running a 5 hp motor on a "3 hp max single motor" ROTARY CONVERTER

have a saw (unisaw, actually) I need to run. need to rip some 2x4's, to a few various widths)
if I put a BRAND new blade on the saw, 24 tooth, what do you suppose would happen if I try to start the saw up (being a manually-fed saw, it starts with 'no load' of course) then rip those 2x4's with it?
I realize I "should" have a bigger rotary converter for the task (or change the saw to a smaller 3 hp motor), but, as a 'stopgap measure', might this work, or, uhh, what?
I have no -other- three-phase machines here (at the garage of my house) I can start up on my ol' arco HD-3 model rotary-converter, either...
thanks for tips, ideas, and WAGS on this one, guys
toolie
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you should be OK - once the saw starts, the rotary converter doesn't really do much to help (well, it helps a little) - you will hae a bit less horspower than if you had full power to the motor, but you should be fine
the "static converters" basically do this - they get the motor started and then are switched out of the circuit - I use one of these on my 6 hp mill (well, it's 6 hp spindle, 2 hp ways) with no issues, so I am sure that once your saw motor starts, it will be fine.
And a new, versus old blade won't make any particular difference

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Bill Noble wrote:

Uh , Bill , it most assuredly WILL make a difference . New sharp blades take a lot less power to cut than old dull blades . I work full time in a commercial millwork shop , and we can really tell the difference when the blades start to drag from dull teeth . Dave , you'll want to be sure the 2X4's don't warp and bind the blade , that'll raise all kinds of hell with that setup .
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I think the point of his statement, and mine was that the motor startup would likely be the pass/fail point of this setup and the blade makes no difference at that point. Once it is running the blade condition will contribute to the overall current draw but a test before buying a new blade would tell if the entire endeavour has even a chance to work. A 5hp motor even single phasing without any help from the rotary can surely rip construction lumber, once it is up to speed.
As a real world data point I coincidentially needed to rip some 2x lumber on my Uinisaw so I popped open the starter and stuck on my clamp meter.
SINGLE phase 3 hp motor, 240 volt input. Motor full load amps label spec is 12.3 Blade is a thin kerf 24 tooth, probably average sharpness from having been used for light duty reno work on mostly clean old lumber. Wood was old, some more completely dry reclaimed construction grade lumber of an unknown spruce/pine species.
Measured current while ripping was about 8 amps, climbing to about 10 if I pushed a little too fast. Given about 80% motor effiency for calculation purposes I figure the input to the blade was near 2 HP, give or take.
Based on all that I think the OP's setup will in fact be capable of performing the task, provided it can even start the motor unloaded.
Lacking a clamp on amp meter to monitor the input current I would ensure the rotary converter had some form of overcurrent protection before using the setup for any length of time. The saw should have it's own protection for all three legs built into the starter, assuming it has one. At 5 hp it should.
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Charles U Farley wrote:

Good point . I knew that start current was the biggest draw , kinda lost sight of that ...
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And if you buy a narrow kerf blade - thinner than the standard - the cutting faces of the teeth hit or cut down on less wood. Martin
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of course - I was contemplating old, not quite razor sharp versus new - old dull cruddy won't cut anything blades don't count
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dave wrote:

WAG Catagory - and I'll try to muddy your question :)
Would it be possible to put some kind of breaker between saw and phase converter that'll trip out if the actual load is more than the 3hp worth that the phase converter is designed to deliver?
I wouldn't guess that the job is likely to need anywhere near 5 hp.
--
Morris Dovey
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Lew said:
>Are you sure you want to risk that Unisaw motor for a few ripped >2x4's?"
well, motor's 'no good to me' if I can't run it to saw wood. I'm more concerned with ruining the arco, to be honest...understand I can (possibly) convert the saw to take a 'standard' 56 frame motor rather than the 'unisaw-specific' one, with the 184Z frame (if I burn out the saw motor) but 'that ain't gonna happen today'
Charles U said:
>My WAG is blade sharpness won't matter much. The converter will either >start it or not and I'm guessing it probably will.
well, I've used table saws quite a bit, and a dull OLD blade (usually 'gummed up good' with resin and sawdust, to boot) can put quite a load on a motor...and "cut REALLY lousy" too (so I figure a NEW blade, 24 tooth, puts -way- less load on the motor).
I'd actually buy a 16 or 18 tooth or so for ripping, if I could get one local, but doubt I can; I've looked.
ok, guys, thanks, I'll try 'booting' the saw with NO blade first (but the 3 belts on, driving the arbor), mostly to see which way it turns (correctly or not :-)
then put the blade on, cut some dry 3/4 thick pine, then the 2x4's.
if I 'let the smoke out' of course it'll be a bad day :-(
my LAST table saw was a 2hp 3 phase, and, I could, when ripping thick 8/4 oak, 'manage' to (just barely) still hold the board when it would sometimes 'close up and bind' behind the blade, smoking and squealing, in the kerf. but, this ol' new 5hp unisaw thing, well, I'm older now, not as strong as I was then, and it's more than double the power, so, uhh, well, here' we go, guys :-)
better to let the SMOKE out than the BLOOD out, though, I guess :-/
<'foot kill switch' idea brewing...>
thanks again :-) *all* you guys
toolie :-)
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dave wrote: ...

