On Apr 30, 8:37*pm, Andy Champ wrote:
On 30/04/2013 16:30, harry wrote:







On Apr 30, 9:48 am, Andy Champ wrote:

I've never understood why a load on the grid _must_ affect frequency.
(and yet it does...)

A generator is shoving out a certain amount of power into a load. If the
load increases (reduces its resistance) either the power goes up, or the
voltage comes down.

I can see why the easiest way for the voltage to come down is for the
generator to slow down. The coils aren't going through the fields as
fast, so you get less power consumed. In the case of the grid, where all
the generators are synchronously locked, if they all slow down then the
grid frequency drops.

But why can't you just control the current into the field coils to keep
the frequency spot on, and let the volts go up and down?

Andy

The frequency of the alternator depends on it's speed which depends on
the speed of the driving device. To *vary power output/maintain
voltage, the excitation current is varied.

This means that the mechanical input torque has to be increased if
excitation is increased. *If this fails to happen the frequency will
fall.

Yes. And?

If you get an increased load, surely you can maintain speed by reducing
the excitation current - can't you?

But that also reduces voltage. If you reduced it sufficiently the
"generator"
would be run as a "motor".