PTO generator at max torque or max HP?

[nepa]

I just bought a 15K watt generator which I will be putting on a 12K watt circuit. I will be attaching it to a Mitsubishi 180D three cylinder diesel tractor. The tractor has a three speed PTO. Running at the standard 540 setting, the engine will run at 2200rpm which is close to the max horsepower speed. If I run the PTO at the mid setting, then the engine will run at 1600rpm which is close to the max torque speed for this engine according to the owners manual. Which option should I choose for stable sustained operation?

 

[sea2summit]

If I'm looking at the correct tractor you have 15.5 PTO HP so 12KW isn't possible in any configuration. It might support a 12KW surge load but I wouldn't want to plan on it or do that to the engine very often.

 

[RickB]

If I'm looking at the correct tractor you have 15.5 PTO HP so 12KW isn't possible in any configuration. It might support a 12KW surge load but I wouldn't want to plan on it or do that to the engine very often.

I think 12K is a stretch, even a 12K intermittent load. Without any doubt I'd run that combination at the standard 540 RPM speed.

 

[tomplum]

Wouldn't the generator have a specified input RPM to begin with?

 

[sea2summit]

Wouldn't the generator have a specified input RPM to begin with?

He's got ranges on the PTO, so each range should put out 540 but it will have significantly less torque/HP output but higher fuel efficiency. Same as the Kubota 540/540E.

 

[CobyRupert]

I believe you’re thinking of this wrong.
To get 540 PTO rpm out of 1600 engine rpm means the PTO is running in a “higher gear”, which you can think of as a torque multiplier (or divider). So your not comparing apples to apples.

2200 rpm to 1600rpm is 1.375. I think this is a “gear ratio” to keep in mind when comparing torque curve to horsepower curve.

This means at any given HP load demanded by the generator loads, the torque on the 1600 rpm engine is already operating 1.375 times higher than the 2200 rpms engine.
It depends on how the motor’s torque and hp relate to each other.
I think with “some” loads switching on, rpms might drop more with the 2200 rpm motor before the motor’s torque rises to recover; in other cases the torque demands on the 1600 rpm motor will have less rpm effect because your already operating near the sweet spot.

Ultimately, it comes down to the characteristics of the loads being switched on (inrush and their torque curves) that determine stability and voltage dip characteristics as they correspond to the motor hp and torque curves (and rotational momentum of the system, governor response time, etc..).
And how you define stability.

In most cases, I’d venture it’s most likely to be more stable at 2200 rpm.

 

[workinonit]

Don't forget that generator needs to run at 60 hertz. It should have a volt meter on it with a safe voltage band so that you can adjust the throttle to get the voltage and frequency correct. If it doesn't have a volt meter then you need to hang a VOM on it and verify the rpm it takes to maintain voltage and frequency.

 

[Cahaba Valley Farm]

He's got ranges on the PTO, so each range should put out 540 but it will have significantly less torque/HP output but higher fuel efficiency. Same as the Kubota 540/540E.

Well said. I just realized that you don't see stuff like this until you get into higher horsepower tractors. However with that being said, I do believe JD offers a 540E PTO on their 3R series tractors.

 

[Vigo327]

This is inferred from spotty knowledge to begin with but.. I think multi speed ptos were on lots of old Japanese small tractors because A: Japanese implements had a different 'norm' for rpm, and B: lots of tractors that size had mid pto's for mower decks, and the higher speed pto range/ranges usually coincided with the speed of that mid pto. I think ive looked at a few tractors (maybe even my own and im forgetting since i ignore the mid ptos) where if you wanted the slow PTO gear, only rear pto engaged, but if you wanted the mid/high pto gear you also got the mid pto engaged.

The original question would be a valid one on a tractor that made comfortably more power than the generator rating, but in THIS case the answer is: you would HAVE to run it in the PTO range that would let you get the engine closest to its peak power rpm.

