Single Phase Fractional Horse Power Motors

It is a very nice article in this month's MEW about fractional horse power induction motors. The only point that gave me a slight pause for a cringe was the idea of cleaning up the centrifugal switch contacts with sand paper. Almost all electrical switch/relay contacts have very fine coatings of rare earth metals to avoid corrosion and consequently high resistance. Sand paper will strip these coatings off and give a quick fix that rapidly deteriorates.

In the absence of a burnisher (I've only ever seen one once in my career, here's a link to what I mean, http://www.tecratools.com/product615.html) I'd suggest a toothbrush and contact cleaner. It's also useful to set up a slight wiping action on the contact.

There will be motors without special finishing on the contacts and these can be sandpapered but if you can't tell the difference between these contact types then err on the side of gentleness.

A very nice article Ted.

Colin

Used to refurbish motors all day lo0ng, contacts on Cent switches are generally platinum, no rare earth coatings, clean with fine flat needle files, restore points to parralell or domed, depending on how they were originally, sandpaper is frowned upon as it embeds grit in the points, but fine wet&dry to polish up is fine!

double double post post, sorry mods!!

I have read in many places that sandpaper is OK but not emery or silicon carbide as they are hard enough to embed. This is mainly in reference to copper commutators. And today I read in a book from 1923 that emery is not used as it is magnetic, too. Which it is, or at least it includes magnetite in its make up.

If the contacts are coated it is likely to be with 'precious' metals (as distinct from 'rare earth' properly speaking) and if they have become darkened with corrosion the protective surface has surely already failed? In such a case a rub with flour sandpaper will be an improvement, and (like all repairs) a temporary fix but still a useful way of getting more use from a duff motor.

Cheers, Tim

Tim, Slip of the finger. Precious metal, not rare earth. But the precious metal means a coating thickness measured in microns not thous. Any metal working abrasive will rip off that coating and fresh corrosion will shortly follow. If the repair is to be more than temporary then the contacts need cleaning.

The quick fix of the sandpaper (in the old days often the abrasive from a box of matches) was greatly deplored by the technical managers of the Strowger telephone exchange I worked in.

Colin

I very much doubt Platinum is used for contacts these days, other than for high quality installations. Mostly Palladium at best, I would think. For small mass produced motors the contacts are likely a thin coating or surface alloyed with the precious metal. Old magneto contacts were of Pt, but replaceable contact points for the Kettering ignition system have been throw-away items for a long time....

Same with CATs. Early ones contained quite a worthwhile amount of Pt (very popular pikey targets) but now there is far less, most of the (reduced amount of) precious metal in them is Pd.

Having the article again, sand paper wasn't mentioned. In the distant past when refurbishing motors which had a "weld spike" on the centrifugal switch we used a needle file to reshape the contacts and polished them up using Crocus paper. I don't think many home workshop will have such a thing, but may have fine wet and dry as used on there cars. John

Tut!Tut! At least you would have kept ME in work replacing warn out equipment ( construction man)

Note nonBT people will not understand the animosity between maintenance (men) and construction

I would use on of these, intended for cleaning plated tracks, contacts etc on PCB's and other things.

**LINK**

People who repair washing machines etc also use them to clean up motor com's while it's running.

They last rather a long time.

John

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Ah - Strowger switches - Happy Days! (well maybe not - pretty tedious ones actually) sitting in front of rows of twirling selectors burnishing contacts as part of the routine exchange maintenance.

I don't recall the burnisher being anything too complicated, just a piece of very smooth metal on a slim handle - standard PO issue. For occasional use a fairly thick feeler gauge would work pretty much the same I would have thought. I have another version of a burnisher (which is similar in size to a small Swiss file but with no teeth) and it's used exactly like a file to finish/polish metal surfaces. The PO versions were much thinner though, they needed to be to get between the switch contacts without disturbing them too much.

Regards,

IanT