5 Sep
2014
5 Sep
'14
9:44 p.m.
Hi Brent,
On Fri, Sep 5, 2014 at 8:59 PM, Brent Hilpert <hilpert at cs.ubc.ca> wrote:
Yes, that's what a GFCI does.
Inside the GFCI, the hot and neutral are sent through dual primaries (a
turn or two each on a toroid core) of a current sense transformer,
connected such that the currents are in opposition. A third - sense -
winding feeds an an op-amp and thence the circuit-trip relay. If the
currents in the hot and neutral are in balance (equal) they cancel and
nothing is induced in the sense/secondary. If out of balance the difference
induces V in the sense winding, which trips the circuit open.
The same principle works for three phase, with all three (or four)
current-carrying wires going through the transformer.
Hey cool, I actually figured that out on the fly..
The counter-phase windings with the tertiary 'sense' winding makes perfect
sense. I hadn't made it quite that far - to the design level - but
obviously, it's probably the simplest way to get what you want.
Very cool, the things one learns!