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.
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.
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.
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.
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.
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.
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.
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.