Paralleling phase converters?

I have read here that some folks use more than one phase converter in their shops and just turn on more converters as the load increases.
Are these rotary converters connected in parallel? If so, is there a concern about them being in phase which each other? Does this just happen automatically? What about voltage differences between the converters? I'm asking because it seems that with both machines running at top speed there is a voltage drop which causes one machine to alarm out. The spindle drive senses a too low voltage condition and stops the machine. Thanks, Eric
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Basically I use a second idler to feed a second row of machines-it has it's own mains breaker and the only time it is truly "connected" to all three legs of the first converter is through a large single pole contactor when it is initially being started up.
Tomorrow I'll look for the ladder I made when I built the thing, pretty sure it popped up in one of my desk drawers a couple months ago and so I put it into the contactor enclosure.
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On Sun, 20 May 2012 21:32:23 -0700, "PrecisionmachinisT"

Thanks for the reply. By ladder do you mean a drawing of how it's all connected? I'm only familiar with ladders you climb and ladder logic drawings. I know I can parallel just motors, that is what happens when machines are turned on. I don't know if I can parallel the capacitor balanced output from a phase converter though. I suppose I could parallel the extra idler motor to the existing motor and let the caps balance the output of both motors but I'm not sure what that would do to the voltage balance. Eric
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Specifically, the contactor coils, auxillary contacts, overload heaters, start and stop push buttons and so forth.

Pretty sure it's best to balance each converter separately, with no load, adding additional run caps at the actual load.
FWIW, I don't have any voltage correction caps on my system at all.
IIRC I typically I get ~218, 220 and 238; someday, I might burn out a coolant pump or something from the imbalance but since it's been going on 15 years now and so I'm not all that worried.

IMO, best to feed each one from it's own breaker in the mains panel, and have each one supply a dedicated load.
Also, in order that inrush doesn't go clear through the roof, never try to start them both at the same time.
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Sorry, found my schematic but it is incomplete, apparently I made some last minute changes and failed to document them
--I could reverse engineer if you for sure wanted to copy my design but at the end of the day, you're probably better off simply installing a second converter that operates as a stand-alone unit.
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On Mon, 21 May 2012 11:23:53 -0700, "PrecisionmachinisT"

end of the day, you're probably better off simply installing a second converter that operates as a stand-alone unit. The problem is that the extra rotary converter that I have is not enough by itself to power either machine. Both machines will run on the existing converter at the same time but when both spindles are loaded to the max (actually, one spindle is loaded to120%) I get the low voltage alarm. The 120% load is brief but often enough on one particular job that the extra converter, or a bigger one, is needed. Eric
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I used to do this, it is not a problem and works better than one big idler. I no longer do it,m because I moved to a warehouse with 800A 3 phase.
i
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On Mon, 21 May 2012 20:53:36 -0500, Ignoramus20629

Did you parallel rotary phase converters that used capacitors to balance the voltage? Because this is what I want to do. I need only a little more power when running both machines but the extra converter is not powerful enough by itself to run either machine. Eric
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I used parallel idlers and run capacitors on both L1-L3 and L2-L3.
i
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On Mon, 21 May 2012 21:47:00 -0500, Ignoramus20629

Did you switch the idlers in and out as needed? This is my goal, to use the seconc RPC only when needed. Eric
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No, I always ran both. I had a starting sequence. When both were turned on, the capacitors balanced the legs correctly.
You can do it differently.
i
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    They should be. Each additional one adds capacity (including each bare three phase motor running from the same generated line), as long as the wiring from the breaker box is capable of providing sufficient current.
    But this does *not* apply to things like VFDs.

    It happens automatically -- during a moment's high current surge as the extra idler is being spun up. (Note that all you need is an extra three phase motor -- not the starting circuitry which is associated with the primary rotary converter. If you make your own, you know what to leave off. If you are buying them commercially, it is more tricky to know what to disconnect.

    Self adjusting -- other than the fact that adding idlers without tuning capacitors may increase the imbalance in the output voltage.

