# Re: Latest MMC Calculations...

```Hi Jim, Terry, Reinhard, all,

If we use 100 caps in an MMC, will the total lifetime be
reduced by the factor of 100? Well, I think NOT.

Of course, the caps will not fail at the same time, but
they also will not fail evenly distributed over the time
as they ALL degrade more or less evenly over the time.
So the lifetime of an MMC buildt out of 100 caps WILL be
shorter than the lifetime of one cap alone - but how much
shorter?

Statistic is a very complicated thing, especially if you
don't know the exact functions to describe the problem.
I.e. we don't know how the dU/dt affects the lifetime factor.
which depends on the corona.

So we can test each special type of small cap we want to
use in an EMMC-arrangement and calculate the lifetime from
the first dead cap. But this gives only a very bad (rough)
approximation as you need a 'high' number of events for a
'good' statistics.

Seems to me as if the only way to exactly determine the
lifetime of the _COMPLETE_ MMC, is to build a complete MMC
and try how long it will last before the first small cap
fails and how often you have to replace one of them.

One thing we all should have a look at is the setting of
the safety gap (or the static main system spark gap). If
you allow a too high voltage buildup, the MMC will die
very fast because of the 15th power law. Of course one
spike every now and then will not harm, but if your
voltage level is constantly higher than expected, your
MMC lifetime will go down rapidly.
As Terry pointed out, if you apply 1.17 (1.36) times the
voltage, the lifetime will decrease by the factor 10 (100)
due to the 15th power law.

Cheers, Stefan
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----------------------
Jim wrote:
> Generally, you multiply life multipliers... So, say you had
> a part spec'd at 1000 hours, and if you have a factor of
> 2.1 for reduced voltage, and an factor of .2 for pulse rate,
> the overall life would be 2.1*.2 = 1000*.42 or 420 hours....
> This presumes the factors don't interact...

Terry wrote:
>> Hi Reinhard,
>> I think I'll agree with you.  If we have three lifetimes that
>> are all 900 hours, My equation would give only 300 hours total.
>> That is not right. You assumption would give 900 hours total
>> which is much better.  If we knew the distribution of the failure
>> functions we would see that it is really a
>> little less than 900 hours but that detail is meaningless (it is like
>> figuring all this out to 30 digits of accuracy...).  I think your
taking
>> the lowest number as the life is best.

Reinhard wrote:
>>>I donīt know if I can agree with this. Personally, I think
>>>one needs to calculate the lifetime for each situation (L1,
>>>L2, L3) and simply take the lowest number as THE life
>>>factor. I know this is somewhat pessimistic, but at least
>>>you wonīt be disapointed. For an analogy lets have a
>>>look at a normal light switch. Letīs say the life of the
>>>plastic handle (before it gets brittle and breaks) is 10
>>>years, the life of the pivot (before it wears out so far
>>>that it wobbles all around) is 5 years and the life of the
>>>contacts themselves is around 2 years (Of course these
>>>are just imaginary numbers). This means the effective
>>>lifetime of the switch is 2 years and not 1/(1/5+1/10+
>>>1/2) years. <snip>

Terry wrote:
>>>> Hi Stefan,
>>>> I too was wondering how to combine the lifetimes.  I was thinking of
>>>> combining them like parrallel resitors.  Suppose you have three life
>>>> times of 10000, 1000, and 5000 hours.  1/(1/10000 + 1/1000 +
>>>> 1/5000) = 769 hours.
>>>> Seems reasonable to me...<snip>

Stefan wrote:
>>>>>...and you calculate the lifetimes L1, L2, L3.
>>>>>But don't you think that all those three major factors play
>>>>>a role TOGETHER in decreasing the lifetime? In my opinion, the
>>>>>lifetime reduction factors should be multiplied as each one is
>>>>>decreasing the lifetime at the same time.

```