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Varnishing methods for a transformer

Varnishing methods for a transformer

Varnishing methods for a transformer

Hello all,

I try to come up with some good idea of how to varnish small electronics transformers in mass production. My field is electrical, so pardon me for any foolish statements and questions regarding coatings.

The problem is that there is variation in the dimensions of two essential parts of the transformer: the bobbin and the ferrite core. The tolerances for this variation are for ferrite core +/-  0.15-0.3 mm and for bobbin +/- 0.1-0.2 mm. This means, when the core is assembled into the bobbin, some units end up having a gap as large as 0.5 mm in between the bobbin and the core. This in some circumstances leads to audible noise, which doesn't occur in units with a small gap.

The current method, dip varnishing, does not give satisfactory results, since most of varnish flows out from the gap during dripping and curing. However, the varnish viscocity should be more or less optimal, more viscous varnish woulnd't penetrate the gap.

I have been thinking of a couple of alternative procedures:

The first one is making use of thermal expansion. I have observed one side of the bobbin (Phenolic PM9820) expands ~0.07 mm and the core ~0.01 mm when heated up to 150C degrees. When cooling the bobbin down to room temp, it's dimensions seem to have shrunk from the original dimensions before the heating up.

After every 25C...150C -cycle the dimensions shrink more. In the end the difference to the first 150C dimensions is nearly 0.20 mm. For the ferrite core, there's no such effect. Could somebody confirm my findings are for real and explain why? Is it the number of heating cycles or only the time in the high temp that causes this?

So I was thinking of heating the transformer up to 150C (temp. of its glue curing) or more and dipping it in room temperature varnish. Now, I hope, the varnish would penetrate the max. open gap, and cool down the transformer, shrinking the gap and capturing the varnish inside in the process. Then take it out to drip and dry and start new heating up...cooling cycles to shrink the gap more. Do you think this would be feasible?

My second idea was to do somekind of electrostatic coating (is the term right?). I mean using somekind of charged varnish and thus making it stick to the ferrite and the bobbin and remain in the gap. How would this work in practice? Is it feasible for mass production?

Are there any other methods worth trying?

Best regards  

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