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can a 3 Ufd ac start capacitor be replaced with a 3 Ufd 600 volt dc capacitor

I don't understand why you need to ask this when Phil has already answered why 600v caps fail on 240V AC.

I can assist you with repeating the relevant passage here:

"A given 600 or 1000V DC rated cap may well be series wound, impregnated with oil or just use unusually thick film and so last quite well. But the vast majority of DC rated caps last only a short time when used with a 240V supply - then typically explode violently. Failure is due to internal corona discharge across trapped air pockets inside the cap eating away the insulation.

Corona simply does not occur in wound film capacitors at 100VAC, that is why I mentioned the number."

What part of this don't you understand or accept as an answer to your question?

--
Mike Perkins 
Video Solutions Ltd 
www.videosolutions.ltd.uk
Reply to
Mike Perkins
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"Mike Perkins"

** Einstein once commented that his theory of relativity was: " ... quite easy to understand, just very hard to believe ".

The problem of trapped air pockets in metallised film caps is well known to cap makers, but very obscure outside of those few. Internal corona discharge across tiny air gaps is the ticking bomb inside any wound component that has to operate at high voltages. The solution adopted in that past was to vacuum impregnate transformers and many capacitors to exclude air completely.

The old paper, film/foil caps were vacuum impregnated with wax and survived continuous high AC voltages well, until the wax absorbed moisture and they developed severe leakage.

When metallised film caps arrived in the early 1960s, the problem was soon discovered and has not gone away. So cap makers came up with a solution, put two caps in series in one pack and the AC voltage across each is halved = no more corona. Then some cunning engineer came up with the technique used today of making one cap with an internal floating electrode (or two ) that does the same job.

All this drama was explained in an article on capacitors by Cyril Bateman in Wireless World magazine some time in the 1990s. Riveting stuff.

.... Phil

Reply to
Phil Allison

Problem found. The wiring to the base to the motor was bad . One of 3 wires had an oxidized connection and was open. I reconnected the wire and the fan operated with the replacement cap. I then reconnected the original cap and the fan operated fine. The fan is from the 1940's but runs quietly and oscillates and moves a large amount of air. Thank you all.

Reply to
BEN

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