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 04-04-2003, 00:53 Post: 52558
DRankin



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 New PTO Generators

Two new items of note:

Northern Tool has a 8000 watt continuous/12,000 watt surge PTO generator for $1k. They say it will run off a 16 horse tractor.

John Deere has a new PTO generator designed to run off their X series lawn tractors. Those models have 24 gross HP.
I think it is rated at 10,000 watts and it looks like it will work on any Cat1 compact. It has a nifty voltage readout so you can adjust your PTO RPM's to the perfect setting. It retails for $1600 plus the driveline at $175.






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 04-04-2003, 06:45 Post: 52564
TomG

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You sort of have to wonder about specs these days. Watts and HP are both measures of work and 750 watts = 1 hp. I get 16.4 hp for a 12K load. Maybe the 16 theoretical hp tractor actually is beefier or maybe a whopping flywheel is assumed to push an engine passed very brief surges.

It's probably a decent generator for the money but I don't think I'd depend on one for bringing very large electric motors up to speed. Lugging an engine when an AC motor is the load isn't good.






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 04-04-2003, 09:16 Post: 52577
AC5ZO

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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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 04-05-2003, 06:23 Post: 52606
TomG

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Looks like part of AC's message is missing. I think there are a few other chopped ones. I wonder if there is a length limitation on posts as Dennis is trying to manager the problem of traffic and server costs. Maybe I get to blab because I paid my $20. I couldn't find any length limit notices. If there is a limit, it might be good to trap long messages and provide warnings rather than just chopped messages. Even I could learn to shorten up my ideas and not try to get a whole subject in one post, but a warning that I need to shorten things up would be good.

I did miss reading the rest of AC's post. I think it was going to talk more about surge specs. Yes, I think heating is the main limitation, and a another spec (the duty cycle) usually goes along with purge specs to indicate how long surges can be maintained. If there's no duty-cycle speced, the surges talked about may be only momentary load spikes. Duty cycle is important to people who want to use generators for arc welding.






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 04-07-2003, 09:12 Post: 52722
AC5ZO

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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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 04-07-2003, 09:25 Post: 52723
DennisCTB

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 New PTO Generators

Tom,

You should be able to see the full thread of all messages now that you have a "Premium" account.

To invoke this new feature you need to login again to reset your "cookies"

There is no limit on message length, so don't worry about breaking your thoughts into bite size chunks.

Dennis






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 04-08-2003, 04:50 Post: 52763
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I have the whole message now.

My original interest in MarkH's post was if the pto generator with a 16hp tractor would result in a situation where the generator load might be capable of lugging the engine.

I'm aware that if rpm decreases both the frequency and voltage of the output decrease. Lower frequency power creates less impedance in a motor, which would result in increased current at the original voltage. Of course, since the voltage also declines which would decrease current at the same frequency through a motor. I don't know the net result of current change or if there's any potential for low frequency generator power to burn up motors under these conditions. It sure would through the accuracy of electric clocks off though.

I used 746W/HP instead of 750W for the 12,000W-surge rating and I get 16hp and very small change. That leaves me wondering if the surge rating might be theoretical and simply calculated from an assumed tractor HP. I also wonder if the 16 hp is engine, pto or theoretical.

I run my own Honda backup generator with a 6500W surge rating through a bit over 100' of 10-gauge wire to a transfer panel, and then there's the length of wiring from the panel throughout the house. That's basically minimum code adding 2-gauge for the extra length run. If I did it again I'd use 8-gauge. When it's running I juggle loads to minimize one motor starting when others are running. Neither the generator nor the motors like it much when left to their own devices.






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 04-08-2003, 10:47 Post: 52778
AC5ZO

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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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 04-08-2003, 10:59 Post: 52779
AC5ZO

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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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 04-09-2003, 06:29 Post: 52820
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Yes, I think it's more likely that the tractor spec was obtained from a calculation of the max load permitted by the generator's breaker rather than the other way around. I just noticed that the tractor hp and surge specs were too close to think that it was coincidental, and as you say if the specs came from calculations they might not say much about how a generator would actually perform if driven by a 16hp tractor.

About the freq thing; when I was teching in the USAF a long time ago. We had a mystery transformer in surplus. We wondered what its output voltages were so we connected it to an AC source. It burnt up before we could get a VOM on the outputs. Turned out that it was an aircraft part that used a 400 HZ AC system.

I've got the loads fairly well balance across the phases in my house wiring. I put heavy circuits and motors on both phases--'cause a 200A service turns into a 100A one if everything's on one side. Balancing of course is more critical when working with limited capacity backup generators. There is a related issue here for use of generator and especially pto generators. Grounding is crucial and a ground always should be provided for portable generator operations.

Lack of a ground can result in neutral potentials that are different than ground and can create mild to severe shocks from equipment cases. A heavy load on one phase without adequate grounding can produce 220V on the other phase. There's been a few generator powered sound systems with poor grounds burn up when starting up heavy amps on one phase. We should think about that sort of thing if running a 110V arc welder from a generator. A radio plugged into the other side may not fair well.






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Discussion Boards > Active Subjects > Messages as Posted > Tools - Generators Forum

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