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Mike Raiford <"m[raiford]!at"@gmail.com> wrote:
> Frequency shouldn't vary much.
AFAIK, while the voltage can vary quite some (like eg. the stated 10%),
they are extremely careful to have the frequency as accurate as possible.
And we are talking about very tiny fractions of a hertz here.
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. I think it has something to do with energy
loss when two sources produce alternate current not exactly in sync. The
only way to avoid energy loss because of desynced AC current is to have
all sources exactly at the same frequency and phase.
Thus a wall socket is a really accurate and reliable 50 Hz timer.
--
- Warp
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On 4-6-2009 22:52, Mike Raiford wrote:
> Orchid XP v8 wrote:
>> Darren New wrote:
>>> Invisible wrote:
>>>> OK, so what *is* the correct mains voltage in the UK?
>>>
>>> Don't forget it's AC, so you also have to figure out whether you mean
>>> peak, RMS, or what. Plus you should check the frequency.
>>
>> 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.
>
Can't find a reference for the tolerance of the frequency even though
the amplitude tolerance is easy to find. :( Tolerance is at any time a
few percent. Yet, there should be (almost?) exactly 4320000 cycles in 24
hours (assuming 50Hz). Under most circumstances the frequency is
actually very close to 50Hz except at times of (unpredicted) peak
currents, but that will be compensated later.
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Warp wrote:
> Thus a wall socket is a really accurate and reliable 50 Hz timer.
In the USA, it's a very accurate 60Hz, but if it drifts at all, during the
last minute of the day, they adjust the frequency to get exactly the right
number of ticks in. Probably because of the alarm clock thing, if nothing
else.
It might need to be more regulated on three-phase wiring, and I don't know
if the frequency is different between the generator and the wall-socket
(altho I doubt it), so I'm not sure just where the accuracy is enforced.
--
Darren New, San Diego CA, USA (PST)
There's no CD like OCD, there's no CD I knoooow!
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>> > 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] scott com> 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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