Questions about setting up a three phase panel fed by two RPCs.

Currently my three phase equipment all run off of a single 25hp RPC.

It's all hard wired, no load center, no breakers (except the big one in the single phase load center that runs the RPC), etc. Don't scold, poor folks got poor ways, and it's worked just fine for about ten years now.

Anyway, I've got another VMC coming in with a 15hp spindle and I don't think that 25hp RPC is going to be able to handle it all by itself, so I'm thinking of tying another 15hp RPC I already own into the system while also changing over to a load center with breakers for the three phase, to kill two birds with one stone.

My plan is to run out of the RPCs into the mains bus bar on the three phase load center, and then wire each of the three phase machines from there, each on its own breaker, just as if I had three phase from the pole.

Is this a workable plan? I don't see why it wouldn't work, but since I'm not omniscient, I thought I'd better ask some other folks what they thought.

My one question is this: Can I just tie both RPCs into the main bus bars on the load center the same as if they were mains from the electric company? If I do this, is there any special precautions I need to take to prevent one RPC backfeeding from the other if I decide to only run one of them?

I would assume that the contactors inside the RPC cabinet would prevent such a thing from happening, since they'll break all three legs on the output when open, but I thought I'd better ask.

Thanks for reading, and any help you all can lend.

My $.02.

Run single phase input to 2 legs of a 3 phase panel. Install 3 phase breakers to feed the RPC's and a 3 phase breaker for your VMC.

Nice to have some protection for the wild leg.

With 40 hp load you are getting into some amps, you should talk to a electrician, and the power co...Phil

This has been discussed on other PM threads. I recommend against running two parallel RPCs as you will have some wonky synchronization issues between the synthesized leg between the two RPCs. I recommend you get a larger single RPC for this application. And per the previous post, do make sure you have properly sized panels and wiring.

An alternative, would be to run two different 3-phase panels, one off each RPC. In this scenario, you will have no problems.

Ultimately, it depends on what you call an "RPC".

You can run any number of motors on the RPC up to the HP capability. Every one of those is a potential "RPC" if running unloaded, and they do not have issues with sharing. In fact one may be able to start a difficult load by starting another motor first, which will then, if unloaded, act as a helper RPC in starting the heavy load. As long as you have the current capability to supply the load AND the idle current of the two "RPCs", ther is no issue.

Now, if you have two full "RPC" with starters and run caps etc, there might be some issue. But if you essentially connect an RPC without starter circuit, or a plain motor, to the 3 phase from the first RPC, then as long as you have the current capability, you will get helper action from the added motor.

You need to disconnect the start circuit because it has no effect, since the RC idler will be fed 3 phase, and because it will draw extra current unnecessarily. I would also disconnect the run capacitors, but I suppose one does not have to.

Bottom line, you will be essentially connecting another large motor, and not a full "RPC", although the added unloaded motor will in fact provide RPC action (acting just the same as a rope-started RPC, once it comes up to speed).

Doing that, I see zero reason to be concerned. I would NOT suggest connecting a full self-starting RPC in parallel with an existing RPC.

A couple of things I forgot to mention:

1) I have a 400 amp service to the shop, so no issues with not having enough juice. This isn't a hobby shop in my home garage, three phase just is not available from the power company (or I'd already have it). I've got plenty of power to the shop, it's just single phase.

2) Both RPCs are commercially built units with start/run caps and one-touch button start. They have balancing caps inside the cabinets and produce pretty good balanced power.

So what I'm hearing here is that there may be some harmonic issues between the two RPCs tied into the same bus bars?

Can you cite any examples of this? Like what will happen and how can I test for it?

I don't see how the motors themselves would be out of sync, as they're being fed single phase from the same source, which will set the frequency sync for two of the three legs automatically.

I ask, because I've seen two or three shops around here locally that run multiple RPC's into a single load center (that's where I got the idea from) and nobody said anything about harmonic distortion. I just never asked about running only one RPC with the rest turned off, thus this thread.

JST said:

Ultimately, it depends on what you call an "RPC".

You can run any number of motors on the RPC up to the HP capability. Every one of those is a potential "RPC" if running unloaded, and they do not have issues with sharing. In fact one may be able to start a difficult load by starting another motor first, which will then, if unloaded, act as a helper RPC in starting the heavy load. As long as you have the current capability to supply the load AND the idle current of the two "RPCs", ther is no issue.

