Re: x10 - CM12U (CM11) Clock Accuracy

[Charles Sullivan <cwsulliv@xxxxxxxxxxxx>]

On Tue, 25 Sep 2007 16:56:18 +0000, Jeff Volp wrote:

> "Charles Sullivan" <cwsulliv@xxxxxxxxxxxx> wrote in message
> news:pan.2007.09.25.16.18.28.581477@xxxxxxxxxxxxxxx
>>
>> The CM11/CM12 clock is usually kept roughly in sync with the AC
>> line frequency, which can fluctuate. In the USA the power companies
>> tweak the frequency up or down so that AC clocks will be accurate
>> on the average over the long term.
>
> The powerline frequency may be more accurate than you think.  As you say,
> long term is very precise.  Short term can deviate slightly, but peak errors
> are less than 0.1%, and typically better than 0.025%.  The deviation is
> caused by load variation.  Generators have enormous rotating inertia, but
> sudden increases in load will begin to slow them down.  Many power plants
> use the steam turbine cycle, and it takes a finite amount of time to respond
> to the increasing load.  So there can be instantaneous variations in
> frequency as the control loops respond.  The short-term frequency errors
> occur on both sides, and long term average is very accurate.
>
> Years ago the MIT National Magnet lab had a huge flywheel generator brought
> in to provide the instantaneous energy necessary for their experiments.  It
> was impossible to pull energy of that magnitude off the grid.
>
> Beacon Power is a company using flywheel energy to help stabilize the
> electric grid.  This is a more efficient way to handle instantaneous
> increases in electrical demand compared with having excess capacity
> available in a steam boiler.
>
> Jeff

Thanks for the info Jeff.

A CM11A that's doing nothing else seems to keep time within a few
seconds of NTP time, but will drift a little when given other tasks
(like reading its clock?) so I've never trusted it to accurately
reflect the daily frequency accuracy.

Regards,
Charles Sullivan