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[TCML] MOT Measurements



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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