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Thread: 12-20kW PTO-driven 3 point generator for tractor

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    Rancher's Avatar
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    12-20kW PTO-driven 3 point generator for tractor

    A lot of times, I have need for big power far out in a pasture, so I have just started thinking about building a PTO-driven generator that I can mount on the Cat 1, 3-point hitch of my tractor. I'm thinking that 12,000 watts is the absolute minimum for purposes of welding pipe fence and other fabrication. Then, there is the running of big pumps and compressors from time to time. Finally, there's the need for a whole-house generator when weather knocks out the power. That takes me to about 20 kW. Something I can back straight up to the power pole and connect to our Generlink interface. I thought about building an AC/DC stick welder right into the works and this wouldn't be bad if I can do it for "not much money". But, I've got plenty of welders that I can take with me. So, it's not a deal-breaker. My tractor is 30 HP and 20kW is about 27 HP, so I may not have enough horse for the power that I want. I need to look into that, as well.

    Here I sit, thinking about it....

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    Let's start off with your tractor is the 30 HP the stated engine HP the draw-bar HP or the tested PTO HP? For now we will go with the possibility of PTO HP @540 RPM which is pretty much standard What you would first need to do would be to locate a ST style generator head THis is a stand alone generator that has a keyed output shaft and and a bearing plate on the output end it will also have a built on connection box with built in AVR (Automatic voltage regulator). Then you will need agear box to step up the 540 PTO RPM to the required generator RPM usually around 1800 for a 4 pole generator 3600 RPM for a 2 pole generator to give you your 60HZ.
    Next is your tractor is not diesel figure on a maximum of 35% of the PTO HP for the generator size at sustained loads this is to maintain voltage and HZ stability under changing loads if diesel then this can be more along the lines of 65%. This does not mean that you could not use a 25 to 40 KW generator head on your tractor you would only be able to use the amount of power that could be produced at the PTO. the over-sized unit would require more power to turn the rotating mass and larger gear box, the up side would be a more stable output for starting loads up to the max KW you r PTO was capable of.
    So sticking with a 12 to 15 possibly a 20KW generator head would be a better choice.
    Next another way to do this would be to locate an engine driven generator with a bad engine remove the engine and machine an end plate and bearing for the generator bolt that to the gear box make a 3 point frame and you would be good to go
    l since there are several factory made PTO driven generators on the market already set up the one thing you mentioned that may be your deciding factor and that was the comment for not much money, you might be better off finding one on one of the for sale lists or watch the auctions.
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    Thank you. There's a lot of good information in your post. My tractor is a diesel Kubota L2800 and the specs look like this:

    Engine (gross): 30.0 hp [22.4 kW]
    Engine (net): 28.6 hp [21.3 kW]
    PTO (claimed): 25 hp [18.6 kW] (gear late)
    24 hp [17.9 kW] (gear early)

    Mine is about a 2005 model (can't remember at the moment). So, the power available at the PTO is within my needs, but not at the upper end. I probably won't use it at the top end very often, since it's really not cost effective as a whole-house generator. But, it would suffice to keep the electric heat and refrigeration going in the winter for a day or two, if the power goes out. We've lost power for a week here, before. But, it's only happened once.

    Thanks for the information on generator heads. I have an electrical engineering background in my past, but as I constantly have to tell friends and neighbors, that doesn't make me an electrician by any stretch of the imagination. Power generation is NOT my forte'. It looks like I could have the head I need for $1500-2000 new on eBay. So, that's not in the cards for the moment. But, it does give me an idea of how high to go at some of our local farm auctions. We have one that I go to, that is pretty large, so it might turn up there. Not long ago, I scored about 600 dollars worth of equipment for the staggeringly low price of $5. Then, will come the gearbox. That might be a little more difficult. Thanks, again.

    ETA: Just saw a new one (15000 W) from Harbor Freight for $1300. Not really a fan of Harbor Freight for expensive things (and they aren't conservative on their specs... LOL). But, it is food for thought.
    Last edited by Rancher; 11-08-2017 at 11:35 AM. Reason: additional info

