From: John De Armond Subject: Re: Linking 2 generators Date: Fri, 12 May 2000 11:40:18 EDT Newsgroups: rec.outdoors.rv-travel Jim Pattison wrote: > > For "fun" sometime, try parallelling two generators that are out of phase. > They WILL get themselves in phase very quickly. The stress on the prime > mover/drive train is something else. > > The key here is having a phase meter and then the necessary hardware to > quickly bring one generator into a live circuit. Then ther is the little > problem of keeping the load balanced so one generator doesn't try to take > the entire load, and so on. As usual, a whole thread of people posting who don't know the answer and instead scold the guy for asking. Bill, here's how it's done. Two AC generators operated in parallel must be not only at the same speed but also the same phase to be paralleled. Doing so is not at all difficult but you do have to know what you're doing. If the neutrals are connected together and then the voltage between the hot legs of the two generators is measured (hot leg to hot leg and not hot to ground), the following conditions will be noted: In phase: zero volts 180 degrees out of phase: the sum of the two voltages. Since the voltages must be the same, this will be 2X the nameplate voltage. The generators can only be connected together when the voltage between the hots is zero, e.g. in phase. This suggests a phasing method. The standard power plant method of synchronizing two generators is to use a very expensive instrument called a synchroscope. This instrument indicates whether the incoming generator is faster, slower or in phase with the bus. Since the rotors of power plant generators weigh many tons, the phases must be extremely accurately matched or else the rotor will be forcibly yanked into phase, possibly wrecking the generator. For small units, we don't need to be so precise. We can use a pair of lamps. What you'll need is a couple of lamps hooked in series and connected between the hot lead of the running generator and the hot lead of the newly cranked generator. You'll also need a switch of some sorts to parallel the units. If the generators are 180 deg out of phase, the voltage across the lamps will be 2X the nameplate voltage (240 volts in the case of two 120 volt generators.) and the lamps will burn full brilliance. If the generators are nearly in phase, the lamps will be out because there will be no voltage on them. If one generator is faster than the other, the lamps will flicker on and off as the gens are in phase one moment and out the next. The procedure is as follows. Start the second generator. The lights will be flickering or slowly coming on and off. Manipulate the throttle of the incoming generator which ever direction is necessary to slow the flickering. As the speeds become almost equal, the lamps will stay off for a long period of time and then slowly start lighting, slowly get fully bright and then slowly dim again. You want to manipulate the throttle until the lamps are off for as long as possible. You want to close the breaker when the voltage between the generator is the least. Since the lamps will go out before the voltage reaches zero, you'll want to mentally time the period between going out and coming back on again and close the switch about in the middle. Once the breaker is closed, the generators are locked together. Indeed, you could close the throttle of one engine and the coupled generator will motor the engine at precisely the sync speed (3600 RPM for small gens). If the generators are just a little bit out of phase, then they will be yanked into phase as momentary heavy current flows between them. And if you close it out of phase, then you have a double voltage short circuit. Usually there is severe mechanical and electrical damage. (I heard and saw the results of a 50 MW diesel genset being synched 180 degrees out as the result of reversed leads on the synchroscope. Literally ripped the stator out of the foundation and twisted the shaft.) Once the generators are in parallel, the load accepted by each generator is governed by the governor setting. The generator with the most throttle will accept the most load. On larger generators, the field excitation is manipulated to control VARs but you don't have that control and so you have to accept what you get. Actually, the idea of using more than one generator is very good if you only occasionally have a heavy load to drive. If you bought a generator large enough to run this occasional load, then most of the time it would be running very lightly loaded and thus very inefficiently. Cranking the second generator for only those occasions when the large load is needed is a good solution. Here are some problems you may encounter: * Unequal voltages - This will cause heavy circulating current to flow between the generators. Will cause overheating and excessive load on one or both generators. * Unequal voltage slope - one generator regulates voltage vs load better than the other. Same result as above but