From: Larry Tighe (larryradio@worldnet.att.net)
Date: Sun Jul 18 2004 - 18:19:17 PDT
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.
This is the case only for purely resistive loads like heaters and lamps.
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.
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.
Your figure of 5865 watts is correct of the power factor is 1.
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
(Plagurized from my friend Bill Lewis)
----- Original Message -----
From: "J. Forster" <jfor@quik.com>
To: "Military Vehicles Mailing List" <mil-veh@mil-veh.org>
Sent: Sunday, July 18, 2004 9:01 PM
Subject: Re: [MV] Electric current ? (Ohms Law)
> rertman@ix.netcom.com wrote:
>
> > In AC power circuits where the frequency is
> > low (60 Hz) the impedance is essentially the same as the DC resistance.
> >
> > Dick
> >
>
> Absolute not true. An inductor can have a DC resistance of an ohm or two
and a reactance of
> tens or hundreds of an ohm at 60 or 400 Hz. An inductor with a reactance
of Z ohms at 60
> Hz will have a reactance of approximately 400/60 x Z ohms at 400 Hz..
Also, reactance can
> be either inductive or capacitive. Motors are much more complicated
because their
> reactance is a function of speed.
>
> -John
>
>
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