Alan B
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| posted on 7/10/08 at 11:41 PM |
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Simple electrical question
OK so DC always flows one way so positive and negative terminals on a battery for instance are easy to understand.
So what about AC, which keeps changing direction?...how can one wire always be live and the other neutral?...surely they switch at the AC
frequency?
Always puzzled me that one....
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zetec7
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| posted on 7/10/08 at 11:47 PM |
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The short explanation is...it's all BL@@DY BLACK MAGIC!! The only thing I understand about AC current is that if you make a leak in the
insulation, you'll let out the gray smoke (that's what's in the wires), and once the smoke is let out, it won't work any more.
Once you get to doubled-up voltages, then you apparently have two hot wires, one that's hot only half of the time (but in two directions!?) and
one that's neutral...but not always!
Go on...tell me that's no black magic!
http://www.freewebs.com/zetec7/
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owelly
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| posted on 7/10/08 at 11:47 PM |
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Does it help if you think of the neutral as the ground/earth/negative?
BTW DC current flows from the negative to the positive!
http://www.ppcmag.co.uk
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l0rd
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| posted on 8/10/08 at 05:15 AM |
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I can explain but will be £19.99 p/m +VAT.
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nz_climber
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| posted on 8/10/08 at 05:30 AM |
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as posted above, DC does flow negative to positive..
When talking about AC, the live line is the one connected to the supply. And neutral is usually connected to earth (at the switch board) or maybe
seperate completely) Any switches and fuses should always be in the live wire. Every half a cycle the polarity of the supply changes - But this
doesn't mean the neutral becomes the supply thou.
not sure if that makes sense.. Easy to think of Live as Positive, and Neutral and Earth.
http://aarons7.wordpress.com
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maartenromijn
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| posted on 8/10/08 at 06:55 AM |
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In AC the voltage is going from positive to negative, following a sinusoide. The distance from the neutral to the sinusoide is the voltage. So not
only it is changing from positive to negative, but also the voltage is going up and down.
Typically an AC current is generated by an alternator (hence Alternating Current?). In your car the rectifier transforms AC into DC.
I guess this a bit abstract information. A grafic will explain a lot. But I do not have that 
BLOG: http://thunderroad-super7.blogspot.com/
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David Jenkins
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| posted on 8/10/08 at 08:03 AM |
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The way I was told it (which may be a bit wrong), is
You have 3 wires in a plug.
Live - this is the power coming from the grid.
Earth - The local ground connection (usually within the house).
Neutral - a ground connection at the local sub-station.
Live will be at 230v relative to earth. Earth should be zero, and neutral will be a few volts, dependant on how far you are from the sub-station.
[Edited on 8/10/08 by David Jenkins]
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mistergrumpy
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| posted on 8/10/08 at 08:31 AM |
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Is it not because AC follows a sinusoidal wave i.e. like a roller coaster. It climbs up to the peak and then rolls down and into the negative before
climbing again. I know in simple small circuits you can install a capacitor so that when the current is "rolling down" the capacitor will
discharge and so the current climbs back up thus avoiding or smoothing the sin wave.
My understanding is also that instead of just sending current down the wire so it follows a sinusoidal wave pattern a few waves are sent split seconds
apart so that each wave slightly overlaps and, again, it stops the current from entering the negative side and smooths the whole thing out.
Thats my understanding.
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coozer
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| posted on 8/10/08 at 08:51 AM |
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Hmm, all very interesting.. how about 3 phase? 3 positives adn no negatives? Am I right or do I need to back to school??
1972 V8 Jago
1980 Z750
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MikeRJ
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| posted on 8/10/08 at 09:00 AM |
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The neutral wire is at a nominal potential of 0 volts (or should be anyway!), and the live wire has a sinusoidal voltage that varies between +155.5v
at the positive peak and -155.5v at the negative peak if you are on a 110v supply. This gives 311v peak-peak, and the RMS voltage (which roughly
means the equivalent DC voltage) is 110v.
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MikeRJ
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| posted on 8/10/08 at 09:08 AM |
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quote:
Originally posted by mistergrumpy
My understanding is also that instead of just sending current down the wire so it follows a sinusoidal wave pattern a few waves are sent split seconds
apart so that each wave slightly overlaps and, again, it stops the current from entering the negative side and smooths the whole thing out.
Thats my understanding.
What you are describing is the three phase system, as used in power transmission and car alternators etc. You have three sinusoidal voltages spaced
120 degrees apart in phase, but they are all sent down separate wires (they have to be, otherwise they would short each other out).
When you rectify a 3 phase supply (e.g. alternator in car), you combine all the resulting DC voltages onto one wire, and since at any given time one
phase will always have a non-zero voltage, this means the ripple is much lower.
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MikeRJ
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| posted on 8/10/08 at 09:17 AM |
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quote:
Originally posted by coozer
Hmm, all very interesting.. how about 3 phase? 3 positives adn no negatives? Am I right or do I need to back to school??
3 phase can be used in two configurations, star and delta. With star you still have a neutral wire, but the net current flow in it should be zero if
the loads on each of the phases is balanced. With Delta, there is no neutral connection, all currents flow between the phases.
