MOT tesla coil: Why put the midpoint to ground ?

[paulj]

Hello, I noticed that when several  (quad or twin) MOT were mounted in series, amateur builders connected a midpoint to the earth?

Personally I find it risky because of EMI interference which could come back on by the ground connection

My quad MOT assembly produces a floating voltage so there is galvanic isolation.

Regards.

[Mads Barnkob]

Microwave Oven Transformers are built with one design requirement. Keep the COST down.

So in order to make a cheap high voltage transformer, that also operates at the brink of saturation, one end of the high voltage winding is connected to the core/ground. Then you have less cost to ensure enough insulation between HV windings and core. So midpoint grounding is used to mitigate from that design "flaw" that can make it hard to just put a lot of them series with completely floating secondary coils. You properly need to put them in oil to do this, which they are not in a microwave oven, as it costs money

[paulj]

Thank you Mads for this information, I better understand the incident that I had in 2018, I had among my four transformers two very old MOTs which didn' t have their secondary connected to the core. Following a streamer on the primary coil, these two transformers burned down. The last two most recent have remained intact.

On the microwave oven, I suspected that they were a question of cost, in addition to the villard circuit on the microwave allows to have zero reference point, so less cables

I have  transformer oil Shell diala S4 zx-ig . but finally I will listen to you and put a midpoint to the ground to experiment.

[klugesmith]

If the circuit on HV side of transformer is completely floating (no DC path to ground, even through high resistance)
then what controls its potential with respect to the primary windings?

That potential always has some finite value, which determines the dielectric stress in insulation, even under oil.
I guess in HV applications it could get pretty large, for example due to unbalanced surface leakage, corona currents, or Tesla coil arc strikes.

[paulj]

For now with a floating voltage I haven't new problem, but I take your warnings seriously.