Hydraulic drive for 15 kw generator

[skidsteer.ca]

I have been thinking of constructing a backup up generator to run off the 3rd valve on my skidsteer.
The 15 kw, 1800 rpm gen head calls for a 20 hp diesel engine to drive it
I have 2 skidsteers that could be used to drive it. One has 16.9 gpm at 3000 psi and the other has 15 or 24 (if hi flow is activated) gpm at 2500 psi

I'd like to find a hydraulic motor to turn the 1800 rpm at @ 2/3 to 3/4 throttle on the skidsteer. I was hoping to stay wth a direct drive to avoid belts etc.
If I figuring this right, 14 gpm at 2500 psi would give 20.4 hp
Which would let me run the 16.9 loader rpm at 82% or the hi flow machine at 58% or 1740 rpm (3000 rpm is wide open throttle) Engine is about 45 hp. I think they would produce 20 hp at 1740??

A 1.8 cubic inch hydraulic motor would require 14 gpm to turn 1800 rpm.
All this is based on 100% efficency.
So what else should I be factoring in to make this work right?

Most of the time I would think the gen load would be well below the he max, likely 5 to 6 kw

Ken

 

[RichNJKubota]

What about keeping the generator turning at 1800 RPM (60hz output) while the AC load changes?
Would the speed governor on the engine take care of this? Or would the hydraulics allow a bit of variation in output RPM?

 

[skidsteer.ca]

The engine governor should hold the rpm as well as a tractor does. I did some reading on here last night and it seems with a pto generator maintaining frequency can be a bit of a ongoing problem and a way to test frequency is recommended. I have a hand held tach, and the loader has a tach too. I think my multimeter reads frequency, or I could get one.
The other thing they recommended was not running the 1000 pm pto as the tractors rpm was usually more stable using the 540 pto and running the engine faster. (although harder on fuel) Now obviously if you tractor has more then 2 times the required hp this would be less of a factor.

Following this advice if I go with a 1.8 cubic inch motor, I could run the gen on standard flow, for heavy loads and hi flow for light loads.

The pump and hyd motor both are fixed displacement so in theory the rpm from one to the other should be constant. Just not sure if this holds 99% true in the real world or only 90% true
I have never seen or heard of gen like this, so maybe there s a reason??
Ken

 

[kennyd]

Here is a link some online calculators that may help some:
Baum Hydraulics :: Spec Calculator

Using your 14 GPM @ 2500 PSI figure only gets 13.3, the calculator assumes 65% efficiency.

This is from the Surplus Center Tech help page that say's to assume 85% efficiency?

• Pumping = 1 Hp = 1 GPM x 1500 Psi
  (linear relationship i.e. 2 GPM @ 1500 Psi = 2 Hp)
  
  
  [*]Hp to drive hydraulic pump = Psi x GPM / 1714
  
  
  [*]Efficiency - typically assume hydraulic pump/motor efficiency of 85%
  
  
  [*]Hydraulic motor torque (in-lb) = Pressure (Psi) x motor displacement (in3/rev) / (2 x Pi)
  
  
  [*]Hydraulic motor speed (RPM) = 231 x GPM / motor displacement (in3/rev)
  
  
  [*]Hydraulic motor power (Hp) = Torque (in-lb) x RPM / 63025

 

[JimmyTig]

I have been thinking of constructing a backup up generator to run off the 3rd valve on my skidsteer.
The 15 kw, 1800 rpm gen head calls for a 20 hp diesel engine to drive it
I have 2 skidsteers that could be used to drive it. One has 16.9 gpm at 3000 psi and the other has 15 or 24 (if hi flow is activated) gpm at 2500 psi

I'd like to find a hydraulic motor to turn the 1800 rpm at @ 2/3 to 3/4 throttle on the skidsteer. I was hoping to stay wth a direct drive to avoid belts etc.
If I figuring this right, 14 gpm at 2500 psi would give 20.4 hp
Which would let me run the 16.9 loader rpm at 82% or the hi flow machine at 58% or 1740 rpm (3000 rpm is wide open throttle) Engine is about 45 hp. I think they would produce 20 hp at 1740??

A 1.8 cubic inch hydraulic motor would require 14 gpm to turn 1800 rpm.
All this is based on 100% efficency.
So what else should I be factoring in to make this work right?

Most of the time I would think the gen load would be well below the he max, likely 5 to 6 kw

Ken

Old post - but if you are still around - did you ever work out this problem? I have the same question and plan to build a quick attach 20 kW 3Phase generator system to connect with my Cat 289C, etc.

Best

Jim

 

[IceStationZebra]

This is rarely done because of the poor efficiency and heat generation. A pump or motor in good condition and running a reasonable speed will be 85-90% efficient, so .85 x .85 = 72%. The 28% of wasted energy, plus all the other parasitics of the driving machine (hydrostat charge pump, hydrostat pump oil churning, etc.), turns into a lot of heat so you need a good cooler designed for the continuous load. Not to mention wasted fuel.

The other issue is speed regulation. As the load changes the pressure required changes, which affects the flow losses and efficiency, requiring tweaks to the flow. Same goes for oil temp, though it will be more stable. It is also possible to have an oscillation issue if the hydraulic system and engine governor have similar resonant frequencies, though the likelyhood is small.

In short I would only do this for short durations or emergency use. A direct drive is usually a much better solution.

ISZ

p.s. We had a pto driven generator for our 115hp tractor. It had three LEDs to tell us if the speed was right - green for 60hz, two red for over and under speed.