Re: how does voltage from a wall plug vary with time
From: Nick Rouse (nick_at_rouse.123isp.co.uk)
Date: 02/16/05
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Date: 16 Feb 2005 14:28:39 -0800
alam_seher@hotmail-dot-com.no-spam.invalid (aasiyah) wrote in message news:<42123339$1_2@127.0.0.1>...
> > CWatterswrote:
> "aasiyah" <alam_seher@hotmail-dot-com.no-spam.invalid> wrote in
> message
> > news:420a3a8c$1_2@127.0.0.1...
> > so from this graph http://en.wikipedia.org/wiki/Three_phase you
> would
> > say only one of these three voltages goes through a wall plug? How
> > does this work..
> >
> What do you mean how does this work?
>
> All you need is one live (sine wave) and one neutral
> ("0V").[/quote:1e0309f5e6]
>
> where does the neutral wave come from and why is it there, what's its
> purpose.
> Please correct me if im wrong; the voltage from a wall plug is only
> with one wave- the sine wave- and NOT in three phase. Three phase is
> rarely used. BUT why is not three phase used in ordinary wall plugs,
> since it is not what is its purpose?
>
>
Three phase supplies are not used domestically because of the extra
cost yielding very little benefit for most domestic needs. It needs
three fuses or circuit breakers per circuit, a more complicated three
phase meter, at least three pole switches, at least 3 current carrying
conductors (4 with a balanced neutral) and with neutral and protective earth
5 pin plugs and sockets. A three phase supply is of no benefit to the user
for resistive loads (lighting and heating) and for electrical motors
up to about 1kW the slight gain is not worth the extra cost. For industrial
users there are gains to be made. Three phase motors are smaller, more
efficient, run more smoothly and have much greater starting torque than
single phase motors of the same rating. For multi-kilowatt motors these gains
are worth the cost of the supply installation. Network electricity is always
generated three phase and the generating companies do not like large
imbalances between the phases. Here in Britain and I suspect elsewhere
the generating companies charge industrial users according to the maximum
installed capacity as well as for the energy used. A single phase load
that drew three times the per phase current of a three phase load would
cost a lot more.
Three phase supplies in Europe are different to the normal American
system. The three phases are generated balanced about neutral (rather
than having one phase earthed). Power is transmitted and transformed
up and down in a balanced pattern. In places in or close to urban
centres (In England that is almost everywhere) the power is transformed
down to its final voltage 230 or 240V in large chunks (100kW or more)
and fed along the road as three phase. Domestic and light commercial users
are connected to one phase and neutral with each phase supplying each third
house. The pole transformers supplying just one house that are so common
in America are almost unknown here. Since the voltage is high there
is no need for two voltages and two styles of plugs and no possibility
of connecting two live conductors with different phases.
There are disadvantages of course, it is considerably more lethal.
Nick Rouse
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