Changing NST Input Freq??
From: Jim Lux [SMTP:jimlux-at-earthlink-dot-net]
Sent: Wednesday, June 03, 1998 11:29 AM
To: Tesla List
Subject: Re: Changing NST Input Freq??
Tesla List wrote:
> From: djQuecke [SMTP:djQuecke-at-worldnet.att-dot-net]
> Sent: Tuesday, June 02, 1998 6:40 AM
> To: Tesla List
> Subject: Changing NST Input Freq??
> I dismantled some old telephone gear recently and have a rather large
> frequency divider. Input 60hz, Output 30hz. It was hooked up with a good
> sized transformer and I think it may handle the current draw of an NST.
> So what would the effect be on the NST's output?? I seem to recall a
> discussion that lower the freq may raise the Out current and lower the Out
> voltage, is this correct?
Core losses in the transformer will increase (loaded or not), due to
increased eddy current (this goes inversely as frequency).
The output voltage will remain roughly the same (assuming the core
doesn't saturate, always a possibility, because the NST designer didn't
put any more iron in the core than the minimum required).
The output current will be doubled, because the current limiting
impedance will be halved. (Same effective series L, half the frequency:
Xlim=2*pi*f*L, Iout = Vout/Xlim) ( I neglect the resistance of the
windings, typically a few kOhms, compared to the hundred kOhms impedance
of the L)
This will also double the input current, which will increase the I^2*R
losses by 4! The secondary IR losses aren't too huge (a few kOhms at 30
mA is only a few watts). However, the primary IR losses are pretty
significant. My NST has a primary resistance of a few ohms. At the
normal 4 Amp primary current, it dissipates about 50 Watts. Double the
primary current to 8 Amps, and now it is dissipating 200 Watts, which
will cook it in short order.
So, the upshot is, you'll get twice the watts out of the transformer,
but the thermal load will be quadrupled. If you run it for short
intervals, you won't melt it.