cnc-grade rpc's--inneresting story

Awl--

So I been agonizing over this, reading Fitch's stuff, bunch of other stuff, and everyone seems to have trouble balancing the lines.

So I'm about to bite the bullet, and shell out some hefty bucks for converters that supposedly hold +/- 5% between any leg--which is still not great--but I cain't quite get the straight story from the various commercial sources. Some suggest they somehow load/unload caps with load, which others say is not so great for pyooters/cnc, etc.

Phaseperfect.com has very pricey digital converters, 1% regulation--not rotary, not static, just pure 120 deg 3 ph, they say. With really good amps. But, cnc mfrs I talked to hem and haw about the digitals, but the specs are sure nice. Heavy, but not quite as heavy as rotaries.

But the gents at

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may have let the cat outta the bag.

Forget caps and all the bullshit, the guy said, just get the biggest assed idler you can!!! THIS is what stabilizes voltage!

So, I happen to already have an rpc "bank": a phaseamatic, a Steelcase (commercially supplied rpc's), and regular motors: a big-assed baldor Super-E 10 hp beauty (very expensive), and a current-gluttonous Magnatek, I think.

So I fired up the Steelcase, which gives leg-to-leg variations of about 30 V!! Phaseamtic not much better.

So I used these to fire up the Baldor (just a motor), then *shut off* the Steelcase/phaseamtic, and just measured voltages on the baldor. +/- about

15V, no load, on 240 input. Not bad!!

The magnatek also gave good results, but different.

wow.....

I'll have to measure this under load, but the baldor seems like it will solve a lot of my problems--and likely save me $2K+++.

I would spend the money, if I really had confidence in what I was getting. But if all CNC grade rpc's are is a bigger-assed motor, sheeit, I already got those!

Btw, phase-a-matic uses baldor motors, but specially made, and they run really really not-- over 200 deg!!!

Supposedly "real" rpc idlers are not just motors used for their 3rd leg, supposedly different, but at this point, I don't know who/what to believe anymore.

All I know is, that baldor leaves me sig'ly better off 3ph-wise than I was.

I'm still willing to pay for a "professional" solution, but I'm not sure there really is a better one, unless I go with something like Phaseperfect's digital, which not everyone agrees is great for cnc anyway.

Iny idears/comments?

Reply to
DrollTroll
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My phase converter holds voltage better than 5%. That's not because of some magic, but because the two idlers are big and they have capacitors attached to both legs.

If you have a big idler, and properly sized run caps, that apply to both legs, it will have great regulation.

For $100 or $200 you can have a great phase converter.

Run your 10 HP Baldor just like you did, but put 80-100 uF run capacitors between legs 1-3 and 2-3. Very likely, the output of that, at low loads like 3 HP, would be extremely well behaved.

i
Reply to
Ignoramus20788

Changing IDs again? You're confusing the simple minded folks like me... :)

Reply to
Joe AutoDrill

That's an old nym of his Joe. There was another bit I've forgotten it. LOL He's a pistol.

Reply to
John R. Carroll

How'd you know it was me?????

This is the nym I use when ahm tryna be dignified'n'shit. Cain't hold out too long, tho.

I used Screachy Preachy for a while, which was really probably the most apt. Then I tried Cunning Linguist, but heard that was sort of taken and old. Much better to use that on alt.english.usage et al.

I achieved considerable status over on alt.english.usage (as DT), when I asked for the entomology of "pissante", and nobody caught it for quite some time. AEU is quite a sophisticated My-dick-is-bigger-and-more-well-read-than-yours arena. OK, then, well, My-Scrabble-score-is-higher-than-yours....

90% of aeu is from the Upper West Side in Manhattan, trudging around in flip-flops, in and out of Zabars and Barnes&Nobles/Starbucks, with the NYTimes under one arm--and usually the WSJ under the other. And of course the Atlantic Monthly when it comes out.

I suspect, tho, most of the 'boomers go to B&N/Starbucks for the A/C, a Starbucks sugar high, and the resultant nappypoo. Yeah, and the singles/gays cruise the magazine racks, mostly the muscle/fashion mags....

9% of aeu is from San Fran and Berkeley CA. The last 1% are feriners tryna learn englich good.

Proctologically Violated©® is really my "soul handle".

Uh-oh.... I'm feeling the Violation.....

Hey, Props to Harold!

Reply to
DrollTroll

Hey Ig,

Leg 3 is the wild leg, right?

How big are your idlers?

I forgot to mention perhaps the weirdest thing: the wild leg off the Baldor is LOWER than the 240 V input!! Izzat possible? I'll have to check it again!

In googling diy rpc's, I saw one with 3 panel volt meters. How cool is DAT??

-- DT

Reply to
DrollTroll

Your old sig used to list all the previous IDs... And for some reason,

*that* information stuck. It's amazing how my mind remembers the CUBI but not the CUBI...

CUBI = Completely Useless/Useful Bits-o-Information.

Screachy Preachy is one I didn't know... Great. More CUBI. :)

CLIP

TMI... TMI. :)

Reply to
Joe AutoDrill

I do not like the word wild, but yes.

10 and 7.5 HP

No surprise

very cool, I have one and want to install one too.

i
Reply to
Ignoramus31221

Cupla Q's:

I've got a bunch of caps, and of course none are suitable. Do you have a source for caps? And analog panel meters?

I'd like to monitor all three leg-leg voltages as well, possibly in different locations--at the rpc, and at the Fadal. I'm a little surprised cnc makers don't put voltmeters somewhere on the machine.

fyi, for people curious as to why caps have to be rated at 350+ V, that's because AC voltage is rms, and the zero-to-peak voltage of a 120 V sine curve is 177, iirc. If the 120 V is 180 deg out of phase (which gives 240 V--which means the 3 ph from 240 V svc is right away slightly hobbled, phase angle-wise), you have momentary voltages of 354 V.

120 V (to ground) legs that are true 120 deg out of phase gives slightly less peak to peak, and is what gives 208 V rms. And is true-blue 3 ph.

Have you thought of switchable banks of caps, for ranges of loads? Good idea? For caps in parallel, you just add the cap values, right?

Should the run caps be disconnected at start up? The start caps disconnected on run? Many designs disconnect the start caps, maybe because the start caps are what give that really high generated leg?

I'm also thinking of experimenting with a very light resistive load (few amp draw), connected to all three legs in delta and/or wye fashion. May help initial regulation?

Have you noticed that the generated leg is very high wrt ground? I guess that makes sense, since there is not really a complete circuit between the two.

It's an inneresting trig problem to show the resultant sine curve and rms voltages of two out-of-phase sine waves. May want to put his on your algebra site as a real practical problem. And from this show the intuitively obvious, that if two 120 V legs generate

240 between them, they MUST be 180 deg out of phase.
Reply to
DrollTroll

The caps should not be expensive on ebay. I do not think that I have any oil filled run caps.

You can use a multimeter for your testing.

You start your Baldor from something that is already running right?

If so, then you can leave the caps.

If you want to make a new phase converter, it is a different story.

i

Reply to
Ignoramus31221

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