Re: Primary Cap rms current not a function of frequency

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

Original poster: Terry Fritz <teslalist-at-twfpowerelectronics-dot-com>

Hi Bob,

At 08:55 AM 1/15/2004, you wrote:
>Hi all,
>
>I was calculating the rms current in my MMC configuration for various 
>break rates.
>
>I was surprised to discover a simple equation for the primary rms current. 
>Assuming no R or SG losses, quench at a notch, assuming magic k's 
>and  ignoring charge current the rms primary current in amperes is given by:
>
>Irms=pi x V x C x (break rate) x (number of primary cycles to quench) 
>where V is the charge voltage and C the capacitance in F
>
>For a standard MMC cap of 0.15uF it is   0.113A rms per number of primary 
>cycles per 2000V of charge per 120Hz break rate. (0.094A rms for those in 
>50Hz flicker land using 100Hz break rates)
>
>The value above means the rms current rating of 13.5Arms of the cap will 
>not be exceeded unless the coupling is very low or the break rate much 
>higher than 120Hz. In any case the current rating may be at 85C.
>
>WoW I think this means you can have break rates of 1000Hz  with out frying 
>the caps with 1st notch quench I should add.

In general, the RMS current is proportional to the BPS rate given the same 
primary cap size and firing voltage.  But I am not quite sure what you 
conditions are here.

>
>How close to or over rated can the  maximum allowed peak current and dvt 
>of the cap spec be pushed  for reasonable (in home TC service) life given 
>the rms current is not a limiting factor.

In general, the cap makers give a margin of SQRT(2)X  (twice the 
heating).  But they DO have bad days...  I would not exceed RMS current or 
dV/dT ratings.  We certainly know that exceeding those ratings can 
completely destroy the whole cap.

>
>Comments please.
>
>I still need to double check the above  but I thought I would bounce it of 
>the group first.

It would certainly be a new and cool equation!!  Hope it works out!!

Cheers,

         Terry


>
>Bob
>
>