Re: Maxwell 31159 cap on eBay

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

Original poster: Bert Hickman <bert.hickman-at-aquila-dot-net> 

Tesla list wrote:
>Original poster: Ed Phillips <evp-at-pacbell-dot-net>
>"If a HV pulse cap is not rated for high % voltage reversals, there is
>significant corona formation at the edge of the foils.  This corona and
>surface tracking begin to heat up the local dielectric area (di/dt) and
>will
>eventually produce enough heat to begin melting through the dielectric.
>Dielectric breakdown at the edge of the foil is the number one killer of
>HV
>pulse duty caps (according to Maxwell Sr. Engineer Bob Cooper).  Second
>on
>the list are small voids in the dielectric material, which also produce
>corona in the voids due to the different dielectric constant in the void
>material, and, again, leading to local intense heating effects that
>damage
>the dielectric.
>Pulse caps not rated for high voltage reversals literally "can't take
>the
>heat".
>Dr. Resonance"
>     I understand the breakdown phenomena and also the problem with corona
>in voids under AC operation, but don't understand how the construction
>of the "high voltage reversal" capacitors differs from the "low voltage
>reversal" ones.  Are they built differently or just screened?  When I
>was at Hughes Aircraft we had a group building special capacitors for
>radar modulator pulse-forming networks (where dV/dT can exceed 10^7
>volts/sec and operation is at several thousand pps) and they used to
>screen capacitors after construction by applying a high AC voltage and
>listening for radio noise as a symptom of corona in unfilled voids.  In
>this case they were pitched out because the construction techniques
>including vacuum impregnation were the best they knew about.  Does
>Maxwell do something like this but salvages the rejects by issuing them
>under a different part number?
>Ed

Ed and all,

Pulse capacitors are constructed with a number of equally sized sections 
connected in series or series-parallel. During the design phase, a 
sufficient number of sections is used such that each section will "see" a 
worst case voltage in the range of 3500-5000 volts/section. This reduces 
the peak voltage stress seen (both at the edges of the metallization and 
within any voids) below the point where corona normally forms - called the 
"corona inception voltage". Capacitors that are designed to withstand high 
voltage reversals are constructed with more individual sections than 
simpler DC or lower percent voltage reversal capacitors. A capacitor rated 
for 80% voltage reversal (Q~7 - highly oscillatory) is actually constructed 
with a dielectric system that can withstand 180% of the rated pre-discharge 
DC voltage. For example, compare the x-ray of the 15 KVAC Plastic 
Capacitors BNZ series pulse cap (20 sections in series) with the DC-rated 
80 kV LN series cap with only 8 sections in series in Mark Rzeszotarski's 
recent article, "X-Ray Pictures of High Voltage Capacitors".

Pulse capacitors designed to endure severe voltage reversals sometimes use 
a layer of oil-saturated kraft paper between the foil and the polypropylene 
(PP) film. The oil-soaked kraft paper acts as a higher dielectric constant 
(k ~ 4-6) buffer zone between the foil and the film, helping to even out 
the e-field stresses seen by the film. Because of the lower dielectric 
constant of the PP film layer (~2), the film actually sees most of the 
voltage stress in the cap. The oil-kraft paper layer is more resistant to 
corona damage (if it should occur during accidental overvoltage conditions) 
making for a more robust dielectric system during highly oscillatory 
discharges. The combination of a thicker dielectric system and more 
capacitor rolls/cap also mean that caps rated for high Q discharge duty 
will be significantly larger than their DC-rated or low % reversal 
counterparts.

Best regards,

-- Bert --
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