Phil said:
"Partially let go"? Given that the magnetic force drops very steeply as
the iron moves away from the pole piece, I'd have thought that a very
remote contingency...maybe it just moves too slowly for the arc to be
extinguished? Please expound.
It's an old lesson that I learned after building my first RF power
amplifiers (kilowatt-class). The T/R switch relay had to be controlled
with gusto when doing very fast morse code sequences where you get close
to realtime switching, else its contacts would erode rather fast. IIRC I
went through almost a dozen Potter&Brumfields over ten years back then.
A transformer presents to some (small) extent an inductive load which
likes to arc a bit on anything that mechanically opens and closes. So in
this case you'd need some circuitry that cuts the relay loose instantly,
like a threshold detector. It's all doable but at some point the costs
kind of add up.
The main problem with power relays is often when they want to come back
on. A contact that sluggishly leans into its counterpart can easily
cause a brief "bzzzt". If that happens a lot it's like a pinging engine,
it causes wear. This is why the HV guys spend so much effort to speed up
the action in their contactors. It's a whole science in itself. At the
power electrics institute of my university I had seen some really nifty
prototypes, such as contactors where a pressure capsule was used to give
them gusto. Every time they switched you heard a bang from that
pressure-assist mechanism, to the point where they required people to
wear ear protection.
