PTO generator

alchemy · Jul 8, 2012

I'm strongly considering building a pto generator setup for my belarus 250AS since that tractor (31HP) seems to get unbelievable hours per gallon, something like 5-10hrs per gallon (diesel) it seems.
I want to make my own from one of the 3 generators I have. I would like to be able to run the tractor at slow speed maybe less than 1000 rpm. I also have several gear reductions or "gear ups" maybe, whatever they are called. Don't know if I can use a reduction type backwards or not. I'm not sure what speed generators need to be or what the PTO speed is at lower RPM. It would be nice to get close the first time since it would take some effort to rig everything. Part of the reason I want to go to PTO is because I hate the speed and loudness of those portable generators. Also, they are expensive on fuel and are pretty unreliable. Any ideas out there?

RoyJackson · Jul 8, 2012

First, you need to determine what RPM those generators require to produce the rated output. Then you can determine how you're going to increase the PTO RPM to the required RPM for the genny.

Running your tractor at the low RPM you suggested may not be good for the engine. Extended running at low RPM isn't good for the main bearings and can result in cold stacking. That is, unless your tractor's engine is designed to run at that low RPM (which I doubt).

JoelT · Jul 8, 2012

What are those "gear ups" you're talking about. I have a 12K pto designed to run at 540rpm which is 2600rpm engine speed for my tractor. I'd like to gear down through something to around 1800/2000rpm. I don't think the motor needs to scream along at 2600rpm to hold rpms for the genny.

alchemy · Jul 8, 2012

Thanks for the tips so far guys!
Roy, what do you think a good rpm is. My RPM gauge doesnt work btw so I assume about 3 K is max and maybe 500 kills it, don't remember. If higher rpm is most efficient then maybe it would only be good for a huge KW or to run something else worthwhile too. 4K is way more than I need during power outages and I imagine the engine power necessary would be small.
Joel, that seems real high to me too. I'd like to be around 750 actually, just a "quiet idle" The "gear ups" would be speed increasers, the opposite of gear reduction, but I forget what they are called. Essentially what you are looking for to slow your engine speed and maintain rpm for the generator.

Ductape · Jul 8, 2012

If your gen heads are off of cheap gasoline generators...... they are usually designed to run @ 3600rpms. Unfortunately..... these are usually one bearing generator heads, which use the engine crankshaft to keep the generator shaft centered. It won't be easy nor cheap to make them work being driven off your PTO (unless you are a machinist). I'm with Roy..... you don't want your tractor lugging @ 1000 rpms. I can see why you might not want it running at 2500, but you'll want to be closer to 2500 than 1000. Changing the gearing is reasonably easy with belts / pullery, or chains / sprockets.

SPYDERLK · Jul 8, 2012

What PTO speeds do you have available? As in Roys reply; Is your gen 1800 or 3600 rpm? I run a 12KW gen requireing 540 to its gearbox from a 25HP 2 speed pto. On the 540pto gear setting the tractor and gen are very happy together all the way to max gen output. Gen power is stable even with large changes in electrical load.

On the 1000rpm PTO gear setting the tractor runs at 1300 rpm to drive the gen at correct speed. RPM varys a lot with load. It works well at continuous light to heavy moderate electrical loads, but the tractor throttle must set for the load and it will bog or overspeed when the load changes significantly. -- A load change of 20% is quite significant. Normally, at full pto rpm a tractor governor holds the engine, and thus pto, rpm pretty stable with varying load. Its much less able to do this at lower tractor rpm.
larry

Dana Warner · Jul 8, 2012

I put this together just before the October New England storm. The tractor was run in high pto speed to cut down on the tractor rpm. When I built it I measured the circumference of the pulleys to calculate the needed diameters of the pulleys to get the correct 540 rpm to the generator speed of 1800 rpm. It is a 7kw onan generator. We did not use anywhere need full output and checked the voltage often. Once set the rpm/voltage was very constant. We ran it 16 hours a day for 7 days.

alchemy · Jul 8, 2012

Lots of great info guys !! Learned a ton already. I guess the clincher for me may be not wanting the tractor engine screaming. That's what I hate about the portable generators already. I cringe just running them an hour the way they sound. Thanks to the info, I need to find some specs on what speed they need to run. I also need to know a good rpm for the tractor. Then I probably need to fix my rpm gauge to see what my rpm is. 1800 may not be bad, don't know. I think I only have a one speed PTO.
My generators are a 2500 homelite with a 5hp B&S, 4000 coleman with an 11HP B&S, a 7-12K, probably onan, natural gas automatic, wisconsin motor. I'd like to use the 4000 because I always have trouble with that motor anyway. I'm not opposed to looking for another used generator to use.

RoyJackson · Jul 8, 2012

alchemy said:
I also need to know a good rpm for the tractor. Then I probably need to fix my rpm gauge to see what my rpm is. 1800 may not be bad, don't know. I think I only have a one speed PTO.

Your tractor is designed to run at what ever RPM is used to operate the PTO. For example, my Deere 4400 PTO speed is about 2600 RPM.
Granted, this seems like the engine would be screaming, but if you ever use off seat operation (tractor would be a stationary power source...such as I do when I'm running a chipper), you get used to the RPM pretty quickly.

Dana Warner · Jul 8, 2012

As Ductape said sounds like your generators are designed to run connected directly to an engine. These will be difficult to adapt to a pto.

alchemy · Jul 8, 2012

Dana Warner said:
View attachment 272556I put this together just before the October New England storm. The tractor was run in high pto speed to cut down on the tractor rpm. When I built it I measured the circumference of the pulleys to calculate the needed diameters of the pulleys to get the correct 540 rpm to the generator speed of 1800 rpm. It is a 7kw onan generator. We did not use anywhere need full output and checked the voltage often. Once set the rpm/voltage was very constant. We ran it 16 hours a day for 7 days.