varies with load. Can fool you into thinking the generators aren't "putting out" enough. * Unstable voltage regulation - for generators that use electronic voltage regulation (most any brushless design), you may find that the regulator cannot handle the new dynamics and either malfunctions or oscillates. This can damage the control, the field windings and the other generator. * One generator faltering - if one engine falters - out of gas or low oil cutoff - its alternator will motor the engine in order to preserve the phase lock. This will probably burn out one or both generators and if the engine is low on oil, damage the engine too. This is protected against in larger installations with reverse current relaying. Fairly inexpensive solid state relaying is available (TimeMark Inc and other mfrs) but you do need to be aware of the need to use them. * resonance-induced hunting. The electrical phase lock between the two rotors is springy. If the governor on one engine is a bit unstable and the frequency of the hunting happens to resonate with the electrical coupling, the generator can go into very severe hunting that can result in an engine stall. It will sound like a hotrodder racing his engine up and down. Again, heavy circulating current will flow between the generators which may overload them. If one engine stalls, then the other generator will force full current into the stalled generator. Unless there is overload protection (breaker, fuse, etc), both units will be damaged. I didn't cite all these potential problems to discourage you. I've run small generators in parallel with great success. I just want you to know what you're getting into. At the minimum, you need an ammeter in each generator lead. You really should have reverse current protection on both generators. A carefully selected breaker as the intertie switch is a decent substitute. Arranging so that a low oil signal on either generators kills BOTH engines is more good insurance. If I was going to do this often, I'd get a large metal pullbox and mount the lamps, the relays and switches in it and run cords out for the generators and another cord to the load. Clear christmastree/nightlight lamps are excellent synch lamps because they're small and since they're clear, one can see the filaments to seen when they're even dull red. Feel free to drop me a line if you have any more questions. John
From: John De Armond Subject: Re: Linking 2 generators Date: Sat, 13 May 2000 22:10:53 EDT Newsgroups: rec.outdoors.rv-travel Hugh wrote: > > Look, let's not try to get cute with this. I'm talking about the > EU1000i/3000i generators by Honda. Frankly I don't care what the > "proper" terminology is. You know what I mean when I say they are > "linked together. My EU1000i is designed by Honda to be paired with one > other like unit. They will be in phase with each other and will supply > +- 120 volt ac, 2000 watts. Now what are you saying, Honda EU_i sets > don't work as advertised, sold, used whatever. John's post does not even > mention the small new units. I have one and it specifically states it > can be "joined" (my words) to a second unit. To the best of my > knowledge, these two units are the only consumer generators designed > this way. > Hugh You are absolutely correct, Hugh. The intertie syncs the inverters' clocks so that they output in sync. The engine speed is irrelevant since the generator actually produces DC to drive the internal inverter. John
From: John De Armond Subject: Re: Linking 2 generators Date: Sat, 13 May 2000 22:21:51 EDT Newsgroups: rec.outdoors.rv-travel email@example.com wrote: > > Thanks for all the feedback. > > Here's a link to what got me started: > http://www.campingworld.com/shop/products/index.cfm?type=product&skunum=10262 > > I was wondering if you could use adapters to make this work with the > plugs on my generator, but judging from the posts, that may be a bad > idea. My guess is that the one at Camping World works because both > outlets are coming from the same power source? If you used that same > adapter across 2 generators, they may not be in phase (alternating the > current at different times)? I'm just guessing here with my layman's > terms. IF and only if your 50 amp service never ties the two sides of the line together ( pretty good bet unless there is some sort of paralleling switch in the rig to allow all loads to run on 120 volt service. Never seen one, but have to mention the possibility.), then this adapter will work just fine with two generators. Each generator will be running at its own speed and will not be in phase with each other so no 240 volt appliance operation will be possible. In effect, you're feeding two independent power sources in using a common neutral. Only possible concern is that there will be some interesting currents flowing in the neutral as the generators come into and out of phase. You might get some slight, very low frequency light dimming if the neutral is inadequate. Won't really harm anything but it could be annoying.
