why are electric motor/gas engine HP requirments different for pumps

[Egon]

I don't need to ask anybody, I size power units almost everyday and know what works and what doesn't. And I fix a lot of other peoples errors in sizing units. What the numbers say and what works is not usually the same. CJ

Empirical does work.

Electric motors are rated and run at one constant RPM. (Yea- variable speed )
Engine HP is also derived at a specific RPM. The engine may/may not run at constant RPM or the rated speed.

 

[SPYDERLK]

I don't need to ask anybody, I size power units almost everyday and know what works and what doesn't. And I fix a lot of other peoples errors in sizing units. What the numbers say and what works is not usually the same. CJ

... But it is not apparent from your post(s) that you know why.

Not once have you addressed torque rise characteristics of each prime mover.

,,,Nor a further key related point on the sensitivity of each. -- An IC engine running at full power is beyond its torque peak. Otherwise it would not be at its full power capability. Torque fell off above some rpm, causing the power rise to decline and finally stop. This all happens in a range of 20-30% of the engines peak power rpm. Thus for an ICE, torque rise happens gradually over hundreds or thousands of rpm loss. Power declines immediately because the torque rise per rpm is so slow.

,,,The situation with an electric induction motor is exactly the same but effectively different. This is because the torque rise happens almost instantly; over only 1 or 2% of its line frequency locked rpm [~3600 or a sub multiple]. Slip from the line freq causes the motor to admit much more current with a close to linear torque increase. Thus rpm drops ~ none as torque rises ... = MORE power - almost straight up.

... Kinda like a hyperactive turboed engine with no fuel limit and no waste gate. :confused3:

 

[SPYDERLK]

old tired motor or did it do that with a brand new engine? Whats the pressure?

Our splitter that has the 11gpm has a 6HP motor, and 3000psi confirmed with the gauge. It dont stall when deadheaded. Using everyones "general rule of thumb 2:1" would indicate a 3HP electric is adequate. It is not. The 3HP motor has a FLA rating of 8a. When making 3000psi deadheading it pulls 12a. THATS not adequate.

And using the formula for 11gpm pump (~3gpm @ 3000psi) would indicate the need for a 6 HP motor. (judging by the amps drawn by the electric motor, thats pretty accurate), so again....2:1 rule......says I would need a 12hp gasser. So why does the 6HP run it just fine without stalling?

It was a brand new Honda motor. I think the pump was 2700 psi on high pressure. I think it was about 700 on low pressure. Unless your motor was 3 phase it is not a real 3 hp motor. I used to drive the same pump that stalls the Honda with a 3 HP electric. The electric motor could run it dead headed for a short period of time, but tripped the thermal overload.

Much better to follow the [correct] convention of distinguishing engine (a self sustaining prime mover), from motor (always parasitic to an engine somewhere upstream). -- That way youre not always falling all over clarifying.

 

[CJONE]

I don't always agree with the some policies that BCP, LD1 and Spyderlink hold. However they are all educated, smart and have real world practical technical experience .
Not once have you addressed torque rise characterics of each prime mover.

I understand what everyone is saying, with everything being equal and sized in the 'meat' of the torque curve if you set up a hydraulic pump to develop 5 GPM at 1500psi on both a electric driven unit and a gas engine with a 5hp rating, the electric motor will happily give you the rating with out over amping itself. The gas engine will fall flat on it's face at about 800-900psi. I have to pay particular attention to the torque curves of diesel engines because some manufactures are loosely following guide lines when they rate their engines and their torque curves look more like a mountain than a nice flat curve like most diesels are known for. Heck even Honda quit rating their gas engines by horsepower, that is real fun to figure out when sizing a engine! So I don't get to wordy about torque curves and the like, most don't care about it they just want it to work! So what I have said was not meant to offend anybody, just the results of close to 30 years of doing this. CJ
Heck, I even have hydraulic pumps with HP limiters that will reduce flow as the pressure increases so as to give the max flow that the prime mover will allow at a specific pressure. I am very hesitant to use something like this because of the set up difficulties down the road when the pump requires servicing. It is usually well over most customers heads. With electric motors I have way better success talking a electrician through the process of set up with the compensator. Anyway cheers!

 

[CJONE]

old tired motor or did it do that with a brand new engine? Whats the pressure?


And using the formula for 11gpm pump (~3gpm @ 3000psi) would indicate the need for a 6 HP motor. (judging by the amps drawn by the electric motor, thats pretty accurate), so again....2:1 rule......says I would need a 12hp gasser. So why does the 6HP run it just fine without stalling?

sorry, I misread this. Does the gas engine lug down? I would bet it does. The pump is rated a 3gpm at 3600 so if the engine lugs down to 1800 or so it is only putting out half the gpm and actually a little less, so 1.3 or so at 3000 is only 2.64 hp. Is the engine a Honda? They have a heavy torque curve that goes fairly low in the rpm band. That would be the reason it does not stall. CJ

 

[LD1]

I don't find the 2:1 rule to be close at all