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Posted by GORF on July 21, 2008, 10:14 pm
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> 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.
>
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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>
> 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