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.

 

[Sawyer Rob]

I have a 15kw pto generator and I've run it with a 12.5 pto hp tractor quite a bit. I just don't pull more than 6kw out of the generator when on THAT tractor.

No problems at all doing so, it works perfectly...

SR

 

[Chilliwack Murray]

Just a note on a previous post - you can be 10% undervoltage and most things won't care but motors will run hot and likely trip their overcurrent protection - if there overcurrent doesn't trip however you may we damage the motor. Up to 10% overvoltage isn't good but it's not much of a concern.

10% under or over frequency however is damaging to all sorts of modern electronics. Steady state frequency should be maintained between about 59 and 62Hz to prevent damage. Many electronic power supplies and UPSs won't function outside of 59-61Hz. Momentary excursions when load changes are not a huge concern but you'll find some devices may restart and UPS's will be switching on and off battery supply and eventually go dead.

One other item when using a PTO generator on a smaller tractor... Smaller and especially older tractors are not designed for unattended operation and have no automatic engine protection shutdown. If you lose oil pressure or overheat and you aren't there to stop it you lose an engine. It's cheap and simple to add a low oil pressure switch and a high engine temperature switch to shut off the fuel solenoid like a standby engine has There are also premade protection units and switchgauges like Murphy gauges that have built in adjustable contacts for this.

 

[tbarrick]

Choosing not to go full nerd tends to cause someone to be full mechanic.....

 

[bavrit]

V is spot on with the HP rating. I run PTO generator all the time. I found when a big load turns on, like a well, or AC unit, I need the higher RPM. Otherwise the voltage will have a sag because the tractor will drop too many RPM.

 

[norshud]

I have always understood you must run at the rated input RPMs of the generator and it takes 2 PTO(not engine) HP per 1 KW of electricity.

 

[bdhsfz6]

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?

I ran an 18KW PTO generator behind my old Kubota L3430. The tractor put out 27 PTO HP at 540 RPM. It worked fairly well with loads less than 12KW. At higher loads, the engine would slow, dropping both the voltage and frequency. As others have mentioned, this caused problems with motors & electronics. The combination was workable but I had to constantly monitor the engine RPM when large loads like pumps, oven, stove, A/C etc. were on line.

IMO, your setup will work using 540 RPM as long as you monitor & regulate large loads. I wouldn't push it past 10KW though.

BTW, I'm 20 miles east of you in Honesdale. I'm frequently up your way riding my bike on the rail trail. What is all the excavation going on just east of Forest City??

 

[nepa]

Yes Woodstock Walt, it is an old Winco that was never used, just sitting in a barn, no pto shaft attached, that I bought for $500 plus about $200 for a new pto shaft. I tend not to put much stock in what manufacturers call horsepower. This 180D outperforms my Kubota BX2660 26 horsepower diesel tractor in every way. Plus it is close to 25 years older. I had a 28 hp Jinma that had twice the power of either of them. I tend more to consider engine displacement. I have a stand-alone that is rated 7000 watts running and 8750 surge with a 420cc single cylinder gas engine. The 180D has 3 cylinders and over twice the displacement in a very torque rich diesel. Time will tell when I actually need to put it to use - if ever, and with luck will not need to. Look next week when I attach pics.
For other people who I greatly appreciate you replies, there is no meter for cps, just voltage. I hope not to go over 10k watts anyway.
Hi to bdhsfz6. I have not seen the excavation and do not know anything about it. My parents lived in Pleasant Mount and are burried there.

 

[workinonit]

A voltmeter is likely all you would have. That's what mine has. Freq and voltage are directly proportional if a gen is not syncronized anyway.

 

[Mudfarmer]

My PTO generator can produce more electrical power than my tractor is capable of producing mechanically. That simply means my tractor is the limiter, not the generator. I still have plenty of power to spare if I am a little careful. So quite rationally I do things like turn off the heat pump and rely on the gas furnace and wood stove. Turn off the freezer and refrigerator at startup then start them separately to avoid combined surge loads. I check the frequency meter on the generator and adjust the RPM of the tractor after each refrigeration unit is restarted. Since all my lights are LED at this point their power consumption is negligible and I cook with natural gas. Don't use the big screen TV on generator (too expensive). Use a laptop instead which is already set up to take variable voltages (look at its brick). And I turn the tractor off at night (seldom gets that cold in Western Washington). What I don't have to do is fiddle with a small engine all the time to make sure it is still running when I need it. Nor do I have to keep gallons of expensive no-alcohol gas sitting around getting stale. I always have diesel. My generator sits in the back of the shop with stuff piled up on it. I pull it out every two or three years when I need it, check the oil (it's always fine), plug it in and power up. Way easier than pulling the rope on my Powermate.

 

[Ken Kegris]

I have a CK25 with rated HP is 22 at 540 But with the pump turned up it easy will handle a 15KW gen set. The CK25 has a gov drop of about 200 and I dont run out of power but I have to bump the throttle up slightly to keep the freq correct. I have tried to run the CK25 on the high speed power take off and it dont seem to have when I want full power, If you are using a small amount of power its a way to save fuel The rpms is about half. I also have a CK2510 which handles the genny better than the CK25 and has a gov drop of about 50 RPM and you are not bumping up the throttle and I have not messed with the pump and it carries the 15000 watt load just fine. 746 divided into 15000 is 20.10 so that is power needed for 15000 but there not much allowance for surge I have some but you have too manage loads.