[TCML] Power Factor Correction
skip at ladybughell.com
Tue Jul 15 13:52:27 MDT 2008
Where are you connecting the PFC capacitors and what value are you
using? The PFC caps should connect to the 120VAC input connections
of the transformer. If connected properly, the capacitance makes
absolutely no difference to the current in the transformer.
For the trial and error method follow these steps:
1. connect an amp meter between the variac and the NST input.
2. Power on the system and measure the current at some voltage near
the expected operating voltage.
3. Connect a PFC capacitor smaller than the expected value and
measure the current. If the capacitance is lower than the correct
value, the current should be a bit lower.
4. Continue adding capacitors until the current starts to rise
again. The best value is the value that had the lowest current reading.
The best way to select the best PFC capacitance would be to use a
current transformer or current probe and an oscilloscope.
Connect the current transformer output to one channel of the scope
and connect a probe to the HOT wire of the AC. If you were to
connect a light bulb to a current transformer, you would find that
the voltage and current waveforms are in phase with each other with
their zero crossings at the same time. If you connect an inductive
load, you would find that the zero crossings are skewed from each
other. By following the basic procedure above, the correct value is
when the zero crossings are together again.
I have made a couple current transformer setups where the transformer
is mounted in a basic square electrical box with a standard line
cord, an outlet, and a BNC output connector to connect to a scope. I
have calibrated them to show one Volt AC per Amp of current. Other
input/output ratios are easily made.
It is also very interesting watching the waveforms of various pieces
of equipment. It can be a fun toy.
If anyone is interested, I could show some photos and give some
plans. For a nominal price, I could also make and calibrate some
transformers. I could even provide a complete setup as I have
described. I will post a couple photos later. I have a few things
to accomplish this afternoon before I can "play".
At 01:40 PM 7/15/2008, you wrote:
>How do I know what direction to go with capacitance? I'm running the NST
>with my Jacobs ladder to find the right value. So far I've only found the
>wrong value, and the very wrong value. Turns out if you get it wrong you
>actually increase [dramatically] the amount of current being dumped into the
>NST. I was running it and all the sudden the aspahlt I neglected to remove
>between the core and the HV windings bubbled up from the oil. When I run
>without the PFC caps I'm drawing about 15A, with the caps I was pulling peak
>currents of 35A! If I'm increasing current does that mean my capacitance is
>too high or too low?
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