From: John De Armond Newsgroups: alt.energy.homepower Subject: Re: Parallel generators Date: Sun, 02 Sep 2007 13:50:24 -0400 Message-ID: <firstname.lastname@example.org> On Sun, 2 Sep 2007 07:51:07 -0500, "Mike" <MAStepelton@SBCGlobal.net> wrote: > Hi !!! I'm also about 35 miles SW of Chicago ( Romeoville, Will Co. ). >Electrically or electronically syncing two gens is very technical and >requires a good deal of additional equipment.. Ya coulda fooled me. I've done it many a time with nothing more than a pair of 120 volt light bulbs in series. This is the standard synchroscope backup in even large plants and an operator must be able to sync without the 'scope. I was certainly trained that way. Hook the neutrals together and hook the bulbs between the hots. Adjust the speed of one generator until the flicker goes away and then trim the phase until the lights remain out for many seconds. Close the paralleling switch that connects the hots together. There you are. The only significant potential issue is with electronic voltage regulators. Some may oscillate or fight each other. There are solutions but they're not generic and are application-specific. The easiest small generators to parallel are the harmonically regulated type, followed by the transformer-regulated type. The two generators don't have to be the same size. They'll share the load proportionally. I don't know that it would be worth the hassle for long-term use, as they have to be sync'ed every time the second unit is started. It's handy to know how to do it, though, for ad-hoc situations where more power is needed than any available generator can supply. As far as welding the shafts, that's a dumb suggestion. Ignoring for a moment the problems with metallurgy and keeping them square and then fighting torsional vibration of the resultant assembly, there's a much easier way to mechanically sync two 2-bearing generators. A timing belt, also known as a cog belt. A timing belt and pulleys lock the multiple shafts in fixed relationships, at least until the belt wears and jumps a cog. The belt also absorbs vibration and damps any sympathetic torsional resonance. Two heads on a suitably large engine is certainly viable. John
From: John De Armond Newsgroups: alt.energy.homepower Subject: Re: Parallel generators Date: Mon, 03 Sep 2007 00:14:59 -0400 Message-ID: <email@example.com> On Sun, 02 Sep 2007 18:24:32 -0000, philkryder <alt.google@Kryder.com> wrote: >I love it! > >do simple mechanical governor units - like Generac 8000w - stay synced >once this is done? Yes, absolutely. The same forces which will wreck a generator connected out of phase hold the rotors in-phase. If one generator tries to "run" significantly faster than the other then reactive current will circulate between them but that is of little consequence as long as the total current remains below specification. In the limiting case of one engine being turned off and assuming the field excitation on the turned-off generator remains active, the generator on that unit will act as a synchronous motor, spinning the engine at the same speed as the driving generator. For fairly obvious reasons, this is undesirable so installations of parallel generators that are un-manned normally include reverse power trip relays that open the generator breakers when power flowing back toward the generator is detected. Probably the worst problem to deal with when a generator gets overloaded sufficiently to suffer field collapse. when this happens the units fall out of sync and any of a number of unpleasant things can happen. The least unpleasant is that the generator breaker will trip. Blue smoke leakage can be the worst result. > >What do you use to "trim the phase" on such a unit? I don't recognize that term in this context. Do you mean how do I trim the speed when syncing? Hand manipulation of the governor. Not hard to do with a little practice.
From: John De Armond Newsgroups: alt.energy.homepower Subject: Re: Parallel generators Date: Mon, 03 Sep 2007 11:15:33 -0400 Message-ID: <firstname.lastname@example.org> On Mon, 03 Sep 2007 06:52:53 -0000, philkryder <alt.google@Kryder.com> wrote: >On Sep 2, 9:14 pm, Neon John <n...@never.com> wrote: >here is the quote out of context from your earlier post: >"....Hook the neutrals together and hook the bulbs between the hots. >Adjust the speed of one generator until the flicker goes away and then >**** trim the phase **** until the lights...." > >I didn't know what you meant either - hence the question. >Phil Oh, sorry. My mind sometimes outstrips my fingers and I leave words out. I should have said "trim the phase between the two to zero". Gentle manipulation of the governor. Usually all it takes is the merest touch on the governor spring. You can actually close the breaker as the two pass zero phase as long as the rotation isn't too rapid. The two will be forcibly yanked into sync. With large generators this can do things like rumble buildings, crack foundations, etc, but with small generators, the worst risk is probably field de-excitation, forcing one to start the sync process over. John
