----- Original Message -----
From: "Chuck Grandgent" <[email protected]>
To: "drakelist" <[email protected]>
Sent: Tuesday, May 03, 2011 4:20 PM
Subject: [Drakelist] off topic - motor start vs electrolytic
caps
I know this is off topic,
but I also know someone here will certainly know.
Air Conditioner guy was out today for annual maintenance,
the air mover had
a 15uF motor start cap that he measured at 7uF, so we
replaced it.
No quarrels with that, but here's what I couldn't
understand.
I tested it on my cheapo DVM, which I rarely use for cap
testing, and the
capacitance reading just kept going up and up and up,
20uF, 30uF, 40uF,
etc., clearly not right.
I grabbed a 22uF electrolytic, and that quickly stabilized
on my DVM as
19uF, on the tech's meter (better than mine) 23uF. So
that was believable.
Now I really don't know much about motor start caps, but
they are
"unpolarized", compared to electrolytics ?
How to account for its anomalous behavior with my DVM ?
Chuck, K1OM
I think they are unpolarized but probably the reason for
the difficulty in reading was that the old one had a very
high dissipation factor or lots of leakage, maybe both. Most
simple cap testers can't compensate for either series or
parallel resistance. It really isn't measuring capacitance.
The fact that the cap had lost half its value suggests the
dielectric had changed.
FWIW capacitor checkers or bridges that check using
only AC don't check for leakage and that is sometimes more
important than series resistance. Leakage can be tested
using a small variable DC power supply capable of putting
out something like the rated voltage for the cap. You also
need a decent DC micro-ameter to measure the leakage
current. The power supply and meter are put in series with
the cap and the voltage brought slowly up. For electrolytic
caps the current may go down as the cap charges and and the
electrode forms. If it begins to rise the cap is no good.
Good electrolytic caps should have no more than perhaps 15
_micro amps_ , Paper and film caps should have no more than
about 5 microamps and good modern film, ceramic or mica caps
should have vanishingly small leakage. I have encountered
caps that measured OK on an AC bridge but were no good
because of leakage so leakage tests are good to do on a
routine basis.
--
Richard Knoppow
Los Angeles
WB6KBL
[email protected]
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