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

[CJONE]

Not so. If you don't believe me ask BCP , LD1 and Spyderlink in thier posts.

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

 

[4570Man]

I dont believe there is a 2:1 difference. Sure, there may be "some" difference. But HP is HP. If you overload a gas motor, RPM drops. With RPM drops power also drops. So if you arent running at the rated RPM like may people wrongfully do, you arent getting the rated HP. And electric motor that is overloaded a bit wont loose RPM, it will just draw more amps. Even the calculator and recommendations on SS website is suspect at best. Here is a perfect example.......22 GPM DYNAMIC 2-STAGE PUMP Same pump I have on my splitter. 7GPM @ 3000PSI in the high pressure stage. Says right in the description that 12HP minimum for gasoline engines. Plug those same numbers into their calculator and it is calling for a 13.6HP (electric). Double that for gas would indicate a need for a 27HP engine. But why in the description does it call for a 12HP? BTW, I am powering it with a 16.5HP briggs motor and can deadhead the pump and the motor hardly slows down. So their calculator that says I need a 27HP is WRONG. Here is another example. Dads splitter uses a 11gpm 2-stage pump. 3GPM @ 2500PSI on high pressure stage. Calls for 5HP. So wouldnt that mean it needs a 10HP gasser? Uh....how many of the 20-22ton logsplitters that have the standard 11gpm pump you see being sold with a 10HP motor? None. But see a lot being driven with a 5, 6, or 6.5 HP motor..... Well, we converted this splitter to electric. I didnt have a suitable 5HP/3600RPM motor, but I did have several 3HP motors. Picked the one with the highest rated FLA and 1.15 SF. The 3HP motor is too small. When building a full 2500PSI deadheading the motor, it pulls around 11.5-12 amps and the motor is only rated for 8 FLA. Fortunately, with splitting wood it almost never reaches 2500PSI, and if it does it is only for a split second. But you would never be able to run 2500PSI continuous without trashing the motor. It needs a 5HP for that. Same as a gas motor would need. So I think everyone saying 2:1 is being overy cautious and I think alot of it has to do with gas engine MFG's not being honest with HP ratings.

I have one of those 2 stage pumps on my wood splitter. It is driven by a 5 HP gas motor. If I deadhead the pump it stalls the motor.

 

[buickanddeere]

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

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.

 

[LD1]

I have one of those 2 stage pumps on my wood splitter. It is driven by a 5 HP gas motor. If I deadhead the pump it stalls the motor.

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?

 

[4570Man]

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.

 

[bcp]

This reminds me of all those 3 to 5 hp shop vacs, with 15 amp plugs, you see in the hardware stores.

 

[4570Man]

This reminds me of all those 3 to 5 hp shop vacs, with 15 amp plugs, you see in the hardware stores.

I have seen some rated as high as 6.5. Actual hp around 1.5.

 

[LD1]

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.

Yes it is a 3 phase running off a VFD.

 

[4570Man]

It was still probably a pretty generous 3 hp.

 

[LD1]

No. That's what the Fla is of a typical 3hp.

Beyond that, I have no idea what you mean