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  1. #1
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    Default Is this generator big enough?

    G'day everyone,
    Well I've finally got a shed (6mx6m) and my mates were right "It'll never be big enough". It's already filling up with stuff that my wife, my parents and the in-laws want storing

    Anyway, until I can afford to put power to the shed I was thinking of using a generator. I can get my hands on a 2500 Watt (2800 W peak) genny. Would this be big enough to run say a 1HP jointer or 1 1/2 HP table saw?

    Thanks,
    Geraint.

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  3. #2
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    NSW
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    Aaaah, yes. Murphy's second law of shed sizes. Murphy clearly states that possesions will acumulate to a volume of 110% of the space available to store them. This means that any storage space can NEVER be large enough for all the things us blokes need!
    Now, the generator......2500 watts is a bit on the small side but you should get by.
    Induction motors can draw anywhere up to 10 times their rated power when they start. This means that your 750W (1HP) motor will draw something like 4,000Watts, perhaps more, for a fraction of a second when you switch it on. The genny will slow down for a second as the load comes on, then pick up revs again as the electric motor settles down to it's running current.
    The 1 1/2HP saw could be a bit diferent. If there is much of a load on the motor from the drive belts and saw spindle, as the motor starts it could draw more current than the little genny can handle and this could be a real problem, both for the genny and the motor on the saw. Too much load usually means the voltage the genny can supply falls dramatically and eventually you could, and I repeat could, not will, burn out either the motor or the generator.
    Speaking from experience, I have to run all my tools from either solar power or a petrol generator, I would sugest you go for something bigger. My generator is 6KVA which works out to be about 5,000Watts in the real world. If you get the 2,500W genny for free, take it. If you are going to buy one, spend a couple of hundred $$$ extra and go for the largest one you can afford with electric start.
    Buy a generator made in Australia with a genuine Honda motor, not a cheap Chonda motor from China. Fuji Robin (Subaru) are good and the Briggs "Vanguard" were, last time I heard, made in Japan. Kawasaki are pretty good also. If you can find a motor with an oil filter on it, so much the better and follow the manufacturer's recomendations for the type of oil the engine uses. Many small engines need a straight 30 weight oil, not the 20W50 that is used in cars.
    Good luck with your shed, you can always do an addition later!
    Dennis.

  4. #3
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    Default

    Thanks Dennis,
    I appreciate the help.

    Geraint.

  5. #4
    Join Date
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    Garvoc VIC AUSTRALIA
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    Default

    The rule of thumb for sizing a generator is that the genny needs to have 3 times the load power to enable motors to start and run.

    Electric motors are generally only 40 to 60% efficicient so a true 1hp motor ( ie a motor that produces 1hp at the motor shaft) will actually consume about 1500 watts under full load and be aware some elcheapo chinese machines claim their 750 watt motors are 1hp whereas they only produce about 1/2 hp.
    Regards, Bob Thomas

    www.wombatsawmill.com

  6. #5
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    Location
    Gold Coast
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    Default

    Quote Originally Posted by Old Hilly View Post
    Aaaah, yes. Murphy's second law of shed sizes. Murphy clearly states that possesions will acumulate to a volume of 110% of the space available to store them. This means that any storage space can NEVER be large enough for all the things us blokes need!
    Now, the generator......2500 watts is a bit on the small side but you should get by.
    Induction motors can draw anywhere up to 10 times their rated power when they start. This means that your 750W (1HP) motor will draw something like 4,000Watts, perhaps more, for a fraction of a second when you switch it on. The genny will slow down for a second as the load comes on, then pick up revs again as the electric motor settles down to it's running current.
    The 1 1/2HP saw could be a bit diferent. If there is much of a load on the motor from the drive belts and saw spindle, as the motor starts it could draw more current than the little genny can handle and this could be a real problem, both for the genny and the motor on the saw. Too much load usually means the voltage the genny can supply falls dramatically and eventually you could, and I repeat could, not will, burn out either the motor or the generator.
    Speaking from experience, I have to run all my tools from either solar power or a petrol generator, I would sugest you go for something bigger. My generator is 6KVA which works out to be about 5,000Watts in the real world. If you get the 2,500W genny for free, take it. If you are going to buy one, spend a couple of hundred $$$ extra and go for the largest one you can afford with electric start.
    Buy a generator made in Australia with a genuine Honda motor, not a cheap Chonda motor from China. Fuji Robin (Subaru) are good and the Briggs "Vanguard" were, last time I heard, made in Japan. Kawasaki are pretty good also. If you can find a motor with an oil filter on it, so much the better and follow the manufacturer's recomendations for the type of oil the engine uses. Many small engines need a straight 30 weight oil, not the 20W50 that is used in cars.
    Good luck with your shed, you can always do an addition later!
    Dennis.

