Re: tesla-d Digest V99 #16
> Subject: Pig current limiting
> Date: Thu, 07 Jan 1999 12:34:07 -0700
> From: Tesla List <tesla-at-pupman-dot-com>
> To: tesla-at-pupman-dot-com
> Original Poster: "Marco Denicolai" <Marco.Denicolai-at-tellabs.fi>
> The tank supply I am designing uses a 3-phase pig transformer and a 6 diode
> bridge to charge a filter capacitor of 15 uF. After that there is a reactor
> that charges the primary capacitor usign resonant charging.
> Of course, every time the primary capacitor gets charged the filter (15 uF)
> capacitor voltage drops a little down: I need a device to limit the current
> used to re-charge the big capacitor.
> A suitable resistor will delay too much the recharge (and develop losses
> and heat!) and the capacitor will never reach full voltage.
> I was thinking to some limiting reactor on the 400VAC (primary) side.
> My questions:
> - if I use a simple autotransformer, it will limit the voltage but the
> current will be still orders of magnitude too high, right?
> - what about using a non-linear reactor, one that allows the current to
> rise until a value X and then will keep it about to that value. I mean like
> a MOT with secondary shorted? Who manufacture/sells these devices?
If you really feel that a big 15uF filter capacitor is necessary, then a
filter choke (about 2.5 henry for 15uF) is required between the 3-phase
rectifiers and the filter capacitor. In addition, a charging diode
(with an RMS current rating 3X higher than the main stacks) must now be
placed in series with the charging reactor, in order to trap the charge
on the primary cap. The 15uF cap will also require personnel protection
suitable for lethal energy.
Avoid the big DC filter cap if at all possible.
If you really need 1% stability, active regulation
is far simpler, and more accurate.