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loggyboy
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| posted on 8/10/08 at 09:27 AM |
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Its quite simple really...
You plug the appliance in to the socket and turn it on... No need for ANY further explanation.
SIMPLE
Mistral Motorsport
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02GF74
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| posted on 8/10/08 at 10:46 AM |
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quote:
Originally posted by nz_climber
as posted above, DC does flow negative to positive..
When talking about AC, the live line is the one connected to the supply. And neutral is usually connected to earth (at the switch board) or maybe
seperate completely) Any switches and fuses should always be in the live wire. Every half a cycle the polarity of the supply changes - But this
doesn't mean the neutral becomes the supply thou.
not sure if that makes sense.. Easy to think of Live as Positive, and Neutral and Earth.
No it doesn't.
Current flow from positive to negative but inside the wire, the electrons flow from negative to positive.
About live, neutral and earth.
Earth is taken as a reference from which to measure voltage. And it is called earth because in pratice there is a large copper plate buried in the
ground (i.e. the earth) connected to a wire so that any current will flow into the ground.
The live and neutral are a power supply. For AC, think of it as having a car battery with some one swapping over the wires 30 times a second .
(ignore the waveform!)
So using our earth to measure voltage, sometimes the live has voltage above earth (current flow from live to neutral) and other times it is below
earth (current frlow from neutral to live).
So although the live supplies power, the direction of current flow changes. The neutral is to complete the circuit.
You don't need earth for an item to work, it is there for safety to sink any current that is where it should not be.
So if you connect a bulb to live and neutral, it will light up as this is the supply.
If you connect a bulb to neutral and earth, it should not light up as the live is the supply not the neutral.
If you connect bulb to live and earth, it should light up as the earth will sink the current.
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Alan B
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| posted on 8/10/08 at 11:23 AM |
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Good debate...not many convincing answers though...
I'm not actually electrically clueless (despite the question LOL), I did C&G 236 pt2 many years ago so I'm pretty au fait with 3 phase
and most industrial/machinery type wiring....it's just this fundamental concept that kept me awake at night....(not really)
Thanks for the input.
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02GF74
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| posted on 8/10/08 at 12:40 PM |
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wiring up a 3 phase machine or a 3 pin plug does not necessarily mean one needs to know the physics behind electricity.
electricity is very mysterious, I don't think anyone, even Lucas, can really say they know what it is but we can obxerve and measure its effects
and form theories about it which can be tested.
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b16mts
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| posted on 8/10/08 at 03:51 PM |
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Thank god others have responded first!
I'm 26 and in my second year of my electrical installation apprenticeship (in the first year i got distinctions in all 6 modules, the only one
to do so in the college).
my plan is to finish the apprenticeship, then go straight into teaching.
as regards AC, i'll use one of my many anaolgies that make the 'boys' at college laugh:
imagine the 'live' (though subsequently called phase, and they now call it line) as blowing down a hose pipe. if you blow and suck 50
times per second, but with the end blocked, then the energy goes no-where. however if you put say a fan in the way and a release the end, the fan will
turn (albeit one way, then the other). Basically, all the neutral is is a way of conpleting the circuit, or unblocking the hose pipe.
the problem comes if someone cuts the pipe halfway down, then the air escapes, the same as current does if there is a short circuit.
hope that's helped to confuse some people.
Martin
PS. I still have no clue how to wire my Tiger!!!
who says you can't drive a kit car when you're 6'5"?
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MikeRJ
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| posted on 8/10/08 at 05:40 PM |
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quote:
Originally posted by Alan B
Good debate...not many convincing answers though...
Hmm, ok how about this. The outputs of an isolated (i.e. non grounded) transformer will wiggle up and down in anti-phase at the line frequency, just
as you describe in your opening post.
If you tie one wire to ground (neutral), you stop it wiggling up and down (since it would otherwise have to make the entire planet wiggle up and
down). In order for the total voltage to remain unchanged, the other wire (live) wiggles up and down twice as much.
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Liam
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| posted on 8/10/08 at 05:45 PM |
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^^^
As a holder of an electrical engineering degree, that's my favourite one. 
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Alan B
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| posted on 8/10/08 at 11:26 PM |
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quote:
Originally posted by Liam
^^^
As a holder of an electrical engineering degree, that's my favourite one.
Agreed......thread closed...
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nz_climber
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| posted on 9/10/08 at 02:53 AM |
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quote:
Originally posted by 02GF74
Current flow from positive to negative but inside the wire, the electrons flow from negative to positive.
These are not to be confused.. there is too distinct theories on this, Electron Current Flow vs Conventional Current flow
For Electron Current Flow, current flows negative to positive to match the electrons (since current is electrons/charged particles past a point in
given time)
For Conventional - its just normal positive to negative.
Electron current flow is hardly used unless trying to work out current flows etc in tranistors, FETs etc
Also yes Neutral goes back to the supply - but if you cut it and hold on you won't get a shock (thats what I was try to get across above) and
alot of the time earth is tied to neutral (NZ uses MENS distribution system)
http://aarons7.wordpress.com
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