Another PTO Generator Question

[RonR]

I'm thinking of building a PTO driven generator to power stuff around the house during winter power failures. Mainly the frig/freezer/furnace and lights/fans.

I have a 22HP tractor and could get a generator matched for that size. I'm sure I'd have to run the tractor at PTO speed to maintain the voltage.

Would it work to buy a smaller generator, which would be belt driven to increase the RPMs, but then I could run the tractor at half throttle.

I'm thinking the generator won't put that much load on the tractor, and if I have to run it very long, I'd just as soon run it at 1800RPM instead of wide open.

Anyone tried this? Your thoughts? THANKS.

Ron

 

[Tom_Veatch]

I have a 22HP tractor and could get a generator matched for that size. I'm sure I'd have to run the tractor at PTO speed to maintain the voltage.

Would it work to buy a smaller generator, which would be belt driven to increase the RPMs, but then I could run the tractor at half throttle.

I'm thinking the generator won't put that much load on the tractor, and if I have to run it very long, I'd just as soon run it at 1800RPM instead of wide open.

Anyone tried this? Your thoughts? THANKS.

Ron

Consider that one HP is about 750 Watts (not exact but close enough for back of the envelope stuff). Actually, considering the various real world losses and inefficiencies don't count on getting much more that 500 Watts out of each developed HP of the tractor engine.

I used the term "developed HP" instead of rated HP for a reason. You may have a 22HP engine in your tractor, but it only develops 22HP at a particular RPM. Slowing the engine below that RPM reduces the maximum HP it will develop. Likewise, running it above that RPM also reduces the maximum HP it will develop. The actual maximum HP developed at any given RPM depends on the characteristic HP vs RPM curve for your particular engine. But, whatever it might be, that is the maximum HP developed at the RPM you want to run the tractor and is all that is available to run the generator

With that said, you should be able to run the tractor at whatever engine RPM you desire if you:
1) Size the generator at no more than about 500 Watts Continuous per "developed HP" at the RPM you want to run.
2) Figure the actual PTO RPM as the ratio of 540 X (Running RPM/Engine RPM for 540 PTO RPM)
3) Fabricate/purchase some sort of power transmission to increase the actual PTO RPM to provide 540 RPM to the input shaft of the generator. Be sure to size the transmission to handle the "developed HP" (with the appropriate safety factor) from 1) above.

Now, about the generator not being much of a load on the tractor. I'll agree that the tractor frame, wheel bearings, etc aren't stressed by the generator load. However, it that generator is requiring 22HP from the tractor to satisfy the electrical load on the generator, the tractor engine is working just as hard, under no less load, than if it were developing 22HP to drag a plow through the ground or a rotary cutter through the brush.

OK, bottom line. Yes, you can engineer a solution to do what you want to do.

Should you do it? Probably not.

My opinion on what you should do?

Figure the electrical load based on what circuits you want to power and the electrical load and duty cycle on those circuits paying particular attention to the size of any motors fed by those circuits. Electric motors drink a lot of amperage until they come up to design speed. Take the total wattage and motor starting requirements and look for a generator that satisfies that load. If you need more than about an 11 to 12.5KVA generator, you have too small a tractor and need to scale back on what circuits you expect to power.

When you've identified a generator size that adequately supports the electical load and doesn't overload the tractor, hook it up direct to the PTO and run the tractor at the 540 PTO speed. The tractor was designed to run for hours on end at that speed. You really aren't going to be doing it a favor by running those hours in an "off design point" state.

Just my opinion, take it for whatever you think it's worth.

 

[RonR]

Thanks Tom,

Guess I'll buy the PTO one and be done with it.

Ron

 

[Soundguy]

I agree 100% with tom. Most places selling pto gensets, figure 2 pto hp per 1kw and that takes into account all the losses.. etc. Thus.. your 22 eng hp machine may only make 20 pto hp.. thus.. only making about 10kw.

I think I'd still get a 11/12 kw genny.. then you know that you were not taxing it, vs using a 7-8 kw genny, and running it at max all the time.

Also.. the engineering to get from belt to genny head has it's own problems. belt drives are ineficient, and then you have side load problems with bearings that may not be made for side loads.. etc.

Soundguy

 

[lwalsh]

One thing I would recommend Is undersizing your generator for the available
pto hp. I went the other direction, I have an 11K gen head, and 18pto hp.
The problem is that under heavy load the generator will slow down the tractor rpm's before the current draw is heavy enough to pop the 50 amp breaker on the generator. This in effect reduces the available voltage which can cause electric motors, pumps, etc. to fail. So I have to be carefull. What I need to do now is switch to a 40 amp breaker on the generator. Most newer generators will give higher peak output than rated for a few seconds but only if you have suffecient hp to keep them spinning at rpm.

 

[Tom_Veatch]

Ron, I really think you'll be much happier with the results doing it that way. Assuming of course, that's all you're looking for.

On the other hand, if the challenge of designing/building your own custom solution is what floats your boat, (it does mine, BTW), buying a ready-made solution just isn't very satisfying.

 

[Tom_Veatch]

<snip> Thus.. your 22 eng hp machine may only make 20 pto hp.. thus.. only making about 10kw.<snip>
Soundguy

You're right, Chris. I slipped up in my discourse by assuming that "Engine HP" is equivalent to "PTO HP". Obviously it's not.

Since my grey market Yanmar is "advertised" or rated by PTO HP (and not even real honest 'murrican" HP at that) I frequently miss that distinction when talking tractor talk.

BTW Ron, what part of Kansas do you hail from?

 

[Soundguy]

I don't see how having more generator output capability is a bad thing. If you only have 18 pto hp.. just plan your electrical loads to fit into 9000w. This means you are not running your genny at max.. actually more like 75%. i'd think the service life on the unit will go up... less stress, electrical and mechanical. Your other option would be a 7.5k head that is commonly sold.. that robs you of about 1.5k of usable power, and may mean you are running the head at capacity all the time.

Soundguy

One thing I would recommend Is undersizing your generator for the available
pto hp. I went the other direction, I have an 11K gen head, and 18pto hp.
The problem is that under heavy load the generator will slow down the tractor rpm's before the current draw is heavy enough to pop the 50 amp breaker on the generator. This in effect reduces the available voltage which can cause electric motors, pumps, etc. to fail. So I have to be carefull. What I need to do now is switch to a 40 amp breaker on the generator. Most newer generators will give higher peak output than rated for a few seconds but only if you have suffecient hp to keep them spinning at rpm.

 

[ZackaryMac]

With some careful planning, electrical requirements can be met without a large generator.
In my own case, a 3000w generator will do anything I need, as the water pump is the biggest draw I have. My 1500w home-made diesel genset won't start it up, but it will run the fridge, furnace, freezer, microwacve, lights...just not all at once. Taking turns with major loads works for me. In my case, the power typically doesn't go out often, or for long. A day and a half was the longest in the last 15 years. I know everyone has different needs, but my point is planning can reduce hugh surge draws, therefore reducing generator size.

Just my 2¢ worth.

 

[Soundguy]

A common practice to conserve power is to power up your well to make sure your hot water heater is full, then swap the hot water heater on for a half hour, then swap the pump back on for water pressure, and take a warm (quick) shower.

Soundguy