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  1. #1
    wippin' boy is offline Diamond
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    Question electrical HP vs gas motor

    so if i'm building a hydraulic power unit
    can a system that runs on a 5 hp electric motor run on a 5 horse gas motor
    and if not then how do we decide what we need?
    no real system in mind, just got a wonder when i saw the newest northern hydraulic spam

    what about governing,do little crate motors have there own?

  2. #2
    mobile_bob is offline Stainless
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    the little crate motors do have governors, some better than others
    but all of them are generally adequate enough for most stuff.

    as for a 5hp engine pulling the same load as a 5hp electric?

    in theory they should both pull the same load, however in practice
    they probably won't

    the electric is rated at 5hp at a specific speed or slip on a continuous duty, but can provide a bit more hp intermittently

    the gas engines on the other hand seem to be rated at maximum load intermittent and probably couldn't put out an ounce more power
    under any conditions. sort of like the sears peak hp rating we all know about.

    if it were me, i would probably spec a 6.5 hp gas engine to do the work of a 5hp electric motor, minimum

    ymmv
    bob g

  3. #3
    alcosteam is offline Cast Iron
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    I think you will find that alot of claims made in advertisements are alittle (ok maybe alot in the case of vacuums and air compressors - think Sears) exaggerated.

    If you look at compressors that have a gas or electric version to them you generally find the gas version has AT LEAST twice the HP as the electric version. A 5 hp sized electric compressor will have a 10-13 hp gas engine on it.

    tim

  4. #4
    ray french is offline Titanium
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    Northern Tool used to offer a formula for just this very thing.Don't know about now-a-days though.

  5. #5
    wippin' boy is offline Diamond
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    cool
    i'm gonna have ta build me somthin'
    next is to decide what

  6. #6
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    Toms Wheels is offline Titanium
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    The biggest difference is life, a 5hp electric motor can produce that 5 hp forever, until something fails anyway, a 5hp gas engine not so, it is rated as 5hp max HP, but a continuous hp of about than half that. In addition a 5hp gas engine would be at 3600 rpm, not the common 1750 of a electric.

  7. #7
    Jon_Spear is offline Hot Rolled
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    Default how gas engine hp is figured

    Wikipedia actually has a good section on this:
    Horsepower - Wikipedia, the free encyclopedia

    For a gas engine, the hp rating is calculated by doing real-time pressure and displacement measurements of the pistons. They calculate (integrate) how much force was put on the piston over the distance of displacement, times the repetition rate of the combustion cycle. That gives a value of how much work is being done by the exploding fuel-air mixture.

    But the actual mechanical power produced is less than the motor hp rating, because there are losses due to internal friction. The important quantity, to compare against gas vs. electrical motors, is called the "brake horsepower." Unfortunately, that number is not what is listed for most gas engines.

    For electrical motors, rating on the motor is supposed to be the true mechanical power output. But there has been some discussion in the archives here about bogus ratings being listed for electrical motors, (particularly regarding shop vacuum cleaners).

    In summary, a 5hp gas engine will have a slightly lower power output than a correctly rated 5hp electrical motor. This is due to gas engine hp ratings not taking into account the internal friction losses.

  8. #8
    Hdpg is offline Stainless
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    You can get some indication of the difference between internal combustion engine horsepower and electrical motor horsepower by looking at a gas(or diesel) driven generator. A genset rated at 2000watts, about 2.5 electrical horsepower, will be driven by a 3.5 to 5.0 horsepower gas engine.

  9. #9
    Jeff_M_PA is offline Cast Iron
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    The rule of thumb that I've been told and used is 2:1, whereby you'd need a 10hp gas engine to do the work of a 5 hp electric motor. It has never made much sense to me, to be honest (though Jon_Spear's explanation may be the answer), but it seems about right in practice. I've generally been converting to electric from internal combustion but have gone the other way too. My experience has been on stuff larger than 5 hp though. For example, we converted a tree-chipper to electric from diesel, and it worked quite well at around 50% conversion.

    So my answer on the hydraulic power unit is probably not, I'd go 10 hp if practical and accept maybe 7 or 8 if I happened to have that engine available.

