Hi All,
I took a a few minutes to measure the characteristics of a MOT I had
on hand and I've found the microwave transformer rather odd. All this
stems from Herwig Roscher looking at his MOT power supply determining
if resonance is or is not a problem for his SISG circuit. Herwig
fried his first transformer and posted on the subject, but almost no
replies other than the initial emails when the transformer died. This
was not a MOT tranny that died, but now he is looking at his new MOT
supply. During those brief comments, Terry mentioned the sisg circuit
was designed for resonance and possibly that could have played a role.
Well, since then, Herwig has been measuring and running simulations
for the new MOT supply. I've also made some measurements of my own
MOT and came across some interesting observations (well, interesting
to me).
Basically, I'm looking at the MOT and if the transformer resonance is
an issue. Here is a list of my typical measurements for any transformer.
Measured dc resistances:
L2 R=53.2 ohm
L1 R=0.204 ohm (low dc voltage with series resistor for .053V/0.26A).
Measured inductances with LCR (knowing this is not real, but just a
reference as why it's not).
L1 Inductance via LCR = 0.044H
L2 Inductance via LCR = 15.75H
Measured open and short circuit currents and voltages in 10V increments:
Vin Vout IpOC IpSC IsSC(A) VA K
10.14 182 0.178 1.72 0.0960 17 0.947
20.0 365 0.220 3.31 0.1500 66 0.966
30.2 563 0.270 5.00 0.2400 151 0.973
40.2 747 0.310 6.62 0.3200 266 0.976
50.1 928 0.360 8.42 0.4100 422 0.978
60.2 1123 0.430 10.63 0.5300 640 0.980
70.2 1321 0.510 13.46 0.6800 945 0.981
80.0 1476 0.600 16.60 0.8600 1328 0.982
90.1 1681 0.780 24.00 1.1200 2162 0.984
100.0 1812 0.980 29.00 1.3200 2900 0.983
110.1 1971 1.600 36.00 1.6000 3964 0.978
120.2 2071 2.760 43.00 1.8800 5169 0.967
I've graphed this data in the following files:
http://www.classictesla.com/temp/mot-vin-vout.gif
http://www.classictesla.com/temp/mot-scc.gif
http://www.classictesla.com/temp/mot-k.gif
This MOT does not have specs labeled on it, so I needed to look at
turns ratio. For accuracy here, I chose a low voltage midterm value
of 50.1V.
Turns ratio 928/50.1 = 18.5. For a 120V input transformer, 120 x 18.5
= 2220V out.
At the normal 120V input, open circuit primary current is 2.76A, thus
primary reactance is 43.55 ohms. Therefore, the following "real"
inductances for L1 and L2 can be found:
L1 Inductance = 0.1155H (Reactance / (2 * pi * Hertz)
L2 Inductance = 39.5H (Primary Inductance * Inductance ratio)
Note: inductance ratio is turns ratio squared (18.5^2 = 342.25).
I then simulated in Microsim the transformer with L's above and at
0.967 k (and of course the R's) using a linear transformer. I
inserted a cap across the loaded side of the MOT and adjusted C until
resonance was found. Transformer resonance occurred at 2.75uF. This
is equivalent to a 5kva transformer with an impedance in the range of
970 ohms. I verified both 1e3 and 1e12 load resistors (resonance is
not affected, only the output value). But, was this large C value for
transformer resonance real? I then looked at Terry's MOT.
I inserted Terry's MOT values as shown in the Piranha schematics in
place of my own. Terry's transformer resonance occurs at 1.5uF and is
equivalent to a 2.5kva transformer. My MOT is twice the power, and my
chosen measurement MOT is rather large (compared to other MOT's I
have). So, Terry's MOT resonant C value is no surprise compared to my
own.
My conclusion here is that unless one is using a very large cap
value, there is no transformer resonance issue. Terry's Piranha
circuits using 165nF is no where near resonance. There is no need to
simulate the full circuit. If the resonance won't occur until 1.5uF
in a basic transformer circuit, it certainly won't make any
difference when rectification is inserted. I feel confident that
transformer resonance with a MOT is not a problem for the LC values
which we use in spark gap and ss switching designs.
But, the MOT cores are small for the odd inductances. This is really
the wierd thing with a MOT. There's no doubt that if a MOT is run in
continuous mode at it's "max kva ability", the MOT will heat up
tremendously and likely see a heat caused death. But as we are
pulsing the MOT, we can certainly get away with some abusive currents
(at least for a limited time). MOT's from my measurements seems to be
quite the transformer! But due to the core size, I suspect it's
"power" is rated for it's thermal capabilities. My particular MOT is
probably rated at about 1000W considering it's size.
Has anyone else made detailed open and short circuit current
measurements of a MOT? I'm curious how my measurements compare?
Bart
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