Generator Will 27HP run generator?

[MarkLeininger]

I want to buy PTO generator. The one I want is 15kw, suggests 30hp tractor. I have a 27hp (Kubota B2710) that is at my house all the time, so that's the one I'm most likely to use. I have a 45 hp JD that is not always here so might not be available when I need it. So the question is, if I run the generator will the only effect be that I might not get the full 15kw with my kubota, or is there some other effect resulting from using a HP a bit smaller than what's recommended?
thanks

 

[Kyle_in_Tex]

I believe it will run it, but, when a large load (like a big A/C unit) is turned on, it may bog it down if the demand exceeds HP. It will be up to you to manage which loads are placed on it simultaneously.

Some loads like water heaters or heaters are very great but don't need the extra oomph that a starter motor (like a big A/C unit) would need.

 

[Jimbrown]

Well according to my calculator 1Hp =.746 KW and 27Hp =20.134KW
so you should be OK unless that generator has a lot of internal losses. But you realise that the 27 hp is probably at full throttle. And, you may have to run the the PTO at less than full power to get the correct cycles.

 

[MarkLeininger]

Well according to my calculator 1Hp =.746 KW and 27Hp =20.134KW
so you should be OK unless that generator has a lot of internal losses. But you realise that the 27 hp is probably at full throttle. And, you may have to run the the PTO at less than full power to get the correct cycles.

Jim,
I don't understand why they don't make the recommendation based on PTO HP. Then, as you point out, since my PTO HP is 20: 20*.746 = 14.9KW. The rated PTO HP is presumably at 540RPM.

 

[John_Mc]

A good rule of thumb is 2 HP (at the PTO) per KW of generator power (that's continuos KW rating, not the "surge rating"). That rule may break down as you get into really large generators, but seems to work pretty well from a few KW up into the range you are talking about.

For example, Northern Tool advertises a 13 KW surge, 12 KW continuous PTO generator that says it requires 24 PTO HP to get full rated output.

Note that many manufacturers have taken to advertising the surge rating of their generators. The Northern Tool unit mentioned above is shown under the title of "13 KW PTO Generator". Some times the difference is 10% or less, others can be significantly larger difference.

John Mc

 

[Tom_Veatch]

Jim,
I don't understand why they don't make the recommendation based on PTO HP. Then, as you point out, since my PTO HP is 20: 20*.746 = 14.9KW. The rated PTO HP is presumably at 540RPM.

The 746 conversion factor is for an ideal process, i.e. 100% efficiency, which is never realized in real world processes. Not only will you have mechanical losses in the power transmission from the engine to the pto output, but you also have significant mechanical and electrical losses in the generator itself. Oh, considering the generator as a thermodynamically closed, isolated system, you might get pretty close to an ideal case using some sort of magnetic levitation for the bearings, and running it in a hard vacuum with supercooled/superconducting coils, but I wouldn't want to have to either pay for, or maintain something like that.

If you use the rule of thumb of 500 W per Engine HP, you've effectively assumed an overall process efficiency of about 67%. That won't be exact for any given tractor/generator combination, but is in the ballpark of what you'll actually see.

In your calculation, you tacitly assumed a generator efficiency of 100%. You'll actually achieve something on the order of (20 PTO HP X 746 W/HP X ~90% Assumed Generator Efficiency) = ~ 13.4 KW. Your tractor is rated at 27 Engine HP. The 500 W/HP guide applied to your rated engine HP gives ~13.5 KW.

Remember, in any case it's a SWAG and the accuracy in any specific case is open to question. It's usually spec'ed in terms of 500 W per tractor HP instead of something like 670 W per PTO HP because it's easier to calculate in your head and gives a fairly good ballpark result.

 

[MarkLeininger]

Thanks Tom. So I don't care if my Kubota can only turn this thing enough to generate 10KW instead of the full 15KW. I just wanted to confirm that there isn't some reason that would prevent my Kubota from producing any output at all, or that there is some other problem related to the engine being slightly underrated in size that I can't imagine until I try it. If I need the full 15KW for some planned project I can always wait until my larger tractor is available.

 

[Tom_Veatch]

I just wanted to confirm that there isn't some reason that would prevent my Kubota from producing any output at all, or that there is some other problem related to the engine being slightly underrated in size that I can't imagine until I try it.

No reason at all. In fact, there is a pretty widespread philosophy that recommends a generator oversized relative to the tractor. The idea being that the larger, heavier generator isn't worked as near to it's limit and the greater rotational inertia of the larger generator results in a more stable output during surge loads. I have no data to either confirm or refute that theory, but there is no question that a larger generator provides a growth path.

The tractor may not be able to develop the generator's full load, but if you sequence or otherwise restrict the load so that, at any given time, you don't try to draw more from the generator than the tractor can support, you'll be fine.

