Cleaning Modified Sine Wave

eddief1

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OP, what sensitive electronics are you talking about?

All your equipment will work just fine, just don't plug in devices with an induction motor into the thing (fridges and washing machines)

I got a 42" Samsung LCD which really does not sound healthy running of modified sine wave, I have been advised to not take the risk and will definitely take the advise. If you hear it you will know why.

The inverter I got now has got a changeover etc, a nice big charger so really want to try and utilize it...

I was thinking of buying a small true sine wave inverter to run with the modified sine wave one. Modified sine wave handles the charging and true sine is to just run the LCD. But this would mean the inverter running the TV will have to run 24/7...are they designed to do this ?
 

Alacrity

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655
I'm very sorry, but I'm very, very, very sceptical about a transformer outputting a true sine wave. How does it do that?

To have a square wave at the output all the harmonics needs to be present at the output which is not possible since the transformer will act as a filter for most of them.
 

Gnome

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TLDR: Why not get a true sinewave inverter?

My thought was to maybe feed the modified sine wave from the inverter into a true online UPS, so in theory I should get nice clean power out from the UPS.
Possible but truly wasteful. You would save a bit of money just going for a true sine wave inverter in my opinion...

I was thinking of soldering in some capacitors to filter the "noise".

Not sure if it will work.
Definite nope, a capacitor is like a blur tool. It makes edges rounder but it doesn't change what is essentially a square. The edges of the square get slightly more round but it still has a flat top and flat sides (mostly). Even really, really big capacitors (in terms of farads) won't make this look like a sine wave

The only way to try and smooth out the output from a modified sine wave, is to run it through a 1 to 1 ratio transformer.
Problem is this transformer will run very hot and cause electrical losses.
Rather spend your bucks on buying a decent pure sine wave unit.

To have a square wave at the output all the harmonics needs to be present at the output which is not possible since the transformer will act as a filter for most of them.

Definite no.

"Modified sine" is a fancy word for pulse width modulation. The output a square wave and the size of the "square" varies with load and input voltage.

The total harmonic distortion, for all intents and purposes of our discussion, is simply a measure of how close you are to true sine, so naturally a modified sine will score very badly on such a test.

Square wave will still be square wave after a transformer. You can think of a transformer as follows:
Two buckets of water. We call one bucket the primary and another bucket the secondary.

The primary bucket and secondary bucket are always the same (due to magic, called magnetism). So if you fill up the primary bucket, the secondary bucket will have the same level of water! Magic! Some buckets take more water, some less. So think of the water as voltage. But their LEVEL is always equal even if the amount of water they have isn't (so voltage can differ but it is at a ratio). The way we fill up these buckets can change: Therefore sine wave is a different way of filling the bucket from modified sine.

This is how sine wave works:
We fill the bucket slowly and the water level rises in both buckets gradually and then drains again down gradually. This looks like a beautiful wave, like you would see in the ocean. It raises and falls gently.

This is how modified sine works:
We suddenly have a full bucket that stays full for a while, then suddenly it is empty again. There is no in between gentle rise and fall. Because both primary and secondary are "mirrors" of each other, the level of the water is the same, it is just the amount of water that can be different (what we think of as voltage).

Because both buckets are the same level at all times, the wave doesn't change after we suddenly put it through the buckets.

Does this metaphor make any sense? It is simplest way I can think to explain the function of a transformer. It is obviously not that simple because the change in a magnetic field is actually what induces a current, but this explains the idea behind a transformer more clearly I think. (Therefore modified sine saturates the core instantly, then burns some current as heat, and the magnetic field collapses suddenly whereas sine wave has a gradual rise and fall. In both cases the magnetic field induced in the secondary will look the same. There is no way it will suddenly change its shape except that it may have a shorter width due to the core saturation in the modified sine case. Which is why your electronics run at much lower power and the core saturation means lots more heat)

Edit: If you are really interested in how this stuff works, the electronics we are most familiar with we know as the electronic circuit, which we use Ohm's law and etc for. Every electronic circuit has a magnetic circuit. Transformers operate on this magnetic circuit. Every formula we have like V=IR, P=IV, etc. has an equivalent magnetic circuit formula. There are also rules in the magnetic circuit, just like the electronic circuit. Very fascinating read and must learn for anyone that truly wants to grasp electronics.
 
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Thor

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TLDR: Why not get a true sinewave inverter?


