Re: PFC for neons and sytem comparisons
> From: Richard Wayne Wall <rwall-at-ix-dot-netcom-dot-com>
> To: tesla-at-pupman-dot-com
> Subject: Re: PFC for neons
> As a newcomer to this sport, I probably missed the previous threads on
> PFC. My understanding is the primary load of the transformer
> introduces inductance which shifts the phase angle causing a portion of
> the power to be converted to reactive power.
Close, but not quite. The resistive part of the load is the power that
is actually consumed by the circuit in watts. The reactive load is that
which is caused by inductive reactance of the coils in the transformer
measured in volt amperes reactive. This reactive power is not power that
is consumed, or that you even pay for, it just circulates in the circuit.
If you read the voltage and amps on this circuit and multiply the two you
get the load in volt-amperes which is the vector sum of the resistive and
reactive load. The resistive power is not "converted" to reactive power
or vice versa. They are seperate entities, which add up to the total power
drawn in the circuit.
> Capacitance may be added in series with the primary and at the driving
> frequency of the mains, a tank circuit is formed with capacitive
> reactance balancing inductive reactance. The power factor is thus
> "corrected" and power appears resistive rather than reactive. Please
> correct me, if this is incorrect.
PFC caps are normally put in parallel with the line. When the capacitive
reactance is equal to the inductive reactance at the given frequency
(60 Hz in this case), the two will balance each other and the reactive power
will be zero. This can be achieved without any fancy calculations by
adding capitance across the line in small increments. As you add capacitance
the current in the circuit should go down, keep adding until you see an
increase in current. At this point you have have overcorrected and
the extra current is now feeding the capacitive reactance instead of the
inductive reactance. The point where the current is lowest is where
you have the best power factor.
> My question is how does this PFC influence the transformer secondary?
> I assume there is higher secondary power output? How does this work?
> Does mutual inductance between the primary and secondary play a role?
> If mutual inductance does play a role, does addition of a PFC capacitor
> in the primary circuit help protect the secondary against kickback?
How does all this come into play? No you won't get a higher secondary
output, just by PFC anyways. It depends upon how you look at it. If you are
balancing the streamer length against the volt amperes, then yes it will
seem like an increase. In reality, by removing the reactive load and then
running the equipment up to the same volt-ampere level you are driving the
coil with more power.
The only effect PFC has is to keep the amount of current feeding the circuit
down a little. The resistive load is going to remain the same, but you won't
be needing a few extra amps to feed the reactance. Power factor correction is
hardly worth the effort unless one of the components you are using is running
at its max current level, such as a variac. By removing the reactive load you
can use that current to drive the coil at a slightly higher power level without
exceeding the current limits of the power feed. If none of your equipment is
taxed to its limit (at the moment I'm using a 28 amp variac to feed a 1KVA neon
supply) power factor correction isn't worth the trouble.
People in this mailing list tend to describe their coils by saying "it puts out
this much spark at this KVA level". Now the next fellow looks at this and thinks
"man, what am I doing wrong, I don't get that much at that power level". Comparison
by KVA doesn't amount to a hill of beans without knowing the power factor. The
first fellow may have a perfect power factor so almost all the measured power is
actually feeding the coil. The next fellow may have a piss-poor power factor
and half the measured power is feeding the reactance. IMHO I think we should all
be comparing our systems in watts. If a fellow runs neons, it's easy, whatever is
stamped on the can is pretty close to the true power (if they are driving it at it's
rated voltage). For the fellows running PTs or pole mount pots, start scrounging
the salvage yards and hamfests for panel mount wattmeters or power factor meters.
There are such creatures out there.