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> Date: Wed, 15 Dec 1999 14:19:28 +0800
> From: "Visser, Martin (SNO)" <[EMAIL PROTECTED]>
> Subject: RE: [SLUG] UPSs again! some questions
>
> VA = Watts only when there is no reactive power, that is
> when current (Amps) is in phase with the applied voltage
> (Volts).
WRONG! VA = Watts only when the power is DC. As your power outlet
provides AC power, you need to determine the maximum voltage to
determine the VA rating. For a sine wave, the maximum voltage is
1.4 times the RMS voltage. In Australia, the power is 240V RMS.
You are thinking of Power Factor, a different subject altogether.
> Reactive power occurs when there is capacitive or
> inductive load (like a big motor).
Nope, power factor problems occur when a capacitive or inductive
load is applied to the power lines. Purely capacitive loads are
rare, but large motors are highly inductive, and this is why
they require a capacitor to start correctly.
As a side-effect of either a capacitive or inductive load
affecting the power factor, your metered power usage actually
falls. This is why motors connected to mains require capacitors
and capacitive loads require an inductor.
> For this purpose (a computer UPS) it probably can be
> assumed to be the same, (unless of course most of your
> VA is going into driving a 1/2 horse power motor that
> runs the CPU refrigeration unit!!!!!).
For this purpose, multiply the Watts by 1.4 to obtain the VA
rating. Please get it right. A 250VA UPS is badly underated for
a 250W computer.
Aussie
[insert derogatory compaq comment here]
:-)
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