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Re: AC-rating for MMC caps // EMMC vs the GTL-WIMAs

Hi Stefan,


At 11:49 PM 5/24/99 +0200, you wrote:
>Hi Terry, Coilers,
>
>part one: AC-rating for MMC caps
>===============================
>
>Today I've read all those MMC mails in the May-archive on pupman.
>Terry wrote some very interesting scentences:
>
>TF> The size of the cap indicates how much heat it can 
>    dissipate.  That is why they rate dissipation only on size.
>    I think Panasonic has a bit of an advantage because their 
>    thin epoxy covering does not hold heat in like WIMAs thick 
>    potted caps do.  I would think if a cap got too big it would
>    not be able to pull heat out of the center well.  Commercial 
>    poly caps I have seen my have a problem there??  Of course, 
>    MMCs have far greater dissipation do to their many little 
>    radiating surfaces.
>
>This got me thinking of the graph I created once from the WIMA
>data sheet FKP1. I've plotted the AC rating over the DC rating of
>one special cap size (constant capacitance for all data!).
>The factor AC/DC decreased with increasing DC rating.
>
>This becomes very clear if we read Terrys mail and consider that 
>a 6kVDC-cap of say 10nF is far bigger than a 600VDC-cap of the 
>same 10nF. The generated heat simply has problems to go through
>the fat roll of dielectric to the outside of the bigger cap.
>
>
>BUT(!!!)
>========
>as Reinhard wrote:
>RWB> 4.) The only real clue that Panasonic gives, is the 
>     following statement: 
>     "If a capacitor of DC rating is used in an AC circuit 
>     (except....), the maximum voltage is limited by heat 
>     generation or electric discharge"
>                ^^^^^^^^^^^^^^^^^^^^^^
>
>So there STILL seems to be the problem that in some caps the 
>AC rating would be made due to corona problems. I don't know 
>where this aspect was mentioned first, but concerning the fact 
>that in most data sheets the maximum allowed AC voltage is 
>constant (!) up to 1kHz or even 10kHz. This could be NOT due 
>to heating effects, else the value should decrease with the 
>frequency in this 'low frequency range' also as it does at
>higher frequencies. So there MUST be any other effect we are 
>not aware yet and the corona problem seems to be a logical 
>explanation. The strange thing is that this is not mentioned 
>clearly in many datasheets (I fact I've read it in none of 
>them). Perhaps the philosophy of the manufactureres is not to 
>worry the customer too much with too detailled data. Any 
>'normal' customer should use the derating curves given in the 
>data sheets and be happy. I think they haven't thought of
>us crazy coilers who will doubt anything and stressa cap
>beyond any limits given ;-)

Since the fields are contained in the poly dielectric.  Corona should not
be a problem.  I was more concerned that having a cap say 10kV above ground
may cause external corona to the air that may degrade the covering or
eventually cause an arc from the cap to some external object.   This is why
I was concerned about putting it in oil.  However, now, I think that this
will not be a major problem  The ozone and degradation of the epoxy coating
will not be a factor in our limit time uses.  A few data sheets mention
some mysterious AC effects at high voltage but nothing beyond armwaving
guesses.

I quote from WIMAs data sheets "Furthermore the r.m.s. voltage derived from
the peak voltage shall not be greater than the normal AC voltage rating of
the capacitor to avoid the ionization inception level:  V r.m.s. <<V a.c.
rated."

It's anybody's guess as to what that was supposed to mean... 


>
>The answer from Wima on my question (how much the lifetime 
>will decrease if a cap is stressed beyond the AC-rating) still 
>hasn't arrived :'-((  
>
>Terry wrote:
>> Let me know if more info is needed.  Since we use zillions 
>> of WIMA caps, we have "leverage" with them when we need 
>> questions answered...
>Perhaps you could ask them if there is any truth in the 
>'corona-fear' (constant AC-rating at 'low' frequencies)?
>
>

We use WIMAs at the DC/1.4 level all the time.  As long as they do not get
hot they last forever over (15 years of using them).  WIMA would probably
just base their answers off our data anyway.  I bet somebody wrote some
paper warning of doom once, or something like that, which caused this
concern.  No one I have talked to sees any problem with using them at the
DC peak levels as long as the ESR heating does not raise the body
temperature above 5 degrees C.  I doubt WIMA could explain "ionization
inception level".  I'll try to check around for a definition,  guidelines,
equations, and data... However, I don't expect a darn thing....

