Re: x10 - CM12U (CM11) Clock Accuracy

["Jeff Volp" <JeffVolp@xxxxxxx>]

"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