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Do motor capacitors (i.e. start, run capacitors) change in value with respect to age or other environmental factors? I understand they fail, as observed when the motor will no longer start. Do they exhibit a change in capacitance prior to failure?

  • You might also be interested in equivalent series resistance (ESR) which is another (usually undesirable) property of capacitors besides their capacitance, which can also change. – Kevin Reid Aug 7 at 14:24
  • When my AC-run capacitor failed ; it had obviously swelled , we did not measure electrical properties. – blacksmith37 Aug 7 at 14:58
  • maybe; failure can be incremental or catastrophic. – dandavis Aug 7 at 18:13
  • Look at the spec sheet of the capacitor. Basically every number on that capacitor will tend to get worse through age - capacitance, max voltage, ESR, even physical dimensions. If caps got better through age, then "shelf-aged caps" would be a premium item. – Harper Aug 20 at 0:33
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Yes, capacitors such as motor start capacitors can change in capacitance value due to aging and high temperature. The change in capacitance could actually be the cause of the motor no longer being able to start but there are probably additional factors as well. One of these being ESR of the capacitor (mentioned in @KevinReid 's comment) can change (usually increase) which seriously degrades the function of the capacitor. Usually the change of ESR also relates in some way to the net change in capacitance as well.

Other common capacitor failures include:

  1. Breakdown of the insulation layer between the plates in the capacitor can result in a short circuit in the capacitor. Often when the short occurs a lot of current will flow and this can cause internal connections to blow open like a fuse and render the capacitor open circuit. If the capacitor design does not support this internal fusable connection type the excess current can lead to overheading and the capacitor physically exploding or catching fire.
  2. Over temperature operation can cause the seals on a capacitor case to fail which can be catastrophic for certain types of capacitors that have an internal damp nature due to an electrolyte. If this dries out the capacitor can lose most of its capacitance.

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