>
>part two: EMMC vs the GTL-WIMAs
>=================================
>
>BTW, did I understand right, that your 'EMMC' is just a new
>type of cap used in a configuration where it is stressed
>by using 'real peak AC = rated DC'? The data I cite is from 
>your mail 
>   Subject: "Enhanced" Multi Mini Capacitors 
>   From: Terry Fritz <twftesla-at-uswest-dot-net> 
>   Date: Fri, 21 May 1999 22:42:08 -0600 .

Don't get to excited about my silly acronyms :-))  I was just jokingly
trying to name caps that were used beyond their manufacturer's rating (with
a good degree of understanding).  Manufacturer's rate their caps so anyone
will be able to use the caps within that "rating" without a problem.  The
parts can often be pushed much harder.  So why would we ever want to use a
part beyond the ratings in our precious coils??  Because we want to save a
bunch of our precious money if we reasonably can...

My "EEMMC" cap was meant to take the rating to very near destructive
levels.  It is right on the edge where my best guess said it "probably"
would survive.  If my guess was wrong, it would have instantly fried.  The
fact that it lived is an indicator that I must know a little something
about caps :-))  I don't recommend the EEMMC at all (although I'll use it
until blows).  It was just a torture test "ginny pig".

>
>If so, our (the GTL community) WIMA caps will still be a better
>(cheaper) choice (if they will survive...). If the goal is to 
>achieve 5.5nF, just wire 6 of the WIMAs in series. Total price 
>would be about 5$US if purchased in quantities above 1000 
>(which we currently did here in Germany), only 1/4 of your price.

I pay higher prices through my source.  They have excellent service which I
am personally glad to pay for.  No doubt, caps can be found at a FAR
cheaper price...  

>Now lets compare how hard we'll drive them:
>15kV/(6*700V) = 357% of the AC rating
>15kV*1.41/(6*6kV) = 59% of the DC rating 
>111% of the risetime  rating (assuming 250kHz for your test system).	
>
>If I remember correctly, your cap is rated 600VAC. So lets 
>calculate how hard you drive them (10caps in series):
>15kV/(10*600V) = 267% of the AC rating
>15kV*1.41/(10x1600V) =  132% of the DC rating 
>risetime is a bit complicated (I don't know if I interprete your
>data the right way here):
>at 250kHz, I=C*dU/dt is 117A, which is 39% of the rated 300A.
> 

Yes!  I do drive them hard!! :-))

>Not to rain on your parade, but I do not understand where 
>the 'enhanced' thing in your EMMC is. If we look at the data 
>calculated above, the WIMA caps ordered by the GTL community 
>cost only 25% of yours (when ordered in the same quantity)
>if we shoot for an equal capacitance of 5.5nF in a comparable 
>setup. Ok, that is not really comparable as the maximum stress 
>is not equal. So lets do another calculation:
>If we'll use 9 WIMA caps in series, we'll overstress the 
>AC-rating in the same way as you do, but the DC stress will be 
>4 times lower than it will be with your caps (only 39% of the 
>rating) and the risetime will be equal. For equal capacitance 
>(5.5nF), we need '1.5 strings in parallel' which works out to 
>14 caps or with other words about 11.5$US which still is half 
>of your price but with better chances to survive (equal AC 
>stress, but 4 times lower DC stress and equal risetime).
>
>

And you now see why the manufacturers have so much darn trouble rating
capacitors :-)))  It just isn't easy....  Take your best guess as they do....

Cheers,

	Terry


>Stefan
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