I recently watched This video about an air purifier made from an old oscillating fan and at 5:20, he explains that the motor's speed has decreased over the years because the capacitor "isn't as good as it used to be". So he replaced it.

Why does a clapped-out capacitor reduce the speed of an AC motor like that? An alternative perspective on the question: Why is the capacitor there?

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    \$\begingroup\$ Basically, I have no idea about electrical engineering. \$\endgroup\$ – OmarL Oct 14 '18 at 13:12

As old oil-filled capacitors dry out, the capacitance goes down and the can't pass as much AC current.

This type of motor is called "capacitor run induction motor". In order to create a rotating magnetic field, the capacitor is there to create a phase shift for one of the two motor windings.

  • \$\begingroup\$ Am I correct in assuming that it is a 3 phase motor design but one of the phases have been replaced by a capacitor? Or am I totally off? \$\endgroup\$ – Harry Svensson Oct 14 '18 at 13:45
  • \$\begingroup\$ @HarrySvensson: No, it's just two windings and two phases. The second winding is just to create a preferred direction of rotation -- a motor with just a single winding rotates equally well in either direction. \$\endgroup\$ – Dave Tweed Oct 14 '18 at 13:55

Certain AC Induction motors cannot start without a cap (start Cap switched) and a RUN Cap.

These are necessary to shift the current phase 90 deg and start turning in the correct direction. The caps these days are all metallized plastic rolled foil in cans with high Vac ratings made from PU, PE and are Y rated for safety.

Oil filled caps tend to leak when aged from electrode contamination and are less reliable and less common.

Due to Arrhenius Law , temp. Rise and aging, caps have a limited life span, although much more than electrolytics.

Reversing the primary winding would result in opposite rotation.


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