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Starting and using a slightly over-sized RFC

Jollygrn74

Plastic
Joined
Nov 1, 2017
So a friend gave me a project that is pushing the limits of my personal experience, leaving me somewhat unsure of best solution. I was give a diagram including 3 motors and later one including showing a possible 6. I believe that the fellow sending these document is somewhat confused. Now the two main ones are 1- designated Main Motor @7.5kW 400V and another being labelled Hydraulic pump @5.5kW with a servo motor, cooling motor, a conveyor motor and another "forming" motor on an encoder. Only the two show sizing on these schematics. The unit itself is is to bend up roofing metal into preset panels. He wishes to setup this unit in his personal garage that has a 100A 120V/240V household panel.

It may confuse with so many things but I am going to start with the initial 2 sized motors with the premise that the other motor well not be significant as was essentially told to me to start.

In sizing a RFC I boil it down to requiring a 15HP sized unit as no loads would start at the same time and the loading is assumed to be medium. In looking for possible used units I found one that doesn't look bad only in local area but it is a 20HP system which I know if it can work is in a way better as less wouldn't be working it as hard as a smaller. The issue is that there is only a 100A service supply (which will just do a 15HP system) and the 20HP is showing 134A input. First, can I soft start the phase converter to reduce initial loading? Now if I am only using a the shown load that will not cause a large draw, is it ok to use it on the undersized service? Obviously I would size disconnect to prevent overload to main.

Thanks
 
I understand that, that is simply standard practice to do in a normal scenario. I've never considered this option as a normal practice though. I just wasn't sure if it was a faux pas to put something on obviously larger than the service without being an issue overall. I would hate to tell someone we can do this than find it not an option at all. I must say that the garage is sub fed from 400A service at house. Could upgrade garage but that is more work and money which at the moment is less than ideal.

Can a rotary drive be soft started anyway or is it already a part of the phase converter control box? It would help prevent any issues at house caused by the inrush. I would intend on including it as soon as possible if not immediately if it is an option.
 
I understand that, that is simply standard practice to do in a normal scenario. I've never considered this option as a normal practice though. I just wasn't sure if it was a faux pas to put something on obviously larger than the service without being an issue overall. I would hate to tell someone we can do this than find it not an option at all. I must say that the garage is sub fed from 400A service at house. Could upgrade garage but that is more work and money which at the moment is less than ideal.

Can a rotary drive be soft started anyway or is it already a part of the phase converter control box? It would help prevent any issues at house caused by the inrush. I would intend on including it as soon as possible if not immediately if it is an option.

Simplest way to "soft" start an RPC for reduced effects back at the rest of the "local" grid, your (and your neighbour) side of the last utility transformer?

Start modestly. Drop supplementary idlers on at intervals. "stagger start" style.

Ex: Need 20 HP? Start a ten, then add another ten. Or start a 5 HP to 7.5 HP, add another, then another, then...

A downside is more total parts, more complex controls. Even so, it isn't THAT complex. Only the FIRST one needs an RPC "control box". The rest are started on already-existing 3-P power from it, not 1-P.

An upside is not all hours of all days need the entire "chain" online.

Lots of situations exist where the "extra" is only needed to START one of the heavier loads, doesn't even have to remain online to serve the running load. Even more situations exist where the "big guy" isn't to be run at all that morning, afternoon, or entire day - only one or several lighter load motors, and even those not necessarily all at once.

Further, I'm a believer in "pony" starting RPC idlers over 10 HP. Rest of your own household, nor those neighbours possibly sharing the same utility transformer should not be bothered by startup effects.
 
Further, I'm a believer in "pony" starting RPC idlers over 10 HP. Rest of your own household, nor those neighbours possibly sharing the same utility transformer should not be bothered by startup effects.

I'm a firm believer in the "salad spinner" approach to starting a RPC.

The references to RFC should be changed to RPC.
 
I understand that, that is simply standard practice to do in a normal scenario. I've never considered this option as a normal practice though. I just wasn't sure if it was a faux pas to put something on obviously larger than the service without being an issue overall. I would hate to tell someone we can do this than find it not an option at all. I must say that the garage is sub fed from 400A service at house. Could upgrade garage but that is more work and money which at the moment is less than ideal.

Can a rotary drive be soft started anyway or is it already a part of the phase converter control box? It would help prevent any issues at house caused by the inrush. I would intend on including it as soon as possible if not immediately if it is an option.

When starting high inertia loads like conveyors, this IS standard practice. I am not sure what you want, the cheapest way to get the 20 Hp on line or keep from flickering the lights?

Tom
 
Start the RPC, then bring the transformer online, then bring the machine online. 123, 321 for shutdown . It Is that easy?
 
I'm a firm believer in the "salad spinner" approach to starting a RPC.

I am not sure of this reference. I know of pony starting as described before and where you use a single motor to bring up to speed the larger before starting. Is the latter what you are referring to? I always like to ease a load on if possible. Soft starting (or VFDs) is just something that I know, I've never played with RPCs.

