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

[memphismark]

Hello everybody. I've been on the Surplus Center website (again!) and their tech section has a quick calculator to plug in GPM, H.P. numbers etc., and it will calculate everything from needed HP to cubic inches for you. Easily. What's puzzling me is that just under the HP calculation total is (in red letters) a statement that has me confused. Basically say's that a given HP number that is punched out by the calculator is for electric motors, and that you have to double that number if your going to use a gas engine to drive your pump. Can anyone offer a plausible explanation for this? Horsepower is horsepower, right? Thanks in advance for your input. memphismark

 

[davesl708]

Horse power is horse power but torque is what gets the work done. Electric motors have more constant torque.

 

[bcp]

From:
https://evmc2.wordpress.com/2012/04/06/electric-motor-hp-ratings-and-other-secrets-of-the-universe/

Here, in one sentence, is the crux of the matter: “When the load on a petrol motor increases and rpms drop, torque also falls. For an electric motor torque increases.”

A gasoline motor has a peak torque value for a given RPM (3600). It’s easy to peg that as a HP rating – measured torque at a fixed RPM. An electric motor produces torque in response to load. You load it, the RPM drops, and it draws more current and produces more torque. It’s more of a moving target, so you have to rate it, rather than the actual horsepower it produces, the horsepower it can produce at a given load before it melts.

I think it goes back to the whole idea of a gas motor “producing” power and an electric motor “handling” power. Since an electric motor responds to load by drawing more current, it really is a completely different animal, ratings-wise, than internal combustion.

 

[memphismark]

Fascinating. I'm driving a .73 cubic inch pump with a rating of around 6-11 GPM. (Surplus Center pump 9-7790-B) This is for a 2 yard shop-made concrete mixer. A 22 HP, V-Twin gas motor will spin the pump which turns my Bobcat auger drive gear motor that steps speed down to a manageable 35 or so RPM'S. I need intermittent/start up mixing pressures of around 3000 PSI to get the sand/gravel/cement/water slurry going, then things drop to around 1,200 PSI for the total mixing to occur. Should I really expect a near 50% drop in torque for the gas motor on start up from 22HP to maybe 12HP? 15HP? Thanks for the article by the way. I'm a hydraulics novice at this, but learning quickly. Thanks memphismark

 

[CJONE]

Nope, horsepower is different depending on what you are talking about. 10 EBH [electric brake horsepower] is equal to 20hp from a gasoline engine. It is also equal to about 14hp from a diesel engine. Here is some more to confuse you. A Honda 20hp gas engine is stronger than a 20hp briggs. A 50hp cummins is stronger than a 50hp Isuzu. I know this from the school of hard knocks. You have to start to consider torque curves, displacement, turbocharging ect ect. Not fun to size engines for hydraulic drives with so much inconsistency. CJ

 

[memphismark]

I got a feeling this may be a pump 'trial and error' thing, or worse, a motor trial and error thing. I've been driving the mixer with a Bobcat 753 thus far, but I can't do that anymore. If the 22HP doesn't quite cut it, I may have to get a cheap Volkswagen motor/clutch setup for cheap. I just can't spend anymore on expensive motors for this project. My wife keeps pointing to the 'doghouse' as it is! Anybody know of maybe a 40hp or so motor that might work and not cost an arm and a leg? What are the names of some. Thanks. memphismark

 

[bigdeano]

The surplus center calculator is very conservative with gas engines because most people don't want to run a gas engine at max power output continuously. I have a gas pressure washer that puts out very close to the calculated value with a 6 hp tecumseh engine. Small gas engines running 3600 rpm should have a torque rise when loaded so the rpm drops. That's why B & S rates the torque at 3060 because it's higher.

 

[LD1]

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.

 

[Sawyer Rob]

A Honda 20hp gas engine is stronger than a 20hp briggs.. CJ

Like some other parts of your post, that's waaaay too broad of a statement...

Both Honda and Briggs sell several models of motors of the same HP, both making them in different configurations and in different countries. BOTH have their motors made in Japan, China and other places...

I would easily put my Japan made Briggs 20hp up against any gas powered Honda made... That's from the school of "experience"... of owning both.

The Honda is good, so is the Briggs...

SR

 

[bigdeano]

I just bought a 24hp Briggs Professional vertical shaft. Briggs says it's identical to the former 27hp model. The feds got on them and made them rate their engines more accurately.

From the hp curve, the torque figures out at 35 lb-ft at 3600, 38 lb-ft at 2800.