Unbalanced Mecce Alte generator???

Sorry to intrude here, but I have a problem that I hope some of you can help me out with.
My wife and I are building a small, remote cabin in a swamp in south
Alabama. We're about .75 miles from the nearest dirt-road and the power company wants a 30' swatch cut in the trees to run power. Without a lot of discussion, we're going to be off-the-grid, relying on battery-bank, inverter, a couple of pv-panels to help things along (not much solar sky in the trees), and a generator. The problem I have is with the generator.
We purchased a new Kubota diesel/7.5kw Mecce Alte generator-head gen-set. Supposedly good for around 30amps per 120v leg. The problem that we're having is one 120v leg of the single-phase setup appears to be weak. Here's the scenario:
One leg shows 120.5v at 61.7hz while the other one shows 117.1v at 61.7hz. No fancy meter equipment, just a "Watts-Up" plug-in meter along with a Radio Shack meter.
The circuit connected uses only one leg of the generator which I understand creates an unbalanced situation. The circuit is comprised of 3 feet of romax going to a receptacle. This circuit is intended to power saws, drills, etc.,.,
The first thing we loaded the generator with was a 13amp skil-saw. Upon energizing the saw the generator made a very loud, continuous noise which I've since been told was actually the rotors hitting the stators inside the genny. We quickly stopped the saw and shut the engine down. Just to check things out we hooked up a 75w light bulb and it worked fine...much smaller load. I checked the wiring and all is well. We tried it several more times with different sized loads, but nothing over 15amps. We had the same result...loud noise and apparent weak power.
I next switched the circuit to the other generator leg. This leg reacted very differently from the other leg. When we loaded it with the skil-saw and later a small compressor it would "bark" briefly upon being loading, smooth out quickly and, from what I could tell, power the equipment fine.
We switched legs back and forth a couple of more times to be sure of what was going on and the above results were obtained each time.
Upon talking with the seller, GeneratorQuotes/GeneratorSales/Central Maine Diesel, they had me recheck my wiring with the data they gave me...all checked ok. Bill Perry, Sr., the owner of Central Maine Diesel, called Mecce Alte and they stated that what I'm seeing isn't unheard of in the generator-head that I've got. There recommendation was to rewire it to 120v only and that that should balance the generator and solve the problem. I'm hoping to hear from some of you before I do this.
Even if this works, I'll still need two 120v legs later along with the 240v for the well pump. My idea with the generator was to power the battery charger on the Trace 4024 along with the pass-thru circuits on one leg and to run a small window A/C unit on the other leg. At times, though, the A/C unit won't be running and I'm wondering if even though the "wired to 120v only" fix will cure things for now, will I have a problem later on when I wire back to 120/240?
I can't understand why if it's a balance problem why one leg works fine unbalanced while the other one makes the genny sound like it wants to come apart at the seams. Also, the fact that the "problem leg" shows 117v while the "good leg" shows 120.5v makes me wonder a bit, too.
I guess it comes down to wanting your opinion on whether you think it's an "out of balance problem" that is normal, or if something is screwy with the generator. It was a large investment for my wife and I which we hope to use for many years to come. But, for now we're scared to use it to work on the cabin with. As a side-note, we've been using a little 3000w Coleman for the last couple of months to power the equipment that we loaded the Kubota up with...though strained at times it runs everything we've thrown at it.
Thanks for your help and feedback, Ed Welch South Alabama KF4KRV
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I'm no expert, but I have been running my 5kva propane genset for 10 years unbalanced, and never had a problem like you describe. It's a Gillette generator head, and a B&S engine. Nothing special.
A 13amp skill saw is not what I'd call an excessive load on a 7.5kw genset. I've run two Milwauki 7-1/4 saws at a time. It they would both start at the same time the generator would bog down a bit, but I think it was harder on the saws than on the generator.
I able to run a 4hp compressor off it, and the generators engine is only a 9hp Even when it is way overloaded, the engine may bog down a bit, bit it never makes any rapping noises..
Your problem sounds screwey to me, and a level of abnormality that I would not accept. There is no way that the rotor should ever hit the stator unless it's a defective design or put togather wrong.
I'd demand at a minium that the seller swap it for another one. If there is nothing wrong with it, as they seem to be claiming, then they have no reason not to let you swap it for another of the same type. Take it back to them and demonstrate the problem.
If that doesn't correct the probem, them I'd probably ask for my money back and buy a different genset.
my $.02

