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Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
I have a NST (900VA) 15kv/60ma. I have calculated that full PFC would require about 160uf cap. I have placed a 100uf cap. The primary current draw when I short the HV ends without the capacitor is about 8-8.9A. When I used the PFC capacitor the primary current draw is about 3.1A.
Now, I know that if I have a close to resonant parallel LC circuit the TOTAL current draw drops as the inductor draws current as the capacitor gives it back. The currents are 180 out of phase. I know this part. My confusion comes when I start applying this to transformers.
My question is that if I am still drawing off 15kv*60ma (900watts) at the secondary why don't I see this power being drawn at the primary? With the simple LC circuit there is no power being drawn off and the current is minimal because current is being shuttled back and forth between the two reactive components. With a transformer under load power is being taken away so I would think I should see the same amount of power going in: in this case 120v*7.5A.
EDIT: Or, is it that under full load (60ma being drawn) I am not actually at 15kv, but at a smaller voltage? This would reduce the output power which could then equal the current draw from the primary that I am seeing.
EDIT: Mystery solved. I connected my transformer to my voltmeter and slowly raised the variac until I had 600v on the secondary. Next I connected a 10k resistor across. This should give me the full 60ma current (60ma*10k = 600v). I turned the variac on (didn't touch the voltage dial) and the secondary voltage was 24v. I measured the current to be about 2.4ma. So, the transformer isn't putting out 900W when loaded. It is putting out much less which explains the lower current I am seeing on the primary. This is the current going to the load. The balance is being taken care of by the LC tank on the primary.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
The rated current for a NST is the current it delivers into a short. And the rated voltage is into an open circuit. If you draw half the rated current, the voltage sags to about half the rated voltage. (or 0.7 times, I forget) So, the maximum real power a NST can deliver to a neon sign is about one quarter to one half of (rated voltage*rated current)
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power factor correction and transformers
