Combining portable generator output

I do not think that best solution for you problem will be more than one generator.

I think you should look for the generator that can give you the power you need in the smallest sice you could find.

The simple answer to your question is NO!

The generators would have to be synchronised with each other in frequency as well as phase, otherwise they will be working against each other...

Now if you could convert the outputs to dc and then with a bit of fiddling you could add them and feed the result into a powerful dc to ac invertor. But this would be expensive compared to using the right tool for the job in the first place!

John.

Yes. The answer is called a battery.

Many small generators have a 12VDC output in conjunction with 120VAC outlet. Since information provided was not complete, you forced all of the answers to make foregone conclusions which may or may not be accurate.

If AC is required, John correctly recommends a properly sized inverter to convert the dc to ac.

Assuming the power requirement is very short term - select a battery or ultracapicitor with adequate amp-hour output and voltage potential to complete the largest of anticipated jobs.

Assuming the power requirement is intermittent, select the generator from stock sized properly to maintain battery charge with the generator is running full time capable of supplying the amp-hr for the intended job.

If the above assumptions are not accurate nor complete, perhaps John has advised wisely. Select a boat large enough to float the boat, O&M personnel, a single sized generator - or

select tools properly that comply with the assumptions stated above. For example, replace one large oversized pump with two small pumps in order to make utility of the above assumptions.

Hi

As stated above, the simple answer is no. Nevertheless it is interesting to think about. The scheme I'm going to submit is probably unworkable for various reasons and would probably destroy both the generators and whatever is hooked up to them.

It does occur to me however that if frequency and phase could be mechanically locked together you should be able to double the wattage available. Same peak-to-peak voltage times double the instantaneous current. Wired like batteries in parallel.

If both generators are IDENTICAL, and if they could be mounted side by side such that they could be mechanically coupled together through a non-slipping bicycle chain, perhaps to the engine pulleys with identical sprockets...and if both generators were carefully rotated to the same EXACT point in their rotational phase relationship before locking them together...it should be theoretically possible to start both engines (probably by starting one and it spinning the other) and keep the generators in pretty good synchronization. Both engines will tend to reduce RPM under load, which will be picked up by the governors and that will tend to balance the load.

Some appropriate large resistors arranged in a pad to give a little bit of isolation between the two sources might help the load balance and protect the generators a little bit at the expense of some wasted wattage. Calculations required here.

Many power tools, pumps, etc. are fairly tolerant of waveforms that aren't perfect (otherwise we couldn't run them on inverters), and many times the power factor (phase relationships) even off the utility grid are not perfect.

Don't be trying this and burning things up and then blaming me. I just said that it was interesting to think about. Comments from others with more experience than me?

Lonnie

These people seem to be a little out of date.

Honda and other manufacturers make what are called inverter technology generators, these have constant output but vary the engine speed according to the load and with some models, a number of them can be combined to give an aggregated output.

Look up Honda eu series generators which appear to run from 1000 to 6500 watts.

They have been rather more expensive but are raidly coming back in price and I think that in Australia they are now often available on special at a competitive price.

Good luck

P.S. The moral of this story is don't believe too much that the self appointed experts on web sites tell you!

As an expert in such matters, I can state unequivocally that the the final statement in Andrew's post is correct.

Not believing in Web Experts? Next you'll be telling me there are no WMDs and Fox news isn't the whole truth!

I'm going to my cupboard to hide.

Andrew:

Thank you for your informative post! Leave it to Honda.

This is a link to that generator line:

http://www.hondapowerequipment.com/gensup.asp

Regards,

Greg

You might be able to take three units and tie them together into a three phase coupling. You would need some large capacitors and a transformer, just like you would see at a general power substation. They bring in the power, run it through some capactors and then a distribution transformer. Use a buck transformer to increase the output of the supply. Good luck and check out the general capabilities of an industrial power distribution station.

Good luck with that! Aircraft gen. systems might be a good starting point.

It is a normal process paralleling generator of the same frequence and voltage but for great power systens. The problem is the cost.

