PTO generator at max torque or max HP?

[JP_Austin]

Ive been looking into the same stuff lately. You need approximately 2HP at the PTO per killowatt of power produced. The OP said they had a Mitsubishi 180D and that has about 15.5 hp at the PTO. That means the most power you can expect to get is about 7.5KW of power. Not a lot and certainly not even close to the 15KW they desire. As far as matching the torque curve to the ratio... Will help but only slightly. One setting you will have more parasitic loss and another setting will be less parasitic loss. Still I'd be surprised if you produced over 7.5KW at the end of the day.

In my case I have 60HP at the PTO. I sized the genset for 31KW. I could purchase a 50KW version but I won't have the tractor to spin it to that level.

 

[Erik M]

The danger with these PTO gensets is that it is so easy for them to go off spec and then you get over voltage or brownout which may not be too much of a problem with incandescent lights and electric baseboard heaters but it can certainly damage motors and any electronics. Most fridges, stoves, washers, etc all have electronics in them now so be very careful to keep your genset within spec or it will get expensive quickly.

 

[Sawyer Rob]

The danger with these PTO gensets is that it is so easy for them to go off spec and then you get over voltage or brownout which may not be too much of a problem with incandescent lights and electric baseboard heaters but it can certainly damage motors and any electronics. Most fridges, stoves, washers, etc all have electronics in them now so be very careful to keep your genset within spec or it will get expensive quickly.

I haven't had that experience at all, you just need to put enough tractor on the generator to cover the load in the first place.

The tractors governor takes care of the rest.

SR

 

[CobyRupert]

…. 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 think that is true unless the torque curve drops off significantly between 1600rpm and 2200rpm. I think most diesels have relatively flat torque curves, but (what if) if torque dropped off more than 73% (the inverse of the 1.375 gear multiplier), would it still have more torque at that 2200 rpm?
2200 rpm would have more horsepower yes, but in this scenario I don’t think it would have more torque?

 

[ROUSTABOUT]

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.

We not looking at it wrong. It's like climbing a mountain in overdrive. Bog down or overheat is your two options.

 

[Hall00]

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

For some, sure.

A standalone also costs significantly more than a 3pt attachment. You are buying the driver and the attachment. If the fuel source isn't there already, the cost of adding that alone will probably surpass the price of a pto genset.

And then there is added maintenance of a standalone. On my 15kw pto gen, my only maintenance is 75w-90 oil. Have that on the shelf for trucks already. Regular tractor maintenance is already done so the "engine" is taken care of.

 

[workinonit]

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.

True. Most if not all PTO gens have some sort of volt or freq meter to set PTO speed by. I simply verified my meter was in cal by checking it with a VOM. A calibrated VOM.

 

[workinonit]

The danger with these PTO gensets is that it is so easy for them to go off spec and then you get over voltage or brownout which may not be too much of a problem with incandescent lights and electric baseboard heaters but it can certainly damage motors and any electronics. Most fridges, stoves, washers, etc all have electronics in them now so be very careful to keep your genset within spec or it will get expensive quickly.

I've run my gen on both of my JD tractors and never had an issue with drift or hunting. Of course it's totally dependent on how stable your tractor's governor is.

 

[NCL4701]

I haven't had that experience at all, you just need to put enough tractor on the generator to cover the load in the first place.

The tractors governor takes care of the rest.

SR

Agree you need enough tractor. It concerns me a bit when people put an oversized generator on an undersized tractor with a plan to use only part of the generator’s rated capacity.

That’s one of the things that concerned me when I bought a PTO generator. I am NOT an electrical engineer or electrical genius.

My thought was the capacity of the setup would be limited by the generator capacity or tractor HP but, as a practical matter, wouldn’t both reach max capacity simultaneously. In a case where someone (such as me) messed up and exceeded the capacity of the setup, if the tractor ran out of HP first, it wouldn’t be able to maintain RPM causing voltage and frequency to fall out of acceptable range. Alternatively if the generator ran out of capacity first, it would trip the breaker on the generator but frequency and voltage wouldn’t go out of range so no potentially damaging brown out. Closest I got to an expert opinion was the electrician that installed the transfer switching on the house and he agreed, so maybe that’s not totally off base.

Having about 38 PTO HP to work with that put max kW at about 19kW. I didn’t find any 19kW PTO generators so I went with a 16kW, which has worked well. So far we haven’t overloaded it so we haven’t tested what actually happens in an overload situation and have no plans to do so.

That’s the only thing that concerns me a bit when people set up a combo with the tractor HP as the limiting factor. If the max output of the combo is accurately calculated or tested, and the load is kept within that limit, should work. If the max load (which is something less than the generator rated capacity) is exceeded causing the engine to bog down, seems like some bad things could start happening with voltage and frequency.

 

[LouNY]

I wouldn't worry about it much.
Use a Kill A Watt meter in the house to keep an eye on voltage and Hz.
Monitor your loading, voltage dropping or frequency dropping you will be aware of
when your lights start strobing or being slightly dimer then usual.
You can easily be 10% off with frequency or voltage with no adverse effects,
You wouldn't want to make a habit of it but most equipment is vary tolerant.
If it wasn't many people with long runs from transformers or at the ends of
long primary runs which see considerable fluctuation would have major problems.