oil-filled electric motors, topping up?

[Jules Richardson]

I've got an electric sewer pump which uses an oil-filled motor - the lower seal had a tiny leak which I fixed, but it could use a little topping up before I use it in anger (the upper bearing's *just* protruding above the oil surface, so it doesn't need much to cover it again).

Now, I think that the correct stuff is termed dielectric oil (which I've been unable to find locally) - but that this *might* be the same as transformer oil? (I've not seen the latter locally either, but at least it lets me broaden my search!)

I also saw TNP mention recently that big transformers are filled with mineral oil; is that true? Mineral oil I can certainly get (and indeed I have a bottle somewhere) - unless of course not all mineral oils are created equal, and the consumer-grade stuff is a completely different animal :-)

cheers

Jules

[harry]

About twenty years ago many were filled with PCB, a carcinogen. You can't use mineral oil but transformer oil should be OK. The oil has to be compatible with the insulation on the windings.

The oil is there primarily to cool the windings, it carries the heat to the water being pumped.

[The Natural Philosopher]

'mineral oil' is a very broad category. AFAIK transformer oil IS a mineral oil, but only one of many.

But I agree, transforner oil is a safe bet.

[newshound]

Not sure how easy it will be to buy in small quantities though.

Just to clarify, mineral oil just means oil which has been refined from crude oil. Just carbon and hydrogen plus whatever impurities have been left in (typically organic sulphur compounds, perhaps some fatty acids).

What you need in your case is something with broadly the same viscosity as the existing oil. Not too many additives, so I wouldn't use motor oil. The most pure mineral oil which is readily available in small quantities is Liquid Paraffin BP from any chemist. Baby oil is nearly as pure, but it has an added smell. (Baby oil is thinner than liquid paraffin). If you are just topping up existing oil (say, less than 10%) I'd probably go with liquid paraffin, but I think baby oil would work too. (It works on sewing machines).

I expect it is a brushless motor. The electric fields are likely to be trivial compared to conditions in a high voltage transformer, so the selective formulations found in transformer oils give no benefit.

[meow2222]

Tansformer oil often describes a toxic carcinogenic organ killing liquid, s o I'm wary of any such stuff. A thin mineral oil with no additives should w ork nicely, whatever the original is. The easiest way to get that is to hea t up some baby oil to drive the whiff off.

NT

[Grimly Curmudgeon]

Hellfire. Raises a question of just how safe baby oil is. I assumed it was palm oil or similar.

[newshound]

OK I guess some baby oils may be vegetable based, but I'm sure that some used to be mineral. But "liquid paraffin BP" is considered safe to drink so I'd expect it to be pure enough not to cause skin reactions even in the most sensitive of persons. Remember that paraffins (in the proper chemical sense) are very unreactive, whether as oil, paraffin wax, or polyethylene and polypropylene (except when set alight).

[polygonum]

Though they did stop using it on things like dried vine fruits some years ago. Can't remember why.

[Phil Wieland]

...

Just to be clear, that's British Pharmacopoeia rather than British Petroleum, who might well also sell Paraffin.

(-;