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Forum Index : Windmills : Calculating hp for a generator
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| maaca Newbie  Joined: 21/11/2006 Location: New ZealandPosts: 22 |
Hi Guys does any-one know of a formula to calculate the required horse-power to drive a generator?. I have an old single banger 4-5 hp Petter engine and the oppurtunity to get a 3kva generator.Problem is I dont know if the old Petter would have enough grunt to drive it. Can some-one help?.Many thanks Maaca. |
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Gill Senior Member  Joined: 11/11/2006 Location: AustraliaPosts: 669 |
G'day maaca, Now this is not my best subject as I'm better at finger painting, but here goes. 3kVA is equal to 3k watts and my conversion programme tells me that is 4.023HP in the old speak. Now that's output, so we will have to add generator efficiency losses, and transmission losses(direct drive best, V belt or gearbox more lossy). So I will say you'd be on the very limit or over. Remembering that as engines get older they loose HP, but on the positive side, if you were drawing less, say 1.5 or 2kVA then it would cope nicely. I'm sure that's confused you as much as it has me, so will quit while I'm ahead. Good luck. was working fine... til the smoke got out. Cheers Gill _Cairns, FNQ |
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| KiwiJohn Guru  Joined: 01/12/2005 Location: New ZealandPosts: 691 |
I think it is a reasonable match and the generator is big enough to handle all the power of the engine. I doubt you will get the full 3KVA but that, IMHO, is not so important as being able to load the engine to its optimum. Actually, I am pretty good at finger painting too.  |
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| maaca Newbie Joined: 21/11/2006 Location: New ZealandPosts: 22 |
Hey Thanks Guys,2kva would do the old whitebaiting batch but leaves a bit of a concern as to if the fridge or freezer would fire up.Stupid things seen to take a lot of starting current to get up and going all be it that they are not large.Thanks for your comments. Maaca. |
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| petanque don Senior Member Joined: 02/08/2006 Location: AustraliaPosts: 212 |
I would agree with Kiwi John that the engine will spin the generator ok but if you put a large load on the generator the engine will struggle at there is likely to be voltage drops. Equally if you are only likely to ever place a 500w load on the generator the mismatch may not be an issue. Would it be an idea to do a search on Google and try and find what size engine the people who make commercial 3kva generators use. Why reinvent the wheel if someone else has done a lot of research and development? How much life has the “old Petter” got in it? Is the “old Petter” reliable enough to give satisfactory service? |
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| maaca Newbie Joined: 21/11/2006 Location: New ZealandPosts: 22 |
Thanks petanque don. The old Petter came off a fire pump on a Royal Navy ship that was scrapped or sold or whatever.A vintage engine enthusiast has assured me that it has been done up. When starting if you dont get the decrompression lever accross at the right time then for every turn you did one way it will turn you the opposite way.Compression for a 4-5 hp motor is unbelievable. Thanks Maaca. |
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| KiwiJohn Guru Joined: 01/12/2005 Location: New ZealandPosts: 691 |
I am tending to think that this is actually close to the ideal combination. Perhaps, and especially if you can add another flywheel, the inertia of the engine and the capacity of the alternator will get you past the mementary demand of starting the freezer. |
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Robert_VK2BBR Regular Member  Joined: 31/12/2006 Location: AustraliaPosts: 67 |
Macca i was keen on thjis idea too, there are plenty of single cylinder Listers out there. My mate who has lots of experience in the field told me that they will not maintail revs under changin g load conditions, and hence frequency of the AC output will be all over the place. This may or may not be of importance to you. They will, however, drive an alternator well for battery charging . cheers Robert 1555 W grid connect system since Jan 08 both vehicles running on modified, used vege oil. |
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| Feral Newbie Joined: 03/12/2006 Location: AustraliaPosts: 39 |
It does depend on what the rating of the sealed units in the fridge and freezer are. The motor would appear to be under power for the 3 kva but will not be a problem as long as the frequency and volts are maintained. It should run both units at the same time and start either unit sepeartely but not start both at once depending on the size of the electric motors in the freezer and fridge. The starting currant is only required for a couple of seconds and should not be that noticable unless the electric motors are very large. Is the motor designed to run at 3000 rpms or is it 1800 to 2200 rpm ? |
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| maaca Newbie Joined: 21/11/2006 Location: New ZealandPosts: 22 |
Thanks for that Feral Actualy the engine is either 10-12 hundred revs flat tack.The freezer is medium to small size and the fridge is a combination.It would be highly unlikely but still possible that both would start together. Thanks Maaca. |
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herbnz Senior Member Joined: 18/02/2007 Location: New ZealandPosts: 258 |
