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Old 24-07-2015, 12:33   #1
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Inverter - Modified Sine Wave vs Pure Sine Wave?

I am evaluating inverters/chargers. My 10 year old Xantrex Freedom 20 (2000 watt) has been having fan problems and depending on the cost. I may just replace it. I would like "plug & play" so I don't have to deal with a new remote or any wiring complications, but I'm confused.

Do I want the Freedom 458 with "Modified Sine Wave," or the Xantrex Freedom SW 2012 with "Pure Sine Wave." Defender only carries the Pure Sine Wave unit which costs about $1,500 but the Modified Sine Wave 458 is available elsewhere at $800-$900.

I don't know anything about sine waves, but I assume Pure is better than Modified for electronics. Can someone explain in laymen's language what the practical difference is and if I would be better off paying more for it?
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Old 24-07-2015, 14:24   #2
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Re: Inverter - Modified Sine Wave vs Pure Sine Wave?

Grid power is pure sign wave. The incoming voltage flips back and forth from peak voltage in one direction to the peak voltage in the opposite direction 60 times per second and follows a nicely formed sine wave. (US power but others follow a similar pattern)

The result is the voltage supplied varies gradually.

Cheap square inverters, switch the power near instantaneously from one peak to the negative peak without the gradual change.

Modified sine wave try to replicate a sine wave by making the change back and forth in steps. The more steps the better it replicates a true sine wave.

A true sine wave inverter is a bit of a misnomer. If there are enough steps, most devices won't know the difference, so they lable it true.

So why does it matter?
- If you are doing a resistive load (like the heating coils in a toaster), it doesn't.
- If you are running a motor, an AC motor is basically a reversed AC generator, so it really likes pure sine wave power. If you run a square wave (or in some cases a low quality modified sine wave), you are pumping power in when the motor can't use it and this results in the winding heating up rather than turning the motor. Depending on how bad the power is...it could blow out in seconds or it might run for months before failing...or maybe never.
- Electronics can also get finicky.
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Old 24-07-2015, 15:13   #3
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Re: Inverter - Modified Sine Wave vs Pure Sine Wave?

It really depends on your application. Most electrical devices today have micro electronics installed which run the systems. Even my new toaster has a selection buttons for modes, which requires a pure sign wave. I also have a bread maker and it will not even turn on w/o the PSW inverter.

But single speed power tools and small battery chargers seem to do fine with the modified SW.
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Old 24-07-2015, 15:30   #4
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Re: Inverter - Modified Sine Wave vs Pure Sine Wave?

We have an old (1988) heart Interface 600 W mod sine wave inverter, in constant use since that date. We have plugged everything we own that runs on mains power into it. The only problems have been with older SCR controlled variable speed drills, etc, for they would only run full speed, not lower speeds. Everything else has been ok: computers, power tools, battery chargers (small), lamps, shavers, tooth brushes, soldering irons, and so on.

I don't know how many steps are used in the pseudo-sine wave form, and this does differ between OEMs... could be a problem with less expensive current products. in general, this has been the most reliable bit of boat kit ever purchased... too bad that Xantrex absorbed them!

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Old 24-07-2015, 16:29   #5
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Re: Inverter - Modified Sine Wave vs Pure Sine Wave?

Quote:
Originally Posted by PamlicoTraveler View Post
My 10 year old Xantrex Freedom 20 (2000 watt) has been having fan problems.
I would give that unit a good inspection and cleaning, maybe bench-test it to confirm it still meets its ratings, then replace the fan.
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Old 24-07-2015, 17:48   #6
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Re: Inverter - Modified Sine Wave vs Pure Sine Wave?

Having had several different inverters over the years, I have one of these on mycurrent boat and like it very much:

Tiger Claw 1500W 3000W Pure Sine Wavepower Inverter DC AC Power Converter | eBay

One of these in the home's solar grid tie, and it's first rate:

Magnum Energy MS4024 Pure Sine Wave Inverter Charger 4000 Watt 24 Volt | eBay
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Old 24-07-2015, 18:57   #7
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Re: Inverter - Modified Sine Wave vs Pure Sine Wave?

The only time I ever worried about a pure sine wave inverter was when I was building a solar powered medical clinic in a jungle in South America. Most of our medical equipment required pure sine wave.

Of interest, our first inverter blew up during instillation. We had a non-sign wave inverter for a short time to construction and fancy energy efficient light bulbs. When night fall came, the light bulbs flashed on and off like it was a disco club. Lesson learned...fancy energy lights may not work well without a pure sign wave inverter.

On my boat, I have a series of inverters in multiple locations. There are small inverters purchased at West Marine for just a few dollars. Everything I put on them works great. My phone charges. My flat screen TV works great. I only turn on the big inverter when I break out the blender at sea (the crew threatened to mutiny when the blender broke and they had to drink margaritas on the rocks instead of blended...always keep a back up blender).