...
The problem will more be burning out the converter, I think. If it's on an appropriately sized breaker or it has thermal overloads (heaters), then you should be ok if the saw does try to draw excessive power. If it (the converter) isn't protected, let it run on no load (assuming it starts) for a while and do a sanity temperature check.
--
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RE: Subject
You have a "beats the devil out of me" questions.
The first problem is to find out if the "system" is "stiff" enough to get past the inrush current.
To test, try to start the saw under no load conditions.
If the saw comes up to speed without significant "laboring", you have a chance.
"Ripping a few 2x4's..." could be interesting, especially if it is construction grade lumber.
Be prepared to hit the "kill switch" immediately if saw starts to bind.
Are you sure you want to risk that Unisaw motor for a few ripped 2x4's?
Lew
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Even though I stay out of electrical discussions, for all kinds of reasons, the no-load inrush of a 5 HP 3phase motor is well within a 3HP rated MG set. It might 'kick' a little so make sure the whole thing is mounted on a steady platform.
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My WAG is blade sharpness won't matter much. The converter will either start it or not and I'm guessing it probably will. Trying it is the only option. The running load related to your blade sharpness has some effect but the saw should have enough power to cut that board just fine if you can initially get it up to speed. If the converter has a relay that pulls capacitors in and out during sawing, you're pushing it past it's limits.
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wrote:

It may...may may start up, abit slowly. Cut normally until the magic smoke comes o ut of the windings of your converter.
I did exactly that with my Clausing 15" 1501 lathe, on a 5 hp commercial rotary converter. 7.5hp motor...... Took me 3 hours finding the burned in two wire in the converter and repairing it. Then I mounted a 7.5hp VFD on the lathe and havent had any issues since then.
Shrug
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I would expect it to work. You will not, of course, get full power from it. But it should start. Try carefully, of course. (as in, if the saw does not start without 2 seconds, turn off electricity)

yea
You can buy a brand new 20 HP motor from me and make a bigger phase converter.
i
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I have an old metal shaper with a 7.5HP motor and a clutch. It starts almost instantly on a home-made 5HP rotary converter. My WAG is that it will start fine.
Don Young
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The general rule is that a three phase motor will develop about 2/3 power if running on single phase. This is without any phase converter. With your phase converter it should be capable of slightly more than 2/3 of the name plate horse power. But not a lot more. So I would think it should be fine for cutting some boards. Say cut for 15 minutes and then let it cool for 15 minutes. See if either the rotary converter or the saw motor is getting hot.
Dan
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must be getting TRULY stupider. just remembered I have (at another location) a few OTHER 3 phase motors I 'could, in theory' start and run 'no load' off the same (undersized?) rotary converter, when/while I'm running/cutting with the 5 hp unisaw.
seem to recall reading that running OTHER three-phase motors at the SAME Time does some sort of 'magic electrical thing' where the other motors running act like some sort of 'electrical cushion' or something like that (or running others at the same time has 'SOME sort of beneficial effect', the exact nature of which effect I'm unclear on). what am I talking about? :-)
thanks again, guys :-)
toolie 'still with the smoke in, so far...'
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All the "other" motors, when running, form a network that improves capacity to deliver power to the "third leg".
My phase converter has two motors inside it, for example.
i
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let me suggest that you just hook it up and run it - the saw motor itself will be the "other" 3 phase motor - Honest, you only need them for starting and for a peak load - you could start it and then turn off the converter and it would run fine
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