What's new
What's new

Need wiring advice from RPC to 3-phase sub-panel. Problems with single phasing?

JST

Diamond
Joined
Jun 16, 2001
Location
St Louis
Actually, I fully agree with TDegenhart. I think you are misunderstanding the concept.

I have mine set up that way, and it is pretty much exactly what the App note shows.

The "panel" in question is presumably a 3 phase sub panel. Otherwise, why the RPC at all?

So, in that case, the entire 3 phase system is "off" unless the RPC is active. Makes perfect sense, is safe, and if the "switch" is actually a contactor wired with a start momentary (as mine is), then it also affords protection against restarting in case of a power fail.

That's more important than it seems, as the RPC may not start correctly if it is loaded with one or several motors while it is trying to start. That would result in a single phase condition for the entire 3 phase subsystem.
 

scsmith42

Aluminum
Joined
Jul 28, 2020
Location
New Hill, NC
Actually, I fully agree with TDegenhart. I think you are misunderstanding the concept.

I have mine set up that way, and it is pretty much exactly what the App note shows.

The "panel" in question is presumably a 3 phase sub panel. Otherwise, why the RPC at all?

So, in that case, the entire 3 phase system is "off" unless the RPC is active. Makes perfect sense, is safe, and if the "switch" is actually a contactor wired with a start momentary (as mine is), then it also affords protection against restarting in case of a power fail.

That's more important than it seems, as the RPC may not start correctly if it is loaded with one or several motors while it is trying to start. That would result in a single phase condition for the entire 3 phase subsystem.

I had not looked at it that way regarding the potential for the RPC to be overloaded.

Most of my 3 phase equipment has magnetic starters so if the power fails and is restored, there is no load on the 3 phase load center with which to overload the RPC.

Having said that, my old milling machine does not have a mag starter, which proves that in my own shop you and TD’s position is valid.
 

J_R_Thiele

Stainless
Joined
Jan 22, 2003
Location
Columbia Missouri
Ltk--"The problem, I think, is that the RPC does not have a contactor on the output side at all, and for a $2,200 unit that is absolutely ridiculous. As far as installing one myself I have no idea where to begin or what to buy?"

You are correct about this being one solution. With all the discussion which has occurred, there has been little specific to where you begin, what to buy, or how to install it.

The below is what I would do in your situation. I am not going to claim it is the only way, or even the best way. It is a simple practical approach. I will make links to representative products, but intend them as examples, not an endorsement of brand, size or price.

I anticipate others will have other suggestions and corrections if needed.

You will need a 60 amp 3 pole contactor with a 240 volt coil, A time delay relay rated for 240 volts, a way to fuse the two control lines, some conduit and fittings, and a NEMA 1 enclosure with inner panel. The enclosure needs to be large enough to contain the 3 pole contactor, time delay relay, and room for wires to bend- and for you to work in. If in doubt, get one bigger.

What you will be doing is using the L1 and L2 connections to the idler (A and B in the factory diagram) to power the coil on a 3 pole contactor. The contactor control wires go through a time delay relay located in the breaker box before the contactor coil. L1,L2 and L3 from the RPC to the breaker box now go first to the enclosure, connect to the 3 pole contactor, and then on to the breaker box.

The RPC is started and stopped though its usual control buttons. When the RPC contactor closes and powers the idler, it also provides the current to close the added 3 pole contactor, which then connects the RPC to the breaker panel. The time delay relay in the control wiring allows the RPC to fully start before any current goes to the breaker box and on to the machines. This also allows you to turn off the RPC before it connects to your breaker panel if it does not start as it should.

The use of 240 volts in a control circuit is for ease of construction. If the RPC has a transformer and a lower control voltage system you may be able to use that to control the added contactor. Not knowing if this can be done, the design is for what we know can be used.

Construction
Connect the control wires to L1 (A) and L2(B): It would be tempting to add the control wire to the existing terminal where the idler connects- but unless approved for it, there should be only one wire per terminal. It might be possible to wire in a power distribution block . If not, I would use two Polaris taps. The wires should be fused. If there is room you could wire in a fuse block. If not, use two in-line fuses.

The control wires between the RPC and the enclosure should be in their own conduit, with a separate section of conduit for L1, L2 and L3.

The enclosure will have an inner panel. The 3 pole contactor and timer are mounted on the inner panel, then the panel and components are mounted in the enclosure. The wires you have going to the breaker box will now go to the 3 pole contactor, then on to the breaker box.

