Re: [TCML] Centre tapped mot stacks

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

Thanks Ted.

The reason I asked the question to begin with was because I will be running mine in a 3 phase arrangement which would make running a centre tap impossible (it would short the phases). The lack of a definitive "no" to running floated has given me some hope that I will get my supply to work.

As for single phase supplies, I'm not convinced that setting a reference point for the centre of the winding will do anything to help improve voltage gradients within any given mot in the string. This is something which could be easily modelled if someone can be bothered.

I think it makes us feel "comfortable" knowing the exact voltage wrt ground, but frankly the TC primary circuit does not care. So far all I can see that it provides is increased danger and an increased likelihood of secondary to primary arcing within the mots.

I suspect the concept of grounding the centre tap has simply arisen from the early days when we first began using mots. To the first person who ran two in series, running them centre tapped, as they are in the microwave oven, would have made perfect sense. Then people just began adding additional transformers either side without questioning. 

Anyway, I'm not exactly an expert, and I'm hoping that someone who knows more will enlighten. Appreciate your response.

Cheers,

Greg.


Sent from my phone

On 12/09/2013, at 5:32 AM, Tesla <tesla@xxxxxxxxxxxxxxx> wrote:

> 
> Hi Team
> 
> Like many on the list I've run MOT cores floating and some earthed. In all cases I've grounded the centre MOT cores and HV centre point as well
> 
> I think a ground at this point makes sense to control the voltage gradients in a more predictable manner by making at least one reference point. I imagine when cores are floated the voltage gradients then become the subject of leakage and stray capacitances. I would see the floating of the outer cores as providing the ability to share the voltage stress between the secondary to core and the primary to core. Naturally the primary is referenced to ground in all cases where it is directly connected to the mains.
> 
> IMHO it is probably reasonable to float the outer MOTs in a stack. In my case up to 6 MOT (3 per side) with ALL outer MOT's in oil has worked well whether the outer MOT cores were floated or not. The only risk I see is that a flash over will directly connect the HV supply to mains (via the arc) but given the normal precautions needed with this HV this is not a significant extra hazard. On this point I've not seen much comment and would welcome any thoughts the list 
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