Fw: [Boatanchors] rotor starting capacitor

[Brian Clarke]

Hi Arthur,

Good question. I've asked it myself. The answer I get from the manufacturers
is that
both plates are oxidised electrolytically and the electrolyte is chosen so
that the
periodic reversal of polarisation in an ac application alternately transfers
oxide
between the plates - nett effect over several cycles - effectively zero
change in
oxide layer thickness. In the dc electrolytic capacitor, a different
electrolyte is
used - the main function here is to prevent oxidation of the non-oxidised
plate. As
you know, oxygen is one of the most active elements on Earth and aluminium
is
always oxidised as soon as it gets into an oxygen-laden atmosphere. So,
making
electrolytic capacitors takes place under special anaerobic conditions.

73 de Brian, VK2GCE.

Arthur said:

Thank you for the explanation, Brian. Your argument makes sense, but leads
to
another question.
To wit - what distinguishes an AC rated capacitor from the back-to-back
solutions proposed? In other words, what different manufacturing or assembly
process is involved?
If we can acceptably 'fake' an AC capacitor by using a diode bridge, that's
convenient, but surely manufacturers do not use internal diode bridges in
their
motor-start capacitors.
Arthur Shulman