Possible but truly wasteful. You would save a bit of money just going for a true sine wave inverter in my opinion...


Definite nope, a capacitor is like a blur tool. It makes edges rounder but it doesn't change what is essentially a square. The edges of the square get slightly more round but it still has a flat top and flat sides (mostly). Even really, really big capacitors (in terms of farads) won't make this look like a sine wave





Definite no.

"Modified sine" is a fancy word for pulse width modulation. The output a square wave and the size of the "square" varies with load and input voltage.

The total harmonic distortion, for all intents and purposes of our discussion, is simply a measure of how close you are to true sine, so naturally a modified sine will score very badly on such a test.

Square wave will still be square wave after a transformer. You can think of a transformer as follows:
Two buckets of water. We call one bucket the primary and another bucket the secondary.

The primary bucket and secondary bucket are always the same (due to magic, called magnetism). So if you fill up the primary bucket, the secondary bucket will have the same level of water! Magic! Some buckets take more water, some less. So think of the water as voltage. But their LEVEL is always equal even if the amount of water they have isn't (so voltage can differ but it is at a ratio). The way we fill up these buckets can change: Therefore sine wave is a different way of filling the bucket from modified sine.

This is how sine wave works:
We fill the bucket slowly and the water level rises in both buckets gradually and then drains again down gradually. This looks like a beautiful wave, like you would see in the ocean. It raises and falls gently.

This is how modified sine works:
We suddenly have a full bucket that stays full for a while, then suddenly it is empty again. There is no in between gentle rise and fall. Because both primary and secondary are "mirrors" of each other, the level of the water is the same, it is just the amount of water that can be different (what we think of as voltage).

Because both buckets are the same level at all times, the wave doesn't change after we suddenly put it through the buckets.

Does this metaphor make any sense? It is simplest way I can think to explain the function of a transformer. It is obviously not that simple because the change in a magnetic field is actually what induces a current, but this explains the idea behind a transformer more clearly I think. (Therefore modified sine saturates the core instantly, then burns some current as heat, and the magnetic field collapses suddenly whereas sine wave has a gradual rise and fall. In both cases the magnetic field induced in the secondary will look the same. There is no way it will suddenly change its shape except that it may have a shorter width due to the core saturation in the modified sine case. Which is why your electronics run at much lower power and the core saturation means lots more heat)

Edit: If you are really interested in how this stuff works, the electronics we are most familiar with we know as the electronic circuit, which we use Ohm's law and etc for. Every electronic circuit has a magnetic circuit. Transformers operate on this magnetic circuit. Every formula we have like V=IR, P=IV, etc. has an equivalent magnetic circuit formula. There are also rules in the magnetic circuit, just like the electronic circuit. Very fascinating read and must learn for anyone that truly wants to grasp electronics.

Thanks for the post that really cleared stuff up.

Bookmarked
 

Gnome

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Messages
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Thanks for the post that really cleared stuff up.

Bookmarked

Awesome :) some last notes:
The magnetic circuit works on change so to speak (magnetic flux).

So your transformer actually only "transmits" power when there is change.

So in the water example above the water level is the same but the change in water level causes the level to be the same if that makes sense.

So in Sine Wave your current is constantly changing which means you get this smooth wave. With modified sine you get these sudden changes (steps), and while there is no change the transformer is just making heat.

Knowing all this, the following from wikipedia actually becomes much easier to understand, I think:
Wikipedia said:
A varying current in the transformer's primary winding creates a varying magnetic flux in the transformer core and a varying magnetic field impinging on the transformer's secondary winding. This varying magnetic field at the secondary winding induces a varying electromotive force (EMF) or voltage in the secondary winding. Making use of Faraday's Law in conjunction with high magnetic permeability core properties, transformers can thus be designed to efficiently change AC voltages from one voltage level to another within power networks.
 
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Thor

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Why post if you don't know wtf you are talking about?

And you do? You tested all this etc or do you just Google and copy over what you read, just curious as a lot of folks here quickly dismiss stuff, but don't actually think they tried to see if something works or not.

Regarding the capacitor thing for instance yes by now means is that the way to go, but... It worked for me so I thought I might mention it. Nothing more to it really.
 

