Re: Inrush current and BFC's

On Oct 23, 1:23 pm, John Devereux <jdREM...@xxxxxxxxxxxxxxxxxx> wrote:
Hi,

I am putting together a high current supply and wondering about inrush
current.

The situation is a center-tapped 110V 3kVA transformer, feeding a
bridge rectifier and a large, high-ripple current capacitance:

.
. -------- +70V
. ----| |-------o------> 20A rms >---
.|| / | AC + | | |
.|| / 55 | | ----- 20,000uF [LOAD]
.|| / | | ----- |
.|| /-- | | | |
.|| / | | | |
.|| / 55 | | | |
.|| / | AC - | | |
.|| /----| |-------o------< 20A rms <---
. -------- -70V
.
.

Simulation shows hundreds of amps for the first mains cycle!

Do I need to do anything to stop the local substation tripping out
when I plug it in?

Or will the transformer limit the current to a sane value?

I have looked at thermistor based "inrush current limiters" but they
don't seem to protect against short disconnections.

--

John Devereux

Energy storage in 20K uF at 140 V is 196 Joules (i.e., watt-sec),
which means that charging the cap is probably not going to blow a
thermal breaker rated at 30 amps or more (which is what you'll need
for your 2.8 kW load). You can't count on leakage inductance to limit
the current much, either, because the initial core state can include a
flux offset at the moment that you energize the circuit.
A common way to limit this type of inrush is to add a series power
resistor and a time delay relay. The relay contacts short out the
resistor after a delay. The relay can be electromechanical or SSR
type.
Paul Mathews

.

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