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post #25 of Old 04-21-2012
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Re: 1500 W 12V Real Sine Wave Inverters?

Originally Posted by hellosailor View Post
I took a look at Xantrex's web site, found their "Tech Dr" articles. Found that according to the waveform that they post, their Modified Sine Wave is...a crock. It isn't a modified sine wave at all, it is a modified square wave, no way any kind of emulation of a sine wave. A true "modified sine wave" at least uses multiple steps, multiple pulses, not just a simple square wave with delays. No matter how many vendors pretend it is.
So while their own talk says their MSW will run anything just like a real sine wave...I'm not buyin it. Literally or figuratively. Those guys are talking out of both sides of their mouths. I'll gamble on something else. I've had mixed results with square wave in the past, quite some time back in the past, and if it has a money back guarantee...I may gamble, but not with Xantrex. I don't like smoke and mirrors, and anyone who calls a modified square wave "modified sine wave" can be glad I don't work at the FTC.
HS: Semantics. 'MSW' is rendered as 'modified sine wave' by its proponents, 'modified square wave' by detractors. In the end, it comes down to this, as I said above: NO solid-state, static inverter can produce a true sine waveform. Then it's down to how small & how many are the steps per pi/2 radians, as I said in my post. The basic MSW inverters use about 3 steps per quarter wave. The so-called 'pure' or 'true' sine wave inverters do exactly the same trick, tho at 8x or 16x higher resolution.

If anything, I would accuse them of deceptive advertising, for claiming 'true' and 'pure' when it is no such thing. Educate yourself, and make your choice. ALL static inverters are square wave; some are squarer than others.

My central point is this: don't needlessly buy into the hype around 'pure' sine wave inverters. For most applications, their higher resolution is superfluous. I detailed the loads that mislike MSW. We have now run a full household and professional woodworking shop for nine years on the crudest Xantrex MSW waveform available -- seriously, literally, two steps above square wave. And you know what?

We haven't noticed any squareness.

Really. I'm listening right now to the most pristine remastering of Cannonball Adderley playing 'Corcovado', with an RF-noise-free fidelity to make the heart bleed. We're running the dishwasher, two computers, and some CF lights. Square wave? WHAT square wave?

Point is, MSW (call it modified square, call it modified sine) is good enuf for nearly everything you will plug into it.

Here's another piece of hype to watch out for: don't oversize your inverter(s). Where Wh are precious (as aboard a sailboat), you want your inverter to be large enuf to handle your maximum sustained demands -- but no larger.

Inverters near the top of their range are >90% efficient. With loads below 50% rated capacity, you may see 40-50% efficiency. That means you are expending 2 Ah of battery capacity for every 1 Ah you pull thru the inverter. A rare case where overkill is bad policy. Size your inverter to your median demand. If you exceed its limits, it will let you know -- by kicking a thermal circuit breaker.

So I guess my take-away is this: 1) Don't dismiss crude waveforms because they are crude. Semi-square is probably good enuf for your needs. 2) IF you really want 120VAC power aboard a boat at all (and millions of people get by without), choose the smallest inverter (of either type) that will meet your needs.

Counter-intuitive, but true.

(Honestly, you wouldn't believe the number of people who have told me our off-grid setup cannot possibly work. "You can't run a tablesaw off batteries & MSW. You'd need $10k in TSW inverters." But I researched the hell out of this stuff, chose our systems for robustness rather than bleeding-edge technology, and everything has worked just peachy.)

Buccaneer18, Grainnia
SJ21, Diarmuid
Albin Ballad 30, Fionn

Last edited by bobmcgov; 04-21-2012 at 02:35 AM.
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