Inverters - again

[The Medway Handyman]

Getting to the point where an inverter starts to make more & more sense. Phone charger, drill chargers, light bar charger etc.

Looking at these

mentions in the spec "These inverters generate a modified sine wave, which although not perfect, is considerably superior to the square waves which are produced by most other inverters".

Is that a good thing?

It also mentions Peak power - 1000w Does that mean it would run an 800w angle grinder for long enough to cut off a padlock or wheel clamp?

[John Rumm]

Generally yes - some things are not as happy being supplied with a square wave (lots more harmonic noise).

Probably not - it probably means there are enough FBC's in there to cope with a big switch on surge. So it will get something running that draws less than 300W normally but perhaps needs several times that to start. Something with an induction motor like a fridge for example.

[Andrew Gabriel]

Profile boiler and pump. Pump makes a bit of a strange noise due to the harmonics, but it works fine.

The cigarette lighter plug gets very hot though. I've bought a few high current connectors with a view of replacing it, but they've all looked even more feeble than a cigarette lighter plug when they've turned up, so I haven't really solved that one yet. May just go for 30A binding posts and spade lugs.

You might want to check out a Maplin store. They did have loads of inverters (including this range) going cheap (I suspect cheaper than that TLC price).

Yes, but it's also marketing bull****. I've never seen an inverter which produces square waves -- they all produce a modified sine wave. This one basically adds an extended zero crossing period which is as crude as you can get without being a square wave. Some more advanced ones have a stepped sine wave.

[Dave Plowman (News)]

I'm not surprised - they're rated at 10 amps. ;-) The instructions with most inverters say to use the supplied crock clips above a certain output.

Speakon type works fine.

[dom]

For that purpose, this might be the tool:

[Arfa Daily]

"Is that a good thing ?" Well, it depends a lot on what you are intending running on it. Personally, having seen some of the damage that can be done by these things, I wouldn't recommend running anything 'electronic' on anything other than a genuine sine wave. Some of the low quality switch mode power supplies that are now used in some items - and *with* some items when that power supply is an external type - are less than happy with anything other than a sine wave going in. The main filter cap works very hard anyway in these 'small' supplies, and asking it to work harder still as a result of the raw DC being produced by the reccy being a very rough shape, is looking for trouble down the line.

Also, linear power supplies that use a small mains transformer, often don't like anything other than a genuine sine wave, because a bad waveshape can drive the tranny to core saturation, with all the attendant nasties, including overheating, which that can cause ...

Arfa

[Andy Wade]

And are generally crap. I find it surprising that the automotive industry hasn't come up with a better 12 V accessory connector by now. Most aspects of car electrics have improved hugely since the 1960s, but that and the old car radio aerial plug remain as dinosaurs.

A 30 A rated connector using a system of fused plugs would be good...

[Andy Hall]

Yes, but then there would not be enough energy left in the battery to turn over the engine.

[The Natural Philosopher]

They have lots: but they are only used in caravans.

[Dave Plowman (News)]

Absolutely. I hate the things.

Mine has F-type. ;-)

I suppose that's probably the reason the cigar lighter ones have continued

- easy design to have a readily changeable fuse in.

But how often would you need a 30 amp take off inside the car? The common need is for things like phone chargers and I-pods. So a much smaller connector would be fine. I fitted a standard 2.5mm power connector for the Tom-Tom in the dash top and incorporated the charger out of sight. Having a cable draped over the dash really annoyed me.

[Andy Wade]

Very sensible. That's a DAB radio, IIRC.

Well, whatever figure you choose, someone will want more. 30 A was just my guess at a reasonable limit and it's only about 400 watts. Enough to run small mains appliances through an inverter, or for a one-cup immersion heater for brewing up, or for 12 V power tools such as those little compressors, electric winches, and so on.

These days you could have a current limit set under software control, like USB. 5 or 10 A max., say, when the engine isn't running and 30 A when it is.

Whatever happened to 36/42 volt car electrics, which have been "only just round the corner" for the last couple of decades?