Now, if you have two full "RPC" with starters and run caps etc, there might be some issue. But if you essentially connect an RPC without starter circuit, or a plain motor, to the 3 phase from the first RPC, then as long as you have the current capability, you will get helper action from the added motor.

You need to disconnect the start circuit because it has no effect, since the RC idler will be fed 3 phase, and because it will draw extra current unnecessarily. I would also disconnect the run capacitors, but I suppose one does not have to.

Bottom line, you will be essentially connecting another large motor, and not a full "RPC", although the added unloaded motor will in fact provide RPC action (acting just the same as a rope-started RPC, once it comes up to speed).

Doing that, I see zero reason to be concerned. I would NOT suggest connecting a full self-starting RPC in parallel with an existing RPC.

Click to expand...

OK, this is helpful info.

So are you saying I can tie the 25hp RPC into the bus bars on the load center and then wire the 15hp motor, sans startup gear, to a switch wired to a breaker inside the load center, and then that idler motor will increase the HP capacity of the system when it is running, yes?

I can easily do that. Doing it that way would also eliminate any question about whether one would backfeed the other, since they would be isolated from one another by a breaker and a switch (I'd use the switch just so I don't have to be in the panel using the breaker to turn the second idler on/off).

Do I have your idea properly in my head? If not, please set me straight.

Thank you for your time guys, I appreciate the help.

Yes, that is about the deal.

It "kinda " increases capacity.... it will actually increase "surge" capacity more than straight up all the time capacity, but there will be an increase in that as well. You "pay for" that increase with an increase of input current.

There is no particular "backfeeding" since the second idler just acts like a motor on the system. If the generated leg drops, the second idler will tend to hold it up by generating, getting power from the same two input wires. Motors are pretty much self-regulating, especially if both are (as in this case) driven from the same source.

It is helpful if both the motors are the same speed rating. That will help ensure that they co-ordinate well when loads suddenly increase or decrease.

Each motor (RPC and added idler) "should" have its own motor protector. A properly rated breaker in the load center is good.

An alternative setup would be to reserve the 25 HP for the VMC, and use the other as-is to supply other machines for finishing, 2nd op, etc. The 25 is about the least I'd expect to be reasonable though, and you could be correct that it is undersize for the VMC.

What do the VMC folks say it will need?

The following is an excellent post by DaveKamp explaining the issues with parallel RPC in answer to a more general question about whether one should parallel or serialize the RPCs:

Quote Originally Posted by Grigg View Post
Once or assuming the second rotary phase converter is running what is the difference in connecting it in parallel to the first converter as compared to adding another idler motor?
As I see it the second rotary converter is simply another idler although with it's own starting system; which once running would then be out of the picture.
Because the second rotary converter is developing it's own 'generated' leg which is not necessarily going to have the same phase displacement of another unit. Remember- the rotary converter does not create a phase relationship that is 'stable'... the displacement varies with load, and is a function of the motor's impedance and slip.

Since you have TWO sources of incoming power, you'll have two rotary converters, and where the phases don't match, you'll have current flow to TRY to make the voltages equal. The end result, is that one converter is trying to push the voltage/current in one direction, while the other is pulling it backwards.

Its like having two synchronous generators, where one of the legs of each, are improperly phased, so they're perpetually fighting.

Using a second idler, with the first idler's contactors all capable of powering the second idler, means the second HAS to follow the first in a BENEFICIAL state.. it WANTS to help support the first's phase angle, not fight it.

An excellent demonstration of this, is to put a set of voltmeters on each leg, then start one converter, then start a machine that has a good-sized motor... and just let that machine spin (don't put it to work). Just idling, it'll help 'stiffen up' that generated leg's output. you can start and stop more machines, because the idling machine is CONTRIBUTING to the stability of the primary idler.

JST said:

Yes, that is about the deal.

It "kinda " increases capacity.... it will actually increase "surge" capacity more than straight up all the time capacity, but there will be an increase in that as well. You "pay for" that increase with an increase of input current.

There is no particular "backfeeding" since the second idler just acts like a motor on the system. If the generated leg drops, the second idler will tend to hold it up by generating, getting power from the same two input wires. Motors are pretty much self-regulating, especially if both are (as in this case) driven from the same source.

It is helpful if both the motors are the same speed rating. That will help ensure that they co-ordinate well when loads suddenly increase or decrease.

Each motor (RPC and added idler) "should" have its own motor protector. A properly rated breaker in the load center is good.