The references to RFC should be changed to RPC.

I would fix the typo but it won't allow me to edit first post. Am I missing something? I can edit further posts.
 
I am not sure what you want, the cheapest way to get the 20 Hp on line or keep from flickering the lights?

Seeing if I am missing something in running the over-sized RPC and looking at option to reduce the "flickering of lights."
 
Seeing if I am missing something in running the over-sized RPC and looking at option to reduce the "flickering of lights."

What is your dedicated power load?
What size is your maximum equipment load?
What size HP is your RPC?
Are you expecting to put a CNC online?, if so what is the horsepower?
What size transformer is feeding your facility? And how many other facilities is it feeling?

When you’re done answering those questions, maybe more questions can be answered.

From the sounds of it, it seems like you are on a shared line.

I’m on the shared line. My 4 neighbours respect what I’m doing from the 50 kVA feed transformer.

Be respectful, and respect comes back.

I do dim my neighbours lights sometimes.
 
What is your dedicated power load?
What size is your maximum equipment load?
What size HP is your RPC?
Are you expecting to put a CNC online?, if so what is the horsepower?
What size transformer is feeding your facility? And how many other facilities is it feeling?

When you’re done answering those questions, maybe more questions can be answered.

From the sounds of it, it seems like you are on a shared line.

I’m on the shared line. My 4 neighbours respect what I’m doing from the 50 kVA feed transformer.

Be respectful, and respect comes back.

I do dim my neighbours lights sometimes.

Most of that is in my originating question. It is less likely that it is on a shared XFMR as its 400A on residence feeding off to garage in rural area.
I highly agree with being respectful and considering best plans. In this case, the used RPC unit is what it is. So, to reduce start-up effects, it would be a matter of either, connecting a starting motor if possible or a smaller motor to initialize the process in a pony process from what I gather.
 
Most of that is in my originating question. It is less likely that it is on a shared XFMR as its 400A on residence feeding off to garage in rural area.
I highly agree with being respectful and considering best plans. In this case, the used RPC unit is what it is. So, to reduce start-up effects, it would be a matter of either, connecting a starting motor if possible or a smaller motor to initialize the process in a pony process from what I gather.

Jolly -

And it all depends upon who you impact. My shop is separate building / service entrance. But shares the transformer up on the pole with our house. I have a 20 HP Arco unit - which cannot use a pony motor to start. So I feed it with a 100 amp breaker and appropriate wiring. Funny thing is the lights don't do anything in the shop - but my wife 'complains' the lights do blip in the house. But at least she knows when I'm firing up any of the machines. And as we are 1/4 mile off the road in a rural area with nobody else on the transformer I only impact our family.

Dale
 
Thanks for the response Dale. I certainly appreciate any and all replies as I learn here. It is good to see someone has experienced what I am looking at first hand. Wiring and fusing is more of safety factoring which I know well enough. Actually starting and running such a large motor and piece of 3Ph equip off of 1Ph is just not what I could predict. The main two issues was will it just overload things and blow fuses and if the effects downstream would be tolerable.
Obviously it works.

I have a 20 HP Arco unit - which cannot use a pony motor to start.

Can't pony start? I am guessing that pony starting then requires largest first rather than a smaller started to establish 3Ph. That would mean all smaller idlers which you and possible I would simply have large so no go for pony. Could something 1Ph of reasonable size (~1HP or less ) be able to spin a 20HP up before starting?

Phase Quest boasts soft start design, is this reasonable? If so, can it be added to existing system?
 
Can't pony start? I am guessing that pony starting then requires largest first rather than a smaller started to establish 3Ph.

That's "staggered" start.

"Pony" start uses a separate motor - modest 1-P electrical, sometimes battery DC, could even be a lawnmower gasoline engine - to MECHANICALLY spin-up a heavy RPC idler so there is less starting inrush, electrically, as it is cut-over onto the 1-P line power.

Bespoke RPC idler motors may not have an exposed shaft to couple such things to.
 
Most of that is in my originating question. It is less likely that it is on a shared XFMR as its 400A on residence feeding off to garage in rural area.
I highly agree with being respectful and considering best plans. In this case, the used RPC unit is what it is. So, to reduce start-up effects, it would be a matter of either, connecting a starting motor if possible or a smaller motor to initialize the process in a pony process from what I gather.

You may have think you 400 A. If you’re on a 50 kVA (or 75kva)transformer, and the transformer is if fused properly, do you really have 400 A???

My 20 RPC and 15 kVA transformer start at the same time with one press of the pushbutton. I have seen inrush of 93 A. with my line voltage drop from 241 to 212. At Idle it sits at 5 A.

My 5 hp dust collector is on single phase and does this too. But it hits 121 A for a split second. My main voltage drops to about 208 for that 1/2 second. When it’s up and running moving the full amount of air, it draws 19A.
 








 
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