Inductive Kick Effects (tests), and new ballast
Richie, Terry, Malcolm, Reinhard, Dave, all,
Today I made up a new suplemental ballast to allow for shifting the
firing phase a full 90 degrees past the input voltage peak, and did
some tests (below). I used about 320 turns of #10 square magnet
wire wound on a 4" by 13" pvc pipe, with steel bars inserted inside
the pipe, to complete the ballast unit. This gives me an extra 16mH,
which added to the original ballast of 26mH gives me enough
inductance to shift the phase 90 degrees (to the zero-crossing)
using the .0147uF cap.
Anywhere from 60 degrees to 90 degrees, I don't really see any
difference in overall efficiency or power factor. Of course, the
inductive kick type voltage build up decreased as 90 degrees was
approached. At 90 degrees, the variac needed a setting of 48 to
give me a reference firing level. And of course at 90 degrees, the
voltage does not dip after the firing. When I let the gap fire only 35
after the peak, the efficiency and power factor was worse. I still
have to examine the areas between 35 and 60 degrees. At 60
degrees, the variac was set at 38 to give the same reference level.
I see in a new posting that Terry is firing 75 degrees past the peak,
this fits well within the range of what I'm seeing also as a good
firing area for creating this L-kick effect.
By adjusting the ballast in such as way that the cap charging was
delayed, the charging waveform could be made more like a ramp,
rather than a sine-wave, at least at the part of the waveform just
before the firing (gap is firing before sine wave peak). This ramp-
like waveform resulted in an even greater voltage build up. I was
able to set the variac as low as 29, and still obtain the same cap
voltage that would demand a 40 ballast setting when the cap
voltage was permitted to reach the top of the sine wave,
by reducing the ballasting to let the cap charge faster. However
both the efficiency and the power factor (esp. the power factor),
both got worse with the "ramp" waveform. I still don't have enough
ballast L to create the ramp effect with a 90 degree firing delay.
"The flavored coffee guy", mentioned on the list that a ramp-like
waveform should give more of an inductive kick, and this does
seem to occur as described above. But in some of these cases,
the voltage may dip more after the firing, and this might be
increasing the stored energy also?
In all cases, the charging seems to work best (lowest input wattage,
and best power factor), when the cap voltage builds up to the full
top-of-sinewave peak, and firing occurs at that
point. If the firing occurs before or after the peak cap voltage, the
efficiency and power factor is worse. (The variac is adjusted so
the cap voltage at firing is the same in all cases.)
I would assume these results agree with the simulations? I
haven't looked at the simulations. In some tests, I thought the
power factor may have been slightly better at 90 degrees, but it
was so slight, I couldn't be sure.
In any case, there seems to be a wide firing-phase range which
gives efficient operation, but the ballast should be carefully
adjusted to suit the other parameters in all cases. As the power
is increased, the firing phase can be left unchanged, and the
ballast adjusted for suitable peak charging.
Since the leakage reactance cannot be easily adjusted in most
NST's, the gap firing phase must be adjusted for best results
depending on the size of cap used. So the option of setting the
firing phase, and then adjusting the leakage reactance to match,
is not available, although in some cases it might be helpful to add
some external ballast perhaps.