Most are electrolytic for practical reasons including cost and available capacity+voltage rating. Electrolytes in the capacitors dry out in the hot weather as well as other things like other components going bad and drawing too much current, both of which cause further overheating. Overheating makes the electrolyte dry even faster (making the capacitance plummet and resistance increase, i.e. stop capacitoring) and generate gasses (=>swell/pop).

Tldr hot weather is hard on them. They have a finite lifetime and suffer most when you need the AC the most.

Are more robust capacitor designs not an option because the capacitive properties of the paper ones are better for the use case?

More robust capacitors exist. They just cost more.

In HVAC world, the practical differences between a motor start cap and a motor run cap are price, physical size, and longevity.

A start cap is cheaper, but is meant only for intermittent duty and is unsuitable for use as a run cap.

Meanwhile, a run cap costs more but can serve as a run cap or a start cap.

All things (except for outliers like the McLaren F1) are built down to a price.

Aren't electrolytic caps polarized? That can't be used for AC. Iirc these capacitors are film types instead.

Non-polarized electrolytics are somewhat uncommon, but they definitely do exist. They get used rather frequently in things like crossover networks for loudspeakers[0].

They tend to cost more, and tend to be larger than their polarized kins. They're not advantageous in circuits that always have some DC bias, so they only get used where it is necessary.

0: https://www.bennic.com.tw/en/ec/index.asp

For AC these units use non polarized, oil-filled metallized polypropylene film capacitors. But overheating is a problem for them, as it is for electrolytics.

Would be neat to have an AC to keep the electronics nice and cool.

Indeed, I did some basic double checking before I posted but I guess the info I read was bogus+I was also having a brain fart to not know better because you're definitely right. Thanks for correcting.

No one uses an electrolytic type capacitor for the 60 Hz winding of a "single-phase" induction motor.

It’s not so much that they are fragile, as they’re a part that gets a lot of very heavy duty use and they’re expensive to make invulnerable. Ain’t no residential customer going to pay for a 100mfd 240v tantalum cap (how big would it be even?).

Think of them like a car starter motor or transmission (for old ICE vehicles).

Assuming we’re talking motor start capacitors anyway. For most of them, every time the compressor starts they see a dead short at 240v for a couple milliseconds, typically in the 10,000+ amps range.

And most people use their AC the most when it’s hot and nasty out. Which doesn’t help.

By "mfd", do you mean µF? I have some basic knowledge of electronics and am not familiar with "mfd" in this context but assumed you must mean microfarad.

Apologies, yeah most motor start caps for some reason use mfd to mean micro farads. [https://www.packardonline.com/electrical/capacitor/motor-sta...]

I guess from before the days of Unicode?

µF == the same thing, but tends to be the ‘more correct’ modifier used in electronics and engineering, rather than industrial parts supplier catalogs.

Oh, wow, this is so bad. m usually means "milli" not "micro". At least they could have used u instead of µ, which is a more common replacement when you don't have the keyboard character available.

Just wait until you see the actual dimensions of a 2x4!

I owned a house once that was balloon framed, with 2"x4" black walnut studs in the walls.

They just don't make things like they used to. :)

You can control when a capacitor will blow, by placing it at a certain distance to a heat source. They do not like heat and you can thus determine the lifetime of a device on a bell curve around time * times constant average use. Nasty but legal.

They aren't. The ones you buy nowadays just aren't engineered to last.

Electrolytic capacitors are a perishable good. Age even when sitting on a shelf unused.