My 300uf start cap on my table saw motor (110v) blew today so I temporarily substituted a 220uf (400v) and 80uf polarized electrolytic in parallel which works but I am not sure why.

I am assuming the caps get charged every other cycle from the AC supply. They are only active for a fraction of a second to start the motor. Once it starts the centrifugal switch disconnects the cap.

I've used electrolytics in my rotary phase converter to start the 3 phase motor hundreds of times without issue so I am wondering why the conventional oil filled metal encased start capacitor is normally used.

 

I've used electrolytics in my rotary phase converter to start the 3 phase motor hundreds of times without issue so I am wondering why the conventional oil filled metal encased start capacitor is normally used.

Because electrical engineers know more about electrical stuff than you and you have just gotten lucky so far in having DC electrolytics that have stood up to being used in an AC circuit for fractions of a second at a time without literally blowing up.

 

Electrolytic capacitors fed with AC:

 

I used to do that when I had to work on the 4th of July.

 

AC motor run caps are motor run rated and are usually oil filled for continuous use.
Start caps, because of the necessary high value, are mainly Bi-polar electrolytic, two electrolytic's back to back in the same can.
These are only rated for very short duration in this application and if operated for any longer, heat rapidly, ordinary uni-polar electrolytic's will usually blow.
Max.

 

Start caps, because of the necessary high value, are mainly Bi-polar electrolytic,

I saw one of those fail on a seized motor. It was putting out a stream of white smoke like it was trying to signal a rescue plane.

 

AC motor run caps are motor run rated and are usually oil filled for continuous use.
Start caps, because of the necessary high value, are mainly Bi-polar electrolytic, two electrolytic's back to back in the same can.
These are only rated for very short duration in this application and if operated for any longer, heat rapidly, ordinary uni-polar electrolytic's will usually blow.
Max.

Got a link to that design?

I've scrapped hundreds of those types of motor start capacitors over the years * and never found a single one designed with two electrolytics mounted back to back.
What I have found is they are all made the same sn any other non polar capacitor but with thinner aluminum foil and lower grade oil or mystery paste saturated paper type dielectric insulation in order to get their capacitance values up at the voltages they work at given their size which for the trade off to do that gives them the bad high ESR issues that cause them to overheat when ran for very long.

(*Scrap yards won't take them in stock from due to the antiquated and over encompassing PCB laws but if disassembled and unraveled they see them as aluminum foil roles and are thus okay.

 

I have read of numerous examples over the years, but only currently have one, the electrolyte is usually identical to any uni-polar electrolytic, except the construction resembles two back to back.
Max.

P13.

 

I've scrapped hundreds of those types of motor start capacitors over the years * and never found a single one designed with two electrolytics mounted back to back.

I think we're talking semantics here. Two electrolytic caps, back to back, don't have to have two metal cases and wires connecting them. They can be done with layers which superficially look like a single capacitor.

 

I think we're talking semantics here. Two electrolytic caps, back to back, don't have to have two metal cases and wires connecting them. They can be done with layers which superficially look like a single capacitor.

Standard single can units are what I was referring to and I consider that a single nonpolar capacitor not two back to back electrolytics given they are still just two foil sheets rolled together like any other capacitor or at least of all the ones I have ever cut up and unrolled I have never found anything but two sheets and some paper or plastic separating them.

Anode/cathode - insulator - cathode/anode - insulator. To me that's a single capacitor not two regardless ow what the insulators are made of or doped with.

 

My 300uf start cap on my table saw motor (110v) blew today so I temporarily substituted a 220uf (400v) and 80uf polarized electrolytic in parallel which works but I am not sure why.

.

A polarised electrolytic capacitor has a formed oxide layer on the anode foil which acts as the dielectric.

Reversing the polarity will cause high leakage current that is trying to dissipate the existing oxide layer and plate it onto the other foil. Reversing this process on alternate half cycles is likely to generate enough heat for the electrolyte to become high pressure steam.

It also passes assymetric AC through the start winding, which probably isn't doing the motor any good.