Re: dielectric change

>I want to sink my rewound transformer in oil. Is it unwise to use oil 
>that has been exposed to air, humidity, dust etc for extended periods of 
>time? Will the dielectric constant change to the point of making the oil 
>useless as an insulator? If so are any oils immune to contamination ? ( 
>no fancy oils, just common and easy to find )
>Thanks much ,
>Ron Q.  
>Cambridge Ontario                                                   

The quoted dielectric strength of transformer oil only applies while the
oil is new and is free of moisture. The dielectric strength will decay over
a period of 3-4 weeks if the oil is left in contact with air. For example,
I use Castrol Insulax T which claims 60kV per mm when new. When I build an
oil filled capacitor, I assume that the dielectric strength is only 15kV
per mm. While this results in physically large capacitors, I have never had
a dielectric failure.

Transformer manufacturers get around this by passing the oil through a
silica gel (or equivalent) filter to ensure the oil is both clean and
'dry'. The transformer casings are then filled with the clean oil and the
air space (which allows for thermal expansion of the oil) is filled with
dry air and a small air bladder is fitted to relieve the internal pressure.

I don't know of any common transformer oil which is immune from this problem.

Safe coiling,

Gavin Hubbard