RPC solutions....

Awl--
What I found out today was pretty flabbergasting, about commercially-built rpcs. In a word, they effingSuck. In more words, they seem to suck gratuitously, even negligently.
I have two commercial units, one by Steelman (extremely well built), and a 5 hp Phaseamatic, which 5 or more years ago cost almost $800. Both produce a wild leg of about 275 V with 240 input, which fried everything it could.
The reason for this is--despite their bragging about "custom made" motors by Baldor, etc--is that they load the unit up with big-assed start capacitors *only* between L1 and L3 (L3= generated), which are then *never switched out* or redistributed between L1-L3 and L2-L3, resulting in very high, very uneven voltages among the legs.
My first inkling of this occured when I powered up a regular Baldor 10 hp motor with the two "oh-ficial" rpc's, and after switching off the rpc's, noticed I had *much better power* from the Baldor, all by it's lonesome, no caps at all.
After I cannibalized my Steelman rpc for the caps (4 caps at 50 microfarads each), and putting 100 microfarads on both L1-L3 *and* L2-L3, the voltage was much better, and became even better with another 5 hp motor added. The two motors yielded 239, 232, and 231 volts leg-to-leg, with 240 V input. That's about 3%--not bad!
The more caps you add, the higher the voltage gets. It seems to be a bit of an art to juggle the right hp with the right cap values. AND, it seems peculiarities of the motor design may affect this as well.
The site below gives rules of thumb for cap values: Start caps: 50-100 microfarads per hp; Run caps: 12-16 microfarads per hp.
Altho adding in caps "as you go", from a switching/breadboard-type ditty, may seem attractive ito voltage regulation for various loads, you get some fearsome sparks, and one commercial site advises that this is a no-no for sensitive loads, like pyooters, cnc, etc.
So unless you come up with some scheme to make adding/subtracting capacitances less electrically traumatic, whilst a load connected, it's likely best to anticipate these values and connect them before the load is added, and not disconnect them until the load is removed.
Some commercial units stabilize voltage by capacitor adding/shedding, but this is dicey, as above.
The following site has a very nice summary of diy rpc's, and points out two little known factoids: 120/240 V systems provide an inherently hobbled 3-ph, as two legs are not 120 deg out of phase, but *180 deg*. Which makes the other two phase angles 90 deg!
They also point out that a number of small motors will give as much capacity as one large one, AND are more versatile, easier to start up, but require more wiring logistics. And, I suspect, might give an inherently more stable voltage. There is no such thing as too much rpc capacity--altho a lot of unnecessary capacity will spin your electric meter--ergo the beauty of staged smaller idlers.
http://www.nojolt.com/how-to-build-a-rotary-phase-converter.shtml
"NoJolt" -- how effing cool is DAT?? :)
Googling diy rotary phase converters yields many schematics for automatically switching out start caps, and switching in run caps--but it's proly not that inconvenient to switch them in/out manually. I think cnczone and practicalmachinist have dedicated threads for this, with lots of schematics, and some parts lists/sources.
I would advise being careful about putting rpc-idlers together in parallel, as two generated legs can be out of phase, if I grokked one of my many dazzling short-circuits correctly. Check the voltage diff between them, before connecting them up. If out of phase, maybe switch legs for the input.
Inyway, to sum up, I am dumbstruck that I can kluge something together that is *far* better than something I would otherwise pay thousands of $$ for, and which in some contexts is altogether unusable. And I WOULD have payed the money, if the stuff just wasn't so derelictly designed.
Props to Ig, for his good, albeit super-terse, advice.
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Mr. P.V.'d (formerly Droll Troll), Yonkers, NY
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message

5
a
by
very
no
microfarads
input.
of
two
angles
capacity
stable
it's
with
parallel,
that
Reminds me I need to swap in this 125 amp breaker getting tired of the 50 hp unit tripping the 100 amp breaker happens oftgen if I done forget to get the shaft spinning slowly first with my foot.
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Mebbe you can kluge up a pull-start, like for a lawnmower. I have the pullstart that broke off my weedwacker, iffin you want it...
Hey, I see you have "hackmachinist" in your addy. You can't use that... *I'm* the hack machinist in these here parts.... well, at least according to the butt-buddeez jb and BD.
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Mr. P.V.'d (formerly Droll Troll), Yonkers, NY
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wrote:

