Re: Testing caps (fwd)

To: tesla-at-pupman-dot-com
Subject: Re: Testing caps (fwd)
From: Tesla List <mod1-at-pupman-dot-com>
Date: Sun, 2 Aug 1998 21:53:27 -0600 (MDT)
Approved: mod1-at-pupman-dot-com

---------- Forwarded message ----------
Date: Sun, 02 Aug 1998 11:37:15 +0300
From: Harri Suomalainen <haba-at-cc.hut.fi>
To: tesla-at-pupman-dot-com
Subject: Re: Testing caps (fwd)

>Build a leakage tester that runs on HV (approx 5-10 kV) and check them
for
>leakage current.  If good, then they will work fine.  See a future issue
of
>TCBA for an article on this very item.

Leakage testers have some problems why they should not be preferred. If
you charge the cap with a dc source you might run into some dielectric
absorption related problems. Basically it will lead to a situation where
you
have discharged cap and when you finally connect it to an AC source it
might actually fail.

For this reason it is preferred to test AC caps with an ac source or to
use
a reversed polarity charging with a lower voltage to make the dielectric
problems less severe.

You might also like to make partial discharge measurements for very much
info. Basically you connect the cap to an AC source (eg. mains freq.).
Connect
the oscilloscope X-channel to mains voltage (perhaps by a divider) and
Y-channel to current measuring system. In X-Y mode you will observe
current
peaks if cap is partially discharged (has arced internally a bit). Current
pulses
are there due to rapid re-charging of the cap. Having the figure
syncronized to
mains voltage (X-Y -mode) will help in keeping the voltage peaks at same
place
to make measurements more reasonable.

I'll  not go further into partial discharge measurement details. I'm sure
people
know how to look it in the books if really interested in making very
serious
measurements of HV systems.

--
Harri Suomalainen     mailto:haba-at-cc.hut.fi

We have phone numbers, why'd we need IP-numbers? - a person in a bus