Don't forget, a 15KW generator doesn't have to develop the full 15KW. It will only draw enough power from the tractor to support whatever demand is placed on it (plus process losses and inefficiencies).

 

[BobRip]

I agree with pretty much everything that has been said. I offer another word of caution. You must keep the frequency of your generator close to 60 Hz (maybe +/- 2 Hz, not too sure) . If you are running motorized applicance (wells, refrigators, air conditioners, etc.) and the frequency is off you can damage the motors. I suggest getting some kind of frequency meter (Kill A Watt at $40) to monitor this. You can plug it in in the house or at the generator to set and watch the frequency. I have heard of rashes of equipments failures caused by off frequency settings. I don't know how good the speed regulator is on the tractor, so I don't know the severity of this issue, but it is not costly to monitor.

 

[fastpat]

As others may have said, the rule of thumb is 2 PTO horsepower per KW generated. My tractor has 38 PTO horsepower, so 17KW is my max, and allowing some headroom, I'd not go beyond 15KW for my tractor.

I'd also recommend a survey of actual KW's needed to run your house, then the KW requirement for minimal use, that would give you a range of choice's.

 

[Soundguy]

I agree with pretty much everything that has been said. I offer another word of caution. You must keep the frequency of your generator close to 60 Hz (maybe +/- 2 Hz, not too sure) . If you are running motorized applicance (wells, refrigators, air conditioners, etc.) and the frequency is off you can damage the motors. I suggest getting some kind of frequency meter (Kill A Watt at $40) to monitor this. You can plug it in in the house or at the generator to set and watch the frequency. I have heard of rashes of equipments failures caused by off frequency settings. I don't know how good the speed regulator is on the tractor, so I don't know the severity of this issue, but it is not costly to monitor.

Good advice.. however.. if his tractor's governor is ok.. he should be fine.

I ran my 12.5kw northern genny onmy old 1952 8n. I checked it with a few thousand watts of load.. mostly incandescent, and a few 10+ amp motor starts, plus radio and tv. I had a freq counter and VOM plugged in.

Freq was actually more stable on the gen output than my utility line .. same with voltage.. and that northern uses only capacative regulation.. active would be even better.

Soundguy

 

[aesanders]

Jim,
I don't understand why they don't make the recommendation based on PTO HP. Then, as you point out, since my PTO HP is 20: 20*.746 = 14.9KW. The rated PTO HP is presumably at 540RPM.

Most PTO generators are recommended based on pto horsepower. Sounds true for this one as well 15k for 30 pto hp tractor.

 

[RoyKing]

Heat is what kills generators, melts the insulation of the windings. Generator life is shorten when they have to output anything over 60% for long periods of time. Mecc-Alte owners manual goes through disassembly for cleaning and bearing wear check, and when/how to change the bearing. When buying Diesel genset the motor and Alternator will have kva ratings for peak and continuous. Oversizing the Alternator is a good practice like a 36kva Alternator and 30kva engine. As diesel run at 75% will outlast a Alternator at 75%.

I just finished a tank breather/dryer model after the expensive cartridge blue silica gel type. Has two check valves at 1/4 pound opening pressure with 2lb of gel in a clear 4" pvc pipe. The tank inhales and exhales through the gel and when changes color you can remove the gel and reheat it. Cost the same for parts as buying one, but don't have buy new gel cartridges. In Florida with tank outside under the gen the day to night temp swings are pretty high which makes tanks less than full breath a lot.

 

[Spiros]

Mark, I run my 13kw Northern generator with an old Yanmar YM1900 19hp engine and no clue how much pto hp. It will run my entire house but i do turn off the 5kw hot water heater and no clothes dryer use. It runs the house AC unit just fine.

 

[Tractor Seabee]

To prevent accidental overloads that could ruin something I recommend you plan all your essential loads and move into a separate panel only the generator feeds and uses one of the manual change over devices from the main panel. Then you can be sure a motor starting will not drag things down or trip a breaker. I have never gotten involved in a PTO generator so have no clue how speed is set and maintained constant.

Is there a device that maintains voltage and frequency by auto adjusting the engine speed. Even my 17 KW generator does not adjust speed rapidly, there is a little lag time but it has an internal voltage/frequency regulator of the engine. All gen sets I have used or worked with have the auto setting capability. Some of our military generators were 500-1000 KW with synchronization of several in parallel.

Does using a 40-50 HP tractor to power a <30HP load eat a lot of excess fuel where a separate gen set to match the load would be more efficient? I have always wondered about that.

Ron

 

[CobyRupert]

Mark, I run my 13kw Northern generator with an old Yanmar YM1900 19hp engine and no clue how much pto hp. It will run my entire house but i do turn off the 5kw hot water heater and no clothes dryer use. It runs the house AC unit just fine.