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Old 24-07-2015, 19:40   #8
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Re: Inverter - Modified Sine Wave vs Pure Sine Wave?

Thanks all. Sounds like for general use either will do fine. I guess Defender has just gone with the "Pure" model for some reason.
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Old 24-07-2015, 20:20   #9
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Re: Inverter - Modified Sine Wave vs Pure Sine Wave?

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Originally Posted by PamlicoTraveler View Post
Thanks all. Sounds like for general use either will do fine. I guess Defender has just gone with the "Pure" model for some reason.

There is also a big difference between cheap MSW and higher quality MSW inverters, like Xantrex. The cheaper ones have fewer steps and are thus more problematic for tools, etc. than the better ones, which more closely approximate a pure sine wave.

One thing I've noticed that is very picky about pure sine wave is microwave ovens. I generally start the generator for that, since a 1000w microwave draws a lot closer to 1800w and that's a lot of current draw off of the battery. Everything else I have, from wireless routers to laptops to media servers and cordless tool battery chargers, toaster and blender all work fine from my Xantrex Freedom 458.

The older units are built pretty well, I'd clean it up, pop in a new ball bearing fan for $5-10 and keep it for years to come.
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Old 24-07-2015, 20:25   #10
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Re: Inverter - Modified Sine Wave vs Pure Sine Wave?

A sine wave inverter requires somewhat sophisticated circuitry to generate a sine wave "pattern" and then a linear amplifier to beef up that sine wave to 120V and hold it there regardless of current drawn, up to the limit of the inverter.

A square wave inverter simply takes 120V and turns it on and off (essentially) instantaneously. Take a low ohm electronic switch and turn it on and off.

Look at the following:

https://en.wikipedia.org/wiki/Square_wave

Just looking at the square wave vs the sine wave it becomes apparent that the actual power (area under the curve) is higher. Additionally a square wave really is a set of harmonic waves. The primary, and infinite third harmonics, of smaller and smaller amplitude. So a square wave fed through many circuits will cause "buzzing" to leak through the power supply into the circuits that use the resulting voltage. Not good, particularly for radio devices. Those harmonics BTW radiate into the environment just through the AC wiring.

Modified square waves are a still poor attempt to emulate a sine wave.

https://www.google.com/webhp?sourcei...0square%20wave

Instead of a simple on / off, they "step" the voltage, but still in a "square" shape. That square shape is still composed of harmonics, however the power (area under the curve) is now a closer approximation of a sine wave.

The bottom line is that an AC sine wave is the real deal, and expensive to implement electronically. It is what everything we buy for shore use expects to see because that is what the power plants ashore provide.

Square wave and modified square wave inverters are an attempt to save money by "emulating" a sine wave with much less expensive electronics.

Any given item may or may not work, or work well and quietly. Some things may work, but radios may buzz, music amplifiers may buzz, TVs may show noise in the picture. AC Motors really don't like square waves.
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Old 24-07-2015, 20:26   #11
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Re: Inverter - Modified Sine Wave vs Pure Sine Wave?

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Originally Posted by socaldmax View Post
The older units are built pretty well, I'd clean it up, pop in a new ball bearing fan for $5-10 and keep it for years to come.


Or give it to me!
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Old 24-07-2015, 20:58   #12
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Re: Inverter - Modified Sine Wave vs Pure Sine Wave?

Yup, fix the fan.

If it's been working all these years and since its a modified sine wave unit, if what you're running works, why pay more?
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Old 24-07-2015, 21:17   #13
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Re: Inverter - Modified Sine Wave vs Pure Sine Wave?

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Originally Posted by PamlicoTraveler View Post
Thanks all. Sounds like for general use either will do fine. I guess Defender has just gone with the "Pure" model for some reason.


modified sine waves are what they used 10 years ago with the tech and cost of the day. today everything is pure sine. I haven't installed a modified sine wave inverter in over 5 years. but I pull lots out. I wouldn't put one it.

also I doubt any current inverter will be "plug and play". exactly to yours. some work or new cabling will likely be needed.
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Old 24-07-2015, 21:36   #14
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Re: Inverter - Modified Sine Wave vs Pure Sine Wave?

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Originally Posted by jwcolby54 View Post

Just looking at the square wave vs the sine wave it becomes apparent that the actual power (area under the curve) is higher.
Lots of great pounts here, but on this particular point, the amplitude of these square wave units are set such that the (avg) power is the same as a sinusoidal ac source. Otherwise they'd cook literally anything you plugged into it!
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Old 25-07-2015, 01:26   #15
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Re: Inverter - Modified Sine Wave vs Pure Sine Wave?

For 10 years our Xantrex modified sine wave inverter ran everything that I plugged into it, including the microwave. However, the microwave took about 20% longer to cook on the inverter than it did on dock power.
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