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Registered Member #3215
Joined: Sun Sept 19 2010, 08:42PM
Location:
Posts: 780
Hi all,
I have a nice MOT which I freshly unshunted, but I'm wondering whether I should float the secondary or not (I have access to large amounts of isolating vinyl sheet so this is not a problem to isolate a new wire connection and even part of the secondary underside)
Could someone who has experience with MOTS explain things a bit?
I have some uses for it which would require phase angle switching for current feedback, but I'm wondering if I would use a 2000V 150mA NST instead...
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
I've removed the secondary ground connection and driven it from another MOT, in air, for quite a while, without problems.
The reason it will take this sort sort of abuse is that it has to be made so that bazillions of these things can be connected to dirty mains, and few of them fail. This means the isolation is specified at rather more than you think it might be. Notice I haven't said "over specified", it is correctly specified for the application. Normal mains frequently has 1.5kV transients on it. For anything that normally connects to mains, you can confidently expect it will survive 1.5kV primary to ground indefineitely. A MOT has rather higher voltages on it however, and tends to operate in a humid, fatty environment, so the insulation is improved a bit on that. As a result, a clean(ish) MOT will happily withstand 1x secondary voltage from the winding inner to the core, and 2x secondary voltage from the outside of the winding to the core, in air, with no more than enough insulation on the lifted ground wire to do the job.
I have heard reports of people putting 3 MOTs in series the way I've put 2, and not having such happy stories to tell. The stories go on to say that submerging them in oil improves things considerably. Plastic sheets poked under the core are not needed at 2x, and not adequate at 3x.
Registered Member #3215
Joined: Sun Sept 19 2010, 08:42PM
Location:
Posts: 780
I've got vinyl sheet which has the same dielectric constant as epoxy, and I have the room to fit 3mm of that material in the void between first layer and the core, so it would be up to the max of winding insulation
Thanks for the tip! I'll have a go at it with the MOT as soon as I have validated my UAA145 solid state "variac"
I plan on using the example circuit with current feedback instead of the tacho, so I can power my HeCad laser tubes with current control...
I'll share the design once it is completed as it's been years since I'm searching for a good way to power them, and I've finally come to tell me MOTs or similar transformers would be the way to go instead of SMPS, and now I have a mean to phase switch them
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Floating a MOT?
