Dual Athlon MP 1U units

Robert G. Brown rgb at phy.duke.edu
Sat Jan 26 16:26:07 PST 2002

On Sat, 26 Jan 2002, Bari Ari wrote:

> VA = amps × volts
> VA = watts ÷ power factor
> watts = VA × power factor
> amps = watts ÷ (volts × power factor)
> "Power factor" is a number between zero and one representing the portion 
> of the power drawn by a system that actually delivers energy to the 
> system. A system with a power factor of one (sometimes called "unity" 
> power factor) is making full use of the energy it draws. A system with a 
> power factor of 0.75 is effectively using only three-quarters of the 
> energy it draws. Typical PC power supplies are not power factor 

Dear Bari,

I liked all of your description except that of the power factor.  As I
understand it, the power factor is the cosine of the phase difference
between the line voltage and the drawn current.  When they are in phase,
the (rms) VA = the actual power consumed, in watts, as it is in a light
bulb.  When they are \pi/2 out of phase (as they are for e.g. a perfect
capacitor or inductor hooked across an AC power supply) the VA can be
quite high (depending on the impedance of the circuit element) but the
power factor can be zero!  No power is actually delivered to the
circuit, on average -- energy is stored in the capacitor and then given
back to the line.  It does not actually appear as heat in the room.

Real loads are generally somewhere in between.  Loads that are "mostly
resistive" have the highest power factors and only the resistive "part"
appears as heat; loads that are capacitive or inductive can have current
that lags or leads the voltage and draw less power than one might think
looking at the peak voltage and current.

The peak voltage and current are still important, of course.  The power
delivery lines do have to be able to handle the peak current as they
burn energy as (1/2) I^2 R_line for that peak current.


Robert G. Brown	                       http://www.phy.duke.edu/~rgb/
Duke University Dept. of Physics, Box 90305
Durham, N.C. 27708-0305
Phone: 1-919-660-2567  Fax: 919-660-2525     email:rgb at phy.duke.edu

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