Re: [TCML] Centre tapped mot stacks

[Greg Peters <greg.j.peters@xxxxxxxxx>]

Thanks Scott, this is helpful to know. Your observation that they fail quicker when grounded makes sense. I think this is why:

Grounding the inner mot cores causes the secondary winding to have a potential difference with respect to ground. Unfortunately the primary windings are also referenced to ground (through the neutral wire). Suddenly the primary coil becomes a nice pathway to ground so you probably get arcing from the secondary to the primary either directly or via the core.

Leaving the HV windings floating removes the reference to ground and thus any potential difference between the secondary and primary windings, consequently removing the appeal to arc to the primary.

The effect is akin to using an isolation transformer to allow a grounded person to safely work on electrical equipment.

This is my understanding at least. Ok, gonna give my mot stack a go.

Cheers,

Greg.

Sent from my phone

On 11/09/2013, at 9:34 PM, Scott Bogard <sdbogard@xxxxxxxxx> wrote:

> Hi Greg,
>     I see no reason to ground them, in fact I never ground my MOT stacks
> anymore, and I've successfully run 3 Stacks as a result.  On the off chance
> they are recommending grounding from a safety perspective, my answer to
> that is no sane human has any business near a MOT when it is running,
> grounded or no, without safety precautions that would make grounding
> pointless anyway (chicken stick and high voltage insulated gear.)  But no,
> I've never seen a performance benefit, and honestly sometimes MOTs seem to
> fail faster when grounded, but that may be in my head.  My advice is let
> them float, hopefully others will comment...
> 
> Scott D Bogard.
> 
> 
> On Tue, Sep 10, 2013 at 10:32 AM, Greg Peters <greg.j.peters@xxxxxxxxx>wrote:
> 
>> Hi all,
>> 
>> Excuse my ignorance - I seem to be missing something obvious. I am about
>> to try a mot stack for the first time and  confused about something.
>> 
>> Most 4/6 pack mot stacks I have seen have the middle two transformer cores
>> grounded and the outer two or three cores floating. I can see that this
>> appeals because it effectively halves the voltage swing with respect to
>> ground, which on first inspection seems safer.
>> 
>> However, I am wondering if we would be better off running the all cores
>> ungrounded, in effect creating one single floating secondary, akin to a
>> pole pig or potential transformer, etc? Is it fair to say that running the
>> secondary in a centre tap configuration references it to ground and hence
>> creates a PD between the secondary and primary windings (which are also
>> referenced to ground through the neutral conductor)?
>> 
>> Is it possible that this would encourage arcing from the secondary to the
>> primary (probably via the core) on the outer mots?
>> 
>> Would it be better if all cores (and hence  secondary windings) were left
>> floating, removing any tendency for arcs between the two?
>> 
>> I know I am missing something but I just can't figure out what it is.
>> 
>> The only thing I can think of is that it increases the voltage stress on
>> any given transformer but as far as I can tell each transformer secondary
>> sees ~2kv across it whether it is referenced to ground or not?
>> 
>> Someone please tell me what simple law of electronics am missing!
>> 
>> Many thanks,
>> 
>> Greg.
>> 
>> 
>> Sent from my phone_______________________________________________
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