    Excellent advice Dennis, spot on.

  7. #6
    Join Date
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    NSW
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    Default

    Just to add a comment to Echidna's Post, not only is the power output of some of the Chinese motors seemingly low, what about the size? Back when I was a bit younger a 1/4HP motor was fairly heavy and you needed both hands to lift it. I bought an el-cheapo benchtop drill press from Woolies a couple of weeks ago that had a 175W motor and I could pick the motor up with one hand. I wanted the motor to run a very small wood lathe and at $49 it was pretty cheap.
    The rest of the machine will come in handy somewhere along the line.
    Dennis.

  8. #7
    Join Date
    Apr 2002
    Location
    Brisbane
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    Default

    Id be very interested to hear how you go

    I've had a couple of experiences with generators and motors.....&.... you never can tell.

    If the generator isn't reated for inductive loads ( that is motors) things often don't go well.

    I was supplied a 5KVA generator to use on a construction site with a 3HP 9" angle grinder.
    We ended up having to hand throtle the generator to get the job done........as soon as the trigger was pulled it would snuff.

    during the SEQEB disputes in the eighties....I ran my 2KVA to run my mum's washing machine.......used a 500W QI to provide some resistive load......no problems........it wouldnt even look at the neibours hoover twin tub.

    give it a try.....keep the leads short and heavy......maybe use a 500QI or two to provide some resistive load.
    Listen to the machines.....if they sound unhappy....stop.

    cheers
    Any thing with sharp teeth eats meat.
    Most powertools have sharp teeth.
    People are made of meat.
    Abrasives can be just as dangerous as a blade.....and 10 times more painfull.

  9. #8
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    Default

    Those induction motors are a real problem for a generator. Most washing machines use some sort of induction motor (the F&P Smartdrive things could be diferent) and they want heaps of power to get them going. In the woodworking area we might be able to cheat a bit by letting the motor come up to speed without any load on it, then apply the load when things have settled down and the generator is running normally.
    As I mentioned before, my generator is 6KVA and is OK for my small workshop but if I had to run a big bench saw or drum sander I might be in real trouble!

  10. #9
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    Default

    I wouldn't worry about inductive loads on a generator. It's electronic loads that they don't like (harmonics).

    I've prepared an example of how to size a generator in the attachment below. If you want long life & reliability from your generator, it must be correctly sized. "Rule of thumb" methods are asking for trouble.

    HP (horsepower) or Watts mean very little on small induction motors. Many small motors are very inefficient & so will draw more current than expected.
    To overcome the "guesswork", you must look at the rating plate on the motor & use the information on it.
    For example; Volts - 230. Phases - 1. FLA - 4.5 amps. LRA - 19 amps.
    Watts (input) - 1400. PF (Power Factor) - 0.8.
    In this example, the motor is about 59% efficient, which is not uncommon for small & cheap motors.
    So, do not use the power rating (HP or Watts) to calculate your current. Use the listed current/voltage ratings on the rating plate.
    “I do not think there is any thrill that can go through the human heart like that felt by the inventor as he sees some creation of the brain unfolding to success... Such emotions make a man forget food, sleep, friends, love, everything.” - Nikola Tesla.

  11. #10
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    Default

    Elcan, that is by far the best explanation I have seen for calculating the load for a generator and from that estimating the size of machine required. I always used to very roughly calculate the load, add about 15% and then buy a generator of twice that capacity and most of the time I got away with it.
    Your calculations will be of great help to me in the future.
    Thanks for your info,
    Dennis.

  12. #11
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    Thanks to everyone who replied to this post.
    It looks like you may have saved me a few bob and a lot of frustration.

    Geraint.

  13. #12
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    Default

    Only too happy to help. Hope it all works out OK.
    Dennis.

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