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Electric versus gas motors

Post in 'The Gear' started by Steamer, Aug 16, 2013.

  1. Steamer

    Steamer Member

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    Bster13 likes this.

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  2. gmule

    gmule Feeling the Heat

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    I don't think the this is an argument about whether or not an electric motor can do the same or more work as a gas engine. I think it is more about portability to a job site that does not have an electrical connection available.
    Beer Belly likes this.
  3. ewdudley

    ewdudley Minister of Fire

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    Electric motors run at about 1725 rpm or about 3450 rpm, unless you're planning gearbox or belt and pulley. If you do slow a 1.5 hp motor down to 1000 rpm it will have 7.88 ft lb of torque, but it will split wood at less than 1/4 the speed of the 9.0 ft lb torque gas motor running at 3600 rpm.

    A 1.5 hp electric motor rotating at 1725 rpm has 4.57 ft lbs of torque.

    A 1.5 hp electric motor rotating at 3450 rpm has 2.28 ft lbs of torque.

    A 6.17 hp gas motor rotating at 3600 rpm has 9.0 ft lbs of torque, FTW.

    Also, when comparing power beware of the Harbor Freight Horsepower, which is not, as it should be, measured according to torque and rpm at the shaft. It is the amount of electrical power the motor draws, calculated as horsepower, measured at maximum load during the moment before the motor catches fire.
    lukem likes this.
  4. TreePointer

    TreePointer Minister of Fire

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    I split on the farm and don't have a quarter mile extension cord. Makes the decision easy.
  5. Steamer

    Steamer Member

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    I just purchased a new splitter and opted for gas for the same reason as I like to be able to split wood at different locations on my land.
    Interesting enough the one I purchased has option of changing to electric also.
  6. Corey

    Corey Minister of Fire

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    'Electric vs Gas' posted by user 'Steamer' - had to LOL - I thought that was pretty novel.

    This has been covered several times, so I'm sure a search would dredge up many hours of reading. As 'dudley points out, if you take a splitter designed for a 3600 rpm gas engine and put in a 1750 rpm electric motor performance will suffer. But if you have a system designed specifically for either rpm, that point is effectively eliminated you're basically looking at power in vs power out. A system designed for a 1750 rpm electric motor would likely have a different pump than a system for a 3600 rpm gas engine.

    The next main factors would be the relatively flat torque curve of the electric motor and the 'peaky' torque curve of the gas engine. As the gas engine slows due to load, if will likely loose torque, so your power is going down due to both reduction of rpm's and torque. As the electric motor slows, it will generally maintain torque, so you are only loosing power due to rpms. But again, in a properly designed system, this shouldn't be a huge issue as the load shouldn't reduce the rpm of either source that much.

    Which leaves the last main factor - do you want a wide area of use, and willing to trade oil changes, gas buying and routine maintenance issues, or do you want 'plug and play' convenience with minimal maintenance, but be limited to use only where suitable cords/outlets are?
  7. jharkin

    jharkin Minister of Fire

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    Only AC synchronous motors are limited to 3600rpm or 1800rpm (or really any multiple of 60hz AC). AC induction and DC motors can be run at any speed.

    The point that ewdudly was making is that you should not get fixated on the torque numbers. Just because a 1.5hp motor happens to make the same peak torque as a gas motor doesn't mean it will drive your splitter. The torque is meaningless here, this is a constant speed application and the hydraulic pressure generated by the pump is a function of power, not torque. Whatever RPM the motor or engine makes peak horsepower at can be simple geared up or down to deliver that horsepower at the desired motor speed.

    Horesepower and torque are derived figures related by a formula. In imperial units the formula is:

    HP = lb-ft x rpm / 5252​


    For example, say you have a pump that needs to run at 2000rpm.
    • That 1.5hp electric motor delivering 9lb-ft @ 1000rpm would need a 1:2 gearing which would half the torque to 4.5ft-lb at the pump. Horsepower stays constant at 1.5.
    • That 6.17hp gas motor that produces 9lb torque at 3600 rpm would get a 1.8:1 gearing and would deliver 16.2lb-ft at the pump. Horsepower again stays constant at 6.17.

