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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


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.

The MOT is feeding a four stage Cockroft Walton multiplier

MOTs normally have the low end of the secondary bonded to the core, and the core is grounded. Have you lifted this bond?

No, the secondary is still bonded to 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.

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.

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.


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?

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