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Re: Oil for Caps (fwd)
---------- Forwarded message ----------
Date: Mon, 12 Jan 1998 22:21:21 -0800
From: Bert Hickman <bert.hickman-at-aquila-dot-com>
To: Tesla List <tesla-at-pupman-dot-com>
Subject: Re: Oil for Caps (fwd)
Tesla List wrote:
>
> ---------- Forwarded message ----------
> Date: Mon, 12 Jan 98 20:56:22 EST
> From: Jim Monte <JDM95003-at-UCONNVM.UCONN.EDU>
> To: tesla-at-pupman-dot-com
> Subject: Re: Oil for Caps
>
> Hi,
>
> Here's another angle to the oil selection question.
>
> <snip of Fr. Tom agreeing that the cap value will not dramatically
> change by using a different oil for caps with thick poly>
>
> >HOWEVER, I believe that the looked-for increase probably WOULD show up
> >if the cap was made using 6 mil poly!!! The reason here is that the
> >thickness of the oil interface will now be much larger in proportion
> >to the poly thickness. Note that this would imply TWO things at least:
> >Higher dielectric constant oil COULD be used to increase the capacitance
> >of caps made using THIN poly, AND such use of high dielectric constant
> >oil WOULD put greater electric stress on the poly, because the
> >proportion of thickness would be quite different with thin poly.
> <snip>
>
> I've been thinking about the electric stress on the poly again, and
> maybe MORE stress on it via a higher-k oil would actually be a good
> thing, regardless of any cap value improvement. Why? Because more
> stress on the poly = less stress on the oil. If the oil has voltage
> breakdown, the poly is doomed. So it may be better to balance the
> load via k so that each dielectric is operating at nearly the same
> percentage of its breakdown voltage, instead of poly at 50% and oil
> at 90%, for example.
>
> For a parallel plate cap, the electric flux is nearly constant between
> the plates, so if Dp= flux density in poly and Do= flux density in oil,
> Dp = Do (1),
> Now flux density is proportional to the electric field via the
> permittivity of the dielectric -> D = e*E. Using the same subscript
> notation and substituting into (1)
> ep*Ep = eo*Eo (2)
> Let the system operate at some fraction f of the breakdown voltage for
> each dielectric, and let ExBD= breakdown voltage (per unit length) of
> dielectric x. Substituting into (2),
> ep*f*EpBD = eo*f*EoBD (3)
> Solving (3) for eo,
> EpBD EpBD
> eo = ---- * ep or ko = ---- * kp
> EoBD EoBD
>
> Since a piece of poly has a breakdown voltage/unit length several times
> higher than oil, to equalize the stress, the dielectric constant of the
> oil should be proportionally higher.
>
> BTW, the oil and poly thicknesses or even relative thicknesses did not
> show up anywhere. It makes no difference if the cap is loosely taped
> together or squeezed in a press and then tightly clamped. These two
> cases would produce caps with different properties, but to equalize
> voltage stress on each dielectric, the same k ratios should be used.
>
> It seems reasonable to want to balance the stress since failure of
> either dielectric will kill the cap. Also, for given thicknesses and
> breakdown voltages, this will produce the cap with the highest overall
> breakdown voltage. Other ideas?
>
> Jim Monte
Jim and all,
Excellent post, Jim! The strategy of using high-dielectric constant oil
soaking a kraft paper layer, and a higher breakdown strength, but lower
dielectric constant, polypropylene (or LDPE in homebrew caps) dielectric
is a key "secret" of professional pulse capacitor manufacturers for the
very reasons you have outlined above. The wicking of the high dielectric
constant impregnant tends to fill in any microscopic gaps, reducing the
possibility of air entrapment and ionization. This higher dielectric
constant layer then "sees" a lower voltage stress, pushing more voltage
across the plastic dielectric which can safely handle it.
With the right "tuning" between kraft paper density & thickness,
impregnant dielectric constant, and multiple, thin, layers of
polypropylene (or LDPE) dielectric, an optimal combination of maximum
capacitance and maximum voltage withstanding capability can be realized
in a minimal-volume roll. While this strategy works with mineral
oil-soaked kraft paper (combined k of between 3-4), it works most
effectively with high-dielectric constant impregnants such as castor
oil, silicone capacitor oil, or proprietary (but non-PCB) oils having
combined k's in the 4-6 range.
This is how Maxwell, Plastic Caps, Condensor Products, etc. construct
their individual capacitor rolls. Series/parallel combinations of these
identical rolls are then used to arrive at the desired working voltage
and capacitance. For further information on capacitor construction, the
following references are suggested:
1. Longland, T., “Power Capacitor Handbook”, Butterworth-Heinemann,
1984, 308pp, ISBN 0-408-00292-1
2. Marbury, R. E., “Power Capacitors”, McGraw-Hill, 1949, 205pp
3. Frungel, F., "High Speed Pulse Technology", Vol I, Capacitor
Discharges, Magnetohydrodynamics, X-Rays, and Ultrasonics, Academic
Press, 1960, 620pp
4. Frungel, F., "High Speed Pulse Technology", Vol III, Capacitor
Discharge Engineering, Academic Press, 1976, 498pp
Safe cappin' to you!
-- Bert --