Jason Kenney jason at ohm.ath.cx
Sun Dec 18 20:54:42 AST 2005

> I'm not going to pass myself off as an expert on electricity.
> However I don't understand the above.  If a UPS produces
> power that is dirtier than from the electrical company, then
> why do we have them?

There are two types of dirty too. I suspect people are probably not too 
familiar with the look of the wave coming off the line.

The amplitude (height) of the wave is changing all the time, as the load 
on the system as a whole changes.

In addition, the shape of the wave is far from a perfect sine. This is 
changing too, but less than the amplitude. (Although I suppose they are 
related - the amplitude of the frequencies components are changing 
relative to each other, which is what changes this shape).

The only thing about the wave that can be even approximately called 
"perfect" is the frequency. The amount of variability of this is 
specificed by law I believe, and is very tightly limited (something like 
no more than .001% deviation over the course of a year).

Frequency is important for time-sensitive applications. The others don't 
matter at all really. Things that take big AC are motors which don't care 
too much about the look of the wave. Lighting and heating don't care about 
either. Most other things are DC, so they just need a frequency/amplitude 
combo they can regulate.



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