PTO GENERATORS

[BuilderML]

I have a question about pto run generators. I understand they are made to run at the 540 that the rear pto produces, which on my tractor would be about 2500 rpm.
Lets use a 15kw generator for example. Running the tractor at 2500 rpm the pto at 540 produces 15kw so if i lower the rpm on the tractor i guess it would lower the speed of the pto accordingly correct producing less KW power? I have not contacted a generator supplier yet i figured i would ask you guys. Here is my thinking i don't need 15kw so is it safe to run the tractor at a lower rpm without damaging the generator or must they run at the 540? Thanks for the info yet again

 

[kennyd]

It MUST run at 540 RPM to achieve the 60 hertz frequency. Ruining slower can damage whatever is connected to it.

 

[BuilderML]

thank you kenny

 

[varmint]

Or, if you have tractor power to spare, usually the PTO has a high speed gearing, too, and by watching the hertz on a meter, you could actually run the tractor engine at a lower speed and still get 540 or whatever you needed at the PTO.

 

[Dave M7040]

If your tractor has a 1,000 rpm pto you could buy a pto generator with the gearing for that speed. I think however you would find that the engine would be running at about the same rpm as you have indicated you need for the 540 pto as the tractor manufacturer needs to have a lot of power available to handle attachments. If you had a 70 hp tractor and only needed 15 kw it would be possible to make a chain drive to get the required rpm for the pto generator while running the tractor engine at lower rpm.

Dave M70400

 

[zzvyb6]

My Winpower will drop out below a fixed rpm (probably a centrifugal contactor). My guess its about 500 pto rpm. Running low frequency would work for a certain type of load (electric heat, laptop or phone DC power supply, for example). AC motors would overheat.

 

[wrooster]

Running the tractor at 2500 rpm the pto at 540 produces 15kw so if i lower the rpm on the tractor i guess it would lower the speed of the pto accordingly correct producing less KW power?

You have a couple of misunderstandings above.

The alternator head on the PTO generator will produce the correct voltage and frequency ONLY when it is turned at it's specified RPM. For a 2 pole alternator head, that required rotational rate is 3600 RPM, and for a 4 pole alternator head, that rate is 1800 RPM.

As you have noticed, neither of these values is the same as your tractor's nominal PTO RPM, 540. Ergo, between your tractor PTO and the alternator head will be a small gearbox -- to "upconvert" the 540 RPM output from your tractor PTO to the 1800 or 3600 RPM input needed at the alternator head.

Let's assume for the moment that you have a 2 pole split-phase (120Vac/240Vac) 60Hz alternator head which requires 3600 RPM; and the gearbox mounted on the face of the alternator head has a 1:6.66 gearing -- each input rotation from the tractor PTO results in 6.66 output rotations to the alternator head.

A reduction in PTO speed from 540 RPM to 486 RPM (-10%) results in a reduction of alternator RPM from 3600 to 3240 RPM (-10%). This will reduce the voltage by 10% (from 120Vac to 108Vac) and the frequency by 10% (from 60Hz to 54Hz) . Neither of these is a desirable situation from the viewpoint of the loads plugged in (such as a refrigerator). An internal voltage regulator on the alternator head will attempt, by increasing the field voltage, to counter the voltage sag from the lower RPM, but a) this is usually bounded by about 5% "improvement", and b) this will not and can not fix the frequency offset.

As you can see, you MUST turn a direct-coupled alternator head at it's specified design RPM -- there is no other option. (Inverter generators are another story). So, a PTO generator requiring 540 RPM input must be spun at 540 RPM (+/- a little bit).

All that said, all we have discussed above is regarding voltage and frequency -- and nothing about power. The output power (in W or KW) of the PTO generator is a function of the input power (PTO HP) and the applied load. As you have seen above, the voltage and frequency of the PTO generator must be maintained for compatibility with the load (e.g., a well pump) but the actual power delivered is according to how much load is attached to the generator. Hence, a 15KW PTO-driven generator makes no power if nothing is connected to it -- although it is making 120Vac/240Vac at 60Hz, there is no net current flow and therefore no net power output.

As others have pointed out above, it is sometimes practical to engage the PTO using the 1000 RPM setting and (nearly) halve the engine RPM to again achieve 540 PTO RPM. However, note that the designer of the tractor set up the internal PTO gearing to achieve 540 RPM just *past* the torque peak of the engine (that is, on the high side of the curve). This means that as the load increases and the engine speed sags slightly against the governor, torque will go UP marginally -- helping the engine recover from the applied load (such as starting a large motor). If the engine is running at half speed with the PTO selector set up for 1000 RPM, torque will go DOWN with applied load and engine speed control will be worse. With enough horsepower available at the engine output shaft, this may not be a problem. You will have to experimentally find out what "enough" means, however.