[meow2222]

If pk output is 1kw, continuous output will be something like 300w, as has been said.

Now an 800w angle grinder will consume a bit over 1kVA running due to less than ideal power factor, and probably several times that during startup. So your invertor is inadequte by a factor of 3 or more.

You can still run it on the inveror by using a dropper - but the performance would be so dire the discs just cease to cut below a certain speed, so you'd be there all day.

So no.

Oh, all cheap modern invertors produce msw, you wont find square waves anywhere unless you go back several decades. MSW increases power dissipation in motors, making matters worse.

For a padlock you could probably use a die grinder + a few discs.

NT

[Dave Liquorice]

Plenty of 12v microwaves and TVs about. Check the truckers web sites...

I have a 12v kettle 2 mugs capacity takes about 20mins to boil. Pulls a good 10A and the cable gets warm...

Got a "heavy duty" compressor as pumping up one of the tyres from 10psi killed the previous ordinary one. This one is continusly rated, fan cooled motor, doesn't have a rating but probably no more than 10A if that.

Ah a decent whinch takes far more than 30A...

KISS is better I feel. There has to be "smarts" in the kit and the car. The current 10A is enough but a smaller better connector really is needed.

[Andy Champ]

All the ones I've ever looked at are made up from a bit of zero volts, a bigger bit of plus volts, a bit of zero volts, then a bit of minus. If you draw a graph with a very blunt pencil then take your glasses off and put someone else on, then turn out the light you *might* think that's a sine wave.

Nevertheless mine seemed to drive my laptop (which was old enough for me not to care about killing it) for a week on a canal boat.

Andy

[Andrew Gabriel]

Yes, that's exactly what this one does.

Most computer PSU's don't care. Actually, many would run better off a square wave.

[Arfa Daily]

On what do you base that ? A computer PSU is designed to run with a sine wave input. It's like saying that my diesel car runs better if I put petrol in it ...

Arfa

[Andrew Gabriel]

Most computer power supplies are designed to cope with sine wave input, because they have to. The first thing they do is to full wave rectify it to DC. Sine wave is not a good starting point for that -- you either end up with power factor well below 1, or you have to use more complex circuitry to compensate (in larger commercial computer PSU's). If you feed a square wave in, that's pretty much as good as DC, and the power factor will be 1. If you knew you were only going to feed the computer with square wave (and/or higher frequency) and you didn't have to design the PSU to run from 50Hz sine wave, you could make the PSU somewhat simpler and more efficient. Actually, some mainframes used this for decades, and required to be fed with 400Hz 3-phase, which made PSU design much easier, and still more efficient even allowing for a motor-generator set to create the 400Hz supply.

[meow2222]

All true nuff. On an invertor its a bit more complex though. An invertor doesnt put out a square wave, its msw, ie there are 0v periods between each rectangular pulse. During this 0v the reservoirs partly discharge. Now with a sine input, V_mains climbs at a certain rate, and this rate of climb limits the peak i flow during reservoir recharging, along with C size and the various stray Rs around. But when the reservoir is recharged from an msw wave, the waveshape does not limit the recharging i. And since the psu is designed to run on sine, there is nothing but an rfi filter to limit this i. The result is a _very_ peaky current waveform. If wanted this could be tamed with a small inductor.

NT

[Dave Liquorice]

Er, I've never heard of an alternator that can produce a squarewave output... Surely the physical nature of the beast (coils and magnetic fields) means that the waveform is sine (or close to it).

Isn't the improved efficiency down to the higher frequency thus less discharge of the reservoirs for a given load current?

[meow2222]

I dont know the alternators or psus in question, I just wondered if the cost & efficiency gain was down to smaller transformers running at 400Hz, and of course smaller reservoir caps. 400Hz TFs have much lower resistance windings for the same throughput.

Alternators dont produce square output of course, but they can produce non-sine by using a commutator, fwiw. In principle this could remove the need for a rectifier, but what actually was done I dont know.

NT