turn them on, and maybe local "balance" capacitors. They would start from the 3 phase on the "master" unit, which I assume was the one in the pics.

But it still is odd that there seems to be no provision for automatic starting of the "extra" idlers, nor even a set of switches for them on what appears to be the master panel.

 

[lws09]

I agree. I keep coming across a thread on here from 21 there was a user jessebo that had two identical to these that each had their own control panel but there weren't any pictures of the inside of the panel.

 

[hvnlymachining]

It appears that the panel and idler likely are from two different set ups, according that motor rating data plate it appears no other motor could have been hooked in tandem without overloading the panel during start up since no sequential starting system appears to exist.

A motor or "idler" should have capacity to run an equal sized motor or combination of motors within the stated amperage per the data plate. ( This does depend on how it's wound however) but most will come very close.
As mentioned, when multiple machines are started on a single RPC they should be started in sequence, and must NOT be all at once.

If in serious doubt, a DIY panel can be purchased fairly cheap and build your own out of a surplus motor. I ran machines on one of those for years before having to go larger.

 

[lws09]

It appears that the panel and idler likely are from two different set ups, according that motor rating data plate it appears no other motor could have been hooked in tandem without overloading the panel during start up since no sequential starting system appears to exist.

A motor or "idler" should have capacity to run an equal sized motor or combination of motors within the stated amperage per the data plate. ( This does depend on how it's wound however) but most will come very close.
As mentioned, when multiple machines are started on a single RPC they should be started in sequence, and must NOT be all at once.

If in serious doubt, a DIY panel can be purchased fairly cheap and build your own out of a surplus motor. I ran machines on one of those for years before having to go larger.

Thanks for jumping in!

I’m defiantly not opposed to building my own it’s just rare for me to come across any decent sized used motors that are still worth using. Although I’m usually just eyeing equipment that people are throwing out looking for freebies.

Correct me if I misunderstood. From the data plate on that motor it should do what I want to do without any real trouble.

I was alway under the impression that if I built a rpc out of a used say 20hp motor that I could only safely run a 10hp piece of equipment off of it.

I’m not opposed to adding a couple smaller idlers to help boost my output if that’s what I need to do. I’m just not 100% sure the proper way to that.

I’m assuming that the proper way to add idlers would be to have the main rpc fed off of and feed back into a 3 phase sub panel and each additional idler be fed from that panel off it’s own breaker. This way makes the most sense to me.

 

[hvnlymachining]

You can generally only START half the size motor, but running capacity is usually closer to equal. Again it depends on the manufacturer.

 

[JST]

The starting capacity can be anywhere from 2/3 of the idler to half the idler. Depends on the load motor, the harder to start, the larger the idler needs to be.

Some right here have mentioned starting and running with an idler the same size as the load motor. That would have to be with an easy starting load motor.

And, some lathes, and some compressors might work that way, or at worst at 2/3 the idler. If the motor is clutched to the drive, then it starts unloaded by anything but the drive system up to the clutch. That may be a fairly easy start. Bigger lathes are often made that way.

Likewise, compressors that start with an unloader have just the inertia and friction of the pump at start.

I'd not count on those, but if both conditions apply, your RPC may not need to be as large as it might seem.

 

[SomeoneSomewhere]

There was a thread here recently where someone was having issues with a compressor, even with the unloader. Unless you have an unloader with a large delay, they usually still require the compressor to come up to speed very rapidly because it starts to build pressure as soon as it starts turning.

15 hp RPC struggles to start 7.5 hp 3 phase air compressor - add cap?

I have had my RPC up and running for ten years or so without a hitch. Confident, I decided to rebuild a 3 phase air compressor despite persistent rumors they can be hard to start with an RPC. I spun it up today for the first time. It started fine with the tank empty. (There is no pressure gauge...

www.practicalmachinist.com

 

[JST]

There are different kinds of unloader. Some are timed, and those can be whatever time is needed.

Others have a bleed port, but build up some pressure when the flow rate is high enough. At that point, the pressure forces a plunger to move, cutting off the bleed, and connecting the compressor to the tank. That type is heavily dependent on matching the bleed etc to the motor and head. If wrong, the delay can be zip.