[X4U] OT: Belkin UPS

[Stroller]

On 12 Jul 2006, at 19:12, Doug McNutt wrote:

> 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.

Wow!

I see!

What an excellent explanation! It was truly the answer I was looking  
for - I knew there had to be a decent reason for Belkin's 750VA  
supply to offer only 400 watts, and your explanation both was  
understandable and answered all (well, nearly ;) my questions.

I appreciate your posting.

Stroller.