An alternative setup would be to reserve the 25 HP for the VMC, and use the other as-is to supply other machines for finishing, 2nd op, etc. The 25 is about the least I'd expect to be reasonable though, and you could be correct that it is undersize for the VMC.

What do the VMC folks say it will need?

Click to expand...

It's a 1998 Fadal 4020, the VMC folks say they've been out of business for ten or fifteen years. IIRC the full load amp demand is 40amps @ 230V, but I'd have to go back to check that to be sure.

I don't need to support 40hp worth of motors running all at once, I need to support, at most, half that. The VMC I already own has a 5hp spindle, and the 4020 has a 15hp spindle. I very much doubt I will ever see either machine pulling its maximum rated hp, never mind both at once. I just don't want the other machine to brown out over voltage drop when the big machine's spindle kicks on from a stop.

Everything else in the shop is manual equipment, the biggest motors there being 7.5hp.

If you tally up every 3ph motor in my shop it only comes to 38hp, but that's five machines. Being a one man band, the most I can ever conceivably use is both VMC's and one of the manual machines, but in all the time I've been here, I have never attempted to run my single VMC at the same time as one of the manual machines, so I don't see that ever coming up.

So, bottom line, can my 25hp RPC with the added 15hp idler reliably power 20hp worth of spindle motors that run from VFDs?

The VMCs both have VFDs to control the big spindle motors, so it's not like they hard start at full load like a gear head lathe does.

Ideally, I'd just pop for a 50hp RPC and be done with it, but I'm already biting off a pretty good chunk with the new machine, so I don't want to spend another three grand on a phase converter if I don't have to.

Plus, a bigger RPC can be substituted in place of the 25hp unit pretty easily later on down the line when finances allow.

Don't need it to be perfect or the best ever, I just need for it to work reliably until I can afford to do The Right Thing™ and get a bigger converter.

Thanks for your advice so far, and thank you to the fellow that explained the parallel RPC is a bad idea thing to me, I appreciate you both taking the time to respond.

Probably be worth trying running on what you have, you will probably be fine.
The 15hp Fadal won’t draw much when it starts(it’s not a gear head lathe with big chuck), it will draw the most when taking a big cut. The majority of the time you won’t be any where near 15hp
It doesn’t sound like you really plan on running multiple machines at once anyway??

I run my Fadal VMC 20 with 10hp motor on a 10HP American Rotary converter. It’s fine most of the time, although I have stalled the spindle before.


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ripperj said:

Probably be worth trying running on what you have, you will probably be fine.
The 15hp Fadal won’t draw much when it starts(it’s not a gear head lathe with big chuck), it will draw the most when taking a big cut. The majority of the time you won’t be any where near 15hp
It doesn’t sound like you really plan on running multiple machines at once anyway??

I run my Fadal VMC 20 with 10hp motor on a 10HP American Rotary converter. It’s fine most of the time, although I have stalled the spindle before.


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Click to expand...

Glad to hear that.

I'm already going to be ~$600 into the panel and breakers, I really don't want to spend on a new RPC right now.

Won't be doing the install until the end of August, but when I get it in and running, I'll report back with my results, if for no other reason than to act as a cautionary tale for others.

Hopping on to the thread here ...

I am in a very similar boat and would like some clarification. Just got a new to me 15hp vmc, and previously ran a 7.5 cnc from my rpc no problem.

Clarification on where to wire in the additional idler in my current set-up

200 mains single phase, 50 amp breaker leading out to 50 amp fused disco. Disco leading out to phase craft panel, 10 hp 3 phase motor. Line from RPC feeds into a 3 phase nqob panel box, and my machines are all breakered separately from that box.

Should I wire the additional 10 hp idler motor from a breaker in the panel box, and then add a new breaker from panel box to the VMC?

Or should I add the idler in before the panel box, with an extra starter and a disconnect between the rpc and the 3 phase panel?

I know i’ll have to change out the feeding wire, breaker and disco to the proper size.

Thanks!

An added idler is connected same as a load... to all 3 wires output from the existing RPC, just like any other motor. And it cannot drive anything, no load should be on it's shaft if you expect it to be an idler.

Be aware, it is not "free", it draws input current, so you actually can have a lower overall capacity with it in place due to the extra draw from the single phase source.

Thanks! I can pick up the additional 10hp idler very reasonably, so I plan on wiring it all in and then taking measurements to see where it all takes me.