Dual Athlon MP 1U units

Bari Ari bari at onelabs.com
Sat Jan 26 21:58:11 EST 2002



Robert G. Brown wrote:

> 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.
> 
>    rgb
> 
> 


True. That's is why Watts are used rather than Volt Amperes to determine 
the amount of heat generated by a system.

A power factor corrected power supply will match the capacitive loads of 
the semiconductors on the motherboard to raise the power factor closer 
to 1. Resistive loads account for very little on a well designed 
motherboard.

Motors are inductive loads that can be corrected with capacitance across 
the load. Power companies may place large capacitors across the power 
lines outside large factories that have high inductive motor loads to 
correct the power factor.

Bari




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