Question re Inverters

[boatmike]

Heres one for the technical whiz kids out there.
I want to use an inverter to supply power to run power tools, vacuum cleaner (little one) charge mobile phone, computer etc.
The highest current draw will be 800W continuous but only for very short periods.
I am told it is best to have full sine wave but modified sine wave inverters are a lot cheaper.
Question: why do I need full sine wave and what are the problems running stuff from modified wave inverters. Thought one about 1000W would be more than adequate which I can buy for about £130 but I see others at a much higher price and am not sure of the wise course of action. Alternatively I do have a 2 stroke generator that will run power tools etc and could make do with a little one for computer etc.

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[TrueBlue]

It's not only the continuous power consumption that's important it's what capacity it has to provide the surge power to start the motor. As there are no standards, some manufacturers fudge the figures to make the devices look more powerful.

As to the sine vs. modified square wave output, I have found that nearly anything I have works adequately with the latter - large hoover vacuum, Hitachi power drills large and small, Home cimena, two sorts of laptop, video recorder, 21" tv,Freeview STB.

The only things that didn't work was a recent philips video (bad design), and a very small moulinex food processor (cheap motor).

Because the "modified sine wave" - really a stepped square wave doesn't give true RMS power you will notice a slight diminution in power when using motor appliances - but it's only slight. Modern units are very efficient so there's no real disadvantage in running a tv or laptop from a high power unit.

I have a Sterling 1800w unit and it's 4 years old. Works for me, but then I have 600aH capacity at 24v, so I've got load o' grunt.

As with any power you don't get owt for nowt, so 800w at 12v = 66 amps, say 70amps allowing for inefficiencies. Make sure that you have at least 150aH capacity in your battery bank.

Hope this is useful / or stimulates feedback from others.

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[[2574]]

Interesting. Thinking about power delivery using your example, I imagine that to deliver 70A one needs a wire as think as your forefinger!? Otherwise I can imagine a fire pretty pronto - Am I right or is it Wattage that determines cable thickness not Amps? (ie is it irrelevant to cable diamieter whether 800W is delivered at 240v or 12v)

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[boatmike]

Thats pretty much how I saw it Trueblue but I am a Mechanical Engineer not Electrical so like to ask first! Figure if I have a 1000W inverter I can run power tools et al for short periods and other stuff for longer. I have a 300AH capacity at 12v for service bank (engine start seperate) and have smart charging on engine + solar which keeps them pretty well topped up. Should be alright I guess but other contributor also has a point that it deserves some pretty heavy battery cable too.
Hmmm. Might take my little portable genny for power tools and stuff and buy a little inverter for computer and charging mobile phone etc. That alone sounds silly though as I can buy an in car 12 volt charger for that can't I?
Anybody else have any thoughts on the subject?


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[boatmike]

No you are right Robih it's Amps. The higher the voltage the smaller the wire that's why 24 volts is better than 12. This much I know!

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[TrueBlue]

Further thoughts:-

Whatever cable you use, the inverter should be near the battery as practicable, perhaps no futher than 2m. in order to reduce voltage drop. Yes, you will need a fairly chunky cable, but you could use a reasonable jump start cable set (merely for the ease of buying). A decent motor factor should be able to supply you with proper starter cable. You must be able to isolate the circuit and a battery isolator from the same source should do. I fitted a sort of circuit breaker rated at 100amps in case of overload - the sort sold to youngsters who insist on cooking their hearing by fitting 500W audio amps in their cars.

Perhaps a genny would be easier, and yes, a cheap 300W inverter will do the other jobs. Cost from £25 - £40, but they do come with some awful cheap cable and croc clips.

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