OT Inverters

Probably.

Most ordinary domestic ones don't. It only becomes a serious issue when you have lots of 'em like in a factory or office. Even then the power factor correction is most likely done in the wiring cupboard rather than in each fitting.

Nick- In my experience these cheap inverters fall down if presented with anything other than a purely resistive load. Even if they do work, the rating is very much reduced. I guess it depends what application you were intending to use one on.

Regards

Philip T-E

"philipte" wrote (snip)

I'm thinking about a small stand-by power set up to run central heating pump, fridge freezer and a few lights in the event of a power cut. Power source to be old car batteries (I know they are not intended for deep discharge applications but they are cheap/free) kept trickle charged from mains and/or wind, solar etc genny, with diesel (perhaps even veg oil) driven automotive alternator to be brought into play if 'outage' persists beyond battery capacity. All a bit of a pipe dream at the moment, but there's no harm in planning!

BTW just done a little lunch time research myself on my original question, and it seems it may be the high harmonic content in the output of cheap inverters which gives problems with fluorescents and also induction motors (so may be no good for fridge). I notice 'eurobatteries' does one stated as ok for fluorescents at about the same price as Maplin.

you can pick up pallete loads of computer ups's in the 3000va range for as little as 20 quid each, sans batteries.

AIUI their waveform is nothing like a sine wave, more like a square or sawtooth I believe, and the ballast (choke or electronic) in the fluorescent light unit would probably not be too keen on it.

An inverter is an expensive transformer, some almost equally expensive power transistors, and control electronics that costs chicken feed in comparison.

Cheap inverters use a simple H bridge and produce a crude near-square wave (+ / 0 / - ). by switching each transistor either fully on or fully off. Some really crude ones just use a simple +/- square wave. It's rare to switch them part-way so as to generate a more accurate sine wave because that increases the power lost in the transistors and thus the power rating (and cost) required. Modern designs use high frequency PWM (pulse width modulation) then a low-pass filter to simulate a sine wave before feeding the transformer. This is the best compromise because modern power transistors are easily driven at high frequencies and any extra complexity in the control is compensated for by

Square wave designs have an output that's rich in high frequency harmonics. These cause problems for any inductive load. For fluorescents then they work well enough with newer solid-state ballasts but for the old iron-cored ballasts they increase the heat losses in the ballast. You can generally work them, but you have to de-rate the inverter by a factor of 2 or 3.

FWIW, Nick, I bought one to run a small CRT TV that someone gave me. After a

200 mile run, an hour's use left the battery too flat to start the car in the morning.

Regards,

Kim Siddorn.

"Kim Siddorn" wrote:-

Ones I've looked at claim around 90% efficiency, though I guess that is at full rated output. How thirsty was the TV?

Nick & List,

These cheap 12V input inverters often have a high idle current. Some draw 150mA or more, on no load.

The immobiliser in my wife's Peugeot 106 diesel, also has a heavy draw on the battery, if left in the "drive" condition. It also will flatten a good battery in a few hours (especially in cod weather) - effectively immobilising the car.

There are many of the 12V inverters available, some even claiming 5kW continuous and 10kW peak power. Clearly drawing 5kW plus froma car battery is impractical from the large currents involved, which would need very heavy duty cables.

IMHO, anything more than a 3kW inverter should have an input voltage of at least 48V, in order to keep the input current manageable. Drawing several hundred amps from any sensible sized 12V battery is a sure fire way to an early battery demise.

My 5kW continuous rated inverter has a 108V dc input, but then it was UK designed and built in the early 1990s, before the advent of cost-cutting cheap chinese junk.

At the end of the day, you get what you pay for. A well engineered, continuously rated 3kW inverter designed specifically for solar or wind power off-grid applications (such as SMC WindyBoy) will cost about =A33000, and not the $599.99 as seen on a well known online auction site.

Ken

Your central heating will want something in the order of 100W (assuming a gas boiler not oil). I don't think that CH circulators have a nasty switch on surge. How ever your fridge freezer will and that may kill a small invertor or make it shut down. Those small 650W

2 stroke generators can struggle to start a big fridge/freezer. Motors need two to three times their rated capacity to start.

If you are going that far I think it would be better to get a proper 2 to 3KVA genny, little point in faffing about with batteries, invertors battery charging etc. Your fridge freezer will be fine without power for 12+ hours. Heating well, call the power co and see if they have an estimate on the fault (you may find you are the first reporting it other than the automatics, I frequently am) then decide if another jumper or dragging the generator out is the best course of action.