Great Job! If I could just snap my fingers and one would appear it would be alot like that. The only thing is I think I could get by with a much smaller generator.

daybreak1998 · Jul 8, 2012

Howdy,
The alternator portion of a generator come in a few different flavors.

Of course the main one here would be a single phase. There is of course 3 phase as well.

There are 2 pole alternators which need to spin at 3600rpm to make 60Hz 240volts current. That is why so many small engine which run in the 3600rpm range are direct connected to the alternator. (no gearing) Then you have a 4 pole alternator which spins at 1800rpm. (more expensive, considered heavy duty, commercial grade) Which is why so many generators which are diesel, have 4 pole alternators attached.

PTO generators = have gearing, smaller units known as residential spin internally at 3600rpm. So that 540rpm pto needs gearing to get it to 3600rpm. The larger pto generators have 4 pole alternators and spin at 1800rpm. They still have gearing to get that 540rpm pto to 1800.

The main thing to consider is this. Anything running at 3600rpm is going to make noise. Think about a car on the highway doing 60mph in second gear. Its redlined and sounds like it going to blow up. Imagine driving across country like that. Will that engine make it? That is why the slower spinning engine and alternator will last longer. They are also more expensive. The only quiet generator spinning at 3600rpm is probably the Multiquip Whisperwatt 7000. Small diesel running at 3600rpm and alternator head at 3600rpm.

A PTO operated implement can be run at different speeds. A PTO Generator needs to spin at the proper speed to make that 60Hz 240volts.
You can take a look at this you tube video which shows them putting load on a smaller PTO generator. It lugs down the engine pretty good.

Small tractor with 15KW pto generator which spins at 3600rpm. So you have the tractor noise and the generator noise.

alchemy · Jul 8, 2012

btw, may need corrected, but it looks like pto generator manufacturers are recomending about 1Hp per 600 watts. (20HP for a 12KW generator)

alchemy · Jul 8, 2012

Dana Warner said:
As Ductape said sounds like your generators are designed to run connected directly to an engine. These will be difficult to adapt to a pto.

yeh that was a great point and it was something I would have never thought of. I have some roller bearings and mounts for pulley shafts if I needed them. If it required some type of true tolerances not easily acheived with that then one could actually just use a case from an identical engine. Even though I probably have cases for that I may not go that far cause it would probably look stupid.
I hate giving up and I haven't yet but I'm starting to wonder. That bigger one I have probably doesn't have the single bearing.

PioneerMan · Jul 9, 2012

Another point I haven't seen made is your 31hp engine will not be making the 25ish hp required to 'fully' run a 12kw genny at under 1k rpm. I guess it depends on how light of a load you're looking at...

Soundguy · Jul 9, 2012

keep this in mind. running your tractor at a low rpm is running it OUTSIDE it's prime power band. that means you will have less hp.

also.. you will need to get the gear ratio correct to run the genny head to get correct output freq.

also.. fuel usage will DIRECTLY depend upon kilowatts of electricity being used ( load )

doesn't really matter what 'unloaded' fuel economy you are getting.. when loaded.. that will be the difference. nothing is free.. fuel has 'x' amount of energy.. and it will be less than perfectly converted to rotational, then electrical.. in a lossy process that will roughly consume 2pto hp per KW generated.

soundguy

alchemy said:
I'm strongly considering building a pto generator setup for my belarus 250AS since that tractor (31HP) seems to get unbelievable hours per gallon, something like 5-10hrs per gallon (diesel) it seems.
I want to make my own from one of the 3 generators I have. I would like to be able to run the tractor at slow speed maybe less than 1000 rpm. I also have several gear reductions or "gear ups" maybe, whatever they are called. Don't know if I can use a reduction type backwards or not. I'm not sure what speed generators need to be or what the PTO speed is at lower RPM. It would be nice to get close the first time since it would take some effort to rig everything. Part of the reason I want to go to PTO is because I hate the speed and loudness of those portable generators. Also, they are expensive on fuel and are pretty unreliable. Any ideas out there?

alchemy · Jul 14, 2012

someone tell me if this calculation is right. If you ran a 540 RPM PTO to run a 3600 rpm generator you would need about 7 to 1 and a 14" inch sprocket to a 2" sprocket. I say sprocket because it would probably need chain driven because 2 " would be too small for a pulley with a load like that. Seems like a cheap 3600 rpm generator might make the project ridiculous. If that calculation method was right then an 1800 rpm generator would be about 3.5 to 1 or 14" and 4" pulleys or 12" and 3". That doesn't seem right. If it is right then maybe 14 and 3 would be good so you could run the tractor slower? Is tractor rpm linear to PTO rpm? In other words if it's 540 at 2600 rpm would it be 270 at 1300?

alchemy · Jul 14, 2012

Looking back at the one Dana Warner built it does look close to that. Maybe 12" to 2" ? Can't see the lower pulleys. He said his generator was 1800 rpm.

Todd727 · Jul 14, 2012

You have to count teeth if using a chain. For example, a 60 tooth sprocket would match up with an 9, which is a little tight and would probably wear the chain prematurely. A 12 tooth would be about the smallest, which would perfectly yield an 80 tooth. You need to find a ratio that is equal to 6.666667 between the smaller and larger sprocket for exactly 540 to 3600.

Here is a calculator. It's made for motorcycles. If you set the standard to 12 front, 12 rear, 3600 rpm and set the speed to anything, because it's not useful to your problem, then you can try different size sprockets in the new section to reduce to 540 rpm. Remember that you will be putting the large sprocket on the front or driving shaft.

http://users.tpg.com.au/jono794/tools.htm