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Neutral in electrics?

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TchmilFan
782615.  Tue Feb 01, 2011 2:36 pm Reply with quote

And to answer why not a square wave... noooooooooooo!

It's not a matter of just closing some kind of electronic gate and chopping the waveform off.You'd have to make the square from lots of little harmonics. The sine wave is what's actually coming out of the generators (kind-of)
What would happen to all of the signal that doesn't quite make it into the square form? You lose it.

There'd be much more heating of all systems if they were wanging straight up and down between plus and minus - it would break stuff - and you would have to convert it back to a smoother AC sine wave for transformers to work efficiently, after all you wouldn't want 110,000V being fed into your home, eh? (My memory is saying hysteresis curve - you can't just chop a magnetic field about without suffering some kind of consequence)
Hmmm... would a normal AC substation that's fed a square wave instead of a sine shake itself to bits before it melted?

The sine wave is just the easier way, no extra boxes, er, I'm sure I could think of more stuff if I concentrated but I've not needed to think about this for over 20 years! Bugger! Someone with more up to date and accurate science type knowledge please step in.

 
Posital
782708.  Tue Feb 01, 2011 5:52 pm Reply with quote

Generally, it's easier and more efficient to create a sine wave from generators. I also suspect that during the transmission over distance, the signal would degrade into a sine. I don't want to think about the kind of magnetic fields that would be unleashed at a substation...

 
aTao
782721.  Tue Feb 01, 2011 6:15 pm Reply with quote

TchmilFan wrote:
And to answer why not a square wave... noooooooooooo!

It's not a matter of just closing some kind of electronic gate and chopping the waveform off.You'd have to make the square from lots of little harmonics. The sine wave is what's actually coming out of the generators (kind-of)
What would happen to all of the signal that doesn't quite make it into the square form? You lose it.

There'd be much more heating of all systems if they were wanging straight up and down between plus and minus - it would break stuff - and you would have to convert it back to a smoother AC sine wave for transformers to work efficiently, after all you wouldn't want 110,000V being fed into your home, eh? (My memory is saying hysteresis curve - you can't just chop a magnetic field about without suffering some kind of consequence)
Hmmm... would a normal AC substation that's fed a square wave instead of a sine shake itself to bits before it melted?

The sine wave is just the easier way, no extra boxes, er, I'm sure I could think of more stuff if I concentrated but I've not needed to think about this for over 20 years! Bugger! Someone with more up to date and accurate science type knowledge please step in.


ahh, and now to the fun stuff (if you like calculus that is...)

What TchmilFan is saying is that any repetitive waveform can be expressed in terms of a sine waves: a fundamental frequency plus harmonics (all sine waves). So, for a high power square wave you would need some high power, high frequency harmonics. Not good, not good at all.
So, why sine waves, why not start with a square wave fundamental and square wave harmonics? Well ,it simple, isnt it? No, not the problem, a sine wave, sine waves are simple, easy to make and kind of pure. A weight on a spring will describe a sine wave (over time).

heres the fun bit:
a weight on a spring (set horizontal on a fictional frictionless surface with all sorts of other conditions to bring out the basic physics)
if the weight (mass in proper terms) is displace from it resting position the restorative force is proportional to the displacement.
dv/dt = -a * s [v= velocity, t is time, a a constant and s displacement]
gives
d^2s/dt^2 = -a * s which solves to,... yup, a sine wave, see here for more.. http://en.wikibooks.org/wiki/Physics_with_Calculus/Mechanics/Harmonic_Motion,_Waves,_and_Sounds

So, sine waves are simple and have no harmonics.

Also in electronic circuits certain components act as mathematical integrators and others as differentiators...
Consider a capacitor, the voltage across it is proportional to the charge that is stored in it, the stored charge is the sum of all currents that have ever flowed into it. So, V(on a capacitor) = a* Current.

Quote:
Hmmm... would a normal AC substation that's fed a square wave instead of a sine shake itself to bits before it melted?


If the substation was fed a square wave, it would be similar in effect If not scale) to a car ignition system. The transformers in the substation are inductors, if you try to change the current through an inductor it will exert a back EMF proportional to the rate of change. In a square wave that change is instant and therefore infinite rate.
Tricky to decide if it would explode mechanically or vaoprise from the resultant sparks.

 
TchmilFan
782735.  Tue Feb 01, 2011 6:26 pm Reply with quote

Imagine it... booster stations to square up the signal every few km. Oh lordy, I've just realised that I could actually do the maths to work out the optimal distance if I tried hard. It's just basic attentuation.

I must admit that I was skipping the whole "well that's what comes out generators" bit as it wasn't quite as interesting a visualisation as the transmission lines between the pylons turning into long, glowing bar heaters and loud, scarily humming subs.

 

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