CAO

The main problem is synchronizing the generators to each other initially. After the generators are running they will effectively phase lock themselves together. About 25 years ago I worked with portable generators on a military contract. They had phasing lights on the control panel. The way they worked was that a lightbulb was connected between the output legs of both generators. One generator was brought on line with a load, and then the speed of the second generator was varied until the lights were turning on and off as slowly as possible. Then with careful timing you could switch the second generator on line when both lights were out. We used to use this method during field maneuvers when we needed a constant power supply for weeks at a time. One generator would supply all of the power requirements, but when it needed maintenance (filters, oil change, fuel, etc.) The other generator would be brought on line, pick up the load, and the first generator would be brought down without losing power. On critical loads, such as when we were running large mainframe computers, we would wire 3 or 4 generators together and keep two running at all times so when one died unexpectedly, power would still be available.

bright bulb and dark bulb methods. we have done this in college many times . will be cheaper.Better to use dark bulb method.

"After the generators are running they will effectively phase lock themselves together."

Of course they will! And, given decent equipment, likely waste an imperceptible bit of energy doing it. Why is the simple way always the best way?

You have restored my faith in web experts.

it is simply paralleling the generators. The cost of making arrangement for synchronising will be greater and operationally troublesome than buying a generator of sufficient rating for the purpose.Technically possible but commercially not advisable.

I parallel large generators, 8-10 MW (15,000 hp diesel engines 248 tons in weight) for a living, designing the systems to do so, and I wouldn't even try this with 4-5 of my little 'put put' generators around the house. Well, maybe if Greg G. helped me and my wife was nearby to video it:

"Hold my beer Greg while I throw the switch ! Honey . . . did you call 911 and make 2 reservations ?"

The Honda idea seems pretty cool. They must be generating from a permanent magnet and regulating frequency with magic boxes (inverter) to have a variable speed engine and nice wave form. Does anyone know how it works? The brochures are "Home Owner" level.

To parallel two AC generators you must have sophisticated governors, voltage regulators and what is called 'damper windings' inside the generator to help keep them from pulling out of sync once in. The phase angle synchronising is not that complex (can do it with an analog multimeter) but even with 30+ years doing this, I still wouldn't do it at my house.

No No No No !

I'm glad someone else see's the problem sufficiently well to know that its not as easy as Honda make out it is.

Two independant petrol engine driven generators running at different speeds, different phase angle.... and someone is still thinking its possible to tie their outputs together?!!

John.

Hi PetroPower,

The OP never defined what he meant by small, and I am thinking along the lines of one cylinder units of around 3-8 kW, and because weight is a factor for them, 3600 rpm. (Why are the questions so very often short on details?)

I love the bulb idea, and had forgotten all about it! We had used that when I was in the military (Corps of Engineers) many years back but I am very much in agreement with you about using it on small generators with their relatively crude governors/voltage/frequency controls, no frequency meters, and normally no means to easily and accurately adjust their speed which is necessary when synchronizing. Like you, I would vote NO! The other crucial difference was that the larger generators had much more inertia, and were sized much bigger than any individual motor starting loads than the small units. The small units tend to take a big momentary hit in terms of both rpm and stability when you throw a sudden large load on them. Therefore I could easily envision them being synced with difficulty, but then going haywire when you threw a large load on them.

If I had a couple of old generators I could spare around, the bulb method might be fun to play with but I certainly wouldn't try to use it.

Something I have wanted to play with is "boosting" a generator with a second engine driven AC induction motor, for a potential application I have. It turns out that many, but not all, standard AC induction motors will generate useful amounts of power if driven and once energized. You have to keep some minimal current (load) on it to keep it energized, but up to its maximum output (where it loses its magnetism) it functions as a generator. This method can pretty much free up the frequency synchronization problem occurring with two actual generators if the generator is much bigger than the AC motor. I have the plan details I had gotten for an AC induction motor generator around here someplace, but they are very simple. In any case, this scenario would have no relevance to solving the OP's problem.

The Honda units are cool for sure and I don't know how they work but I suspect they use a variation of an idea (breakthrough) I had read of some years back where the rotor had no windings but conceptually was a cylindrically shaped magnetizable material. The fields on the rotor were inductively coupled to stationary coils that created the poles on the fly as it rotated, sort of like a write head on a tape recorder, where the rotor would be analogous to and endless tape loop. This method was said to make frequency separate from actual RPMs (within limits of course). At the time, I remember wondering how practical it was, with the extra cost and complication .... but that was before a lot of advances in electronic circuitry, in terms of capability and reliability versus cost. That approach only requires low power control circuitry for the field (rotor), instead of having to deal with the much larger output (stator) currents.

Greg

Why not?