Polaris tap: NSI Industries Polaris Black IPL4-3A Insulated Multi-tap Connector - 3-Port Single-Sided Entry for 4-14 AWG Wire Range - Dry Locations - Dual-rated for cooper and/or aluminum - 1.188-inch width, 1.375-inch height, 1.55-inch length - Hex size 1/8-inch: Wire Terminals: Amazon.com: Industrial & Scientific
Time delay relay: McMaster-Carr
Power distribution block: https://www.mscdirect.com/product/details/84720432
In line fuse holder: McMaster-Carr
Fuse block: https://www.mscdirect.com/product/details/54002720
3 pole 60 amp contactor with 240 volt coil: https://www.mscdirect.com/product/details/51532364
 

Attachments

  • 20211211_231822 (2).jpg
    20211211_231822 (2).jpg
    82.1 KB · Views: 26

Ltk

Plastic
Joined
Sep 24, 2021

I have never seen a more in-depth response to a question, I sincerely appreciate the time and effort you put into this. I have contacted 3 industrial electricians who have been not helpful at all. I will follow your advice and try to make this work properly so I do not run into this problem in the future, my sub-panel will allow me to run 13 machines (42 slot) and I plan to continue adding to my shop.

Seriously, thank you for your kindness and helpfulness! The only question I have; is there a way to wire a wall-mounted green light that comes on when the time-delay relay has released power? As in the system is ready to go? If this would be very difficult to include I will just forgo it.

To everybody else: thank you also for your help and support!

Not sure if this helps but here are some pictures of the inside of my rotary phase converter as well as a wiring schematic. Could not find a schematic for the internal control wiring, although I'm sure there is one online that I could not locate. It is an American Rotary AD20.
I'm going straight into the 3 phase sub-panel from the 3 output lines. As you can see, the inputs are tied directly to the outputs in this configuration. According to American Rotary's technical support team, single phasing your equipment is completely normal..:eek:
rpc2.jpg
rpc1.jpg
 

Strostkovy

Stainless
Joined
Oct 29, 2017
My RPC was set up with a contactor for starting idler motors, and another contactor that you turn on for outputting to the panel. It was powered by the generated leg so if you lost that leg it would shut down.

You can also have the line input contactor fed by the generated leg, so that you have to hold the button until the idlers have started, much like starting an old clothes dryer.
 

J_R_Thiele

Stainless
Joined
Jan 22, 2003
Location
Columbia Missouri
Ltk Glad to be of help. For the pilot light all you need to do is wire something like this-- McMaster-Carr --to both of the coil terminals of the 3 pole contactor -or to the wires going to the coil terminals, so long as the one connects after the time delay relay. The other option is to wire one or more in after the 3 pole contactor, between any of the 3 phase lines. Some people wire wire in voltmeters between L1 and L2, L2 and L3, and L1 and L3. This way they can observe the actual voltages when the machines are on. McMaster-Carr

To do as Strostkovy did, you would wire the time delay relay to C or T3 instead of b or T2. This means the 3 pole contactor cannot be closed unless the generated phase is present at whatever voltage the coils require. This protects against phase loss. The disadvantage to this is that starting a motor will drop the voltage in the generated leg for a short time - and if it drops enough, the coil is underpowered and the contactor opens.
 

Strostkovy

Stainless
Joined
Oct 29, 2017
Ltk Glad to be of help. For the pilot light all you need to do is wire something like this-- McMaster-Carr --to both of the coil terminals of the 3 pole contactor -or to the wires going to the coil terminals, so long as the one connects after the time delay relay. The other option is to wire one or more in after the 3 pole contactor, between any of the 3 phase lines. Some people wire wire in voltmeters between L1 and L2, L2 and L3, and L1 and L3. This way they can observe the actual voltages when the machines are on. McMaster-Carr

To do as Strostkovy did, you would wire the time delay relay to C or T3 instead of b or T2. This means the 3 pole contactor cannot be closed unless the generated phase is present at whatever voltage the coils require. This protects against phase loss. The disadvantage to this is that starting a motor will drop the voltage in the generated leg for a short time - and if it drops enough, the coil is underpowered and the contactor opens.

I personally found that the generated leg has to drop below 160-170 volts to open a 240V contactor, which at that point is probably a hefty overload.
 

J_R_Thiele

Stainless
Joined
Jan 22, 2003
Location
Columbia Missouri
Strostkovy That is helpful to know. I am revising a pony motor design Madmachinest and I have been working on so that the RPC would not continue to try to start if the idler was not spinning (broken or thrown belt). I was going to use the generated leg to "take over" after a time delay relay cut out.

Ltk. I found a PM post is 2008 with a similar complaint, so it is not new. It also appears to be in noncompliance with the NEC section on Phase converters Redirecting... Specifically:

455.21 Start-Up
Power to the utilization equipment shall not be supplied until the rotary-phase converter has been started.

That was also true in 2008.