    The torque curve of electric motors is interesting in variable speed applications, such as powering a car, because an electric motor makes peak torque at zero rpm and then it slowly ramps down to nothing at peak rpm, whereas gasoline and diesel engines make peak torque somewhere in the middle of there usable rpm range. You can use that electric torque to produce spectacular acceleration from a stop or in a hybrid system to complement a high rpm engine that has little low end torque.
  8. Steamer

    Steamer Member

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    That's one of the motives that influenced me to purchased a splitter that has interchangeable power sources i.e. electric or gas.
    Currently gas power suits me but if I alter my preference I can shift over to electric power.
    Glad I was able to give you a laugh- Laughing is healthy.
    Cheers
  9. DennisR

    DennisR Member

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    sigh......Much misunderstanding here. First off, electric motor themselves DO NOT develop a flat torque curve from zero RPM to some top RPM. That flat torque curve is a result of current being limited to one value as the motor is accelerating a load and the motor is controlled by an ELECTRONIC CONTROLLER that controls torque (by limiting current) and speed of the motor. A motor hooked directly to the power source will exhibit its true torque curve. For DC this would be maximum at zero and them falling off as the motor increases speed. For AC motors it is low torque at start up and then huge jump in torque that then goes back down again to a settling point that is determined by the load.


    Now that these common misconceptions are out of the way, lets get back to Steamer. Steamer, the motor to replace your 5 horsepower gas engine would be a 3 horsepower AC induction motor that turns close to 3600 RPM. In reality, it will be about 3450 RPM or so, but that is still enough to run the pump correctly and get good cycle time. The reason it only takes a 3 horsepower AC induction motor is because AC induction motors have a unique ability that no combustion engines will ever have, and that is breakdown torque. This breakdown torque is the max torque that motor can put out when loaded beyond full load torque that if you exceed this torque, the motor will start to lose torque and stall. But no worries because the breakdown torque is about twice the full load torque. Another beauty of AC induction motors is the ability to hold speed very well over large load change swings. So, as motor torque increases, so does the horsepower. A 3 horsepower AC induction can easily output 5 horsepower under overload conditions. For log splitters this overload will be a tiny fraction of the overall load dynamics that the motor will experience. It does not take 5 horsepower every time to split a log. The horsepower is all over the place when splitting logs, but a 3 horsepower gas engine would stall if a part of the cycle needed 5 horsepower to overcome a knot. Hence, why gas engines need to be sized for worst case scenario.

    Here is a list (All motors are 240 volt):

    3 HP E = 5 HP G
    5 HP E = 8 HP G
    7.5 HP E = 12 HP G
    10 HP E = 15 HP G
    15 HP E = 21 HP G <<---- 15 horsepower single phase motors are very rare, but Baldor still makes them.
  10. DanCorcoran

    DanCorcoran Minister of Fire

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    The electric vs. gasoline discussion for splitters is the same as for cars: less maintenance vs. long extension cord! ;lol
  11. billlippold

    billlippold New Member

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    Looking for some basic help here folks...looking at the GreenWorks G-Max 40V 16-inch just to keep around the house for odd jobs. have always had gas powered. Any thoughts or experience with the electric?

    Thanks!
  12. DanCorcoran

    DanCorcoran Minister of Fire

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    Use the search box in the Gear forum...look for "electric splitter". There have been many discussions and all of the models are still current (although I don't remember the GreenWorks splitter).
  13. JayD

    JayD Member

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    I have an electric,RamSplitter. I Iike it alot. I live in a neighbourhood I can split at 7am if I want, I have the 220 2hp tapped into my a/c power breaker, 16 ton i think. Split a lot of wood with it still going strong . Jay

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