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8 mots ?????

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jameselectric

Fri Apr 27 2007, 03:47AM

Registered Member #535 Joined: Sun Feb 18 2007, 06:03AM
Location:
Posts: 65

anyone have any luck doing this for a tesla power supply or know how to go about doing this?

Dr. Slack

Fri Apr 27 2007, 07:33AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

MOTs are ill-suited to TC use. Basically more current, less voltage, and less inductance than is ideal for our normal power levels.

It is possible to series the secondaries and parallel the primaries of several MOTs, if proper attention is payed to the potentials of the cores, to limit the winding <-> core difference. I have successfully used 3 series MOTs in air, first two cores grounded, last core to first MOT secondary output, which limits any winding to core to *only* 2x rated voltage of a single MOT. Two antiphase stacks of 3, for total 6 in series, has survived so far in JL use.

If you want to go to 8 MOTs, then you either have to tolerate more winding to core voltage with increased risk of breakdown, or you need to float the end primaries, using a pair of back to back MOTs as an isolation transformer (with due attention to the potentials of its cores), which means more than 8 MOTs.

The lack of inductance means that you will need to add extra to match your primary cap and to limit the current. Primary ballast will raise the output voltage, further stressing the 'not really meant for this duty' insulation. Use in oil should improve this problem. Secondary ballast would be a lot kinder to the insulation, but would take many components. The ConnerMeister has used 4 audio chokes in series (for a DC coil, but the principle is the same) at 2.5kV each. When I get round to it I will try some flourescent chokes in series in much the same duty, rated 900v each so I expect 2kV without problems. But all this ballast iron detracts from the tidiness and simplicity of an NST, which is what a MOT coil + ballast is emulating at a much greater weight.

Go google, there are some MOT coils out there. Try it. Control your voltage differences. Maybe use oil. Expect flashovers. Don't bolt it all to the same board and then expect to lift it.

Coronafix

Fri Apr 27 2007, 09:32AM

Registered Member #160 Joined: Mon Feb 13 2006, 02:07AM
Location: Melbourne, Australia
Posts: 938

As far as i've seen out there, 6 MOTs seem to be the limit
for series secondaries, and definitely under oil.
The insulation just isn't cut out for those voltages.
Even 6 MOTs lead to failure.

Nik

Fri Apr 27 2007, 09:47PM

Registered Member #53 Joined: Thu Feb 09 2006, 04:31AM
Location: Ontario, Canada
Posts: 638

I have used 4 (total) mots stacked in open air with no problems for the past 2 years in TC and JL use with no problems. However I have several (8) replacement mots incase I have one die. To be safe I would put any stacked mots under oil if you intend to use them as a serious power suply.

sparky

Sat Apr 28 2007, 07:00PM

Registered Member #530 Joined: Sat Feb 17 2007, 07:56AM
Location: Victoria BC, Canada
Posts: 178

I have an engineering friend of mine who did build a 8 stack MOT psu. Unfortunately the MOTs have to be severely modded to prevent flashovers - the cores have be be cut apart and excessive insulation has to be placed on the secondary. - there are only a few select few MOTs that have special core to secondary clearance. Tabuchi Electric manufactures these transformers - they are $200 each new... or look for older Sanyo microwaves from the early 80's. I have been told that older sears model microwaves from the same era might have them too.

I recommend staying with 6 MOTs in series MAX under oil.

Thomas

Sat Apr 28 2007, 07:37PM

Registered Member #120 Joined: Fri Feb 10 2006, 07:07AM
Location: Westchester New York
Posts: 83

Why do they all have to be in series? Why not have two sets of series' mots then parallel the two? Of course they will need to be in a large bath of oil and kept well cooled in order not to melt the insulation. Surly this is more practical then having them all in series to get a voltage wayyy larger then their intended operation range (the wire's insulation will break down).
The setup i suggested will result in other problems however, such as trying to extinguish the spark gap (quenching). I suppose a blower and rotary spark gap can be used.

Coronafix

Sun Apr 29 2007, 02:41AM

Registered Member #160 Joined: Mon Feb 13 2006, 02:07AM
Location: Melbourne, Australia
Posts: 938

Wiring the MOTs in parallel increases the current whereas wiring in series increases the voltage
as you probably know. In TC operation we want as high a voltage as possible into the primary.
The MOTs 0.45A current is more than enough to charge the cap bank, so increasing the supply
current only causes problems such as quenching the spark. This is why you will mostly see rotary
spark gaps on MOT powered coils, and that is only at 0.45A.

Nik

Sun Apr 29 2007, 02:42AM

Registered Member #53 Joined: Thu Feb 09 2006, 04:31AM
Location: Ontario, Canada
Posts: 638

If you put any number of them in parrallel you will only increase the current not the voltage.

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