phase converter questions



    This is a problem with a "static" phase converter, but for a reasonably sized rotary converter -- especially one which has been tuned and balanced by tweaking capacitance on the windings -- you should be able to get full power out of the driven motor.

    No experience with this, but I can't see any serious reason to expect problems running it from a RPC. Here, I am not sure what will happen from a VFD -- given that you have a switching regulator synthesizing the output of the VFD, and possibly another one converting the input to the welder into DC, so they might interfere with each other. (I would also expect similar problems running a typical computer from a VFD, too.)
    But welders (and computers) don't need the variable frequency, so I see little reason for using a VFD for this.
    Enjoy,         DoN.
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On 26 Aug 2004 00:17:46 -0400, snipped-for-privacy@d-and-d.com (DoN. Nichols) vaguely proposed a theory ......and in reply I say!:
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I am probably thinking of the fact that the rotary part of the converter needs a driver that is at least 1.5 times the power of the driven gear, because of the trouble with the fact that it running as if started by a static...on two legs only.

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    [ ... ]

    This -- I agree with.
    But the driven equipment should run very close to full power unless the balance of the rotary converter is really bad. :-) (And going seriously oversized on the idler motor size will eliminate a lot of that, too.
    Good Luck,         DoN.
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This is news to me. Where did you read that? For a reasonably sized rotary converter (at least 50% larger than the driven motor) you can develop full hp on the driven motor.
Jim
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vaguely proposed a theory ......and in reply I say!:
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D'OH. Now I have to find it again! <G>
I do take your and Don's point that the big muvva idea and some tuning sound OK. It would appear that I was getting muxed ip with the static converter.
http://www.metalwebnews.com/howto/ph-conv/ph-conv.html
"The static converter is basically only a start circuit that once the motor starts, disengages and lets the motor run on single phase power. The disadvantage of this method is that the motor winding currents will be very unbalanced and the motor will not be able to run above about two-thirds its rated horsepower. The rotary converter provides current in all 3 phases and although not perfect, will allow a motor to provide all or nearly all its rated horsepower. If the motor has a service factor of 1.15 to 1.25 then you should be able to use full rated horsepower. The service factor can be found on the motor nameplate and is usually abbreviated S.F. "
http://www.deselectric.com / "Either a static converter or rotary phase converter can make it possible to run a three phase motor on single phase power. The Static Phase Converter does not actually generate three phase but only starts an electric motor and then allows the motor to operate on only two of the three windings in the motor. This being the case, the motor then is running at 2/3 of its rated Hp.
A Rotary Phase Converter does provide full rated three phase power to operate equipment without this loss of Hp. Rotary Converters are always the best way to go, if you size them correctly for the application."
Apart from that, I can remember discussions here about it.
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I sort of suspected that. The thing is, most of the techies here (and that is not a derogatory term) love to do stuff the *right* way, the best way. And they have the design princples down to an iota for rotary converters like that.
The deal is though, if you simply oversize the idler motor a lot, you don't need to do any tuning or power factor correction at all, if you are only running a couple of 1 hp or so machines in your shop.
In my case it's a five hp idler that runs small machines, and there's no tuning or fancyness involved.
Sure you can run the same machines on a two hp idler motor, but once again, the 5 or or so hp motors seem to be easier to come by for free, at least around here.
Jim
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Old Nick says...

Jim
I have a little dyno with which I can measure the efects of changing idler size on a 3 phase motor's ability to deliver power. The improvement in "tool motor power delivering ability" is almost undetectable for NO idler or for an idler 1 1/2 times tool motor HP when the tool motor is loaded up to 2/3 of its name plate Max HP.
It impressed me that the 3 HP 3 phase motor that I use produces 3 HP at 1725 RPM to the load even when fed from single phase without idler.
It is probably obvious to those who have thought about it, that a heavily loaded (near full name plate HP) 3 phase motor does run alot smoother when it is fed thru a big-big idler.
It might be concluded that 3 phase motors that arent loaded heavily for long periods can be run from single phase without an idler. But, a 3 phase motor will produce its full name plate HP even when fed single phase with no idler.
Jerry
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Right, though they can't start on 1 ph. You need either a rotary converter, a static converter, or a pony motor to start the 3 ph motor. It also won't run smoothly on 1 ph power because torque will go through zero twice a cycle.