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    Rancher something else to think about Obviously for a gear box when stepping up the output speed from 540 to 1800 the best way to go would be an eplicylic (planetary gear) box but a good adhock way of doing this would be to locate a used car or truck rear end. you would want one to be at least in the Ford 9" A Chevy 12 bolt or a Dana 60 range of size and strength because you would be powering the ring gear to drive the pinion gear. You would cut the axle tubes off and cap one then lock the spider gears install a bearing and seal in the other tube and machine the axle to accept the inboard bearing and seal and to accept a tractor PTO shaft. connect the generator directly to the generator if you had a ring / pinion ratio of 3.33 or install a belt or chain drive between the generator and the rear end to achieve the proper ratio. Rear ends can usually be found on the cheap in salvage yards especially if you search for one that has the tubes damaged from an accident. I've used them successfully to make many things out of both as reduction and step up gear systems. Back in the 60's the most popular thing to make with them was self driven brush hogs just take one that had full floating axles mount 2 or a set of duels facing outward make a 6 ft mower deck connect a free running sprag on the prop shaft end facing down then mount a 30" disk with the blades these were great for tractors that did not have a PTO but without the sprag the mower would push the tractor unless you could get 1 tire off the ground
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    Since you mentioned having EE background you probably know that you could take a 20 HP 3 ph motor and turn it into a generator. You would want to create an exciter to initially energize it and construct an AVR but do-able
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    you can also use an auto small car/gearbox turned end for end so that the output (drive shaft) of the gearbox is connected to the PTO and the clutch end drives the generator. Pick the gear that gives the output rpm you need.

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    Quote Originally Posted by MiTasol View Post
    you can also use an auto small car/gearbox turned end for end so that the output (drive shaft) of the gearbox is connected to the PTO and the clutch end drives the generator. Pick the gear that gives the output rpm you need.
    Yes you could use a manual transmission gear box providing you could find one with a 3.33 to 1 ratio however the same reason applies for my suggesting using the heavier rear ends applies when running any gear system designed to run one way in the opposite rotation it was intended for you have to take into account for the input torque ratings which would be used as the output in this case which greatly reduces their rating hypoid gears of a diff are made to apply large amounts of torque in primarily one direction over long periods of time with intermittent use in reverse. the Helical cut gears of a transmission are designed for the same purpose. Also the thrust bearings are not designed for these extended reversed loads. To expect long service life from either a diff or a transmission in my opinion the original input torque rating should be de-rated by a factor of not less than 4 more if I were making it for myself.
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    What is the maximum draw you use at once? I am on 240V 1~ and average 12kWh a day. That is for heating water, cooking,fridge, T.V., radio,computer, lathe,mill, power tools, compressor and welders,(arc,tig &mig). On a very heavy use day this may get upto 14kWh a day. My biggest draw would be my compressor.
    For regulated power stand alone Diesel gensets are the best as they are tightly regulated between idle and full load with a very short fuel ramp. I have friends who live off grid who have Solar, wind and/or water power, the couple with big workshops get by with 8-10kVa gensets for when they need a lot of power. Modern machinery is pretty efficient.
    Having said that I do have one machine that has a Parker - Crompton 5HP 1~ motor that has a start inrush of just under 19kVa! One day I will set up a bank of capacitators to stop the garage lights dimming.

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    NortonDommi, my main need is for portable power in the field, powering large compressors, pumps, welding equipment and plasma cutters and the usual assortment of tools (grinders, chop saws, etc.). The whole-house part of it would be a side benefit, giving us one more option in an emergency. As with any emergency, we would adjust our consumption to the power available to us. So far, it sounds like about 12 KW (which is about twice as large as my current standalone gasoline generator) is going to be a reasonable expectation from my current tractor, although I do have plans to buy a larger tractor in the future, so I'll probably build my generator with that in mind. But, should the need arise, I do have a neighbor that collects tractors and heavy equipment that he freely loans to me, when I need it. I just don't like borrowing things like that, unless I absolutely have to. He also has a 20 KW PTO generator and a tractor that can drive it to full output very easily. I'm using that generator as a general guide for construction, although his is built to sit on the ground, as opposed to mounting on the 3-point.

    ETA: Most of my equipment is pre-modern... LOL. A few of my welding machines are new or newish. The newest equipment I have is the stuff I've built myself and most of it is not necessarily efficient from a power standpoint, but very efficient from an economical view.
    Last edited by Rancher; 11-09-2017 at 10:13 PM.

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    MiTasol, thanks for the idea. I think I'm going to try to find or perhaps make a planetary gearbox for it. It greatly simplifies the design, keeps it balanced and compact and (I think) transfers the power efficiently. But, it's good to have a fallback plan if that doesn't work out. I have to admit I'm not much of a "car guy". So, I don't know much about what might be available in that area. I rebuilt my own motorcycle engines and transmissions back in my teens, but never messed around with car transmissions at all.
    Last edited by Rancher; 11-09-2017 at 11:11 PM.

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