From: J. Forster (jfor@quik.com)
Date: Sun Jul 18 2004 - 20:03:35 PDT
Larry Tighe wrote:
> Ohm's law applies for impedance.
>
> But here you're talking Watt's law, which is applicable to AC if the power
> factor of the load is 1.
There is no "watts Law". A watt is a unit of power.
> This is the case only for purely resistive loads like heaters and lamps.
If you look at the links I posted, Power Factor is EXPLICITLY included in the
calculations. I also suggested that 0.9 was approximately right for this
application.
> When the power factor is less than one, it means the current is not in phase
> with the voltage sinewave. And for cases like lightly loaded power supplies
> and switching power supplies may not even be a sinewave. This
> causes more current to be drawn when the voltage is lower, than would be the
> case with a resistive load.
Military AC's of the vintage we are discussing are unlikely to have switchers or
SCR controllers.
> The results being heavier wiring required to get the same "power", and is an
> inefficient use of the wiring. (the power company doesn't like it either)
>
> This is the reason modern computer stuff is rated in VA (volt-amps) instead
> of watts. VA doesn't care about the power factor.
The power company does care about VA and VAR (Volt-Amp-Reactive) For large users
they bill on BOTH VA & VAR. The power company absolutely does care about Power
Factor.
> Your figure of 5865 watts is correct of the power factor is 1.
ONLY FOR A SINGLE PHASE CIRCUIT. The original question was about a 3 phase
circuit. The wattage is higher by the Sq. Root of 3 = 1.723 See the links I
posted earlier, if you don't believe me.
-John
> The 3-phase aspect doesn't affect the max current. When it's at its max in
> one leg, the vector sum of the others make up the same number.
>
> Lar
This archive was generated by hypermail 2.1.4 : Sat May 07 2005 - 20:33:53 PDT