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Re: [TCML] MOT Measurements
Hi Herwig,
Yes, I overstated my comment as in the form of an absolute, and I should
not have. There is "always" action of this type with "any" Cext value
and the leakage reactance of the transformer. Even for a C as small as
1nF in comparison to 2nF, one could say that "resonant charging is
occurring as there is a difference in voltage". I was referring to where
resonance was found along the curve compared to where I saw the circuit
values occur.
However, I also found an error today as I graphed out the resonance
curve for you using Terry's MOT. I originally used 165nF for each C (I
took Terry's .sch file for Piranha 3 and modified to the Piranha 2
circuit), but I apparently did not change C when I was looking where C
values occur in comparison to the resonance peak. Anyway, I ran
simulations for resonance using the correct cap values this time, and we
do begin to enter into some resonant charging which accounts for the
voltage seen at the MOT. I used 50nF intervals in the simulations.
Reference the following graph:
http://www.classictesla.com/temp/mot-cres.gif
Red line = MOT and cap alone looking at output of MOT.
Blue line = Full charging circuit looking at output of MOT (diodes, R's,
primary L, primary R).
First X to the left represents the 165nF cap values (which I consider
insignificant).
Second X to the right represents the 330nF cap values (well, more
significant now).
With the larger caps, we start to enter into enough resonant charging
that it is worth noting. The graph I think also helps to see why one
should never consider a Cres value with a single MOT.
Does this graph help?
In order to protect it from overvoltage/kickbacks I
soldered 4 MOVs S20K625 (connected in series of course) directly
across the secondary winding. The no-load-voltage of the MOT is 2232
Vrms.
Now we know why the MOV's got switched constantly. You'll need at least
6 to 7 if using Terry's MOT. However, your not. Are you able to throw in
your particular MOT values and run the same type of resonance simulation
that I did above? If not, give me your MOT values and basic charging
circuit parameters and I can do that for you. From that point, you can
figure out the expected voltages.
BTW, when you disconnected the diodes from the tranny, did you also
disconnect the cap?
Take care,
Bart
To my surprise they got so hot within seconds that they became
unsoldered. Then I disconnected the diodes from the tranny and
repeated the test: no heating at all. To find out the amplitude of
the overvoltage I added MOVs in series until they stayed cold: 3675
Vrms!! A note: I used unused MOVs, as used ones could have reduced
their breakdown voltage. S20K625 means max AC voltage is 625 Vrms.
Bye Herwig
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