On Sat, 22 Aug 2020 16:11:03 -0700 (PDT), wrote:

On Friday, 21 August 2020 18:27:09 UTC+1, wrote:

I know modern caps are much better than old caps. However, why do you
suppose it is that motor run caps are so big for their voltage and
capacitance ratings compared to other non-polarized caps? I'm serious.

That is not a bad question at all:
a) Because they are AC-rated caps.
b) Because the nature of a motor starting puts a tremendous load on the cap for a very short time, so they must be robust enough to absorb that shock, repeatedly and reliably, in addition to the actual voltage involved. So, whereas the actual operating voltage may be say.... 240 VAC, the start-cap must be rated (at a minimum) of 150% of the operating voltage.
c) Typical NP caps are used a great deal in audio applications, where the voltages are small, and the frequencies high.
d) Motor Run caps start around 370V, or so, are typically of much lower capacitance than a start-cap (and whereas replacement voltage may be higher, replacement capacitance should remain very close or identical to OEM).

Hope that helps.

Peter Wieck
Melrose Park, PA

Of course motor rated caps have advantages. So do higher rated non-motor caps.
But the key point the op appears to have not noticed is that his crude paper cap has already lasted over 70 years. That is more than good enough.


NT
So you suggested that I stuff the old cap with a new cap(s). I knew
new motor run caps were too big to fit in the old cap case. You
suggested I use modern caps with the same ratings. I wondered why
motor run caps are still so much physically larger than caps with the
same specs but which are not motor run caps. Then you post the answer
above. I am not sure why you even bothered.
Eric

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