Ie, if you have a small diesel rated at 15hp, it's probably rated to do 15hp at 2800+ rpm. If you put it in the pto high range and the only way to get down to 540 pto rpm (to maintain 60hz output on the generator) was to drop the engine to 1500rpm, you'd probably only be making 8hp at that rpm. It would severely handicap the already mismatched setup.

In a perfect world where the tractor had plenty of power 'overhead' for the generator load, my answer would be to run it at the lowest engine rpm where it will comfortably absorb the 'starting loads' on your powered circuit, because there's a good chance you'll be close enough to this tractor to have to listen to it and if you're going to have to listen to it for hours, optimize for lowest noise.

And for anyone wanting to do their own math on the hp vs kw, 1hp = 746watts, 1kw =1000watts, 1000watts = 1.34hp. So 15hp would be ~11kw.

 

[sea2summit]

And for anyone wanting to do their own math on the hp vs kw, 1hp = 746watts, 1kw =1000watts, 1000watts = 1.34hp. So 15hp would be ~11kw.

Never go full nerd

 

[RickB]

And for anyone wanting to do their own math on the hp vs kw, 1hp = 746watts, 1kw =1000watts, 1000watts = 1.34hp. So 15hp would be ~11kw.

And that at 100% mechanical efficiency which is unattainable. There is always some level of parasitic power loss.

 

[nepa]

Thanks all for the good information. The generator has an analog voltmeter with a green zone for proper operating speed. In addition, I will have a plug-in digital voltmeter to set the engine speed to get 120v output. I have not yet put the generator into practical operation but did try it out to check voltages. The higher the rpm the higher the voltage. I knew but did not apply the fact that gearing means torque multiplying. Even though the engine might have the highest torque at 1600rpm, you can multiply that torque upward with gearing. Thus the 2200rpm at the engine geared down to the 540 output speed at the pto will give me the highest torque. I will also have convenient plug-in voltmeters in the house to monitor the generator output.

 

[WranglerX]

And that folks is why a stand alone generation system designed to spec is much more simpler to operate....

 

[Woodstock Walt]

I just bought a 15K watt generator which I will be putting on a 12K watt circuit. I will be attaching it to a Mitsubishi 180D three cylinder diesel tractor. The tractor has a three speed PTO. Running at the standard 540 setting, the engine will run at 2200rpm which is close to the max horsepower speed. If I run the PTO at the mid setting, then the engine will run at 1600rpm which is close to the max torque speed for this engine according to the owners manual. Which option should I choose for stable sustained operation?

Nepa, just out of curiosity, what did you end up buying for generator? Sounds a lot like my 15k Winco, which the manual states requires 29 PTO horsepower to run. Do you know how much output your Mitsubishi has in it's lowest (highest engine rpm) PTO gear?

 

[Woodstock Walt]

And that folks is why a stand alone generation system designed to spec is much more simpler to operate....

DL, while I agree with you that stand-alone systems are much simpler to operate, there are a few advantages of PTO generators that help offset the disadvantage of their complexity.

Just sharing from my own experience, but we used to own a Generac 11K propane stand-alone unit to power our property during outages. A couple seasons ago we sold it and replaced it with a 15K Winco PTO and it's been great for us. While we have some extenuating circumstances due to our relatively remote location, the main reason for our switch to a PTO generator was fuel type and long-term reliability.

After calculating the fuel consumption rate of the 11K propane Generac at ~3 gal./hr., I determined that our 500 lb. tank topped off with 400 gallons of propane could run the Generac for 5.5 days before exhausting the propane supply. While this seems like plenty of time for most people to have their line power restored, we often go without power for 2-3 days during an outage due to our remote location and low priority. During the infamous East Coast ice storm of 1998, we were without power for 16 days. Furthermore, our location prevents fuel trucks from being able to reach our house in anything but the best conditions.