  10. #10
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    SAG 180 is offline Titanium
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    Another complication is how some manufacturer's actually build their motors: Crompton Parkinson in their many incarnations made a one HP single phase that was actually their three HP continuous motor with a smaller circuit breaker.

    It was easier for them to just make the one motor style and derate them with the built in breaker ratings, this meant that for certain applications with inertia like a disc sander with a 12" disc you could measure 20 Amps peak on start up or 6.4HP for an instant. You would have lots of problems replacing an old 1HP Crompton with a newer style 3HP Crompton as the overload would keep popping on start up. The solution was to fit a higher rated overload breaker.

  11. #11
    ray french is offline Titanium
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    I have a complete hydraulic unit motor/pump and tank 3PH,don't remember the HP that several folks told me would be good for a hydraulic press.I am scepticle because it's only 350PSI.It's yours for a price+shipping if you can verifiy that it will do what you need.If you are interested I'll verify the HP and other particulars at your request.I also have another unit much like those used on car lifts that had a 3/4 HP single phase motor with an(unknown to me) valving system.Negociated price+shipping.

  12. #12
    gbent's Avatar
    gbent is offline Diamond
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    I'm in the 2:1 camp, one lung gas vs electric. Most of the power in the gas engine is in about 120 degrees of crank motion, and then it coasts for 600 degrees. (even worse than coasts, it needs considerable power for the compression stroke). The electric motor gets almost continuous torque.

  13. #13
    Jeff_G is offline Hot Rolled
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    Default its a question of ratings...

    Quote Originally Posted by Jon_Spear View Post
    .... The important quantity, to compare against gas vs. electrical motors, is called the "brake horsepower."
    John is correct - we need to compare apples to apples. I would add that the only rating that we are generally interested in is continuous brake horsepower - the power that the motor or engine can produce for hours of continuous operation. Reputable manufacturers provide continuous brake horsepower on the nameplate. Anything else is meaningless unless your application's duty cycle matches the duty cycle at which the engine or motor was rated.

    The continuous brake horsepower rating depends on rpm. An engine is rated for x hp at y rpm. If the application results in the engine running at a different speed, chances are the power available will be less than the rating.

  14. #14
    Dave D is offline Hot Rolled
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    Default So many variables

    All things, (well most things being equal) you are comparing apples and oranges, an engine to a motor. So many different variables. I'm in the 2 to 1 camp, most manufacturers I can recall, even old ones using "real HP" used this ratio or close to it.

    Dave

  15. #15
    Phil in Mt is offline Cast Iron
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    Cool well almost?

    The way it works, hp is rpm times tork. The gas engines will be rated at some unreal speed like 4200 rpm and the electric motor will be rated at speed related to the poles of the motor (at 60hz a 2 pole will turn 3600 - the slip-about 5% or a more common motor will be a 4 pole or 1800 - the slip) A gas engine will have a lot of derating due to alt, temp, ect also a gas engine will have a service factor you can not load a engine at max for hours on end, if you do it will fail very soon. If you want to do the converson right you will need the hp curve of the engine, match the rpm of the electric you want to use. If you are going from gas to electric you can go hp for hp AT RATED SPEED. If you are going form electric to gas use rated speed of both, then use a 1.5 mult to get engine hp ( dont forget to derate for alt, temp ect)...Phil in Mt

  16. #16
    Dave D is offline Hot Rolled
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    Talking

    Phil are you trying to say that if I rate the HP of my motor/engine by measuring the "peak HP" (read as it's about to burn out) instead of continuous that it would be misleading? Why as a kid we used to tie horse chesnuts to a string, swing them around and we likely generated 1/4 peak HP, 1/2 HP on the bigger chesnuts. RPM was not high but if they hit you in the face you felt the raw HP, more if you didn't peal the husk off first.

    Dave

  17. #17
    wippin' boy is offline Diamond
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    forever more referred to as the raw chestnut horse power rule of thumb

    sounds as thought the consciences is about 1.5 to 2x

    thanks

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