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Ed Welch wrote:

I have no experience with that particular genset, so what I have to say is opinion only: If it's making a noise like it's going to come apart, the damn thing is defective. Wiring it for 120 only doesn't solve the problem, it hides it.
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Is this the unit?
http://www.generatorquotes.com/order/KB750.asp?page=KB750
Assuming it is, the behavior that you've experienced is not abnormal. The Mecc Alte line of generators is a rather low end one that is not robustly engineered enough to sustain unbalanced operation. Characteristics include self-excited, capacitor regulated, untwisted rotor. These characteristics in combination lead to low frequency radial vibration in the rotor when operated unbalanced. At best the generator makes significant vibration. At worst, you experience the rotor/stator interference you've seen. You'll usually also experience voltage oscillations. Plugging a light into the same leg, or even the other leg should show some flicker.
Better designs involve twisted rotors to avoid the "cogging" vibration and a non-self-excited field, even if brushless, that doesn't sustain excitation oscillations when unbalanced.
Unfortunately you have a very nice engine coupled to a rather cheap alternator.
What the factory recommended is one solution if you only need 120 volts. If you need 120/240 as with regular grid service, the cheapest solution (if you can't return the generator and get a better one) is to employ a 120/240 volt dry transformer between the generator and loads. These transformers are used in industry to supply relatively small quantities of power at a voltage other than the distribution voltage. Say, a little 120 volt power from a 480 volt distribution. You can get a transformer with split 240 volt windings on both sides. One side would be strapped straight 240 and connected to the generator. The other side would be wired split 120/240 and would supply the loads. The generator always sees a 240 volt load regardless of what is hooked to the transformer secondary.
Dry type transformers are not expensive and are widely available used since they are so common in industry. A 6kva one would do, since they normally have a very high service factor and can be overloaded. Here is one style of transformer:
http://www.federalpacific.com/fb.html and another http://www.federalpacific.com/fh.html
The FB type (resin cast) type is quieter than the air cooled one but more expensive.
Given the number of used transformer outfits on the net, I wouldn't expect to pay more than a few hundred dollars for a 6kva unit.
here is an example from graingers:
http://www.grainger.com/Grainger/productdetail.jsp?xi=xi&ItemId11603346
This $600 price is the worst case, full-boat retail price for such a transformer. I'd expect to get a new one in the $4-500 range and a used one in the $200 range.
You'll note the hookup - one side is 240/480 which would be strapped 240 and connected to the generator. The other side is 120/240 and would be strapped split 120/240.
This transformer will completely solve the noise and unbalance problems you're having. You should not, however, expect very good voltage regulation from that type of generator. You will get some light flicker with this type of generator. Goes with the territory.
John
On 4 Mar 2004 15:48:25 -0800, snipped-for-privacy@hotmail.com (Ed Welch) wrote:

--
John De Armond
snipped-for-privacy@bellsouth.net
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Neon John wrote:

With replacement generator heads selling for the same kind of money you might want to consider getting the head changed out to a quality unit that can provide better voltage regulation and higher tolerance for load imbalance. -- Tom H
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If you go the "replace the head: route you might consider...
http://www.gillettegenerators.com/beltkit/specs12.html
I've had good luck with mine.
The Kubota is an excellent engine. Can't understand why they would mate it with a crappy head. You really sould not have to do this on a brand new generator.
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Did you ground the unit correctly? If you did there is something definitely wrong with the gen part. Rewiring for 120 is nuts. Get them to repair/replace the unit under warrantee.
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