It may, for a short period, but it will be drawing current in excess of its ratings on the driven phase. A 3 ph motor driven by 3 ph power has its current divided 3 ways. In other words, its windings are sized to each handle 1/3rd of its rated power. If you try to draw full nameplate HP from it while it is driven by 1 ph power, the driven windings will be overloaded.
That's why there's a rule of thumb that a rotary converter needs to be sized 1.5 times the HP of the load motor, so the driven windings of the rotary converter aren't overloaded when delivering full 3 ph power to the load motor.
Example, 1 HP load motor should be driven by a 1.5 HP rotary converter, 0.5 + 0.5 + 0.5 = 1.5, so that the current drawn by the load motor, 1/3 + 1/3 + 1/3 = 1 is the same as the maximum permissable 0.5 + 0.5 = 1 of the two driven windings of the rotary.
Of course the rotary does the *transformation* from 0.5 + 0.5 to 1/3 + 1/3 + 1/3 by its rotary transformer action. But the key concept is that power in must equal power out no matter how it is transformed, and the currents must not overload any windings in the process.
Gary
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Right Gary! You explained it very well: "It may, for a short period, but it will be drawing current in excess of its ratings on the the driven phase" -- and further, when you said, "If you try to draw full nameplate HP from it while it is driven by 1 ph power, the driven windings will be overloaded."
This apparent ability to produce full power on "2/3 of a motor" is a function of the motor's service factor. Surely a motor driven to full name plate output on 2/3's of windings will draw excessive input current, eventually overheating to failure..
Bob Swinney
wrote:

converter,
run
no
its ratings

divided
rated
sized
converter
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This is an unexpected, yet fascinating result. It may be that the hp ratings on some motors is fairly conservatve ("anti-sears power rating...") which could account for some of this.
I wonder if the same 3 hp motor would actually do 4, if it were run on utility 3~?
Jim
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Jerry Martes says...

Jim
All the 3 pjase motors I've tested will produce more than their name plate rated HP when fed good 3 phase. A 3 HP motor will deliver 5 HP to the load when fed good 3 phase. It does get exactly as much hotter as you'd guess so you'd want to put an external fan on it. Heating of the motor is the principal factor that limits the amount of power that an induction motor can safely deliver. That could be improved by external fans and aditional heat sinks. Since the motor conducts heat away from the windings thru its frame, the entire frame becomes a significant factor in determining the max power rating. The 2/3 name plate HP 'rule of thumb' for guessing at the max power available from a 3 phase motor, fed single phase, must be altered to allow the fact that the heat sink and fan is working as though it is a 3/3 HP motor.
It might be interesting to note that many 3 phase motors that are used in Rec Craft applications are often commercial tool motors that will not be loaded to their original ratings so the need for producing 'good 3 phase' is often misunderstood.
A tool motor can easily spun up with an idler much smaller than the tool motor's name plate HP, contrary to the implication that Big idlers are needed. Most tools that use 3 phase motors can be unloaded while being started, so even a switch and capacitor can spin up most 3 phase tools.
As I noted above, the 3 phase motor will provide the load with very nearly the same HP when fed either single phase or 3 phase, when the motor is loaded to only 1/2 its name plate HP. At those "lightly loaded" conditions, I've been unable to descern the difference between the performance of the 3 phase motor fed either 3 phase or single phase. The instanteous loss of torque every 1/120th of a second exist in either a single phase motor or a 3 phase motor fed single phase. That "pulsing" doesnt become measurable when the motor is loaded to less than about 3/4 its rated HP. For loads under about 3/4 its name plate rated HP (2 1/4 HP for a 3 HP motor) a 3 phase motor fed single benefits little from the use of in idler. The pulsing is probably the first noticable result of having no idler (or small idler) when using single phase to drive a heavily loaded 3 phase motor.
I plotted (with Excel) the HP vs RPM for the 3 HP 3 phase motor, fed single phase, with a 5 HP idler and with no idler. I'll send you a copy if you'd want to see it..
Jerry
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Jerry Martes wrote:

Jerry, I have a hard time believing a bigger fan will get you very far. If this were true, motor manufacturers would do it, especially on Sears compressors. I'm sure extra air will get you somewhere, but before long the hot spot will move inside the windings, where your fan can't reach. Extra air (and lower than rated ambient temps) might get you 10-20% (just a guess), but not 50-100% continuous duty. Intermittent, sure.