I have done it, same way we did it back in school. We tied the neutrals together and put a light bulb between the line sides. Watching for a few cycles to estimate the center of the bulb darkness period ( or watching the analog meter) I throw the interconnect. They seemed to stay in phase I assume because it was easier to loaf a little rather than working to handle a load induced by slipping out of phase. Didn't worry too much about burning up the gens as both had CBs on output.

But the easier solution was to re-wire my biggest load, an air compressor, for 220V so it used all the generator capacity instead of half. Many motors have the option especially when you get to 1hp or greater.

I agree with you. Why not? Two roughly similar paralleled generators will "want" to stay in phase. If one attempts to lag behind the other (lazy SOB) it will be given a swift kick in the pants by the other, as the two alternately act as motors to speed up or slow down their connected engines. (Of course, if one runs out of gas, you could expect some mighty warm windings if the CB failed to trip.

The new Hondas take it several steps further, with the two paralleled generators now sharing the pulses (through the third wire) that start each and every synthesized sine wave (which are entirely independent in frequency from gas engine speed) It could hardly be more precise or simpler for the user. (In fact, the Hondas could be expected to keep your old electrical-motor-driven clocks running more accurately than the power company can, with their fancy-schmancy millions of dollars of equipment for keeping things synched.)

If I could muster up enough fear of power outages to actually by a generator, I'd get not one but two of these Hondas, just to play with them. NO... wait... I'd get 3! Then I'd design my own connector cord with a little tiny black box in it that would put each out of phase with the next to provide three phase power. Then, while my friends run their tools on wimpy-little-girly-single-phase-washing-machine motors, I could run mine on the three phase industrial real deal! And have three gas engines running, to boot!! (But I'm worried: would it sound like my old three-cylinder Daihatsu -- wimpy -- or like a Triumph Trident motorcycle -- macho?)

Hi you guys, things have moved since you were at school 30 years ago, how about getting up to date, in the solid state era!

Honda and other's generators are "Inverter Technology". They rectify the current and then synthesize a sine wave, digitaly controlled synchronization between units is a doddle!

Is this innovative? Not particularly at this moment there are millions of Variable Speed Drives being installed in industry and domestic airconditioners and pumps, which use the same technology. Look up VSD's there are dozens of manufacturers.

What I would like to know is: Is this an Engineers forum or just a gossip shop for pensioned off has beens!

I have some complex questions I would like to put, but the response to this thread has been far from encouraging.
Andrew Read

Such a spoil sport! Are not plausible lies always better than complicated truth? Is it not more fun to make things up out of thin air than to simply follow the conventional wisdom?

Go ahead; post your complex question. You will be entertained by both gross oversimplifications and endlessly convoluted answers relying on heretofore undiscovered physical principals.

Ken,

(Laughing here)

All too often true.

I'm surprised no one mentioned some sort of perpetual motion generator design that could provide infinite amounts of power and was smaller than a breadbox, but was squelched by the electric utilities and oil companies!

Greg

Andrew,

I have worked with many VFDs, as have many here, but I don't think that is the technology they are using (although it could be a variation of it). Probably none of us deals with small portable generators as part of our vocation, but only as consumers and occasional users. So it is not surprising in the least that we would not be up to date on all that's out there.

As to your insulting comment:

"What I would like to know is: Is this an Engineers forum or just a gossip shop for pensioned off has beens!"

All I will say to that is if you think you know everything then you are certainly the fool!

We are here to learn as well as share what we may have to offer.

Since you seem to know so much about the Honda style generators would you be so kind as to enlighten us with a more detailed explanation? Also, does connecting them in parallel require connecting the electronics, besides just the output terminals? I would think it would.

Greg

support!

I guess it does, I believe they provide a neat little connector with plugs, that's where the profit margin is, as with all modern technology. Apart from that it appears to me to be a fairly basic adaptation of the VSD (variable speed drive) concept, what else is there to say?
Although some of the later vector sensing VSD technology wouldn't even need the connecting control wire I believe.

My actual trade is "farmer" and I don't find it that hard to keep across most technology that comes by, but then hay seeds between the ears probably help to create an open mind.

Andrew,

You are absolutely right, it is a straightforward adaptation of VFD (Variable Frequency Drive) technology. Not the field control I thought it might be. Those things (VFDs) are amazing, getting smaller and cheaper all the time. And, with the advanced "switching" technology they use, when you open one up, you can't pick out the rectifier stage, its so tightly integrated into the inverter output (on the earlier models, you could).