yes
Wahoo!
bullshit
bullshit
Thank you PV!!!
I think that you are on a very right track.
The two motors that are in parallel, are in parallel and, therefore, there is only one "third legs" that is produced. There may be currents from the third leg contact of one motor to that of another, as they form a dynamic system, but they would not be short circuit currents.
The nice thing about two idlers is that you can make a RPC with only two contactors, that would be nicely balanced AND mave a manageable starting current. That's how mine is. I have two idlers, 10 and 7.5 HP. First one starts, then another, and the caps on another one connect another pair of legs so that they are symmetrical.
Our idea of starting your baldor from your cheaply built RPC, with caps properly connected, and then disconnecting your RPC's third leg, is very solid.
To further refine it, you need to put more caps on the side of Baldor that is opposite to where your RPC's caps are (say if your RPC has a cap on 1-3, then Baldor would have a cap on 2-3). Then your Baldor with your commercial RPC, would form a balanced phase converter network and would have increased capacity.
By the way, I am looking for two 15 HP three phase motors, anyone got them?
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Sorry, Procto, evidently you didn't read the rest of the sentence which went . . . ." those without relay-switched starting capacitors". I tried to make that clear, but guess I failed. Your rambling missive seemed to describe both types, best as I could tell.
Bob (knows RPCs) Swinney
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Keep in mind that the notion of "voltage" or electric potential requires two points. So the two legs of "single phase" are 180 degrees out, when measured relative to a point that happens to always have the exact opposite potential relative to the two "legs".
Now if you take three phase power source, with "legs" A, B and C, then voltages between A-B, B-C and A-C will be 120 degrees apart. And if you picked a point X connected to the middle of transformer supplying A and B, then points A and B would be exactly 180 degrees out of phase relative to X.

Your reference points now are three legs. Not the middle between two of them.

The key is "I fixed it".

Even when two motors are running, there is still too much starting cap?

Yep
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On Jul 22, 8:29am, "Proctologically Violated"

PV,
Think of a graph. You can lay out vectors from -an- origin that are 120 degrees apart from each other. So, from -that- origin, the legs are 120 degrees apart. But, you can assign another origin point, that is in between two of those legs, and holy smokes, the two legs are 180 on either side of this other origin! And even more amazing, from this new origin, it is 120 volts to either of those two legs, and surprise! it is 208 volts to the "wild" leg"
Since those three vectors are still symetrical about the original origin, there is 240 volts between any one of those two legs.
Now you realize I tricked you, that was in reverse order. So that you get unstuck of not seeing it....
The original origin (neutral) from single phase puts the two legs 180 degrees apart. They always are 180 apart... From >>THAT<< perspective. You add the "wild leg" and it is 208 from that origin (neutral) but trust me, there is a point on that chart where alll three phases are 120 degree from each other.
And if you are loading from phase to hase to phase (three loads) then any one of those loads is 120 degrees apart from the other two loads.
It's all done with creating a new, unconnected "neutral" at a new point on the graph.
Is that better?
Next, we could think about the re-circulating currents in the rotor, and what frequency they are... But that might cloud this burst of illumination.
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I question this "little known factoid:
Proctologically Violated wrote:

If you are talking about the 120 volt/240 volt systems that provide power to most of our homes, I don't see where there is any "phase shift" at all. All that's out on the pole is a center tapped secondary from one phase of the utility's power. We get 240 by connecting to the ends of the winding and 120 by going from either end of the winding to the locally grounded neutral. Since you are only using the "240", I don't see where there's any more that one phase to contend with.
I wonder if the "factoid" producere wasn't talking about a wye connected system which gets its "208" by connecting to two phase leads producing 110 or so each. This is NOT what we have at home.
Pete Stanaitis
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When we talk about "phase shift" between voltages we have to be talking more than one voltage and about more than two conductors. Only one voltage can exist between two conductors. (And also, no voltage can exist at only one conductor!) In the 120/240 system the third conductor is the center-tap or neutral. From "that" reference point, the two other conductors are 180 degrees out of phase regardless of whether they are connected to a RPC. From a reference at one leg, the neutral and the other leg are in-phase, which is how "two 180 degree out of phase voltages of 120 can equal 240." When we change the reference point, then magnitudes and directions change, just like when talking about lengths and directions of a ladder sitting halfway in a hole. However when the three leads of the RPC are considered, the neutral is usually no longer the reference point. The only reference point we can measure each of the three conductors to and get 120 degrees (and approximately equal voltages) is a point such as we would have in a Wye connection. In 3 phase we commonly measure between each pair so we do not have a common reference point for the three measurements. It is true that the generated leg is 90 degrees out of phase with the voltage across the other two legs (the input power), but it is nothing different than being 120 degrees out of plase with the other two legs when all three are referenced to a common center point. Check on "Scott connected transformers" to learn more about it.
One solution to the varying capacitor situation is to add capacitors to the loads so they are switched in and out with the load.
Don Young
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