From: John De Armond Newsgroups: alt.energy.homepower Subject: Re: Parallel generators Date: Mon, 03 Sep 2007 21:46:04 -0400 Message-ID: <email@example.com> On Mon, 3 Sep 2007 11:27:51 -0400, "daestrom" <daestrom@NO_SPAM_HEREtwcny.rr.com> wrote: >Trouble with driving two units from a common shaft (either welded, chained, >or timing belt) is that when you start them up you can't adjust the phase >angle between the two. Why in the world would I ever want to change the phasing of two generators mechanically phased from the get-go? Presuming one arranges the rotors to be in phase at the time of coupling (after all, what other reason would there be to couple them?) then there would never be any need to change said phasing - unless one wanted to watch two generators fight each other. This is one of the most patently dumb arguments that I've read on this subject. >They may be close enough in phase to make your >synchronising light appear 'dark', but they could still be out by 30 or 40 >degrees (and that would mean nearly full load current between them without >any external load). Light bulbs are a poor second for a synchroscope. One >you can use a voltmeter across the bulb as an 'in-between'. Best to shut >the tie when the voltmeter reaches zero. I get the impression that you've never actually synched a generator using a light bulb bank. If you had then you'd realize that the voltage (phase) at the point where the bulbs go dark is unimportant. The reason is simple. If the dark interval is, say, 10 seconds, then one knows that the zero voltage point will be 5 seconds into the dark interval. One simply observes several cycles of light and dark and closes the breaker in the middle of a dark interval. Light banks work for even large machines. Probably the largest actual machine that I've seen synched using bulbs was a 50MW machine at a paper mill power house that I was automating. The operator was quite good, even compensating for the approx 1 second delay between twisting the breaker handle and the oil breaker actually closing. Since the plant only had one synchroscope for 5 generators, using lamps was the norm, as each generator had its own lamp bank. I've "synced" the 1200 MW generator on Sequoyah NP's training simulator countless times, using lights and synchroscope in combination and separately. This type of full-scale simulator is indistinguishable from the real thing, including sounds and building vibrations when one does something wrong. Back to the discussion at hand, a synchroscope is certainly nice but I hardly think that one is going to spend a couple thousand dollars for a 'scope that costs as much as the generators. >In my 30+ years working with all sorts of marine and commercial power >(including several years in a rewind shop), never saw such a setup. Closest >thing I've seen is two turbines driving one genny through a gear system or >two diesel gen units with the two generators tied together when starting so >they come up to speed already in-sync. I think the 'common shaft driving >two heads' is bad, bad, bad, and be more trouble than it's worth. Again, I'm getting the feeling that much of your experience was doing things like sweeping the floor or maybe checking oil levels. While not extremely common, two generators driven by one prime mover is not a rare configuration. Usually there is a space or cost or transportation constraint that makes a pair of smaller generators more economical than one large one. Steel and paper mills are two types of places where I've seen them in person. If you'd like to see some photos, might I suggest a couple of books? New book: Electric Machinery 6th ed - A. Fitzgerald, C. Kingsley, S. Umans (McGraw-Hill, 2003) Old book: Coyne Electrical Encyclopedia. I have the 1947 and 1952 editions. Both of these books are available on-line as scanned PDFs. John
From: John De Armond Newsgroups: alt.energy.homepower Subject: Re: Parallel generators Date: Wed, 05 Sep 2007 17:43:55 -0400 Message-ID: <firstname.lastname@example.org> On Wed, 5 Sep 2007 10:41:42 -0700, "Ulysses" <email@example.com/> wrote: >"Neon John" <firstname.lastname@example.org> wrote in message > >> Ya coulda fooled me. I've done it many a time with nothing more than a >>pair of 120 volt light bulbs in series. > >Why two light bulbs and not just one? Do they need to be any particular >wattage? When the generators are out of phase 180 degrees the voltage is the sum of the two. If both are 120 volt generators then the maximum voltage is 240 volts. Any wattage is fine. Lower wattage bulbs typically emit some glow on lower voltage so you'll have light throughout more of the cycle but that's really a minor detail. Of course, a voltmeter could also be used. >What about ground? Should the grounds be connected on the two generators? Only if you want 'em grounded and the neutrals bonded. I don't, especially when running stand-alone situations such as power tools on a job, lights at a camp site, etc. The reason is this. If the generator is grounded and the neutral bonded to ground as is usual with utility power, if you're grounded and you touch something hot then you get shocked. If the system is floating, you're grounded and you touch something energized, nothing happens, as the circuit is not complete. Actually you will feel a tingle because there is capacitive coupling between the generator winding and ground (and probably within some appliances too) but it won't knock yer pecker in the dirt. Just a warning to check things out and find out what's wrong. The usual argument against this is "well, what if the hot leg gets accidentally grounded?" Answer is that on a floating system there isn't a hot and a neutral. Both legs are equal. If one leg gets grounded then by definition it becomes the neutral. To get shocked, one leg would have to be inadvertently ground AND you'd have to touch the other. This is a situation where you play the odds and make your choices. In my book, the odds of getting shocked on a grounded system are so much higher than with a floating system that I choose floating when I'm using a generator. John
