UPS: Compensating for a high input voltage.

[Paul]

Alastair Stevens wrote:
> Man, I'm still trying to find a solid, touchy feely explanation of how 
> AC power really works. Dammit, I'm an energy engineer (though not 
> electrical per se) and I still can't quite get my head around how power 
> is transmitted in an AC circuit, or how grid balancing works, or how 
> frequency vs voltage are affected by load etc, as you're discussing here.

an alternative view I'm making up as I go along.

an alternating electromotive force (voltage) causes electrons (current) 
to flow in a wire; because it's AC they move backwards and forwards. the 
movement of the electrons thus allows transmission of energy to a remote 
location. when the remote location draws power, it becomes harder for 
the EMF to move them, and hence the power source has to work harder 
(supply more power). the movement of the electrons in the wires also 
requires power to overcome (resistive losses, generates heat). there's 
also inductive effects (magnetic coupling of the wires) and capacitive 
effects.


> Surely the frequency is ultimately governed by the rotational speed of 
> the generators? Unless power electronics and phase compensators are used 
> on a variable-speed driver, as in a wind turbine?!  Ho hum, any 
> electrical engineers out there?

traditional generators generate three sine waves out of phase, each one 
being 240V peak to peak.
if you put on an electrical load, the generator will slow down, slowing 
the frequency and also reducing the voltage (voltage being a function fo 
the speed of a magnetic field moving relative to a wire). also, the 
higher current will, due to resistance in the wires, cause the voltage 
across the load to sag more. to compensate you thus mechanically drive 
the generator faster which increases the frequency and voltage.

if I've mangled the explanation, sorry, but it is way past my bedtime!