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Posted by Ignoramus14558 on July 21, 2008, 11:07 pm
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wrote:
> 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.
yes
> 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!
Wahoo!
> 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!
bullshit
> 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.
bullshit
> 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.
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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Posted by Robert Swinney on July 22, 2008, 11:46 am
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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
** Posted from http://www.teranews.com **
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Posted by Ignoramus2245 on July 23, 2008, 12:01 am
Please log in for more thread options >>> 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!
>>
>> bullshit
>
> Well, in 120/240 V systems, the two legs *must* be 180 out of
> phase--unavoidable trig of sine curves.
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.
> What the rotary then does with this may be open to spec--Sam (GORF) in fact
> agrees with you, but someone would have to explain to me HOW separate
> non-120 degree sine curves all of a sudden become 120 deg.
Your reference points now are three legs. Not the middle between two
of them.
>>> 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.
>>
>> bullshit
>
> I would have agreed with this assessment, had it not happened to me!!
> I was in autopilot mode, so I really didn't explore it, so all I can do is
> report the proverbial facts.
> In hooking up my rpc "bank", I was hooking up all the generated legs to one
> busbar (made out of flattened 3/4" copper pipe <g>), and BANG, short
> circuit. I muddled around with the phases of one motor or another and
> fixed it, not sure how.
The key is "I fixed it".
>> 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.
>
> The only problem with this is that start caps have a higher value than is
> necessary for run mode, sig'ly elevating voltages apparently.
Even when two motors are running, there is still too much starting
cap?
> What I am trying to do is create a "run capacitor matrix" that can easily be
> switched in/out/matched to a "motor matrix", ideally a mix of 2 and 3 hp
> motors, totalling 20 hp. I may need only one small motor with one set of
> start caps, switched out upon running, to stage everything else in.
>
> May be more trouble than its worth, but we'll see.
>
> PhasePerfect's 20 hp silent "true 3 ph" digital sure is tempting--except for
> the $5K part....
>
>>
>> By the way, I am looking for two 15 HP three phase motors, anyone got
>> them?
>
> Have you noticed that shit is getting harder and harder to find, as everyone
> is just scrapping shit for a few $$, likely destroying a lot of really good
> stuff.
Yep
--
Due to extreme spam originating from Google Groups, and their inattention
to spammers, I and many others block all articles originating
from Google Groups. If you want your postings to be seen by
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posting on Usenet.
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Posted by Half-nutz on July 23, 2008, 12:55 am
Please log in for more thread options On Jul 22, 8:29=A0am, "Proctologically Violated=AE=A9"
>
>
>
> >> The following site has a very nice summary of diy rpc's, and points ou=
t
> >> 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*. =A0Which makes the other two phas=
e
> >> angles
> >> 90 deg!
>
> > bullshit
>
> Well, in 120/240 V systems, the two legs *must* be 180 out of
> phase--unavoidable trig of sine curves.
>
> What the rotary then does with this may be open to spec--Sam (GORF) in fa=
ct
> agrees with you, but someone would have to explain to me HOW separate
> non-120 degree sine curves all of a sudden become 120 deg.
>
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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>
> 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.