RE: control panel measurments

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Luke" <Bluu-at-cox-dot-net> 

Thanx for the input.
It rings a bell from my electronics school days but I seem to have lost
most of it.

Out of curiosity if the load seen by the source were purely inductive
and the pf was zero can the inductive load do any work?  Like say it
were a motor or .............

Seems that if the true power were zero then there would be no power to
be put to work.

Thanx

Luke Galyan
Bluu-at-cox-dot-net
http://members.cox-dot-net/bluu

-----Original Message-----
From: Tesla list [mailto:tesla-at-pupman-dot-com]
Sent: Saturday, February 28, 2004 9:12 AM
To: tesla-at-pupman-dot-com
Subject: Re: control panel measurments

Original poster: "Gerry Reynolds" <gerryreynolds-at-earthlink-dot-net>

Hi Luke,

Good question.  Power and VA, in general, are not the same.  In certain
cases, they can be the same.  VA,  as the name implies, is the simple
product of volts (rms) times current (rms) [I'm stating this for AC].
How
much power is being delivered depends on the phase relationship between
voltage and current.  If the two are in phase then the product of VA
will
also be the power delivered.  This special case will only occur if the
load
that the source sees is resistive (more on this later).  The load can be
resistive, reactive, or a combination of the two.  If the source sees
any
reactive load, a phase shift will be introduced between the voltage and
current.  In general, the POWER will be:

Power = VA * cos (phase angle difference between the two)

This cos (angle) is refered to as the power factor (PF) and the power
can be
restated as:

Power = VA * PF    (0<= PF <= 1.0)

Examples:  If the line voltage is connected to a pure inductor,  the
current
will lag behind the voltage by 90 degrees.  The PF will be zero.
Current
will be drawn but no real power is being delivered (and yes, your
electric
meter will count this for your electric bill - it is really a VA meter
even
though it is sometimes called a power meter).  If a pure capacitor is
being
driven, the current will lead the voltage by 90 degrees and again, the
real
power will be zero.  If the load is inductive with a resistive
component,
only the resistor will dissapate power, yet the inductor will contribute
to
limiting the current.

In TC service, the load that the line sees, more commonly, will be
inductive
with a resistive component.  The current will lag behind the voltage by
a
certain amount resulting in a PF less than unity.  As a result, the
actual
power will be less than the VA that we measure.  For use in John Freau's
spark length equation, for example, it would be incorrect to use the
measured VA.  One need to use the real power delivered from the line
cord to
use in his equation and thus you would need a watt meter.

Power Factor Correction (PFC) is a concept that allows the source to see
a
resistive load.  The idea is to add some parallel reactance to the load
that
will conteract the reactance in the load.  Since the TC has a inductive
reactive component,  it is common to add some parallel capacitive
reactance
to counter the inductance.  This will improve the power factor and
reduce
the line amperage needed to deliver the power drawn.

Gerry R


  > Original poster: "Luke" <Bluu-at-cox-dot-net>
  >
  > First what technically is the difference between VA and watts?  I
thought
  > it was the same thing.
  > Or does one have to do with power factor?
  > Is power the apparent power (not taking the power factor into
consideration
  > and VA the real power or vice versa?
  > Or is there something else I am missing on this?
  >