Re: PFC Question

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "Malcolm Watts" <m.j.watts@xxxxxxxxxxxx>

Hi David,

On 2 Nov 2005, at 8:41, Tesla list wrote:

> Original poster: "David Rieben" <drieben@xxxxxxxxxxxxxxx>
>
> Hey guys,
>
> I have a question about power factor correction of
> the input to a pole pig transformer. I tried adding
> 700 uFd of PFC to the input to my 15 kVA pig for
> my Tesla system, which can be seen at:
> <http://dawntreader.net/hvgroup/david/gm.html>http://dawntreader.net/h
> vgroup/david/gm.html After I added the (7) 100uFd, 370 VAC PFC caps in
> series with the output of the variac/input to the transformer, I
> noticed that it pushes up the voltage a bit and actually causes the
> variac to start satu- rating when I turn the variac control knob up to
> near 100 (max). This is noted by the dramatic in- crease in the
> audible "hum" in the variac once the control knob is turned past about
> 90 on the variac dial. Also, the output of the coil starts to become
> erratic when I really turn up the variac knob past about 80 on the
> dial. Even the #10 AWG lead wire from the PFCs to the variac ter-
> minals gets warm to the touch, even without any load on the system.
> Also, the output voltage of the variac assembly is notably higher
> (~5%) at a given variac setting with the PFCs than without them. The
> PFC'd output at 100% on the variac dial is measure 325 volts with PFC
> but about 302 volts without PFC. The variac is (2) paralleled 1256D
> Powerstats w/ 240 input and set for 0 - 280 volts ouput.
>
> Once I removed the PFC caps, the loud humming
> ceased, even with the variac knob turned all the way
> up to 100. I just removed the PFCs this afternoon
> and I haven't had a chance to fire up the coil since
> PFC removal, but I have a sneaking suspicion that the
> "erratic" coil output along with the matching "thumping"
> of the variac/ballast assembly will now be absent. I've
> heard that PFCs aren't even necessary with large pole
> pig systems. Until now, I never used PFC with large
> systems and they seemed to work fine. Once I tried
> adding PFC I started having ferroresonant problems
> so I think I'm just going to leave out the PFCs in my
> big coils from now on. Comments, anyone?
>
> David Rieben

It sounds like the caps were mains-resonating with the primary of
your transformer. PFC caps should be _across_ the output of the
variac, not in series with it and the transformer. The loud hum was
your transformer going into saturation. The voltage hike is explained
by the series-resonant circuit that was created. The same principle
is used in the DRSSTC to hike TC primary voltages and currents.

Malcolm