    You seem to be talking about a VFD here, not a rotary converter. You certainly don't want to try to parallel up multiple VFDs, as there is no way to synchronize them -- unless the manufacturer added it as a special feature.
    And if you are running a VFD for the spindle motors, you should not need a rotary converter. The VFD can simply be obtained with some extra capacity, and (assuming that it is not made to totally give up in the absence of a third phase) it can generate the third phase for your spindle motor.
    And VFDs are not normal loads for rotary converters, so you may have strange things happen there -- especially with multiple VFDs running from one rotary converter.
    More details about what you are doing/trying to do might help get better answers, since I know that I had to try to guess what you are doing.
    Good Luck,         DoN.
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wrote:

Greetings DoN, The machines I'm running are CNC machines. They both use VFD spindle drives that require 3 phases. I have a rotary phase converter running both machines. It's fine most of the time but when each machine is running at top speed one machine, the lathe with the 15 hp spindle motor, will stop because the spindle drive senses low voltage. This actually happens when the spindle load meter reaches 120%. The high load is only a couple seconds in duration during each pass but it doesn't happen when only the lathe is running. So since I have another 7.5 hp rotary phase converter I was hoping I could parallel it's output to the existing 15 hp rotary phase converter. Both converters have capacitors to balance the output and I am concerned that paralleling the outputs after the caps might result in unwanted exitement. Thanks, Eric
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wrote:

Although technically, two 10 hp idlers is equal to a single 20hp one, I don't think that an additional 7-1/2 hp is going to be enough to take care of the problems you are having with voltage sag on the generated leg.
Fairly easy test would be to add the 7-1/.2 hp unit right there at the suspect machinem so that when the mains are thrown, it automatically comes on-line.
Otherwisem you could run a mew 60 amp single phase branch circuit directly to the fadal and install your 7-1/2 hp phase converter again, right there at the bottom of the fadal mains switch, so that when the switch gets thrown, rhe converter automatically comes on....
But this time, disconnect the spindle inverter from the transformer in the rear cabinet, and reconnect it to run off of single phase (directly off of the mains switch, same place where you connected the 7-1/2 hp idler). ( in order to run the inverter from single phase, connect L1 and L2 to single phase and then place a jumper between l2 and L3 ) Then reset the parameter to reflect that the machine is single phase--this will make the accelleration ramp a bit more gradual. The 7-1/2 hp idler should be more than enough to run the computer and servo drives, and although you'll need to derate your spindle continuous horsepower somewhat, at the end of the day you're not going to miss much in the way of lost production, it's emminently reversable, and best of all, costs next to nothing.
Anyways, too time consuming writing this crap out, pretty sure I still have your number and plenty of minutes left--I'll give you a jingle if you can let me know when is a good.
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On Tue, 22 May 2012 09:10:08 -0700, "PrecisionmachinisT"

I'll be in my shop most all of thursday and friday and maybe I'll get lucky and get to spend 1/2 a day in my shop today. Today is one of those meeting with customers type day where you need to see them at their place to discuss their on site projects. Thanks, Eric

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    [ ... ]

    Are you sure that they absolutely *require* three phase input? Most are designed to run from three phase, but will run quite well from single phase, with a little derating. (Say take a 5 HP motor and hang it on a 7.5 HP VFD -- as long as you can keep the VFD from considering that a phase loss. Sometimes, the trick is to connect two of the input phases to the same wire, and sometimes you can tell it to ignore "lost phase" conditions.

    It should not. Connect them together before starting them
    You might check whether a different value of capacitors total will give better balance.
    You might also check whether the balance shifts when you up the load to maximum (or 120% in your case). In that case, perhaps a different tuning of the capacitors would keep the problem from reoccurring.
    Are your rotary converters commercial ones, or ones built from kits and components? You can usually just use an otherwised unused three phase motor in parallel with the existing rotary converter. Once you have it connected, try adjusting the capacitors for better balance. The combination may not start so you will want to start the commercial rotary converter first, then switch the second one on, and then finally start the machine tools.
    But in any case, rotary converters are made to power motors, and a VFD is a very different load, so if you can get the existing VFDs to run from single phase -- or replace them with ones which will run from single phase, you will probably be better off.
    Is there anything else in the machines which really needs three phase, or just the spindle motors?
    Good Luck,         DoN.
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wrote:

Greetings DoN, The VFD in the lathe looks at all three inputs. I do not know if two inpits can be tied together. The documentation for the spindle drive is pretty spare. The servo amps also want 3 phase. It's a Fanuc control so maybe I can find out from Fanuc whether the Fuji spindle drive can be connected as you suggest. My rotary converters are both commercial units. Since I bought the first one I have learned how to build my own and have done so for a few friends. After thinking about it I can see that it really shouldn't cause any problems connecting the converters in parallel. Thanks for your help. Eric
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