From: John De Armond Newsgroups: alt.energy.homepower Subject: Re: Parallel generators Date: Thu, 06 Sep 2007 22:25:34 -0400 Message-ID: <email@example.com> On Thu, 06 Sep 2007 19:34:37 GMT, Bruce in Alaska <firstname.lastname@example.org> wrote: >Ok, we have had a Great Discussion, about the Theories, and >Practicalities of Pralleling Generators, here for a week now. >So lets get down to Practical Issues that a Home Backup Genset >Guy is likely to encounter when he tries to implement such >a Genset Parallel Scheme. > >Has anyone, Neon John, or anyone else, actually Paralleled such >a setup, like a couple of "Identical" Home Depot Genrac 5kw >3600 Rpm Gensets?????? Oh, maybe lets bump the Genset Grade up >a bit and, How about a Pair of ONAN DJB/E's or a similar Diesel >Setup????? > >What is the smallest Twin Genset Paralleling Setup that ANYONE >has actually Run and Used????? No theories now, I am looking for >Actual Experience, here. Smallest? Two of these: http://www.northerntool.com/webapp/wcs/stores/servlet/product_6970_200313480_20031 Frequently, as a matter of fact. Reason: Two of these are lighter and cheaper than any other solution for a specific problem I occasionally have - running a small portable AC to cool the cab of my truck on long trips where I have to stop and sleep. It's a lot cheaper running those two generators than it is to idle the big engine. One genny wouldn't quite run the AC but two do it just fine. I already had one of the generators so the marginal cost of getting that AC running was $99 (caught the second one on sale at NT). A pretty good bargain in my book. These are trivially easy to sync. The rotating mass is so low that as long as I get the two in phase enough that the field(s) don't de-excite, they snap right into sync. I use a simple 3-way "Jesus cord" - two males and a female outlet. Yaaaa, one of the male plugs can be hot. Life's dangerous. Get over it. >I, myself, have run a pair of CAT 343's, 250Kw each, for a total >of 500Kw, that were equipped with really fancy Woodward Hydrolic >Governers, that required the CAT MAN to come spend a FULL Day setting >up, before they would Loadshare properly for any length of time. >This was all 25 years ago, and with the Woodward Electronic >Governers of today's PowerHouse World, it is easier to do and >only takes the CAT Guy, half a Day to do the same setup on our >2.8MW Powerhouse. (1 ea. CAT3516, 3 ea. CAT398's, and a lone >CAT353) We also have a CAT3412, and one of those CAT343's, but >they can't be paralleled onto the Main Buss, because they still have >the older Hydrolic Governers, that can't be matched to the Electronic >ones in the Main Powerhouse. > >The answers I expect to read are, really nothing smaller than 40Kw. >As anything smaller, the operator would just install a BIGGER Genset, >rather than all the stuff required to do a Paralleling Setup. Hmmm, it appears that the reason you can't imagine paralleling small machines is precisely what you're criticizing - no experience with small machines. And trying to extrapolate large machine experience downward, of course. There are several factors that affect the big machines that are of little consequence for small ones. The first is that the reset action (integral) contained in either the hydraulic or electronic governors try to pull the engine speed back precisely to the setpoint. This is to keep the long term frequency stable. If one governor setpoint is 60.00 hz and the other is 60.01 hz then eventually they'll fight each other, one at full throttle and the other at minimum fueling. That isn't the case with small machines. There is no reset action - the governors are simple proportional units. The speed varies a small amount with load and that's enough to allow them not to fight each other. A mechanical form of ballasting, as it were. The second consideration is that most machines are engine-limited and/or field excitation-limited in terms of overload and not generator or thermally limited as large machines are. When overloaded a large machine's limiting factor is generally the temperature of the winding and not the engine's power. Just the opposite is true of small machines. The engine bogs from overload and/or the field de-excites long before the windings overheat. The third consideration, related to the first, is that a relatively significant frequency swing and therefore engine speed is designed into small machines. A typical spec is 62 hz at no load and 58 hz at full load. This sagging speed vs load curve allow each generator to accept load according to its governor slope. They rarely run at exactly equal load but that's of no concern. When one or the other reaches max load then it slows, forcing the other one to slow and the second one's governor applies more power, picking up more load. They quickly reach a new equilibrium. The first one may run at full load or perhaps somewhat overloaded but that's no big deal as long as the governors have similar setpoints and slopes. >So lets hear from ANYONE, who "actually" uses Gensets in a Paralling >Scheme. What do you have, and how big is it? I used a Generac 7000 watt contractor generator and a Yamaha 3.8KW generator in parallel for many