I'm using one in a project I'm working on right now, from Automation Direct, my favorite supplier of PLCs, VFDs, and all sorts of good stuff, all high quality at great prices. I started with them back when they were called PLC Direct (just a little plug for a great outfit).

VFDs are "noisy" yet the inverter generators claim to put out "clean" AC power and from researching a little, it appears both things are true: it is a relatively clean sine wave but there is also some RF noise present, which will radiate out from an extension cord but this is of no concern for normal use.

On the vector sensing VFDs you mention, (which I have sometimes used) I am pretty certain that they can't use that for the simple reason that it works on the basis of detecting phase shifts and reflected waveforms from the (single) driven motor and in parallelling two generators they might chase each other's tail so to speak and with one or more motors as a load, the waveforms would be far too complex for the vector sensing circuitry to sort out.

They are extremely quiet, especially when run in the throttle saving mode, where they slow down for light loads, although you can't use this mode if you have to start some large loads: They won't start properly.

Quite few manufacturers make lines of inverter generators. This was news to me, and I read widely and work hard to keep up, but this slipped right under the radar, and I'm not at all embarrassed to say so, since it has nothing to do with any of my engineering work. I have seen a model or two in stores, but of course they don't explain a thing on the box, and I assumed it was just a high priced small, lightweight generator with sophisticated frequency stabilization and some filtering. But then that's why my footnote byline says what it does.

A link I found that explains how they work:

http://www.electricgeneratorsdirect.com/catalog/pickperfectrecreational2a.php

If you spend some time here Andrew you will get a pretty good idea of who you can generally rely on, who speak beyond their knowledge on a regular basis, who have open minds and who doesn't, and who seem to thrive on fantasies of one kind or another and/or delight in blaming all sort of things on deep, dark conspiracies. And of course we are all capable of making mistakes. But most of us have open minds, something to contribute, and are anxious to learn from each other. Keep in mind also, that the questions here are most often somewhat vague, allowing for a number of interpretations, as in this case where generator size wasn't even mentioned: a trailer mounted 35 kW generator could be considered small in some contexts, is portable, and can be moved around and loaded on a boat, but it is very much different than than a 3.5 kW portable generator.

As a technically astute farmer, and someone who works outdoors you are more likely to be familiar with portable generators and your contribution was appreciated. The multitude of backgrounds here is one of the things that makes this site interesting.

Greg

Hi All, I posted the original question and would like to thank all of you who responded. It had occurred to me at the very beginning that the problem was always going to be bringing the phases in line and I did explain to the person who originally asked me the question that this was usually done by having a great deal of control over the motors powering the gen sets. Not being an expert in the field I did, however, do an extensive web search to no avail and thought that I might not have been using the correct search criteria or that the solution was really obscure - hence the posting on CR4. So once again, many thanks to all those who responded - I now "know" that it can't be done with the existing units but that should we want to pursue this that there is a potential solution from Honda. Cheers - Stefan, Fiji Islands

I have thought quite a bit about this over the past few years, and believe there are a few important concerns, and one critical one.

The critical one is that there needs to be a load drawing current BEFORE you try to parallel.

The important ones are : 1. make sure that the generators run close to the same frequency with same size load applied so that the load will balance well between them.

2. Make sure the phasing matches at the moment you close the switch on the second one.

3. Be sure that the loaded generator is running slower than the unloaded one, because a small generator will run slower as more load is added. (If you have done 1. above, this will naturally occur.)

The Phasing can easily, and quite accurately be measured with a simple volt meter. When the voltage between the hot terminals is maximum, the phases are 180 degrees out. WORSE TIME TO PARALLEL. when the voltage reads zero, the generators are exactly in phase. You can use this to tell if the generators are near the same speed/ frequency. Run both generators, carrying the same size load. Put voltmeter btween output hot of each generator. The faster the voltage cycles, the further out of synch the two governors are.

The theoretical procedure:

1. Start both generators.

2. Load one.

3. Hook up the voltmeter. You may find that there is so much disparity between frequencies that the voltage varies too fast. If this is the case, manually pull the governor linkage of the UNloaded one to slow it down until the frequencies are close, and the voltage varies slowly, as was discribed previously with the light bulbs.