I agree. I've started an unloaded 20HP motor using a 7.5HP rotary converter. My gear head mill, on the other hand (significant starting torque needed) was a 1HP motor and needed a 3HP idler in my rpc. 2HP wouldn't start it in the high speeds.
Steve
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wrote:

I have done this on my single-phase air compressor motor. I rigged up some flexible dryer vent to a good-sized squirrel-cage blower, blowing a good blast of air into one end of the motor. It made a big difference when I was using lots of air, as in DA sanding.
Take a look at heatsink specs as from Wakefield, note the thermal resistance from sink to ambient of any heatsink as a fn of air velocity (or not). . Motors do have fans in them, but they don't produce enough pressure to get much airflow thru the limited spaces inside the motor.
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Don Foreman wrote:

I bow to the voice of experience. I wasn't so much concerned with sink to ambient as I was with inside motor to sink, but as you say, it seems to work.
Steve
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wrote:

Threephase motors are designed to run on threephase power. The designers include enough fan to cool the motor when run as it was designed to run. They work hard to optimize the design for cost, size, and efficiency when run under specified conditions. The fan is a source of inefficiency so they don't use any more fan than necessary.
It has been said many times that 3phase motors must be derated when run on singlephase or unbalanced 3-phase power because of increased I^2R losses. But this is just heat. If we can deal with the heat with better cooling in our misapplications, then they can pull the load. We'll just have less efficiency as the price for our misapplication.
The largest thermal gradient is probably in the insulation between the windings and the stator iron. If we can signficantly increase airflow over those windings then we can greatly improve the rate of heat removal, and therefore increase the power we can get from a motor operating under conditions different than it was designed for.
Sorry if this violates any sacred beliefs, but it does work.
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On Fri, 27 Aug 2004 18:09:20 GMT, "Jerry Martes"
snip
I have a little dyno with which I can measure the efects of changing idler size on a 3 phase motor's ability to deliver power. The improvement in "tool motor power delivering ability" is almost undetectable for NO idler or for an idler 1 1/2 times tool motor HP when the tool motor is loaded up to 2/3 of its name plate Max HP.
It impressed me that the 3 HP 3 phase motor that I use produces 3 HP at 1725 RPM to the load even when fed from single phase without idler.
It is probably obvious to those who have thought about it, that a heavily loaded (near full name plate HP) 3 phase motor does run alot smoother when it is fed thru a big-big idler.
It might be concluded that 3 phase motors that arent loaded heavily for long periods can be run from single phase without an idler. But, a 3 phase motor will produce its full name plate HP even when fed single phase with no idler.
Jerry
snip

A pair of good posts. It really is refreshing to see some solid input on 3 phase phase conversions which is both soundly based and backed up by direct measurement. I hope it will dispel some of the myths on minimum sizes of idlers and the practical usefulness of precision "tuning".
Jim
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You also have the choice of hand starting (rope?), or capacitor start with associated switching for every tool motor you're wanting to run or having one idler to start and then having the self start plus immediate reversibility features available at each tool. Big three phase motors being as cheap as they are, seems simple enough to use the idler.
bob g.
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wrote:

Got a wiring diagram? Ive got a 20hp 3600rpm Western Electric waiting to be turned into a RPC. Pony motors Ive got by the score.
Gunner, who was given a Hardinge DV-59 today. Disassembled but complete....sigh
Anyone need any Federal air gauges? Got about a dozen of them <G>
Schaublin Lever action tailstock with built in collet holder in the nose of the ram?
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    *What* wiring diagram? Switch power to two windings of the idler motor, and to the single-phase pony motor to spin it up, then switch off the pony after it has thrown its belt. (Based on what I remember of Jim's setup.) Wire the three windings of the idler to the three inputs of the target tool.
    You just need a larger idler than with a well tuned one, and since you are not tuning for the power factor, you may blow circuit breakers more often than you would like, depending on the size of the idler motor.
    You also might have problems running electronics from the generated power if it is across the wild (generated) leg. If you can be sure that all electronics are connected to the primary two input lines from the power company, you should be fine.
    Note, also, that this has *no* safety circuit to disconnect the idler when the power fails, so when it comes back on, the motor will sit there humming until it burns up, or trips the breaker. So *always* turn it off when you walk away.
    If *I* were going for a rotary, I would want the safety circuits, self starting, and properly tuned to reduce the reactive current drawn from the wall.
    Enjoy,         DoN.
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Well nobody would be dumb enough to leave the darn thing on all night.
Ahem.
Trouble is, this converter is really really quiet. I simply cannot hear it running inside the little root celler it sits in. The rubber stoppers do a great job of vibration isolation, and I think it's a pretty high quality motor to start with.
I've got it powered off of a 15 amp breaker, through number 12 wire. The fuses in the knife switch are 10 amp, so it's on a pretty short leash. I've had momentary lapses of grey matter on occasion and threw the knife switch *before* spinning the idler up - and the result is that it grunts for about a second before one of the fuses pops.
Still a drop-out contactor is now on my list-O-stuff to do.
Until then I'll just be sure to turn the damn thing off before leaving the shop, religiously. Maybe a note on the cellar door, or a big red light in parallel with the drive.
Ahem.
Jim
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