phase converter questions

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.

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

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

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

Hmm ... you could start out by adding a respectable capacitance (power factor correction) in parallel with the motor to reduce that current quite a bit so you could then tune the converter, and then tune the power factor correction capacitance for the optimum current draw. Note that the optimum will shift somewhat depending on load applied, so if you expect to have the idler running unloaded more than at a respectable load, you probably should tune the power factor correction capacitance for the no-load condition.

I'm not sure about that as far as the power factor correction bit is concerned. It is just that if you have the right number of phases available, you also typically have beefier breakers and wiring available to start with.

Also -- I *think* (but I don't know for sure) that a fully loaded motor would have much less need for the power factor correction.

With both tuned, of course the 20 HP one would draw more current. However a tuned (and power factor corrected) 20 HP motor running as an unloaded idler probably will draw significantly less than an unloaded 10 HP one running as an idler with no tuning or power factor correction. You have a clamp-on ammeter don't you? Check what the 10 HP is currently drawing (I believe that you already have that one).

I don't have anything that big to check, since I actually use VFDs, and lucked into my first one before ever building the planned rotary converter -- or even acquiring the motor to serve as the idler.

Good Luck, DoN.

On 28 Aug 2004 08:00:46 -0700, snipped-for-privacy@krl.org (Dan Caster) vaguely proposed a theory ......and in reply I say!:

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hmmmm...OK. Thanks. I had a quick aquizz, and your description is better thna my first find, although the priinciple of overdriving was stated.

Ok. Again here I come up against the feeling that both the RPC and this device need "tuning" to a specific _driven_ device. in other words they drastically alter their characteristics as you alter their loads? Otherwise you get inefficiencies or imbalances....

Sorry to pound this, but I do feel that everybody says it's simple, and it's not that tough, I can see. But I get the feeling that most of these devices are used for pretty steady-state use.

On Sat, 28 Aug 2004 15:12:33 GMT, Bruce L. Bergman vaguely proposed a theory ......and in reply I say!:

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No! No! I said this once before and was reminded of the title. Sousa's "The Liberty Bell"...Monty Python's theme tune. I suppose the littel crapheads will not know the implications, but the tune is so bouncy and silly when simply heard, which is probably why Python chose it.

On Sat, 28 Aug 2004 05:00:28 -0700, Larry Jaques vaguely proposed a theory ......and in reply I say!:

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nnnnnhhhhhhgh! I have touched one a couple of times, and well.....

Maybe I peed on an electric fence once!

rrgh! Makes me me feel quite ummmm...well, there's a significant pucker factor there! errreghgr!

Actually I was thinking of the guy who was here a while make (Mike I think?) whose SO was lambasting him about not keeping in touch, on this forum. With all these terms popping up, I always thought she had no hope.

Well, you don't *have* to tune it. Motor loads in general don't care much if the phase to phase voltage balance is spot on or not. But you *will* want to use power factor correction to get the reactive currents seen by your supply wiring and breaker panel down into a more comfortable region.

Ok, my tuned 20 HP idler consumes less than 400 watts no load. In other words, the *real* current drawn by it to keep it running is under 2 amps. With a fully loaded 7.5 HP load motor being driven by the rotary, total *real* current draw from the wall is just over 25 amps, so it runs nicely on a 30A motor rated breaker in the panel.

Now without power factor correction, the imaginary current draw is about

If you don't use power factor correction, you're going to need a breaker, and associated wiring, heavy enough to handle that 67 amp draw. So use power factor correction.

The size of the power factor correction capacitor is a function of the idler motor impedance, which will vary from one idler motor to another depending on internal motor construction factors not under your control, how well the phases are balanced, which will vary depending on how much capacitance you use to balance the phases (good balance here will reduce power factor, and hence the needed size of the power factor correction capacitor), and the real load power, which will vary depending on the size and loading of the load motor(s) being driven (this has only a small effect when the idler motor is substantially larger than the load motor, so it can normally be ignored).

In short, you have to experiment to find the right size power factor correction capacitor for your particular setup. Just keep adding capacitance across the 1 ph line while monitoring an amp clamp meter *on the breaker side of the line* until you see a minimum current value. (Currents on the motor side of the power factor correction capacitor won't decrease.) For a 20 HP idler, start with a value of about 200 uF, then adjust from there.

Current will decrease as you increase capacitance until you reach a minimum current, then it will increase again as the load becomes capacitive rather than inductive. I like to pick a power factor capacitor value that stays just a little on the inductive side of minimum. That avoids large voltage excursions on the load side with changes in load. (Load voltage regulation becomes touchy right at mimimum.)

You'll need a box full of capacitors to make this adjustment. You can often get an assortment of used run capacitors from the back yard of an industrial air conditioning place for the asking, or for a nominal charge. These capacitors don't normally wear out (oil filled), unlike start capacitors (which you shouldn't use), so good used ones are fine. The A/C places generally use new capacitors when they do a motor replacement, and bill the customer, so they wind up with a bunch of good used ones laying around. Since disposing of them requires that they be treated as hazardous waste, they're usually glad to give them to you.