Hi The power of the engine seems fine to me and large flywheel wiil (inertia ) will supply peaks during stating . I have a freezer and fridge on my system ocassionally if the inverter goes out on some fault the restart can cause problems both starting together and under pressure if restart is after a short time. My answer and would be benifit to this situation is to fit a cheap time delay to both fridge and freezer that delays 5min after power up this allows any pressure to drop if both different starting together unlikely. herb |
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windstuffnow Newbie Joined: 30/06/2006 Location: United StatesPosts: 31 |
I seem to remember the rule of thumb for gas engines was 2hp per kw and for diesel was 1.5 hp per kw. I have an Indian built Lister Clone (GM90 version) it's an 8hp single that I mated up to a 5kw china genny. It has no problem running my whole house. I would think if its a 4hp you should be able to pull 2.6kw from it and at 5hp should do 3.3kw. Gotta love those diesels! I had a gas backup, 5.5kw that burned a gallon an hour while running the house - very expensive and wasteful! My diesel thumper pokes along at 900 rpm and runs for 6 hours on a gallon of fuel.... I'll never - repeat never, go back to gas. Another bonus from the diesel is you can supliment the fuel from other sources. I've run vegie oil in it and it didn't run any different - not noticeable anyway other than the smell. Good luck with your diesel project... you'll love it when your done! |
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| dwyer Guru Joined: 19/09/2005 Location: AustraliaPosts: 574 |
Hi Windstuffnow and all you guy Windstuffnow In Aussie way we call fuel as "petrol" not gas and any motor vehicle run on gas are call "running on LPG" so we call motor vehicle running on LPG is correct words is gas. LPG is mean Liguified Petroleum Gas. or LPG gas stove,gas heaters,gas hot water or LPG use for industrial 
Now next question Neary all Diesel engine including Lister and Petter have rpm control call governer to maintain speed so for sample if diesel running on 1500 rpm suddeny load come on rpm will drop and same time the governer will increase fuel compustion therefor 1450 rpm will go back to 1500 rpm so if is overload the exhaust will puffing out black smoke .Diffrence between single cylinder  and 4 for cylinder so 4 cyl are smoother runnning than single cyl like running house light ficking each cyl fire during every two cycle 
Dwyer the bushman |
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vasi Guru Joined: 23/03/2007 Location: RomaniaPosts: 1697 |
Hello guys, this engines have propeller for cooling? I have a small front-tine tiller that I use in my vegetable garden (a gift from my uncle - Brigss and Straton engine) but I can't run it more than 2 hours because don/t have propeller for cooling. Hello Windstuffnow, great respect for you (Vawt Lenz turbine and your site), I have a proposal for you: please help us with this topic Hobbit name: Togo Toadfoot of Frogmorton Elvish name: Mablung Miriel Beyound Arduino Lang |
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| KiwiJohn Guru Joined: 01/12/2005 Location: New ZealandPosts: 691 |
Hi Vasi I think the Briggs and Straton engine has the fan (i.e. propeller) moulded as part of the flywheel? Maybe you are only missing the cowling and I think a practical person could make one of those? |
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| dwyer Guru Joined: 19/09/2005 Location: AustraliaPosts: 574 |
Hi Vasi Can you able to post some picture of Brigg and Straton engine and how many KW or HP output ? 
Dwyer the Bushman |
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| vasi Guru Joined: 23/03/2007 Location: RomaniaPosts: 1697 |
Yes, very soon. For now, engine have (had  ) 5.5 hp. My brother used it without respect for "user manual" <-- this smiley is too happy for that.Hobbit name: Togo Toadfoot of Frogmorton Elvish name: Mablung Miriel Beyound Arduino Lang |
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| vasi Guru Joined: 23/03/2007 Location: RomaniaPosts: 1697 |
If is one (propeller - maybe connected on starter) is not enough because running time is 40 minutes then 10 minutes pause. 
I will send another one from the other side (this one I had on computer hdd). Hobbit name: Togo Toadfoot of Frogmorton Elvish name: Mablung Miriel Beyound Arduino Lang |
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| dwyer Guru Joined: 19/09/2005 Location: AustraliaPosts: 574 |
Hi Vasi What this engine worried me this American engine Brigg and Straton design to run very hot than Honda engine so they will wear out sighing quicker so when engine slowing refuse start over the time reason is until no compressor left in the cylinder is worn out so anyone will have to force to throw away or replace motor or buy new one and this design by Brigg and Straton to keep the business going and other secret on engine timing is away out of adjust (approx 20 deg out) at high rpm design to wear the engine quicky and of course hotter and this was post on webs site fews years ago Only thing you can do when engine start to get hot simple spray fine mist of water on top of alloy cylinder head while is running until cool down but spark plug cover must be waterproof. Anyone to find out about this horrible Brigg and Straton engine is some where on webs site so good luck . 
Dwyer the bush |
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| vasi Guru Joined: 23/03/2007 Location: RomaniaPosts: 1697 |
Yes dwyer, you are right. I already have starting problems, loosing compression. Few days ago don't started at all. Hobbit name: Togo Toadfoot of Frogmorton Elvish name: Mablung Miriel Beyound Arduino Lang |
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