[Nut-upsdev] Re: CyberPower 685AVR and newhidups

Arjen de Korte nut+devel at de-korte.org
Mon Oct 31 20:51:29 UTC 2005

Charles Lepple schreef:

> VA really isn't going to be much help in the EU where power factor
> correction is required (so VA and watts will be equal).

Not at all. Only for certain classes of equipment there are requirements
for the allowed harmonic content of the input current. And this has
*nothing* to do with power factor, contrary to popular belief. In many
cases, striving for a unity power factor is in fact a waste of effort/money.

It is true that if you have a power factor of one, you'll most likely
meet the regulations. But it is not necessary to have a unity power
factor to meet them. A synchronous induction motor will usually have a
fairly poor power factor, but will meet the directive without doing
anything for instance.

The reasoning behind the the EU directive is mainly limitation of the
current in the neutral conductor. In three phase systems, the third
harmonic will add up in all the phases instead of cancel out (like the
fundamental), so the rating of the neutral conductor may be exceeded in
that case.

> However, we only have a variable for "power", and often UPS manufacturers don't
> distinguish between VA and apparent power.

There is no way to make a conversion between (real) power (W) and
apparent power (VA) without knowing what is limiting either of these in
the device powering the system. In other words, there is no general
mathematic way of calculating one from the other.


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