Re: [CAD_CAM_EDM_DRO] Re: CNC power supply question

At 17:53 09/03/2004, you wrote:

>Tony Jeffree wrote:
>
> >Ken -
> >
> >The following extract from a spec sheet chosen at random, for a range of
> >"long life" screw terminal electrolytics:
> >
> > Shelf life 2000 h at 0V +85°C, or
> > 4 years at 0V +40°C
> > Operational life time 60000 h at +85°C
> >
> >Seems to indicate pretty clearly that the lifetime is severely shortened if
> >stored at 0V.
> >
> >
>But, it may well mean that if you only charge it up once a year, it will
>last
>for a long time. Only by shorting it for 4 years continuously do you
>degrade
>it. I have large electrolytic caps that are over 30 years old (260,000 hrs)
>that are still working fine. The ones in my CNC power supply are drained
>to zero by the wound fields in the servo motors. I seem to have no problem
>with the caps over the last 6 years, and these caps were used when I got
>them.

Well, the manufacturers pretty much consistently say this:

http://www.panasonic.com/industrial/components/pdf/ABA0000PE26.pdf

(see section 1.3 (2) for example, and the comment on long-term storage in
section 5)

Similar comments here:

http://www.cnelectr.com/alum/atten.html

and here:

http://www.europechemicon.de/ecc01/Products/aluelco/Precautions/al-precaution-e-010928.pdf

The general message seems to be that the dielectric in an aluminium
electrolytic, which consists of a thin oxide layer formed on the anode,
will deteriorate if there is no voltage (and hence no leakage current)
across the capacitor. After long-term storage with no applied voltage, you
run the risk of damaging the capacitor if you don't take steps to properly
re-form the dielectric (e.g., by using a series resistor to limit the
leakage current for an initial period before "normal" use).


Regards,
Tony