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Re: [TCML] MOT Measurements



Hi Bart,

I posted some measurements I took of open circuit current draw on two rather
large MOTs.  I think both of the transformers secondary voltage was around
180V with 10V in.

http://www.flickr.com/photos/pslawinski/2885535771/sizes/o/

You're quite right about heat being an issue with these transformers.  This
is especially true when you run MOTs in a VTTC circuit.  I air-cool my MOTs
now.  The two MOTs I have are huge for MOTs.  Perhaps I can make some more
detailed measurements of these transformers, but I'd have to have another
reason to open my VTTC case.
http://www.flickr.com/photos/pslawinski/3119076877/  The transformers are
positioned right next to the slits in the side with two fans blowing on
them.  Even with that the air coming out the other side is still rather warm
after several minutes of runtime.

-Phillip

On Tue, Dec 30, 2008 at 19:08, bartb <bartb@xxxxxxxxxxxxxxxx> wrote:

> 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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