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.