Hi Paul,
I need to have a secondary of between 900 to 1050 to produce between
1200 to 1400 VDC anywhere between 0.25 to 0.4 amps should be
sufficient for the 3cx100A5 it will be powering.
That should be within the capabilities of a typical MOT core. As you
know, they run hopelessly overloaded in ovens, at much higher power, but
only for a few minutes each time, and with a fan blowing at them.
I've cut the welds on the E-I core and removed the primary and
secondary windings. In the process I've nicked the primary which I
was going to reuse.
That's no big loss. Anyway you need a primary with more turns than the
original, to keep the core from getting hot enough to melt solder within
a few minutes.
Since I've nicked the primary, I plan on rewinding the primary but
using the entire surface rather than having the windings separated
top and bottom. I'll then wind the secondary on top of the primary.
Q1. Does this sound appropriate to change the design from top-bottom
to use the full E portion?
Yes, that's fine.
> Q2. What sort of paper insulation should
I use between the primary and secondary?
If you want to be professional, a good choice is nomex-mylar-nomex
laminate. It comes in different thicknesses. After winding, the whole
thing should be impregnated with insulating varnish. The porous nomex
soaks up the varnish very well, while the non-porous mylar provides safe
insulation even where the varnish doesn't reach.
> Where can I get this?
In stores that carry supplies for transformer and motor winding.
Typically there aren't many, so you will have to investigate. In Chile,
where I live, I know just one such store.
> I do have some Kapton tape I could use?
Probably, but I have never tried it, I don't even know the
characteristics of it.
Now for a cheap and ham-like alternative: Painter's masking tape. That
stuff is available in every hardware store, in different withs, it
sticks well, it has a decent ratio between elasticity and puncture
resistance, and it also has decent insulation and heat resistance. Maybe
I should not admit this, but I have wound many transformers, including
high voltage ones, just with masking tape as insulation, and no varnish!
But the NMN laminate and varnish is better. It immobilizes the wire,
preventing vibration, noise and possibly chafing through.
> Q3. I only nicked the first 5
windings of the primary, since I will have a custom ratio, is it OK
to simply remove the 5 nicked windings and adjust accordingly or is
removing 5 windings too much?
On the contrary, the original amount of turns is already too small! You
could remove those 5 damaged turns, and then add quite a few more. To
run a MOT continuously without overheating, you will typically need
about 50 to 60% more turns than it originally had!
Since the magnetic shunts used quite a bit of space, and you will remove
them, I think it makes sense to use the original winding minus the
damaged turns, plus a good amount of additional turns. The total will
use up about half the available space, which is fine. The other half is
for the secondary.
Q4. I won't be grounding the secondary to the E core like was
previously done so I assume I will need some insulation between
secondary windings or will the new secondary enameled wire be
sufficient for the approx. ~ 1KV?
I have often heard people testing the enameled wire and getting
surprisingly high voltages before it shorts. But I wouldn't trust it. My
own practice is to add a layer of insulation wherever the voltage will
be more than about 100V between touching wires.
A lot depends on the type of enamel. There are many different types, and
thicknesses. If you don't know the specs of the wire you are using,
better assume that it's not HV-rated wire.
Also be aware of the sides of the winding. Make sure you can't have HV
flash over at the sides, around an insulation layer. Old trafos were
always wound in layers, with each layer of wire being much narrower than
the layer of insulation. That insulation stand-over avoided creepage.
When I wind with a flexible insulation material, such as masking tape,
and an insulating bobbin, I wind all the way to the edges, but then I
wind two layers of masking tape so that it bulks up against the bobbin
sides. So far that has avoided flashovers. Maybe I'm just lucky. For
bigger transformers, I use NMN, safe spacing, and varnish impregnation.
I know the E-I core of the MOT is not the best, but I'm using it at a
significantly lower power level than it originally was in service
for.
That should be fine. Usually these cores are not poor, they are just
overdriven much above their continuous service capabilities, taking
advantage of the fact that microwave ovens usually run only for brief times.
But you must understand that operating the core at lower loss, to be
able to run it continuously, requires using more turns per volt than it
originally had. Just drawing less current from the secondary won't do
it. What heats the core is loss that depends on applied volts per turn,
not on the actual power delivered from the secondary.
Manfred
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