Gary

It depends on just how picky you are.

If you are going to mostly have the idler running unloaded, and just run the machines for short periods -- tune it for the unloaded state.

If you are going to be running a machine most of the time, tune it for that specific machine -- running about half loaded.

If you are going to be running one machine or the other most of the time, but about equal times, tune it for the average of the two.

I think that you are thinking that if it is just a little off tune, it will draw humoungous amounts of extra current. It won't. It will draw just a bit extra.

Most people who bother to tune probably tune it for unloaded. And the things which really matter for keeping the breaker from blowing too frequently, or the wires from overheating is the power factor correction. The tuning mostly helps things like fast starts with a heavy load (e.g. a gearhead lathe in high), or things like "plug" reversing.

The normal reason for tuning before power factor correction is that the tuning caps do a little bit of power factor correction too. So, if you want *optimum* power factor correction, you apply that

With your big 20 HP motor, you can probably do without *any* tuning, but you *will* need the power factor correction to keep from blowing your breakers.

I don't have enough power to run a 20 HP load in the shop. The best outlet that I have at 220 VAC single phase is a 30 A one, which I calculate to be about 7-1/2 HP. But I *might* be able to spin a 20 HP motor with a pony motor and then connect it as a rotary converter idler, as long as I don't ask it to handle too many HP of load. But I almost certainly would need to do power factor correction, or the breaker would be tripping all the time. I might even use an alternate circuit to power the pony motor.

I don't know what a good starting point for power factor correction on a 20 HP motor would be -- perhaps someone who is using one could chip in with that information.

Mostly -- stop worrying and *do the job*. If you want tuning later, you can apply it later -- and possibly need to tweak the power factor correction. But your machines will be happy.

Enjoy, DoN.

I am with Don. You don't need to tune this anymore than any other motor driving a generator. You do need to adjust the pulleys so the single phase motor is driving the three phase motor.

Dan

uld chip in with that information.

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.

snipped-for-privacy@yahoo.com wrote:

If you weren't so far away - you're in WA, right? - I'd give you a 3HP

Frankly I think all this to-ing and fro-ing about faking 3 phase is fine & undoubtedly necessary for people living in the USA - their 110V power sucks and their tariff rates do too. Here in Australia - just go get 3 phase power put on and stop screwing about. I've had it put on to 3 separate places over the last 30 years without any dramas, just varying costs :-) I've got 3 phase 415V 90A per phase coming into my place and I can plug reverse a 7.5HP motor with only a momentary blink on the light circuit. It also means that I really, really don't need to worry about what size welder I want to run.

Paying the power bill hasn't been an issue - so far.

PDW

On 29 Aug 2004 01:26:22 -0400, snipped-for-privacy@d-and-d.com (DoN. Nichols) vaguely proposed a theory ......and in reply I say!:

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Will the RPC be anywhere near as effective and versatile, under limited power availability and in particular under varying load conditions?

I have been told various answers to this. I reckon a motor gen will be maybe 70% efficient. So I get somewhere around 6.7KVA. I get the feeling that a mogen will alter its characterisitics under varying loads, but not as much as an RPC. Therefore the headroom is a lot less.

That last bit sums up my problem. Sorry about the rant

OK. I have 40A, 240 Volt available. 9.5KVA. I have not built any RPCs. I realise I can do it on the cheap as I have (maybe) a lot of the gear I need already.

I need a big motor, well-balanced and power factored. Bit of fiddling. OK.

I have a rough (foggy with age and 25 years of little- to non-use) idea of starting currents and no-load synch currents of sync motors. I know what happens as you start to load them and they slip a bit and start using power.

But the stopping point for me is that an idler basically MISuses a 3PH motor, and all the rules come under question immediately.

I have googled, and looked at all the articles. The above is how far I have got. I have tried, and sorry if I am missing something.

I know a motor-gen setup is heavy and expensive. I can live with that, because the cost of getting a bigger 33KV - 240V trannie on my line would be a heap. The next step is 20KVA, and probably $4000.

I know I can tweak the RPC.

But after all the tweaking, the RPC _still_ appears to suit a certain load; not given the cost, or the size of idler making it easier, but because if you tweak for no load, then use various loads, you get efficiency losses. Again, this is not a cost issue, so much as limited power supply and making that work. So I realise it takes a lot of power costs to _pay_ for the thousand dollars a motor gen will cost me to set up.

And I say No! No! right back, I can't use the Liberty Bell march.

That's already my Cellphone Ringtone... ;-)

(Not to say that working in a small business can sometimes seem like the Ministry of Silly Walks or anything, mind you...)

Heck, I don't even know the implications to which you imply. Mind giving us the Cliff Notes version to enlighten the unwashed masses?

-->--

With no personal experience with a modern motor generator (the closest are the dynamotors and 28 VDC to three-phase 400 Hz units used in WW-II aircraft) (And also a somewhat more modern "Georator" -- a motor-generator which takes 60 Hz input and produces 400 Hz output.) The Georator is made perhaps fifty miles from here, but as it does not do anything that *I* need, I've not visited them. They can be found by a web search -- but the shipping would kill you, I suspect.

