PTO HP verses electric motor HP

Time to call in the expert. Friend of another neighbor owns a blower-fab-design company. I dont much like experts but this guy is ok - grew up on a farm. He comes out with his calculator, charts and diagrams. Immediatly he points out that the 8 inch pipe is not large enough and we need to make the blower turn faster. Good.

Long story short - the furthest we can go is about 350 feet. So my other neighbor has a small blower we can put inline with a 30 HP electric motor to blow it out the next 300 feet. This is just getting too complicated - either need to run 300 feet of teck out in the field to the 30HP electric or drag a big genset out to it. Did I mention the water table is 2 inches below the surface until about June.

I am ready to can the whole deal. So far this has cost me nothing but time and we farmers never bill out time.

Blower man calls today- got a 40 inch sawdust blower used. If I put 60 HP electric on it - running at 2000 + RPM we can move 1000 yards per hour, 700 feet, through a 12 - 14 inch pipe.
He has my attention again, but I do not have a 60 HP electric motor, not to mention starter, 300 feet of teck etc.

Which finally brings me to my question about PTO HP. Assuming the tractors have about 80 - 85 HP, and we can speed up the blower speed from the PTO shaft with a triple shive, will I have power to match the 60 HP electric motor.

Esentually my question is: How does tractor HP relate to electric motor HP. I am done now. Thanks, Grant

The differences really are what floats your boat. If you have an X HP (dyno'd) PTO and an X HP electric motor initially, you have equivalent methods of consuming energy and producing work. You can adapt whichever you prefer.

The difference is primarily efficiency of the process and for you, convenience. There are losses in all delivery sytems and if you balance out your losses with more HP as required, you will get the same amount of work done.

Pipe resistance is a constant and doesn't care what is blowing whatever is being blown down it. It does care what and how fast but that is a constant in answer to your question.

Want more ask.

Mark

Shaft HP = torque x rpm/5252. Nowhere in this equation is there a qualifier as to what develops the torque and RPM.

It is true that electric motors have wonderful torque reserves. Note that Nebraska MBHP readings taken at the PTO (post 1958) were observed during a two-hour test at rated RPM. So your tractor should be able to run at full power without blowing its innards all over the barn lot.

If an electric motor is rated a 60 HP at a certain RPM, then that motor is capable of giving you 60 HP. If it has an efficiency of 90%, that means that 60 HP output is 90% of the HP coming in which is 60/.90 = 66.67 HP.

Is the sawdust blower you bought specifically designed for sawdust? I guess the question should be: is it more efficient than your silage blower (more swept volume, more enclosed, more paddles?)

Using universal fan laws: If 60 HP is absorbed by the sawdust blower running at 2000 RPM, then an 85 HP input will get you 2246 RPM assuming no losses in the geared-up drive. If you run the blower at triple the tractor PTO RPM, then only 32 HP is required.

This might seem incorrect, but HP requirements on fans and blowers are really skewed by the fact that to double a blower's RPM, it takes 8x the HP.

Now I do not get how you get the 32 HP by running the blower at triple PTO RPM.

It seems to me your last statment says if my 85 HP PTO is turning 1000 RPM blower, I would need 680 HP to run the blower at 2000 RPM (85X8=680).

Thanks, I really find this interesting, sure beats digging ditches in the rain. Grant

This goes for the 85 HP tractor. Just because it's running does not mean it is developing 85 HP. It's CAPABLE of it, but it must be loaded down so that it develops torque. You are assuming that 85 HP is being developed running the silage blower at 1000 RPM. Then again, the silage blower is not the sawdust blower--a different animal.

So, using the fan law equation and the benchmark 60 HP at 2000 RPM, you can say "if 60 HP gets me this, then 85 HP gets me that". You can work the equation backwards for RPMs less than the benchmark 2000.

This all came about my being fascinated with our tractor club's Baker Test Fan. Same principals apply to it according to an engineer. If you want to look at the HP tables I have developed for it, you can check our website. This may make it more clear.

• Test Fan Tables

More than likly the 60 HP electric the blower guy suggested is overkill and therefor the 85 HP tractor should do the job, it just may tack a bit longer.

I guess I need to see what he wants to charge me for the blower and 600 feet of blower pipe.

I will let you know how it works out. Grant

I will be interested in seeing how your project turns out. Nothing is more satisfying than tackling a problem and whipping it.

Suggestion: Is there a way for you to get two 1000 PTO RPM tractors and two silage blowers? Wye connect the second silage blower into the main line halfway down its run? I'm venturing into areas I have no knowledge of now. This may be more feasible than fooling around with a 60 HP electric motor. Your blower friend ought to be able to give you a ballpark CFM capability of the silage blowers. Compare that with what the sawdust blower is capable of.

However in the real world, for a tractor PTO it's good practice to limit the shaft load to about 85% of the tractor's max horsepower. This leaves a reserve to pull through brief overloads, etc. So a 60 HP tractor should be capable of about 51 HP continuous on the shaft.

On the other hand an electric motor is fine for power output up to 100% of it's nameplate rating. And most industrial motors can be run safely at 115% or more of rating. So a 60 HP motor may safely pull 69 HP on a continuous basis - 18 HP more than the "same" 60 HP tractor PTO.

In addition, electric motors have enormous torque reserve capacity - as much as 5 times the normal running torque. That means a 60 HP electric motor will easily pull through an overload that would instantly stall a 60 HP tractor PTO.

Now I don't know the answer to your specific question. But at least you can see how an electric motor can far outperform a tractor PTO of the same rated HP.

If you go with a 60 hp elect motor it should be wired for 480 volts 3 phase. You will need to contact the power company, electrical inspector and a licensed electrician. This motor will draw 75-80 amps.

I do not have a problem running a 60 HP motor, I just do not have one, and I am not sure whether it is worth buying the motor, tech cable, starter etc, for a job I will only do once.

However if I am able to run this blower off my tractor, then I may have other applications for it in other locations without electrical service.

The reason I have this much electric service way out in my field is that I run a 75 HP irrigation pump and various other future pumps are to be installed. Unfortunatly, this 75 HP motor is a vertical pump motor and would not be adaptable to the blower.
Grant