Re: [LUG] UPS, reprise

[Andrew Rogers <andrew@xxxxxxxxxxxxxxxxxxxxxxxxxx>]

Mark Evans wrote:
> > > ===== Original Message From Steve Marvell <steve@xxxxxxxx> =====
> > > PS did we ever find out what was going to go on the end of that 15KVA
> > > jobby?
> >
> > I was reading a rag with ups's in, and it said something like "to convert
> > watts to VA, multiply by 1.4". Is this a ploy to sell more expensive UPS's, as
>
> Most likely 1.414 or ever SQRT(2).
>
> > if I remember correctly, Power (W) == Volts (V) * Amps (A)
>
> It's the difference between the peak and RMS voltage of
> sinewave AC.


Andy is probably correct with his power factor argument. Nearly all PCs
have switched mode power supplies. Just after the mains input is a
rectifier and a capacitor. The capacitor charges just before the peak in
the voltage and discharges after the peak. The capacitor does not
discharge down to zero volts though and as a result there are times in
the cycle where no current is drawn from the supply. It is only when the
mains voltage reaches a voltage above the capacitors voltage that
current is drawn. The result is that the current is not sinewave and
there is a large degree of harmonic distortion, this reduces the power
factor.
-- 
Andrew Rogers
http://www.rogerstech.co.uk

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