Oh I agree, power goes here we have everything to hand to carry on pretty much as normal, sans TV and computers. Two ring and grill camping cooker, gas lantern(*), portable gas fire, light sticks (mainly for the kids comfort), "shaker light" torches to find the gas lantern, other battery torches and lanterns as well. Good stock of tinned/dried food.

(*) Best emergency light source there is IMHO, get one with piezo ignition and you don't have to faf about with matches lighters etc. They have really good run times (8hrs+) on a small and cheap cylinder of gas. Most battery lanterns don't last much over 3hrs if that.

Car batteries really do not like any significant discharge. Whack a few hundred amps into a starter motor for a second or three then get a hefty charge current back straight away is what they are designed for.

"Dave Liquorice" wrote (snip):-

Half the object of the exercise (or at least the thought experiment) is to see what can be done for little or preferably no expediture using recycled junk - I have a suitable engine, old batteries and and an alternator should be no problem, that just leaves an inverter, and it has been suggested elsewhere that it may be possible to press a redundant computer UPS into service in that capacity. An alternative would be to use an old three phase motor as an asynchronous induction generator, perhaps with a small inverter just to fix the frequency.

Don't know where you live Dave, but I don't anticipate having to be quite that 'prepared' in W. London! I get the impression our corner shop would probably still open in the wake of a nuclear attack - mind you, the proprietor did take Christmas day off for the first time in living memory last year ;-)

Colour or B&W? Colour portables take a lot more power. That's why generally, only caravaners with access to mains or generators use them. The later ones may be less power hungry but LCD's are the best. They're also the most =A3 and only available with small screens. Of course, one might also question the age of the car battery you were using. Given a 50Ah battery and assume 50% is needed to start the car. That means the TV drew 25Ah. in one hour, that means 25A. That's rather a lot. I would have thought you would also have noticed the 300W of heat from a little TV.

John

10" black & white. 70Ah battery

The battery was about a year old, but I was at the end of a 200 mile fast run, mostly in daylight & that's why I was surprised when it was as flat as a witch's trivet in the morning. Never had a problem before or since ;o))

IMHO they should have an input voltage of a few hudred volts. When you get to that sort of power it's worth going for a direct drive design so you can avoid the need for a transformer. Assuming any sort of energy capacity at that power, you need sufficient battery capacity in total that the large number of cells isn't such a problem.

If you want multiple kW for low capital costs, get a generator.

Well there is that side, the satisfaction of reusing "junk" and the satisfaction of making building something. Of course you need to balance all that effort against the response from the rest of the household if it fails when called on in anger...

1400' up on the North Pennines. 3 miles from the nearest town with a small Co-op. 20+ miles from any other place with any bigger shops. The area gets cut off by snow at least once a year, normally more frequently. Power is all overhead virtually from the grid to us. If it goes off it's off for at least 6hrs, under normal circumstances. If it went during a storm, that affected a lot of lines, it could be off somewhat longer. This area is not know as Englands Last Wilderness without reason.

There is a flaw with your corner shop thinking. If something does happen every one else will descend upon the shop and it'll be out of stock within hours if not minutes.

When we lived in St Albans there was a Cryptosporidium(sp) outbreak in the water supply. *All* the local supermarkets and shops had sold out of bottled water within hours. Bit blooming daft really as all you had to do was the normal rolling 10min boil to kill the bug off. ISTR that the water company sent us, prehaps everyone affected, a =A310 cheque once it was sorted out to pay for the additional fuel used.

I did buy one of those 650W two stroke gennys, which soon acquired the soubriquet 'Little Stinky' When we had a couple of power cuts it was pressed into service running the house, and it certainly struggled when the fridge or freezer started up. It has since been replaced by a 2.5kW Honda powered set, which is in a different league altogether. Sadly, the 1928 Lister-Light, with its 50 volt DC output can't be pressed into service......unless I get an inverter :-))

Regards

Philip T-E

"Dave Liquorice" wrote (snip):-

There is a flaw with your corner shop thinking. If something does happen every one else will descend upon the shop and it'll be out of stock within hours if not minutes.

Yes, that was slightly tongue in cheek ;-)

No2 son just returned from the local Maplins, they are selling 300W (continuous) output quasi-square wave inverters on special offer for £20 each including 2 sets of leads (croc's and ciggy lighter)

We wouldn't make the box for that!

Peter

-- Peter A Forbes Prepair Ltd, Luton, UK snipped-for-privacy@easynet.co.uk