>> > I was always taught at school that the mains was exactly 250V.
>>
>> I never heard that before.
>
> I have and still say 250 Vac if I don't think about it.

Maybe it was before my time then :-) But the "exactly" part I have never 
heard, and isn't even possible!

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>> 230V RMS at 50 Hz. (I don't see any sources stating what the allowable 
>> variation in frequency is.)
>>
>
> Frequency shouldn't vary much. Some alarm clocks are dependent on the 
> frequency of the power feed.

Yeh, the frequency depends on how much power is demanded and what generation 
capacity is online.  In a grid power system like the UK they control the 
frequency by bringing up and shutting down various power generators to cope 
with the demand.  I don't know what the short-term allowed variation is, but 
over the longer-term (eg 1 day) it is controlled very accurately, enough for 
clocks to run off it.

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>  If I'm not mistaken, this is so severely imposed that an electric company
> feeding the system with even the tiniest discrepance in the frequency or
> phase is automatically dropped.

It doesn't work like that, you don't get to choose the frequency or phase of 
your generator when it is connected to the system.

An analogy is if you imagine 100 cars all fixed end to end on a track going 
at 100 km/hr.  The steepness of the track can be thought of as the current 
demand, each car as a power station, and the speed as the frequency.  It's 
the job of the controllers of the entire system to predict demand and ask 
more or less cars to join the train, and to tell each driver how much power 
to generate.  As an individual you can't set your own speed and be "out of 
sync" with everyone else, by definition of being connected to the system you 
are generating the same frequency and phase as everyone else.

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scott wrote:
>>> > I was always taught at school that the mains was exactly 250V.
>>>
>>> I never heard that before.
>>
>> I have and still say 250 Vac if I don't think about it.
> 
> Maybe it was before my time then :-) But the "exactly" part I have never 
> heard, and isn't even possible!

Well, you don't tell children that, do you? ;-)

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scott <sco### [at] scottcom> wrote:
> It doesn't work like that, you don't get to choose the frequency or phase of 
> your generator when it is connected to the system.

> An analogy is if you imagine 100 cars all fixed end to end on a track going 
> at 100 km/hr.  The steepness of the track can be thought of as the current 
> demand, each car as a power station, and the speed as the frequency.  It's 
> the job of the controllers of the entire system to predict demand and ask 
> more or less cars to join the train, and to tell each driver how much power 
> to generate.  As an individual you can't set your own speed and be "out of 
> sync" with everyone else, by definition of being connected to the system you 
> are generating the same frequency and phase as everyone else.

  Of course you can generate an out-of-phase signal. It's just an AC
generator like everything else. What happens is that if you are out of
sync with the other generators, there will be an energy loss because the
out-of-sync signal dampens the others. The worst possible scenario is,
of course, that your generator has a phase shift of 180 degrees compared
to the other generators, which causes maximal energy loss.

-- 
                                                          - Warp

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"scott" <sco### [at] scottcom> wrote:
> >  If I'm not mistaken, this is so severely imposed that an electric company
> > feeding the system with even the tiniest discrepance in the frequency or
> > phase is automatically dropped.
>
> It doesn't work like that, you don't get to choose the frequency or phase of
> your generator when it is connected to the system.
>
> An analogy is if you imagine 100 cars all fixed end to end on a track going
> at 100 km/hr.  The steepness of the track can be thought of as the current
> demand, each car as a power station, and the speed as the frequency.  It's
> the job of the controllers of the entire system to predict demand and ask
> more or less cars to join the train, and to tell each driver how much power
> to generate.  As an individual you can't set your own speed and be "out of
> sync" with everyone else, by definition of being connected to the system you
> are generating the same frequency and phase as everyone else.

To expand on what Scott has said. (In a past life I used to work on the
electronic governors of Gas Turbine power generators.)

All the generators in a grid will be at the same frequency and phase. As you
bring a generator online you must wait until the oncoming generator is in phase
with the grid before connecting it. If it is slightly out of phase it creates a
nice bang :) As the load on the grid increases the frequency will try to drop
but the individual governors will increase the throttle valve, supplying the
fuel, to keep it steady. The line voltage will drop instead.

cabling.

Stephen

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Warp <war### [at] tagpovrayorg> wrote:
>   Of course you can generate an out-of-phase signal. It's just an AC
> generator like everything else. What happens is that if you are out of
> sync with the other generators, there will be an energy loss because the
> out-of-sync signal dampens the others. The worst possible scenario is,
> of course, that your generator has a phase shift of 180 degrees compared
> to the other generators, which causes maximal energy loss.
>

In RL the other generators will drag the out of phase into phase violently.

Stephen

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Invisible wrote:
> OK, so what *is* the correct mains voltage in the UK?

A related question. why do some countries use 110 and others use 220?

BTW, here in the US I've seen AC voltage expressed as 110, 115 and 120. 
  Generally it's actually somewhere between 105 and 125, devices get 
around this by having voltage regulators. e.g. if the input voltage 
sags, it'll draw more current to keep the output voltage where it wants 
it to be, and vice-versa.. Of course there are limitations, and 
eventually the regulator will output too little voltage, because the 
supply cannot keep up with the current draw, or it'll burn out because 
the input voltage is outside it's operating limits.

This is why battery operated electronics can run until the battery is 
almost flat, even though the battery's voltage continually drops as it's 
drained.

-- 
~Mike

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Mike Raiford wrote:

> This is why battery operated electronics can run until the battery is 
> almost flat, even though the battery's voltage continually drops as it's 
> drained.

Really? That's interesting. I always thought it was the battery outputs 
full voltage until just before it actually dies (which is why battery 
meters are never, ever, under any circumstances, actually accurate).

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Invisible <voi### [at] devnull> wrote:
> Mike Raiford wrote:
>
> > This is why battery operated electronics can run until the battery is
> > almost flat, even though the battery's voltage continually drops as it's
> > drained.
>
> Really? That's interesting. I always thought it was the battery outputs
> full voltage until just before it actually dies (which is why battery
> meters are never, ever, under any circumstances, actually accurate).

Actually, the battery's resistance increases with depletion, which means the
more current you draw and the lower charge of the battery the lower the voltage
you get. The idle voltage will almost always be the same, regardless of the
charge state of the battery.

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