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Most ceiling fans and table fans designed for household use, eventually slows down before completely failing to work.

By close observations I found these fans are using capacitor-split-phase motor and failure of the motor is electrical and not mechanical such as ill bearing etc. When the capacitor feeding one of the two coils of the motor get weak (dropping capacitance), the motor not able to develop and maintain the torque needed to sustain rotor speed.

The type of capacitor used in this fans are generally Metallized Polyester Film. Why this capacitor fails?

Capacitance is depends on dielectric constant of the insulation of polyester, thickness of that insulation film and total area of the metallized surfaces.

I belive the drops of capacitance is due to somehow the metallized surface effective area drops due to discontinuity develops within the rolled film. Polyester is stable thus it's dielectric constant or its dimension will not change over time.

If the cracks that cause discontinuity of metallic surface on the film is what cause the drop of capacitance, why it happens?

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  • \$\begingroup\$ Heat but most probably high voltage spikes do break down the dielectric. The capacitor is self healing, what remains are punctures, thus reducing the area. The spikes could come from different appliances, mostly solenoid valves from washing machine. \$\endgroup\$ – Marko Buršič Aug 18 '19 at 20:42
  • \$\begingroup\$ Loss of capacitance is often due to the self-healing process. Metallized caps can self-heal when a short in the film is cleared by evaporating the metal around the short. Also, the metallization is put down as an array of squares that are connected with narrow bridges that act as fuses. If a short is not cleared the first way, the square than gets isolated and its contribution to the total capacitance is lost. Lose enough squares and you start to notice. Metal foil caps can't self-heal. \$\endgroup\$ – Robert Endl Aug 19 '19 at 0:21
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In my experience, overheating tends to cause bulk failure of the metalized film capacitors. As pointed out by Marko above, voltage spikes (thunderstorms, heavy duty appliances switching on and off, switching on and off the equipment itself) produce spikes that can damage those caps over time b y reducing the effective capacitance. They are called soft-fail because in most applications, they fail open instead of short, so when they are used in EMI filters for instance, that prevents a short circuit across the AC power lines.

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  • \$\begingroup\$ There are many fail modes outside of heating. \$\endgroup\$ – Sparky256 Aug 18 '19 at 23:35
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In my years in surge suppression it is oxidation of the metal film that leads to failure of metal film capacitors. The precursor to this is lack of an airtight seal. This combined with an applied mains voltage can cause rapid oxidation of the metal film.

Measurement of failed capacitors show a value of about 1% of rated value. The seal must be air tight rubber, or a meniscus (lead) seal using the expoy the capacitor is dipped in.

For this reason most film capacitors fail in an open state.

Excessive ambient temperatures and over voltage spikes can exacerbate and accelerate this problem.

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