Wrooster

 

[Copperhead]

Well done, Wrooster.

 

[Rustyiron]

It's nice to read a reply that you don't have to wonder if they (really) know what they are talking about.
I have no question about wrooster's electrical knowledge at all!:thumbsup:

 

[xring100]

As others have said you need to maintain 540 rpm to produce 60 hz.
But your other question had to do with power output. The engine will only produce enough power/ consume fuel to match the electrical load of the generator (and all other losses). So you will only use the the amount of fuel that corresponds to the hp produced not possible.
For example if to get 540 pto requires 2200 rpm and the corresponding brake specific fuel consumption is 0.36 lb s/(hp hr) you need about 10 hp to generate some where between 5 and 6kw depending on generator efficiency

10 hp over one hour will use 3.6 lbs of diesel or about 0.5 gallons / per hour.

For 15kw you need about 30 HP which would use about 10.8 lbs or 1.5 gallons per hour

Diesel weighs about 7.15 lbs per gallon

Dave

 

[Soundguy]

NO

the rpm is tied to the CPS....

you need the rated input rpm to produce the correct frequency. from there as a rough hash.. figure ABOUT 2pto hp per KW generated. paper math says it's closer to near half that.. but real world proof shows otherwise once you get gears and thick oil and spinning shafts and smoke stacks involved.

fuel usage will be based on mechanical load. mechanical load will be based on electrical load.

2 identical tractors and those 15kw gennies running at rated speed, one with a 1kw load, and the other with a 12kw load.. the 12kw load will use more fuel as it is using more hp.

no magic involved..

I have a question about pto run generators. I understand they are made to run at the 540 that the rear pto produces, which on my tractor would be about 2500 rpm.
Lets use a 15kw generator for example. Running the tractor at 2500 rpm the pto at 540 produces 15kw so if i lower the rpm on the tractor i guess it would lower the speed of the pto accordingly correct producing less KW power? I have not contacted a generator supplier yet i figured i would ask you guys. Here is my thinking i don't need 15kw so is it safe to run the tractor at a lower rpm without damaging the generator or must they run at the 540? Thanks for the info yet again

 

[txhillcountry]

I've seen a couple of options to the "high-tractor-rpm" issue (meaning that you're burning more fuel) for running PTO attachments for long periods of time, like generators, pumps, even mowing very large areas.

One is that there are some PTO Generator vendors out there like Power Solutions : PTO Power who will change the gearing on the generator side to allow you to run slower RPMs on the tractor while maintaining the need RPM at the Gen head. Kind of "poor-man's" e-PTO. Somewhat more applicable to the medium to larger HP tractors (roughly speaking, where you have power to spare).

Or, your tractor may have e-PTO, which is an "economy" setting that still gets you 540 rpms out of the PTO while running lower tractor RPMs. My Zetor has this. There's a lever with two settings: Normal and E. On the Normal setting, you set the tractor at 2500 RPMs and you know your getting 540 out the PTO. On the E setting, you put the tractor at 1800 RPMs, and you'll still get 540 out of the PTO, thus saving fuel. Very handy for PTO drive implements that don't need that full "horsepower". My tractor is 65HP.

 

[Soundguy]

wow...

i gues you aren't thinking about the fact that in most cases.. fuel useage is based upon load.. not just rpm.

if your machine is making 10kw of electricity.. it will make little difference if it is doing it at higher or lower rpm.

you have a fuel efficiency and conversion to electricity cal to do.

fuel usage = load for the most part on a modern engine...

I've seen a couple of options to the "high-tractor-rpm" issue (meaning that you're burning more fuel) for running PTO attachments for long periods of time, like generators, pumps, even mowing very large areas.

One is that there are some PTO Generator vendors out there like Power Solutions : PTO Power who will change the gearing on the generator side to allow you to run slower RPMs on the tractor while maintaining the need RPM at the Gen head. Kind of "poor-man's" e-PTO. Somewhat more applicable to the medium to larger HP tractors (roughly speaking, where you have power to spare).

