Re: control panel measurments

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

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?
 >