Re: [TCML] Insulating a MOT

["Kurt Schraner" <k.schraner@xxxxxxxxxxx>]

Paul,

See my comments interspersed.

-----Ursprüngliche Nachricht----- 
From: mrapol@xxxxxxxxxxxx
Sent: Thursday, November 08, 2012 1:56 AM
To: Tesla Coil Mailing List
Subject: Re: [TCML] Insulating a MOT

Thanks for your reply. Sorry if this is not directly TC related, but I
thought the HV content was relevant.

See comments interspersed.

----- Original Message ----- 
From: "David Speck" <Dave@xxxxxxxxxxxxxxxx>
To: "Tesla Coil Mailing List" <tesla@xxxxxxxxxx>
Sent: Sunday, November 04, 2012 10:51 PM
Subject: Re: [TCML] Insulating a MOT

<snip>

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

  in my view, you're right, David. This may be the culprit.

I am powering an ion thruster (asymmetrical capacitor). The MOT is driven by
a 600 watt rotary dimmer switch, along with a 5 uF motor run cap. The output
of the MOT is then run through the Cockroft-Walton voltage multiplier, where
it emerges as HV DC to power the thruster. At higher settings of the dimmer,
I get sparks between the primary coil and the MOT frame. At lower settings,
the arcing stops (though I imagine there is corona loss at the lower
settings I can't see). I thought if I could improve the insulation on the
primary somehow I could eliminate the arcing.

If I understand you right, you're operating your MOT, like an induction 
coil, by means of capacitor discharge with a dimmer and capacitor. See, what 
I mean on:
http://home.datacomm.ch/k.schraner/Dimmer1.gif
This mode of operation is generating a multiple times higher voltage (vs. 
the perhaps 2kV design) in the secondary of the MOT, as a consequence of the 
steep dI/dt current pulses of the cap discharge (i.e. 20kV). I "think" it is 
not possible to insulate the MOT for this sort of overvoltage, even if 
immersing it in oil. If power would not be the problem, you might replace 
the MOT by an autoignition coil. But for a ion truster power seems to be; 
the trust would be very very small, at perhaps 80W! Please excuse, if this 
opinion was just a consequece of my misunderstanding.

<snip>

> 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

Yes, but even oil immersion would probably not help. See above

The MOT, dimmer, and cap are mounted on an insulated board, covered over
with an inverted HDPE box. Using oil would require a complete reworking of
this setup, so I was hoping not to have to do it if an alternative could be
found. Has anyone ever tried spray-on rubber undercoating as insulation?

Spray-on rubber undercoating probably not a solution (only cosmetics), the 
MOT-transformers inside would need better insulation, which IMO ist not 
accessible.

Paul

Regards, Kurt

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