29-05-2012, 06:28 PM
Comments based on my experience.
Rust on the non-laminations is not that serious, provided that is hasn't gone so far as to render the whole item a bit crumbly. Rust on the surface of the lams. is not too much of a worry, but if the rust has penetrated between the lams. that could decrease the transformer's efficiency. However, considering that the lams. are usually bolted down well-tight, the chances of rust between them is reasonably remote.
Dampness and moisture that has penetrated the windings is a much more serious issue. At worst, you could have total breakdown of the insulation between one (or more) windings to earth / lams. / metal frame. If you're unsure about damp in a power transformer, remove it from its host (noting the connections first
) and store it somewhere warm for a few weeks: an airing cupboard that has a hot water tank installed therein is an old favourite. Then give it an insulation test with something like a 'Megger' prior to its re-installation and power-up.
I have met one example of an ancient mains transformer where a low-voltage winding used D.C.C. wire for its insulation. If damp gets into that, there could easily be a problem rendering the whole transformer U/S: the D.C.C. wire 'insulation' fails; you get a shorted turn.
I have never known of the 'green spot' problem in mains / output transformers, but that doesn't mean that they aren't affected that way.
Al.
Rust on the non-laminations is not that serious, provided that is hasn't gone so far as to render the whole item a bit crumbly. Rust on the surface of the lams. is not too much of a worry, but if the rust has penetrated between the lams. that could decrease the transformer's efficiency. However, considering that the lams. are usually bolted down well-tight, the chances of rust between them is reasonably remote.
Dampness and moisture that has penetrated the windings is a much more serious issue. At worst, you could have total breakdown of the insulation between one (or more) windings to earth / lams. / metal frame. If you're unsure about damp in a power transformer, remove it from its host (noting the connections first
) and store it somewhere warm for a few weeks: an airing cupboard that has a hot water tank installed therein is an old favourite. Then give it an insulation test with something like a 'Megger' prior to its re-installation and power-up.I have met one example of an ancient mains transformer where a low-voltage winding used D.C.C. wire for its insulation. If damp gets into that, there could easily be a problem rendering the whole transformer U/S: the D.C.C. wire 'insulation' fails; you get a shorted turn.
I have never known of the 'green spot' problem in mains / output transformers, but that doesn't mean that they aren't affected that way.
Al.
