Moorron
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| posted on 5/12/11 at 01:54 PM |
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Advice on a small petrol genny.
Hi guys im asking here as i think some of you might use generators in the garage. Ive just bought a 2 stroke clarke genny with 700W of power, but it
doesnt seem to power my 600W 240-12v power supply i use to power a radio controlled battery charger. In theory i should be able to pull the full 40
amps @12V on the power supply to feed the battery charger but the genny just drops rpm and nearly stalls when i draw anything over 18 amps @ 12v. The
other reason is the charger pauses every 2 minutes to check the battery's condition before carrying on charging so the load on the genny is
choppy at best.
Have i bought something that i have specified as the wrong type or is the genny faulty? I havent even fun the genny for more than 2 hours yet so is it
running in and tight (so not able to produce full power yet). Can i force the genny to run at full rpm instead of letting it to try and keep up with
the changing demand from my charger?
if not then ive wasted £100. 
cheers
Sorry about my spelling, im an engineer and only work in numbers.
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HowardB
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| posted on 5/12/11 at 02:47 PM |
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In general where ever I have used a genset I've used one with 3x capacity, ie I want 500w, then I've specified 1500w,... otherwise it
maybe that there is a problem of compatability,..
is the charger a switchmode device,. and is the genset of inverter design?
Howard
Fisher Fury was 2000 Zetec - now a 1600 (it Lives again and goes zoom)
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MikeRJ
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| posted on 5/12/11 at 03:20 PM |
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Those cheap 2 stroke generators are very optimistically rated. It might be able to provide 700watts peak into a resistive load for a very short
period, but the maximum continuous rating is likely somewhat below 600Watts. They have terrible regulation as well (self excited alternator), so
driving a constant power device like the switched mode converter that is almost certainly in your charger is very difficult (i.e. as the charger draws
more current, the generators output drops, so the charger will try to draw even more current, voltage drops further etc).
Howard's rule of thumb is about right, you shouldn't select a generator with a peak power rating anywhere near what you want to draw
continuously from it.
[Edited on 5/12/11 by MikeRJ]
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owelly
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| posted on 5/12/11 at 04:03 PM |
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My chum bought a 900kw genny and asked me to have a look to see why it was struggling to run his angry grinder. I plugged a 500w halogen lamp into it
and it almost stalled it.
As said, those cheap chinese gennys must use a different flavour watt to us! Whats the Kva rating of it?
[Edited on 5/12/11 by owelly]
http://www.ppcmag.co.uk
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v8kid
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| posted on 5/12/11 at 04:52 PM |
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Funny I've got one of these generic generators and it's run faultlessly for 5 years! Use it all summer as the garden is too long for an
extension cable.
Last used it on Bro's boat to power a 900w pressure washer and it managed fine although it did make a bit of smoke!
Guess its your donald duck
Cheers!
You'd be surprised how quickly the sales people at B&Q try and assist you after ignoring you for the past 15 minutes when you try and start a
chainsaw
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carlknight1982
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| posted on 5/12/11 at 05:42 PM |
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Generators work on a power factor of .8 so the maximum capacity of the generator is .8 of the rated output, ie 1000w or 1KW is .8KVA or a maximum of
800w, allowing for whasts called load acceptance ie a maximum load jump of 30% you can hit a 900w generic generator with a maxium of 250w at a
time.
as you lower the voltage the amp capacity does increase but bear in mind tha tthe 12V side of the generator is only usually used to control the
magnectic field and give a stable volage Not as an output of its own.
any more help needed drop me a u2u
Carl Knight
Senior Engineer
Generator Power
Southern Division.
Logic will get you from a A to B
Imagination will take you everywhere.
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Ninehigh
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| posted on 5/12/11 at 06:16 PM |
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Also isn't there a wattage difference when you change voltage?
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big-vee-twin
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| posted on 5/12/11 at 06:21 PM |
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A generator is quoted at 0.8 to allow size comparison and this is the standard that all manufacturers quote, if the load is different to 0.8 you get a
different power output.
It actually depends on the connected load p.f. what you get out.
If your p.f. is higher than 0.8 (0.9) then you will get a bit more out.
If you p.f. is lower - in the case of a switch mode power supply about 0.6- you will get much less.
Watts are Watts the voltage or current changes
Duratec Engine is fitted, MS2 Extra V3 is assembled and tested, engine running, car now built. IVA passed 26/02/2016
http://www.triangleltd.com
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dhutch
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| posted on 5/12/11 at 09:03 PM |
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600wa is a lot to ask of a cheap 700w I would say, particually into power supply as said.
Assiming its for hobbie/infreqent use and you dont want to pay the price of a new one I would be looking at secondhand gennys from reputable brands,
Honda is one of the best know brands for suitcase gennys but there are loads of other good ones for sesable money, less if you prepaired to go for a
more bulky frame-genny.
[Edited on 5/12/2011 by dhutch]
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Moorron
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| posted on 5/12/11 at 09:24 PM |
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cheers for the replies guys, i knew you would shed some light on it.
A bit more background to this, my friend who uses an identical power supply and charger bought a clark 1000W 4 stroke genny with 'pure sinewave
tech', this supplied power to both our chargers and seems to work well when we program the chargers to draw no more than 22 amps each (great
chargers too). This only worked when we set the genny to full at full power not eco mode which my genny doesnt have. This is why i thought i could get
the lower model and still be able to run atleast 22 amps and closer to 40.
When i first tried the charger out it worked upto about 22 amps but kept resetting, so i bought one of those units that sits infront of a computer to
smooth the power supply out thinking it would act like a buffer, this made it worse if anything and i could only draw 18 amps max. i understand its
not just a simple change in amps/volts as each unit is only about 85% effiecent but it 12v and 18 amps im not even using 300 watts of power.
i was trying to go the cheaper root and spent £100 on a 700w unit not £300 on the better model but it seems i lost the battle. grrr
The honda ones do look good but for the odd weekend i didnt want to spent £500 on one. 
Sorry about my spelling, im an engineer and only work in numbers.
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