Or, your tractor may have e-PTO, which is an "economy" setting that still gets you 540 rpms out of the PTO while running lower tractor RPMs. My Zetor has this. There's a lever with two settings: Normal and E. On the Normal setting, you set the tractor at 2500 RPMs and you know your getting 540 out the PTO. On the E setting, you put the tractor at 1800 RPMs, and you'll still get 540 out of the PTO, thus saving fuel. Very handy for PTO drive implements that don't need that full "horsepower". My tractor is 65HP.

 

[5picker]

wow...

i gues you aren't thinking about the fact that in most cases.. fuel useage is based upon load.. not just rpm.

if your machine is making 10kw of electricity.. it will make little difference if it is doing it at higher or lower rpm.

you have a fuel efficiency and conversion to electricity cal to do.

fuel usage = load for the most part on a modern engine...

I don't completely agree...
I think what txhillcountry was referring to is if a LARGER HP tractor can sustain a PTO generator at 1800 RPM with an E-PTO (or some sort of gear reduction) to achieve the necessary 540 PTO-RPM's, then you would use less fuel than running that same tractor at 2500 RPM's to attain the same 540 PTO RPM. Again, that's based on the premise the tractor's HP can keep the generator output consistent without adding load.

It's the same as if you are able to run your vehicle down the highway and maintain 60 MPH in high gear at 1000 RPM's... why would you shift into the next lower gear and run it at 1800 RPM's if you didn't need to?

Of course we could get into the argument that a larger HP engine which is 'coasting' uses as much fuel as a smaller HP engine, running at higher RPM's under load, but that's another thread.

 

[Soundguy]

fuel usage is going to be based on load. hard to get around that.

don't matter much if you have a 50 hp tractor, and a 35 hp tractor.. and both are making 20hp worth of electrical load. the larger tractor might use just a hair mor efuel to get mass spun up.. after that.. it's load.. and load is what uses fuel.

as another poster pointed out.. many tractors are most efficient in their power band near normal pto operation speed.. derating to a lower rpm and using an epto may put them in a LESS fuel efficient mode. these things aren't linear.. half rpm is not half hp, and is not half fuel usage.

 

[5picker]

I don't recall anyone saying it was linear or claiming to cutting fuel usage in half. The statement was simply, running at a lower RPM could save fuel.

I've never done it with a PTO generator but I have with other applications.

Again, if you can sustain the generators load at 1500 engine RPM (using a gear reduction or e-PTO) you'll burn less fuel than you will sustaining the load at 2500 RPM.

 

[Soundguy]

fuel usage is based on load. if the load is the same.. your fuel usage is going to be just about the same...

for example.

I can mow my pasture with my 95hp tractor and 15' mower or my 70hp tractor and 10' mower.

takes less time with the 15 mower.. quite amazingly.. by about 30% less time.

also amazingly.. both machines seem to need about 5g to fill back up.

that tells me it takes 5g of fuel in usage to perform the work to cut that pasture.. speed irrelevant.

back before i had the 15' mower.. the largest i had was a 10' mower. at the time I didn't have the 70hp tractor.. but did have the 95 hp tractor. again.. quite amazingly.. it took about 5g of fuel for the 95 hp machine to mow the pasture with a 10' mower... which is what the 70hp tractor also uses.

again.. this tells me it's a load vs fuel usage issue.. other issues are much less of a factor.

less rpm might mean a bit less wear.. though.. my tractor is rated to run at pto rpm for extended periods.. as most are.. som i'm not concerned about using it.. that's what it was made for.

 

[Loadstar]

My Winpower will drop out below a fixed rpm (probably a centrifugal contactor). My guess its about 500 pto rpm. Running low frequency would work for a certain type of load (electric heat, laptop or phone DC power supply, for example). AC motors would overheat.

You can vary frequencies on an AC motor all day long. It is the volts and amps that are critical.

 

[5picker]

fuel usage is based on load. if the load is the same.. your fuel usage is going to be just about the same...

So you are saying if I drive my car down a 2 mile stretch of road at 60 MPH at 1500 RPM's in high gear and drive the same stretch of road in 2nd gear at 3500 RPM's, I'll use the same amount of gas?
Don't tell that to the EPA.

 

[Soundguy]

no.. i didn't say that.

i said equal loads will generally use about the same fuel. you may see some loss difference on a larger machine due to some friction and mass differences.. then again you may see differences the other way based on a machine running outside it's power band and being slightly less efficient.. etc.

we can assume in your 'car' example.. that at least one of the rpm ranges you specify are outside of it's power band ... among other things.. etc..