I have looked at their "UL" listed models, and they appear to be connected the same way. It seems like a huge liability to me
 

JST

Diamond
Joined
Jun 16, 2001
Location
St Louis
"UL requirements" and NFPA 70 are two distinct things. That is recognized when NFPA refers to items which are "Listed for the purpose". That means they have UL or other recognized laboratory approval to an accepted standard.

So, when a UL recognized piece of equipment is installed, there may be ancillary equipment required to install it, and some of that may be needed to comply with NFPA 70.

"Recognition" itself by UL as opposed to "listing", carries with it the idea that the device in question, be it a switch, a capacitor, or an RPC, is meant to be installed and used with other parts, components , or equipment. Only complete equipment, needing only a power connection, can be "listed".

That is not even a question for something like a lamp fixture, where the branch circuit and accessories are accepted as required. The same applies to a Rotary Phase Converter.

So there really is not a liability involved, aside from that incurred by the electrical installer or the operator of the premises.

Short story. The RPC is a piece of electrical equipment. It needs to be installed to comply with NFPA 70, which may (or may not, depending on design) involve added external equipment such as contactors, electrical panels, and so forth.

Ther is a further issue:

The wording of 455.21 as quoted leads to the question as to whether it is intended that the RPC shall be up and operating normally prior to energizing the lines to the connected equipment, or whether the simultaneous starting of the RPC and energizing the lines is sufficient.

In the one case, an added switch or contactor after the RPC would be required, the closure of which is contingent on the RPC being up and operating stably.

In the other, the common practice of having a contactor which energizes the RPC and the connected equipment at once would be sufficient.
 
Last edited:

Overland

Cast Iron
Joined
Nov 19, 2017
As I always, I learn from these threads, thanks.

I'm building an RPC using the attached schematic. (Jim Hanrahan design). Could someone please confirm that if the RPC is not running, then there is no "pass through" of the single phase current ?

By this I mean, to run the idler motor, contactor R1 has to be "in", otherwise it is "out", therefore no single phase passing through, therefore safe.

Thanks,
Bob

3-phase-power-converter.jpg

Link:
Building a Phase Converter | MetalWebNews.com
 

neilho

Titanium
Joined
Mar 23, 2006
Location
Vershire, Vermont
Not quite getting the desire for the separation of 3 phase and single phase panels.

It's code compliant to pull single phase out of a 3 phase panel, so the simplest arrangement would be a 3 phase panel fed by a single phase breaker with a 3 phase breaker feeding the RPC. RPC is switched on with magnetic contactor, supplies 3 phase panel through the same breaker. When the RPC is off, it's isolated by the contactor.

As noted before, if the 3 phase machines wired from the panel have magnetic contactors, they're isolated from the single phase lines. Connecting one side of the equipment contactor coils to the wild leg prevents single phasing the equipment when the RPC is off.

And single phase machines can operate off the same circuit. Saves a lot of wiring.
 

JST

Diamond
Joined
Jun 16, 2001
Location
St Louis
That's "simple" from the point of view of saving wire. And it works fine when you have 3 phase coming in.

Problem is that not all contactors will be happy with reduced voltage. "Most" relays and contactors, once closed, will stay closed (although not "to spec") at fairly low voltages. Not all, however. DC coil contactors are often better for that than AC coil contactors, which themselves have a high inrush current.

Large motors near the RPC limit can really draw down the generated leg when started. So connecting the control to the generated leg can cause chattering and failure to start. Not on every machine, no. But possible with some, and the problem may be different or non-existent depending on how many machines are operating at the same time.

Having a 3 phase sub-panel makes sense for separating the functions.

If you run from a single phase panel, which has no provision for the 3rd wire, then you have the problem that there are two sources of supply for wires in the branch circuits that ....... if you shut off the single phase breaker for that branch circuit, the RPC output can still be "live". The breaker for the single phase does not affect the generated leg. That creates a non-compliant situation, where the branch circuit is not controlled from one overcurrent device.

One way or another, you need to have each 3 phase branch circuit controlled by one over-current device in the panel.

If you have a 3 phase panel, sure, you can tap off single phase loads from a 3 phase branch. You DO then have to have "interesting" ways of avoiding single-phasing the 3 phase loads.

Seems simpler overall to just have a 3 phase only sub-panel, fed from the RPC.
 

neilho

Titanium
Joined
Mar 23, 2006
Location
Vershire, Vermont
That's "simple" from the point of view of saving wire. And it works fine when you have 3 phase coming in....

Just to clarify, it's for single phase in. Dunno why anyone would bring 3 phase power in to a phase converter, btw.

....Problem is that not all contactors will be happy with reduced voltage.....

