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New PTO Generators
While the numerical equivalency is 746 W = 1 HP when you are doing energy conversion you have to include efficiency losses from the electrical resistance and friction.
The electrical efficiency may be 80% to 90%. Speed increasing gears may only run 50% efficient. Belt and chain drives are more efficient getting into the high 80s. And then there are also losses in the driveline if it has much offset. These numbers all multiply together, so it is not hard to see that overall conversion efficiencies can be below 40% and won't get much higher than about 70%.
The differences specified between surge and continuous power generally have more to do with electrical than mechanical efficiency, in my experience. Any good flywheel or high inertia drive train will drive the generator past a couple of cycles of extreme load, but the heating in the generator goes up with the square of the current delivered. Add a long connection cord to this equation and things just get worse, because motors take longer to start with the resulting lower line voltage and draw starting current for a longer period of time.
This is another case where there is no substitute for heavier structures and more power.
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I can read the whole messages. I have to click on the PopUp window to bring up the whole thing. Try that.
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I do not think that the surge capacity has anything to do with the tractor horsepower. It is only related to the electrical characteristics of the generator. Certainly, I can run that same generator on my tractor that has over 30 PTO HP and it would still only generate 8KW cont and 12KW surge.
With respect to efficiency in this case, I am sure that they know that 16HP is the minimum required PTO HP required to drive the generator to full load. Since full load is 8KW the generator/drive train energy conversion efficiency must be approximately 66%. That comes from the 16 PTO HP being equal to about 12KW, but the generator makes 8KW at full load. 8/12= 66%
I also expect that these ratings are a bit optimistic. If you use a 16 PTO HP tractor to drive this generator to full load, then the entire system will be more "load sensitive" than if a larger tractor was used. As you mentioned in your post, frequency and voltage would both suffer leading to higher current and more heating in the generator. If you attach that same 8KW generator to a >30 PTO HP tractor, then the load variation will not affect the engine RPM as much. This will keep the frequency more stable and use less overall current.
My backup generator is a Honda powered 7500 W with 9000 W surge. I think that it has an 18HP engine on it. Since this generator is directly coupled to the Honda engine, there should be little mechanical loss. Compared to the 16HP PTO generator we started talking about, this is a more stable and powerful system. More HP and better efficiency both mean better stability under varying load.
It also helps when using generators and transfer panels to do your homework up front. The ultimate limitation for the generator is the current that it can provide. If you have a lot of 120V loads you may exceed the current limitation without generating the full potential at 240V. If you can balance the loads on each side of the 240V neutral conductor, you will match the loads to the generator's capacity. You cannot account for every motor starting and stopping, but if you do a good job, you can cause yourself a minimum of headaches from managing the loads manually.
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I forgot to mention one thing regarding your mention of burning up motors if the generator system lugs down in speed.
There are a couple things that generally keep that from happening. If you exceed the current rating at any portion or the entire generator winding, there is generally a circuit breaker that will trip. Also, if you exceed the capacity of the generator by a substantial margin, the magnetic fields in the generator will collapse and it will fail to produce more power. Both of these systems will protect a generator in the 6KW-8KW range if a 5HP electric motor attempted to start.
However, neither of these mechanisms will protect the 1/4 HP motor that might try to start if the system is already lugging due to other heavy loads. Many motors are rated 50/60 Hz so that they can be used on Europe's 50Hz grid. It is doubtful that a 50/60 motor will be damaged, but a marginally designed "60Hz Only" motor might be.
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I agree with you about the grounding and swinging potential of the neutral wire. Balancing and grounding are very important.
Years ago, I had a house where the lights would get brighter when the cloths washer would start up. The heavy load of the washer starting would "pull" the neutral toward that phase and the same increase would be seen in the other phase that happened to power the lighting. Except during heavy load excursions, things seemed normal. I traced the problem down to the line actually coming into the house from the power company transformer. It turns out that the neutral connection was corroded at the transformer and a voltage would develop over the corroded connection under heavy load.
When we apply backup generators to our properties, it is even more important to balance loads, because we seldom provide 100A service from a generator. 100A service on a 220V line would be a nominal generator capacity of 22KW and the surge would be even higher.
Most backup generators are in the 10KW range or less, so you have less than 40 amperes to share over all of your needs. Balance helps by distributing those loads on both phases of the power more or less equally, and it means that you probably need to turn off the heavy loads when on emergency power.
Power factor is another thing to consider. While heaters and lighting loads are resistive, most motor and welder loads are inductive. The current and voltage on inductive loads are out of phase. Power companies address this by hanging capacitors on the powerlines as necessary when they find that their power factor is not proper. It is beyond the scope of this email to go into that further, but I even place capacitors on some loads to make them work better and use less wattage. Manufacturers don't put these capacitors in most welders and motors simply because of the cost.
Beyond my career as an engineer, I am also an amateur radio operator. I have seen the problems with 400Hz transformers before. Many hams like to reactivate military surplus radios and some of them need 400Hz. Most we convert to 60Hz using surplus transformers. But, many times the specs of the surplus transformers are not known. Many of us
encounter odd transformers occasionally that are not marked and will burn up when tested. I am sure that you also remember audio transformers used in some USAF equipment. They can also get hot when 60Hz is applied. If you ever have to test something like that again, use a 100W lightbulb in series with the power lead. That will limit the current through the transformer or device to 1A or less. If you want to know more about this, please email me separately.
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