While the fuel consumption rate remains comparable at ~1.5 gal./hr. with our Mahindra 4540 turning the Winco, refueling is a breeze with my 100 gal. pickup-mounted skid tank. Power is typically restored to the fuel stations along the main roads fairly quickly, so I theoretically never have to worry about running out of fuel.

I was also worried about the long-term reliability of the stand-alone unit. A small propane engine that sometimes has to start in below-zero temps and goes straight to operating full-throttle with zero warm-up period doesn't seem like a recipe for longevity to me. Admittedly, restoring power to our home in the middle of the night with the Winco is a much more complicated process. However, having the ability to warm up the tractor and generator before putting them into operation gives me the confidence I need to trust this setup long-term.

Without a doubt it all comes down to your specific needs and priorities. I can imagine that what works for me very likely seems like an over-complicated process to others. Just thought I'd share.

 

[jaydee325]

Thanks all for the good information. The generator has an analog voltmeter with a green zone for proper operating speed. In addition, I will have a plug-in digital voltmeter to set the engine speed to get 120v output. I have not yet put the generator into practical operation but did try it out to check voltages. The higher the rpm the higher the voltage. I knew but did not apply the fact that gearing means torque multiplying. Even though the engine might have the highest torque at 1600rpm, you can multiply that torque upward with gearing. Thus the 2200rpm at the engine geared down to the 540 output speed at the pto will give me the highest torque. I will also have convenient plug-in voltmeters in the house to monitor the generator output.

Does this generator not have a frequency meter? That will be more accurate than voltage in determining required prime mover speed.

Here is a link to some educational material about generators.

 

[dirttoys]

Thanks all for the good information. The generator has an analog voltmeter with a green zone for proper operating speed. In addition, I will have a plug-in digital voltmeter to set the engine speed to get 120v output. I have not yet put the generator into practical operation but did try it out to check voltages. The higher the rpm the higher the voltage. I knew but did not apply the fact that gearing means torque multiplying. Even though the engine might have the highest torque at 1600rpm, you can multiply that torque upward with gearing. Thus the 2200rpm at the engine geared down to the 540 output speed at the pto will give me the highest torque. I will also have convenient plug-in voltmeters in the house to monitor the generator output.

Be careful, the rpm dictates frequency. You need 60-62 hrz, not that volts don't matter, but, RPM ties directly to frequency.

Best,

ed

 

[RickB]

Does this generator not have a frequency meter? That will be more accurate than voltage in determining required prime mover speed.

Here is a link to some educational material about generators.

Frequency and voltage are linear when discussing conventional PTO generators. An inexpensive Kit-O-Watt will verify that. They are a very useful tool.

KILL A WATT Electric Monitor

Amazing deals on this Kill-A-Watt™ Electric Monitor at Harbor Freight. Quality tools & low prices.

www.harborfreight.com

 

[LouNY]

To the OP's first question it will all depend on the load on the generator.
If the household is up and active requiring hot water and cooking as well as lighting and heat the power demand will be much higher then,
in the middle of the night everyone has gone to bed and it's heat and maybe a water pump and a few night lights.
With light power demands you may be able to gear up and slow down the engine, with heavier loading most likely you will need the higher engine speed.
Most pto generators will have a voltage regulator adjustment, it can be brought up to speed to get 60 Hz and then the voltage out trimmed for the desired 120V.
As far as the stability of the pto generator that is total dependent on the responsiveness of your tractors governor and the engine characteristics.

 

[Fordy20]

Don't forget that generator needs to run at 60 hertz. It should have a volt meter on it with a safe voltage band so that you can adjust the throttle to get the voltage and frequency correct. If it doesn't have a volt meter then you need to hang a VOM on it and verify the rpm it takes to maintain voltage and frequency.

If the PTO speed isn't the speed producing 60 hertz, your appliances will not only act funny but you may ruin them. My PTO generator has a gauge showing when the PTO is turning the speed to make 60 hertz.