Hi Team,
I can add some to this, as I've killed a lot of MOTs in my time and
seem to have finally figured it out (a lot of mots, like 30+...) Using a
ballast is everything when stacking them, and I'll wager the better the
ballast the better the life expectancy. When running unballasted, I
immediately kill MOTs when I use more than 2 oil or not, I believe it is
because the unregulated current draw of 6 or 8 or 12 or whatever thousand
volts is too much to handle, the shunts probably exaggerate the heating
effect of the excess current, not to mention disfiguring the already
saturated magnetic flux... The only exception to this was when I ran SRSG,
and my small cap for my system limited my current, then oil made no
difference, they were impossible to kill but my results were lousy. I
theorize in countries where they use 240 V this is substantially less of an
issue, as for the same wattage output they use half the current through the
primary so they can double the wattage before MOT lethal current happens,
thus Peter Terren can run 4 mots unballasted with no trouble, and get 8 foot
sparks. Point blank, at the end of the day, after the fat lady has sung,
there is only so much you can get out of MOTs, and if you exceed that limit
they die, the trick is to make the best use of available power for good
results, the 240V guys will always have a leg up on us for this reason; for
me I will stick with just two, they are bullet proof like this even using no
oil, until your primary strikes kill them by breaking down the insulation,
as was the case on the last several I murdered. Just my two cents.
Scott Bogard.
On second thought, if you ran only 60 volts per primary and used more than
double the MOTS, you might be able to spread out the heat and reduce
saturation enough to get away with more than 4000V (like 6 MOTS for 6kV) I
think I might try this, under oil of course...
On 3/19/2011 4:40 PM, Jon Danniken wrote:
John S. wrote:
[snip]
I destroyed 8 MOTs before I realized the
secret....take the shunts out and make a good ballast. Removing the
shunts made the MOTs run much cooler. In fact, with them in, the oil
would be hot to the touch just after a few quick minute long runs. I
think that the heat contributed to their failure. Now I can run the
coil much longer and the oil never gets warmer than ambient temp. I
haven't lost a transformer since.
Interesting observations, John. MOT shunts are one area where nobody
really seems to be in agreement, at least from I have noticed over the
last decade or so on the list. I have read some suggesting to leave them
in, some suggesting to take them out, and some saying that it really
doesn't matter.
I do know that they are not anywhere near as significant as in an NST,
but considering the intended application of a MOT as only running in a
half-wave double, they don't need to be.
What voltage were you running your MOTs at initially, when they were
burning out? Do you think that perhaps running them with an external
ballast had more of an effect than removing the shunts (ie, what was the
voltage/current at each MOT before and after ballasting)?
It would be nice to nail down the issue of MOT shunts once and for all
and determine the best choice when using them in a TC supply. I've got a
MOT I'll run some experiments with and publish the data when I am done.
Jon
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