Gnome

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Last edited:

wetkit

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Guys, just to clear up a couple of things....
The OP did ask how to "clean" up his modifies sine wave invertor output.
There is nothing in the world that can be added to the output to convert it to pure sine wave.
A transformer, capacitors and inductor coils will all just help smooth out the wave a bit, but the cost is poor efficiency and heat.
Now you say, yes, add a 2nd invertor, but the losses in this system will be extremely bad, and most likely the 2nd invertor will not last very long.
Best option is to buy a new invertor with pure sinewave output.

I had a modified sine wave invertor not too long ago.
It blew all my electronic downlighter transformers. The wirewound ones were fine.
My poolpump started to run extremely hot.
Pure resistive devices like incandecent lamps, toasters, kettle and geyser ran 100% fine.
 

Gnome

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Guys, just to clear up a couple of things....
The OP did ask how to "clean" up his modifies sine wave invertor output.
There is nothing in the world that can be added to the output to convert it to pure sine wave.
A transformer, capacitors and inductor coils will all just help smooth out the wave a bit, but the cost is poor efficiency and heat.
Now you say, yes, add a 2nd invertor, but the losses in this system will be extremely bad, and most likely the 2nd invertor will not last very long.
Best option is to buy a new invertor with pure sinewave output.

I had a modified sine wave invertor not too long ago.
It blew all my electronic downlighter transformers. The wirewound ones were fine.
My poolpump started to run extremely hot.
Pure resistive devices like incandecent lamps, toasters, kettle and geyser ran 100% fine.

Yep, pretty much.
 

Sonic2k

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A transformer, capacitors and inductor coils will all just help smooth out the wave a bit
At most, these will alter the harmonic content and the power factor to a small degree.
 

The_Unbeliever

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Apr 19, 2005
Messages
103,196
Guys, just to clear up a couple of things....
The OP did ask how to "clean" up his modifies sine wave invertor output.
There is nothing in the world that can be added to the output to convert it to pure sine wave.
A transformer, capacitors and inductor coils will all just help smooth out the wave a bit, but the cost is poor efficiency and heat.
Now you say, yes, add a 2nd invertor, but the losses in this system will be extremely bad, and most likely the 2nd invertor will not last very long.
Best option is to buy a new invertor with pure sinewave output.

I had a modified sine wave invertor not too long ago.
It blew all my electronic downlighter transformers. The wirewound ones were fine.
My poolpump started to run extremely hot.
Pure resistive devices like incandecent lamps, toasters, kettle and geyser ran 100% fine.

Par for the course.

Use a modified sinewave for stuff like kettles etc.

Use a pure sinewave for fridges, electronics etc.
 

Archer

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Par for the course.

Use a modified sinewave for stuff like kettles, electronics etc.

Use a pure sinewave for fridges, etc (stuff with AC motors).

Fixed
Electronics (besides really sensitive stuff like medical equipment) do not need pure sine. Perhaps more accurately, anything with a switch mode power supply will not need pure sine
 

Sonic2k

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Most items with switched mode power supplies will get by on square wave UPS units.
This is because the incoming AC is rectified to DC anyway. However, I have reasons to doubt this being such a good idea if the power supply in question includes power factor correction ahead of the actual SMPS.

AC motors, and other items that rely on the sinusoid, will obviously have some detrimental effect. Motors, particularly those single phase ones that use capacitors to run, will probably perform unpredictably and yes, heating will be significant due to the peak currents being much higher, and magnetic flux in the cores being distorted. Basically the core is saturated with a square wave at the top of each cycle, and this also applies to transformers.
 
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Gnome

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I have reasons to doubt this being such a good idea if the power supply in question includes power factor correction ahead of the actual SMPS.

You can actually hear when that is the case, I've found. The power supply will make that characteristic vibration noise. I had a low cost PSU with that "problem". Turned out to be an inducator for PFC that starts vibrating.
 

spiderz

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He is actually talking about a device that converts 220v Modified sine wave to 220v Sine wave.
wonder if it's possible...
 

Sonic2k

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He is actually talking about a device that converts 220v Modified sine wave to 220v Sine wave.
wonder if it's possible...
I know how to do it.
I would use two of these cheap modified sine wave UPS units to generate two DC rails of 150V each, and then use a nice, powerful set of transistors in push-pull to generate a clean sinewave.

I was going to do this for my house but the energy taken out the batteries proves to me that the mains is incredibly efficient at what it does. Although its not difficult to deliver upwards of 6kW continuous from the batteries doing it the Emerson way, it becomes a bitch to charge all of that from solar panels.
 
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