However, unless you have synchronous motors on both ends, you will need some difference in pulley size (an adjustable one) to tune the output frequency to match the input frequency (60 Hz in the US, 50 Hz in the UK, not sure which in Oz).

Those pulleys and the belt are certainly going to contribute to poor efficiency. So -- unless you have a motor and a generator actually sharing a single shaft and set of bearings, I think that your efficiency will be far less than a typical rotary converter.

A heavily oversized rotary converter (something in excess of the

If you want to change the tuning with a single load at a time, you can add tuning capacitors at the load motor (which will effectively be in parallel with the those on the idler motor), so you are tuned for each single load condition. Personally, I would not bother to go this far.

However, if you are running several small loads and one big one (say an air compressor), you might want to tune the air compressor load.

But note that each additional load, once it is started, adds to the capability to start other loads, as the motor of the machine tool becomes an addition to the idler.

So -- build one and try it. If it doesn't cost you much but time, why not see whether it serves your needs.

You need power factored. If the motor is big enough, compared to the loads, there is probably very little reason to tune the balance, other than to feel that you've done the best that you can.

So -- don't analyze it to death. Breadboard up a RPC using what you have and see how it works for *your* needs. If it does what you need, fabricate a housing for the components, so things are cleanly mounted and people are protected from exposure to the high voltage terminals. (Obviously, while it is breadboarded, you should be the only one near it, since you will know what not to touch. :-)

"Trannie"? I presume that you mean "transformer", not "transmission" which is what I normally hear that short form used for?

No -- a *static* phase converter only works with a narrow range of load. A rotary phase converter can easily handle any load from 66% of the idler's rating on down to so tiny that you barely know that it is running. (E.G. you could run a 1/8 HP three-phase motor from a 7.5 HP inverter with no problems. Yes, it would cost you less to *run* a smaller rotary converter to do that particular task, but if you are also using it for other equipment, size it for the largest that you need to run -- or if you have a larger motor, go *beyond* the 1.5X starting point. It will draw a bit more current, even after power factor correction, but it will cost less than building two or three rotary converters -- each to run a different load.

And remember that as you switch on extra machines on that RPC, they are contributing to the conversion job for the next machine you need to start -- until you get enough machines running that the total current trips your master breaker for the RPC.

Again -- *try* a RPC -- breadboarded just to convince you that it will work. Your major cost is likely to be the capacitors to tune the power factor (ignoring any balancing capacitors which are optional, after all).

And elsewhere in this by now massive thread, you have a starting point for the power factor capacitor values. (Not from me, but from someone who is *running* a RPC of the same size.

I think that I will now drop out of this thread, as I seem to be typing the same thing time after time. Come back with more questions after you *try* a RPC with the materials you have on hand. Talking about it any more is counterproductive.

Good Luck, DoN.

Thinking of it as a motor just confuses the issue. Think of it as a rotary

No! Tuning has very little effect on RPC efficiency. What tuning does is improve phase to phase *voltage balance*. As long as phase balance is reasonable (and reasonable can be a pretty rough approximation when you're only driving motor loads, which is why so many people are able to use RPCs with no tuning), efficiency will be good if the RPC had good efficiency *as a 3 ph motor*.

The primary factors which determine RPC efficiency are the winding resistance (wire size), the magnetic path lengths, and the magnetic permeability. The people who designed the motor you use as a RPC determined those things. If it was a good motor, it'll be a good RPC. End of story.

The main reason to tune a RPC is if you have sensitive loads, like control electronics, running off of it. Some (by no means all) electronic loads are picky about phase to phase voltage balance. An example of this sort of equipment would be a CNC machining center. Some, again not all, welders will bitch about poor phase to phase balance. In those cases, balancing the converter, or making sure the sensitive loads are fed by the stiff legs (L1-L2) will resolve the problem.

Gary

Hey Peter,

I checked within the last year, and here it was going to cost $8500 Cdn (approx 6500US or 9300AUD). And that was for the utility company's work only. I'd still have to run in from the "pole", and change the service entrance at my house, and my panels. And it would have been 3 phase at 600 volts, so I'd need a transformer(s) to get

So I have a bag of capacitors and an idler now. It works fine for the lathe and mill, but it dropped out something on the CNC which I have not taken time to troubleshoot yet, and I have not tried the surface grinder yet either, but at least I'm sort of "back in business".

Take care.

Brian Laws>> On 26 Aug 2004 20:06:25 -0700, snipped-for-privacy@krl.org (Dan Caster) vaguely

On 29 Aug 2004 16:44:25 -0700, peter_d snipped-for-privacy@hotmail.com (Peter Wiley) vaguely proposed a theory ......and in reply I say!:

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Umm....I am 3 Km from the nearest 3PH. :-< It cost $8000 to get 1Ph put in just from next door.

On Mon, 30 Aug 2004 03:26:16 GMT, Bruce L. Bergman vaguely proposed a theory ......and in reply I say!:

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Basically that anybody playing those things would say "Monty Who?"