[X4U] OT: Belkin UPS

[Doug McNutt]

At 13:42 +0100 7/12/06, Stroller wrote:
>I would have assumed that VA means "volts x amps"; IE: watts.
>But this doesn't seem to be the case with Belkin's units.

The volt-ampere (VA or often kVA) is a measure of a transformer's ability to pass alternating current (AC) electrical power from one winding to another.

Note that AC voltages and currents change from positive to negative something like 60 times per second depending on where you live.

Power, which is the rate of delivery of energy, is the instantaneous product of voltage and current and it too goes two ways when the voltage and current are both AC. If the current and voltage are always positive at the same time the power all goes the same direction. But, if the current leads the voltage in phase, power goes backwards whenever voltage and current have opposite sign.

Nasty electronics, flourescent lamps, and motors draw current which IS out of phase with the voltage with the result tat some energy flows backwards. It has become a serious problem to power companies and some European laws now require "power factor" correction in the likes of power supplies for computers. Here in the US of A  commercial users are charged for what is called "reactive kilowatt hours" using special meters.

Transformer manufacturers specify their components in volt-amperes because it avoids the problem inherent in the question "do the backward going watts cancel the forward going watts?" The answer is no in a transformer.

VA is AC volts times AC amps

Watts is volts times amps times the cosine of the phase difference. OR

Watts is the time integral of instantaneous volts times instantaneous amps over an AC cycle divided by the time increment corresponding to a cycle.

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