Cleaning up output from a modified sine wave inverter

[tony t]

Morning,

I have a cheap 12v dc 3000 watt inverter ( power jack) that produces a modified sine wave, i was wondering is it possible to clean up the output to make it more acceptable for more electronics.

Or do i have to buy a expensive pure sine wave inverter

I am just starting out in RE

This inverter was one i used for small power tools.

Just received my neo magnets yesterday for building Hugh's 4 foot genny.

Purchased them in the USA and had them shipped to UK. It was still cheaper than buying them in the UK( we get so ripped off!)

Cant wait, also just been given 10 kg of resin ( used in the electrical nindustry for underground joints on high voltage, need to look at the spec a bit closely) this might save some cash.

Anyway going away to work now

Regards

Tony

[TomW]

tony t;

I think that you got what you got there. With the right 1:1 transformer you might knock some of the edges off the waveform. About the time you get it tweaked, it will probably die anyway.

However, I think there is a LOT of misinformation out there on how "bad" cheap inverters are for electronic devices.

I used one for years running computers, televisions, stereos and the like with absolutely no problems. These devices had switching power supplies in them which seem to eat whatever you feed them.

Motors, however, do not like them much and run hot.

Opinions vary on this so I just cite my real world experience running cheap inverters. I now have a sweet 2.5 kw Outback and it runs anything I want it to run within reason. Of course it took a big wet bite out of the budget. Silly expensive hobbies I seem to have.

Cheers,.

TomW

[thunderhead]

I have a cheap 12v dc 3000 watt inverter ( power jack) that produces a modified sine wave, i was wondering is it possible to clean up the output to make it more acceptable for more electronics.

It is.  What you need is a filter.

Unfortunately the filter consists of very large value inductors and capacitors, that are capable of handling the sort of currents produced by your invertor.

That means they are probably going to cost at least as much as the difference between your cheap invertor and an expensive one.

[tony t]

Do you have a schematic that you can post.

I work as a industrial electrician, and we remove a lot of old power factor correction capacitors etc, i thought that would be suitable.

Any further help is greatly appreciated

regards

tony

[drdongle]

  I have had some limited success with cleaning up inverter outputs by using a large value capacitor in parallel with the out put, the perviso is that the out put MUST be transformered. What you then have is a parallel resonant circuit. If your inverter does not have an output transformer than you must add one ( 1:1) ratio for this to work.

If this is some thing you want to try then the transformer will have to be rated for 3KW or better and you will need a scope to watch the out put waveform while experimenting with various values of capacitor to get a relatively clean shape.

Keep in mind as some one else said many, devices, particularly those with switching power supplies ( TV's, Computers, many recent stereos) don't care about the wave form and in fact will work on 60 Hz, 50 Hz, 400 Hz, or even DC.

[Lurker 417]

I, too, have had good luck with modified sine wave inverters, but I always thought that if the need arises, I could build some kind of filter or conditioning circuit for just the load that balked at the inverter's output. For example, a new TV might misbehave on the inverter's output, so maybe clean up the power it uses with a small isolating transformer and a few caps. I have picked up a couple of isolating transformers for cheap on eBay, for other projects. No need to clean up 3kw of power just for one cranky appliance.

[BigBreaker]

It's a big deadly cap you are talking about.  I'm a trained (though not practicing) electrical engineer and I would go VERY slowly through any project involving large capacitors at mains voltages.

[tony t]

in the uk for work on construction sites our tools are 110 volt, therefor we use 230-110 volt step down transformer, rated at 3.3kva these work out about £40.

I might try 2 of these and wait on obtaining some free capacitors.

regards

tony

[Tom in NH]

I spent a lot of time trying to figure out a way to do the same thing when I had a microwave oven that refused to work on anything but a nice clean sinewave. A modified sinewave is basically a square wave. A squarewave is a sinewave with harmonic distortion. A filter designed to remove the harmonics will produce a sinewave. It is not easy to design a filter because the values of the components vary as the load changes. I tried and failed many times.

Other ways to change a square to a sine is with something like a Sola constant voltage transformer. They seem to be able to clean up anything, but there is a cost. They are big, heavy, and noisy. They run hot and use a lot of current to keep the primary coil energized. They're not so bad if you're running at the transformer's maximum load, but efficiency falls off dramatically if you're running less.

Another possibility is to buy the right kind of line conditioner. I have one made by Elgar that is neat the way it works. There is a reference sinewave generator in it and a differential amplifier. The squarewave input is amplified the exact amount to match the output of the sinewave generator. The result is a perfect sinewave. I've not tested the efficiency of this baby, but I imagine it is higher than the transformer. It worked pretty well, but when I tried to run my microwave through it, the internal breaker kept tripping. Darn. I eventually gave up on trying to get my microwave going. Good luck. --tom

[drdongle]

Actualy I have had success with relatively low values. I built a 20 Hz inverter for ringing phones and used a 6.8Uf cap that did a pretty good job, your milage may varry though.

[boB]

>> It is not easy to design a filter because the values of the components vary as >>the load changes. I tried and failed many times.

I have also seen this tried before.  One big problem with just filtering the

output is that when it is all loaded down, the output voltage will necessarily

be lower than it is going into the filter.  I suppose you could use a stepup

transformer before the filter, but after all of this hassle, you might

as well just buy a real sine-wave inverter and be done with it.

After all, Time ~IS~ money.

boB

K7IQ

[Flux]

Yes this is difficult. It may be possible to use a small filter for one critical small load.

Trying to filter the output of the complete inverter will probably result in disaster unless the protection circuits on the inverter are incredibly good.With a parallel filter, with no load the thing will resonate and lots of reactive power has to go somewhere. Many years ago we did no break thyristor inverters with a series element, a shunt element and 3rd and 5th harmonic traps. The filter components cost a fortune and the thyristor inverter was about bomb proof. I think a cheap MSW mosfet inverter would have gone bang with the reactive power circulating.

Flux

[Seaspray0]

you can try filtering with a "tuned tank" which consists of a capacitor and inductor, both placed in parallel on the output.  Info on tuned tank:  

http://en.wikipedia.org/wiki/LC_circuit

If you have a capacitor, then you must choose an inductor to tune it to 60 hz.