To prevent accidental overloads that could ruin something I recommend you plan all your essential loads and move into a separate panel only the generator feeds and uses one of the manual change over devices from the main panel. Then you can be sure a motor starting will not drag things down or trip a breaker. I have never gotten involved in a PTO generator so have no clue how speed is set and maintained constant.

Is there a device that maintains voltage and frequency by auto adjusting the engine speed. Even my 17 KW generator does not adjust speed rapidly, there is a little lag time but it has an internal voltage/frequency regulator of the engine. All gen sets I have used or worked with have the auto setting capability. Some of our military generators were 500-1000 KW with synchronization of several in parallel.

Does using a 40-50 HP tractor to power a <30HP load eat a lot of excess fuel where a separate gen set to match the load would be more efficient? I have always wondered about that.

Ron

Hopefully Mark has his generator by now and isn't still shopping for one. Seeing how he posted this in 2006.

Ignoring inefficiencies/losses for the sake of simplicity:
Tractor Seabee, the power that the generator produces, and the engine driving it produces, is determined by the how much power the load is consuming at that instant, not the "rating" of the generator or the motor. The motor and generator are only producing the power required to match what the load is consuming (and to overcome inefficiencies/losses).


That is, if the load is 240V, 50 amps = 12 kilowatts = 16 horsepower. The generator will only provide 50 amps/12 kilowatts (even if its a "20kw rated generator"), and the engine is only outputting 16 horsepower (even if it's a 30 HP engine). You're not eating a lot of excess fuel.

The details are in how the efficiencies change (worsen or improve) as you move closer or away from the "rating" (sweet spot of the power/torque curve, etc...) of the motor (and generator).

 

[Sawyer Rob]

Using a small tractor to power a bigger pto generator is NO problem at all, I do it quite often with my Winco pto generator, that's rated 15,000 watt continuous...

As long as you follow the rule, 2 pto hp per 1,000 watts, any modern tractors governor, will keep the pto at the proper speed to cover the load.

For bigger loads, I put a bigger tractor in front of the generator, but I can pull about 6,000 watts out, with the pictured tractor.

(for high loads or continuous duty, you should keep the pto shaft straighter than I had it in the above pict.)

SR

 

[attract her]

Interesting topic.

Some SUPERB comments, #3 & #6 included.
To #11 I'll add:

W/ PTO driven generator I gather there's not feedback from frequency (Hz) to throttle. Thus to keep the A/C frequency at 60 Hz may require a tractor throttle adjustment each time the load changes.

Thus I've concluded:
The more sensible move would be a self-regulating generator.

AND !!

While the topic starter may have an ample supply of diesel on hand, I believe there are commercial generators with diesel engines.
But for those that don't have a diesel tank in the ground, Propane (LP) may be the way to go, as it is a relatively safe fuel to store in bulk (doesn't contaminate groundwater).

And if you're going to try that, Generac or some similar unit may make sense, as they're designed to cut in automatically when the commercial power drops.

Do as you wish, of course.
But if you go PTO, bear in mind your emergency power "solution" may turn into a hobby, where you have to hop out of bed and adjust the tractor throttle every time the refrigerator or deep freezer cycles on. I prefer to put the electricity in charge of that.

PS
Neighbors have mentioned to me the manifold benefits of, I think he called them "inverter generators". It seems they're of benefit if a computer, television, or other sensitive electronic unit will be in circuit.

And finally, among my trailer camping friends, it seems to be a consensus that Honda makes the best generators for that purpose.

Good luck. Enjoy our Independence Day !

 

[LouNY]

Of the half dozen different tractors that I have used on my pto generator none have required throttle adjustment for minor load changes.
Mine is a 25 KW pto generator, I have run it with tractors at the 540 rpm pto speed,
I have also used it on 1000 rpm ptos at half rpm, some tractors could handle it with a stable frequency, some not so good.
One of my tractors has the economy mode which allows the engine to be at about 2/3's pto speedd and still be at 540 on the pto,
this is the one that I prefer to use, my second choice is a large tractor using the 1000 rpm pto at around half speed.

 

[TerryR]

While the topic starter may have an ample supply of diesel on hand, I believe there are commercial generators with diesel engines.
But for those that don't have a diesel tank in the ground, Propane (LP) may be the way to go, as it is a relatively safe fuel to store in bulk (doesn't contaminate groundwater).

And if you're going to try that, Generac or some similar unit may make sense, as they're designed to cut in automatically when the commercial power drops

We live at the top of a steep quarter-mile gravel driveway in snow country, so our propane supplier installed a 1000-gal. tank so they wouldn't have to deliver in the winter. Choosing a tri-fuel generator running on propane was a no-brainer. No worries about fuel going bad, and no worries about trying to get more fuel during an outage caused by conditions that make getting more fuel difficult. Ours predates the ready availability of auto-start systems, but it does the job well enough.