The coils engage when the voltage is high and as you note, the majority tend to stay in. (In my experience, never had a problem, but YMV.) Connecting one side of the coils to teh wild leg does prevent single phasing.

...If you run from a single phase panel, which has no provision for the 3rd wire, then you have the problem that there are two sources of supply for wires in the branch circuits that ....... if you shut off the single phase breaker for that branch circuit, the RPC output can still be "live". ...

This diagram doesn't use a single phase panel.

One can install a single phase breaker in the 3 phase panel for single phase loads or pick single phase off a 3 phase circuit protected by a 3 phase breaker. A circuit that can supply both usages has fewer parts than one circuit for each - what I mean by simpler.

....One way or another, you need to have each 3 phase branch circuit controlled by one over-current device in the panel.....

Yes, absolutely.



RPC sketch.jpg
 

JST

Diamond
Joined
Jun 16, 2001
Location
St Louis
Just to clarify, it's for single phase in. Dunno why anyone would bring 3 phase power in to a phase converter, btw.

..................................

You would not.

But with 3 phase, you already have a 3 phase panel, and the breaker for any branch circuit controls all phase wires.

You do not have a specific problem with single phasing due to a particular wire.

And any two wires can supply a single phase tap, two hots, or a hot and neutral. (OK, with corner grounded, there is an issue as to the neutral. Also with a "lighting tap", or "stinger" system)
 

neilho

Titanium
Joined
Mar 23, 2006
Location
Vershire, Vermont
Yes, exactly.

Back to the specific situation here.... One can simplify the circuitry with one 3 phase panel and pull loads and supplies out of that.
 

JST

Diamond
Joined
Jun 16, 2001
Location
St Louis
You "can", but you still may have the single phasing issue for some loads, if they are not of a size to have a contactor.

Sometimes "simple" is not the best plan.

Separating the single and 3 phase reduces the number of "yes, but..." issues, and "hidden gotcha's".

Do whatever floats your watch. Either you will be fine, or you will learn.
 

dalmatiangirl61

Titanium
Joined
Jan 31, 2011
Location
BFE Nevada/San Marcos Tx
What do you do when "code" and mfr's recommendations conflict? According to data tag on my rpc it draws 50 amps, PAM recommends 125 amp breaker, code states "Where the loads to be supplied are variable, overcurrent protection shall be set at not more than 125 percent of the phase converter nameplate single-phase input full-load amperes."

Edit: I have an R-20, PAM specs in link https://phaseconverters.phase-a-matic.com/Asset/RB.pdf
 

Strostkovy

Stainless
Joined
Oct 29, 2017
What do you do when "code" and mfr's recommendations conflict? According to data tag on my rpc it draws 50 amps, PAM recommends 125 amp breaker, code states "Where the loads to be supplied are variable, overcurrent protection shall be set at not more than 125 percent of the phase converter nameplate single-phase input full-load amperes."

The amperage draw of the RPC is just what goes into the idler and out the generated leg. The phase converter can pass through far more amps to the other two legs. If it doesn't pass through and is instead supposed to be parallel, the 50*1.25 amps is fine to protect the idler but will not disconnect the load if the generated leg is lost.
 

dalmatiangirl61

Titanium
Joined
Jan 31, 2011
Location
BFE Nevada/San Marcos Tx
The amperage draw of the RPC is just what goes into the idler and out the generated leg. The phase converter can pass through far more amps to the other two legs. If it doesn't pass through and is instead supposed to be parallel, the 50*1.25 amps is fine to protect the idler but will not disconnect the load if the generated leg is lost.

I understand first 2 sentences, not sure I'm getting the last sentence.

I've been using my rpc on a cobbled up temporary basis, working towards getting it permanent, maybe I should draw out how I think it should be wired and let others critique it. My local sparky is no help, says he has never seen an rpc before, just vfd's.
 

dalmatiangirl61

Titanium
Joined
Jan 31, 2011
Location
BFE Nevada/San Marcos Tx
Here is how I was thinking of wiring it, #3 mag contactor with 220v coil, power the coil with L1 and L2 from single phase side so if power dips or dies (fairly common here) the contactor will open. All my machines have contactors or magnetic switches so if power is lost they have to be restarted manually, the one exception might be air compressor. Control for rpc will be next to rpc, so if it does not start I will know. But if rpc runs, but not generating 3rd leg (internal short???), I see nothing in my current plan that would prevent single phasing, maybe I'm over-thinking this part?

Edit: No single phase loads will be pulled from 3ph panel.

Edit again: Looking at PAM info I should bring T3 from rpc back thru contactor, then to panel.

Third edit: Should T3 also pass thru the 125 amp breaker????
 

Attachments

  • rpc wiring.jpg
    rpc wiring.jpg
    96.6 KB · Views: 10








 
Top