PTO generator at max torque or max HP?

   / PTO generator at max torque or max HP? #1  
N

nepa

Silver Member
Joined
May 31, 2013
Messages
117
Location
Forest City, PA
Tractor
Mitsubishi 180D, Jinma 284 sold, Kubota BX2660, IH Cub, Case 580CK, Minneapolis Moline 4 Star (sold), TYM 574, Furukawa FX-040
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?
 
   / PTO generator at max torque or max HP? #2  
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.
 
   / PTO generator at max torque or max HP? #3  
sea2summit said:
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.
Click to expand...
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.
 
   / PTO generator at max torque or max HP? #4  
Wouldn't the generator have a specified input RPM to begin with?
 
   / PTO generator at max torque or max HP? #5  
tomplum said:
Wouldn't the generator have a specified input RPM to begin with?
Click to expand...
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.
 
   / PTO generator at max torque or max HP? #6  
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.
 
Last edited:
   / PTO generator at max torque or max HP? #7  
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.
 
   / PTO generator at max torque or max HP? #8  
sea2summit said:
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.
Click to expand...
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.
 
   / PTO generator at max torque or max HP? #9  
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. :ROFLMAO:

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.
 
Last edited:
You must log in or register to reply here.
 

Footer Links 1

Top