4. When the voltage reads zero (or actually, just before), throw the second switch and let go of the governor.

Remember, don't run them in parallel unloaded. I don't know if there will be a problem, but let's not find out. When you are shutting down, reduce the load to the level that one generator can handle it, and disconnect one. Then cut the load.

If for some reason you cannot reduce the load (ie an emergency), just disconnect one generator at the same time you trip the load.

REMEMBER: Don't let the magic smoke leak out.

I'm not really up on all this, but I'm assuming that in your procedure, you will have connected the neutrals together to be able to get a circuit for the voltmeter to read? Or am I being dense!

You may well have thought about it, but obviously never done it.

The voltmeter method is a very poor idea indeed, and is never used. It typically can't respond fast enough, where the lightbulb method is not only simpler, but much more accurate for synchronisation.

Well you're all very gracious, I guess coping with the drought here I have become a little irascible and intolerant.

Mostly when it comes to technology we farmers, particularly sheep farmers, are kept like mushrooms, (in the dark and fed . . . fertiliser). I see all this great stuff out there but when I want to try some of it its a hard battle finding someone knowledgeable and interested enough to look at my problem, being as I am on a minimal sort of budget line.

Anyway thanks for your comments

Andrew,

"Apology" happily accepted. We all get carried away here at one time or another ... at least I know I do.

So now, submit some of your complex questions.

Just keep in mind Ken's post# 20, because there is certainly some truth to that going on here too.

Regards,

Greg

Thank's all, Today I have learn something new!!

The matter is not that simple.

1.) The generators are specially designed for sync, Phase-sensing, Output Voltage comparing, Load-sharing, ++++ Throttle-Control & all that are needed.

2.) Can anbody couple Mechanically even 2 generators & sync those to share-load equally?

Why not TO BUILD A NEW GENERATOR OF HIGHER CAPACITY OUT OF THOSE BY COUPLING ON A SINGLE SHAFT & DRIVING BY ALL THE ENGINES OF THOSE GENs?

What a great idea?

Some 52D US Military Generator Mechanics could tell you how to do it. I remember running many generators, 15 kW to 60kW, hooked together, to supply power for a field exercise. The combat engineers hooked them up, not me, I just worked on them. But I seem to remember a load bank, and a frequency adjusting knob, along with a voltage adjusting knob and a light for synchronizing on the front panel. Then again, those gensets were built to be hooked together.

Heli-o,

From my experience depending on the load the neutral doesn't need to be hooked together(prevents oscillating currents), but that is mostly with 3ph and L-L only loads. ALWAYS ground and bond together! You need to first get the no load voltages the same then do as out lined above and load one of the generators first. Seeing as how I have Two Volt meters that read Hz and two clamp on Amp meters (calibrated) I can also balance the load much easier.

There are several ways of hooking up the light bulbs and depending on how they flash would tell you different things. I prefer to hook them L-L, L-L.

I first set up the generator no load voltage and Hz of the two generators. I'll then load the first gen (with a non-reactive load, IE resistive only durring setup)And warm up the second. I'll then turn on the lights and time the closing of the breaker for when the lights go out. At this time the only way to tell if they are sharing the load properly is by checking the current through the respective Phase line. Due to governor differences you may have to minutely adjust the governor speed of one OR the other to bring the load share to the same draw. I've done this with the two 5kw gas gens in my shop at home and larger ones in an emergency in the field. I used and old industrial stove as the load bank for this setup.

My shop runs on 3ph power out in the boondocks of Texas, for most of the work I did in it I only needed the small gen. But when I had to power up the heaters, AC, or welder I would parallel them together as needed. If you do something like that you need to put in reverse power trips or a reed on the governor to trip the breaker in case one stalls. If you don't you may end up motorizing it and overloading the other before the breaker trips. The reverse power relay will protect as soon as one slows down and falls out of sync.

You should have a good understanding of what is going on when setting something like this up, even a couple of 3kw put put units will maim and or kill you if you're not careful and have an understanding of what you are doing. This is also a setup that was done originally for my own education into paralleling. I work on generators for a living at the moment and I wanted to put into practice what I knew. What I learned from that experience has helped me with troubleshooting the computerized systems I now work with. It means it isn't just a black box.

Sorry for the longish wind,

Dave

There is a difference between longish fluffy and longish meaty. Your post was the latter. Great contribution!