Re: [TCML] Insulating a MOT

[David Speck <Dave@xxxxxxxxxxxxxxxx>]

Paul,

You shouldn't be seeing any arcing between the primary and core -- they 
are are separated by only 120 VAC (or 240 VAC, depending on where you 
live.)  The standard degree of insulation is usually more than adequate 
to keep primary voltage away from the core.

MOTs normally have the low end of the secondary bonded to the core, and 
the core is grounded.  Have you lifted this bond?  If you lifted the 
bond to to core, then all bets are off.  If the secondary is floating, 
the you may actually be seeing secondary voltage arcing to the primary 
through the core.

What kind of circuit are you feeding with the transformer?  I wonder if 
something in the downstream circuit could be getting you in trouble.

It's possible that you just have a defective transformer.  Many TC 
experimenters have made 4 MOT stacks running 4 MOTs in series to make a 
cheap substitute for a small pole pig.  Most have been successful 
without oil immersion.  Other experimenters have built 6 and even 8 MOT 
stacks, but they have had to immerse the outermost 2 or 4 transformers, 
respectively, in oil to prevent breakdown.

I don't know of any after-market mod you can do to a MOT that will 
improve the winding/coil insulation capability, short of oil immersion.  
It is a relatively simple technique, and is widely used in industrial HV 
supplies for just that reason.

Dave

On 11/4/2012 3:45 PM, mrapol@xxxxxxxxxxxx wrote:
> I have a single MOT in use as a (moderately) high voltage source. It's 
> working fine, but at high voltages I get arcing between the primary 
> and the grounded core. It doesn't seem to be damaging anything, but 
> I'd like to stop it. Layers of black electrical tape had no effect, 
> and I don't want to dunk the whole thing in oil.
>
> I was thinking if some kind of applied insulation, like liquid 
> tool-dip rubber or the asphalt-rubber spray on undercoating sold for 
> cars and as a stop-leak. Would these be a waste of time, you think, or 
> even a fire hazard? Any ideas how to insulate a standing MOT without 
> using oil?
>
> Paul

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