[Nut-upsdev] Re: CyberPower 685AVR and newhidups
Peter Selinger
selinger at mathstat.dal.ca
Mon Oct 31 20:17:19 UTC 2005
Charles Lepple wrote:
>
> On 10/31/05, Peter Selinger <selinger at mathstat.dal.ca> wrote:
> >
> > Unfortunately, NUT specifies that the unit of the "power" variable is
> > VA, not W. My understanding (partly from the USB HID Power Devices
> > spec) is that W is true power, whereas VA is "peak V" times "peak A".
> >
> > I have converted W to VA by multiplying by sqrt(2).
> > So 390 * 1.4142136 = 552.
> >
> > Perhaps I should round this number to the closest 5, 10, or 25?
>
> well... you can have peak-to-peak VA, and RMS VA.
Yes, and I think that when most UPS manufacturers refer to "VA", they
mean peak-to-peak.
> You're correct that the "W" unit is for true power, and VA is voltage
> times current, though.
>
> For a light bulb, since it is a purely resistive device (for all
> practical purposes), your voltage and current are in phase. I forget
> whether this is "power factor 0" or "power factor 1" or whatever, but
> it's a phase angle of 0 either way.
>
> The problem is that some devices (most notably, motors and switching
> power supplies) are more inductive than resistive, so their phase
> angle approaches 90 degrees. The wattage may be low, because the
> current and voltage are not in phase, but the RMS current times the
> RMS voltage is higher than the RMS value of (instantaneous voltage *
> current).
Exactly right. My multiplication by sqrt(2) is correct only if voltage
and current are in phase; in the other cases, one has to multiply by a
larger number.
Technically speaking, a "VA rating" is really equivalent to an "Ampere
rating", since peak voltage is usually fixed in any particular country
anyway (e.g. 110V or 220V). But manufacturers like large numbers, so
they say it can handle "660 VA" instead of "6 Ampere".
> > In any case, none of these variables have any bearing on the
> > functionality of NUT; they are just provided for information, along
> > with the device name, manufacturer, and serial number.
>
> Right.
>
> I vote for no conversion; if we need a variable suffix to indicate the
> quantity being measured (if not VA), then at least we won't be mixing
> up VA and Watts when displaying values.
Either this, or two different variables. Sooner or later there will be
a UPS that sets both the "ApparentPower" (AV) and "ActivePower" (W)
values. How about
|----------------------+------------------------------+------------------|
| ups.power | Current value of apparent | 193 |
| | power (Volt-Amps) | |
|----------------------+------------------------------+------------------|
| ups.power.nominal | Nominal value of apparent | 500 |
| | power (Volt-Amps) | |
+------------------------------------------------------------------------+
| ups.watts | Current value of active | 161 |
| | RMS power (Watts) | |
|----------------------+------------------------------+------------------|
| ups.watts.nominal | Nominal value of active | 390 |
| | RMS power (Watts) | |
+------------------------------------------------------------------------+
This would need Arnaud's approval, though.
-- Peter
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