# [TCML] NST Measurements

bartb bartb at classictesla.com
Wed Jan 14 23:51:27 MST 2009

```Hi All,

Tonight I measured one of my 15/30 NST's as far as open and short
circuit testing. 10V increments from 10V to 150V. I was pleasantly
surprised! I've posted my Excel spreadsheet of these measurements (has a
few graphs included for the coupling curve, vin vs. vout, linearity of
short circuit current).
http://www.classictesla.com/temp/NST_1530-1_Measurements.xls

I measured as near to 10V increments as the variac would allow. At the
NST's rated voltage the following was measured:
Coupling = 0.909
Pri Inductance = 436.77mH
Sec Inductance = 6824.49H
Leakage Inductance Sec = 1183.34H

This test was not about inductances, but about Cres (how would it fair
with rated impedance). It faired ok and at least consistent throughout
the range of test voltages. There are expected variances, but the
variation was nothing compared to MOT's or even an NST with 1/2 it's
shunts removed.

Cres would normally be calculated with an impedance of rated Vout/Iout.
15000/.03 = 500,000 ohms.
Cres = 1/(2*PI*Hz*500000) = 5.305nF

As you can see, at 120V (rated input), the "real" value for this
transformer is 5.95nF. There is a 12% error in assumed Cres compared to
the real value. Resonant charging will occur. Example: If I calc'd for
at LTR static gap of 1.5 x Cres with normal impedance calculations, I
would end up at 8.025nF. If I use that value with this NST, I would
"really" be at 1.35 x Cres (the point I'm attempting to make). This will
cause increased current and higher bps than expected.

Vin      Vout(oc)     Leakage L(H)     Cres(nF)
-----    -----        ------------     ------
10.0     1250         27.03            6.28
20.1     2680         28.31            6.00
30.0     4130         28.85            5.89
40.2     5590         29.13            5.83
50.1     7030         28.96            5.86
60.0     8380         29.13            5.83
70.1     9770         29.09            5.84
80.0     10970        29.09            5.84
90.0     12350        29.13            5.83
100.2    13550        28.96            5.86
110.3    14850        28.72            5.91
120.2    16390        28.55            5.95
130.0    17120        28.20            6.02
140.1    18320        27.85            6.10
150.0    19290        27.03            6.18

But unlike the MOT and 1/2 shunted NST's, the change in Cres is rather
consistent and doesn't follow the same trend. Still, if your using a
variac here to drive at 140V input, the Cres value is even higher and
this causes even more resonant charging to occur. So it's not just the
fact that higher power is present, it also changes transformer
resonance. In this case it would cause rather significant resonant charging.

Take care,
Bart

```