years as part of my restaurant's emergency plan. In the event of power failure, the two generators ran in parallel to run a three phase rotary converter (10 HP 3 phase induction motor with suitable capacitors to make it produce 3 phase power) that in turn powered my three phase refrigeration equipment in addition to the single phase loads. I never had a sufficiently long power outage to need the rig in an emergency but I exercised my plan a couple times a year. Even though they're different brands they ran fine in parallel. I had to tweak the voltage regulator on the Yamaha (the Generac one is sealed) to make 'em work together properly but that was a minor detail. Again, a simple three way Jesus cord made the connections. Starting with a clean sheet of paper, would I parallel small machines? No, probably not. For most situations I'd simply buy the appropriately sized generator. But in my case I already had both generators and I wasn't about to go out and buy yet another one for this one infrequent application. I made do with what I had. The one instance where I WOULD buy two generators to run in parallel is a situation where the load most of the time could be handled by one generator but infrequently the total load required more. I'd certainly not feed a large machine running at half throttle for any extended period of time. John
From: John De Armond Newsgroups: alt.energy.homepower Subject: Re: Parallel generators Date: Fri, 07 Sep 2007 13:32:41 -0400 Message-ID: <email@example.com> On Fri, 07 Sep 2007 04:45:39 -0000, philkryder <alt.google@Kryder.com> wrote: >wow - one of those - yeah, it's easier than you might think >situations. Yep. A philosophy in life that has served me well is, barrel right in and try it. If it works, great. If not, backtrack and figure out why. There are a few areas of exceptions, of course. Explosives, very high voltage and highly radioactive materials, for instance. > >So.... >That's it - a double ended cord... That's all it takes. I've installed breakers and a variety of outlets on my generators http://www.neon-john.com/Generator/Quiet_pack_55G/GenCart_2.jpg so syncing is just a matter of flipping the breaker at the right time. It'd take a little more coordination to jab a plug into an outlet at the correct instant but it could be done. > >What would it take to make a "semi automatic" product to do this? > >Say a switch controlled by when the two units were in phase. It's called an auto-sync relay. In the trade, such gadgets are referred to as "relays" even though they're now solid state. Originally it was an electromechanical gadget. The simplest one generates an output when the two phases are within a pre-determined phase angle. In TVA when I worked there, that angle was typically +- 5 degrees for main plant generators. That let the operator get the speeds almost the same and then just twist the "close" handle on the breaker. When the two drifted to within that window the breaker closed. Modern microprocessor relays have the ability to look at closing rate and inhibit the breaker closing if the rate is too fast. IOW, if the generators are at different speeds and the phases are rotating past each other. They also have time delays, anticipation capabilities and even the ability to generate a rate signal to go to the governor. The latter allows completely automatic syncing, no operator involved. For small machines like we're talking about, only the simplest relay would be necessary. The operator would have to manually manipulate the governor to sync the speeds and then the phases. The relay would simply close the paralleling contactor when the phase difference was within the window. For a one-off project like we've been discussing, I think that I'd take a look at some of the low end PLC (programmable logic controllers) These can be had in the $100-200 range. It should be easy enough to hook each phase to an analog input and write the logic to generate an output when the difference between them is zero. John
From: John De Armond Newsgroups: alt.energy.homepower Subject: Re: Parallel generators Date: Fri, 07 Sep 2007 23:58:11 -0400 Message-ID: <firstname.lastname@example.org> On Fri, 7 Sep 2007 17:56:56 -0700, "Ulysses" <email@example.com/> wrote: > >"Neon John" <firstname.lastname@example.org> wrote in message >http://www.northerntool.com/webapp/wcs/stores/servlet/product_6970_200313480_20031 >> >> Frequently, as a matter of fact. Reason: Two of these are lighter and >> cheaper than any other solution for a specific problem I occasionally >> have - running a small portable AC to cool the cab of my truck on long >> trips where I have to stop and sleep. It's a lot cheaper running those >> two generators than it is to idle the big engine. >> > >What happens if one runs out of gas while you're asleep? > One of two things. If the fields stay excited then the generator with the out-of-gas engine will become a synchronous motor and motor the engine right along at 3600 RPM. If the field de-excites then the out-of-gas one will stop turning and one of the unit's breakers will trip from overload. In either case, since the other generator can't handle the AC load by itself, it's breaker will trip. I'd wake up to a hot truck cab, one generator stopped and one idling at no load. I don't know which one would happen because I have them rigged up to take fuel from a 5 gallon outboard motor tank. Those two little generators would probably run a whole day on 7 gallons of gas.