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Motor start capacitor size

Don Gitzel

Aluminum
Joined
Jul 25, 2017
I mostly buy capacitors for my RPC’s online. In a recent order the start capacitors although being the same specifications were half the size. What’s with that? Are they expected to do the same job? How do you rate a capacitor for capacity?? I am thinking of a battery rated in amp hours.
 
When they are smaller, they tend also to be capable of carrying less current. For a start capacitor, that is probably less of an issue. They are severely overloaded in the best case, and depend on starts not being that frequent in order to not overheat.

It may be different for something like bus capacitors in a VFD, smaller when that is in a way that has more surface area is often OK. Surface area dissipates heat. High current parts of that type are often fairly long and thin.
 
Capacitor size is a function of voltage rating, capacitance and continuous current rating. Smaller caps mean a lower value in one of those three departments.

At least until you start getting into exotic technologies like supercaps.
 
Capacitor size is a function of voltage rating, capacitance and continuous current rating. Smaller caps mean a lower value in one of those three departments.

At least until you start getting into exotic technologies like supercaps.


Again, the manufacturer has design options. As with nearly any electrical device (barring switches), heat is the major factor in ratings.

The same "volume" of part may more effectively dissipate heat in one physical form vs a different one, and consequently two parts that seem the same other than shape may have different ratings. The one that stays cooler at the same current/voltage/capacitance can have a higher rating in one (or more) of the specifications.

Start caps are a special case.... they are always severely overloaded, so it may become a matter of overload vs lifetime.
 
You can't really iterate much on a plastic tube filled with aluminum foil and electrolyte. Higher temperature-rated electrolyte maybe, but that's about it. Electrolytic capacitors have been around for a century; they've gotten about as good as they're going to get barring some major technological breakthrough. At the end of the day, you need 'X' surface area of 'Y' thickness foil insulated on both sides with 'Z' thickness of oxides and some electrolytic juice smooshed in between layers.

In the case of start capacitors, you can bet they're riding the 'Y' thickness of that foil as thin as they can get away with since they only need to operate for a very short time once in a while - conductivity and 'Q' factor go right out the window.
 
I would check what ratings are considered equivalent, it may be the same uF and voltage, but the temperature rating may be different as well as the number of hours it will survive. Size does matter as to the thickness of the dielectric which effects the voltage rating and durability. I have seen some reduction in some motor electrolytic capacitors but not on the order of 50% for the same ratings. Their have been significant size reduction in electrolytic capacitors used in e;electronics.
 








 
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