Re: Testing MMCs

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Malcolm Watts by way of Terry Fritz <twftesla-at-qwest-dot-net>" <m.j.watts-at-massey.ac.nz>

HI Dan,
       To answer a couple of queries from this post and another:

Tuning: there should be procedures galore mentioned in the list 
archives for low power tuning. To find the resonant frequency of your 
secondary, feed a low impedance sig gen into the base and sweep for a 
strong resonance as picked up by a scope aerial probe positioned 
several feet away. You should know the ballpark frequency from your 
design calcs. For the primary, short the gap, apply the generator 
with series resistor (say 22k) across the now parallel tank circuit 
and find its resonance with a hi-Z scope probe also connected across 
it. The capacitance of the probe is negligibly small beside the 
primary capacitance.

Regarding this:

On 27 Aug 2002, at 12:28, Tesla list wrote:

> Original poster: "Mccauley, Daniel H by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <daniel.h.mccauley-at-lmco-dot-com>
> 
> 
> 
> Another quick question (sorry for all the questions . . .)
> 
> I've already blown my MMC twice and each time the damaged cap was blatantly
> visible (either very black or still on fire), however I still suspect i have
> a bad cap in my MMC.  Is there any other way to adequate test these things
> to isolate a bad cap.
> 
> So far I've:
> 
> 1.  Hi-potted cap to 22kVDC (rated value of cap is 24kVDC).  Only about
> 100uA leakage current so it looks okay.

100uA is *huge* for any film capacitor. I'm wondering whether that 
isn't actually a bleed current caused by the shunting resistors.
 
> 2.  Individually measured capacitance across entire MMC and across each cap
> individually and it seems okay.
> 
> 
> The rest of the system appears to be okay as well, except when I run the
> system, the SRSG seems to run quite at quite low power (much less than
> before I blew the cap)

The caps are polypropylene aren't they? Have you been testing them in 
a tuned system or in just the primary alone? If the latter, you will 
be hammering them to death as they won't get any respite by being 
emptied periodically by the secondary.

Regards,
malcolm