On 8/18/2012 11:09 AM, Ned Simmons wrote:
On Sat, 18 Aug 2012 08:46:46 -0500, wrote:
....
So far, I've found some from TEMCO that are 3PH/AC up to 40A (iirc) at
about $25-$35/ea and one 70A. _BUT_, it appears they also need
heatsinks and very good motor protection because their LRA tolerance is
_very_ short (like 10s of msecs).

Found some SS motor controllers of course but they're $hundreds complete
so don't see (yet anyways) there's any great simple savings to be had...

....
... For low level stuff we used them all the
time in data collection, etc., and for that I'll agree they're the cat's
meow over mechanical...

I think you've pretty well covered it, but I'd add one more warning:
SSRs' failure mode. They quite often fail on. I've used many SSRs in
automation controls to power devices like single-phase sub-fractional
HP motors that don't pose a safety hazard.

SSRs are common in place of the once-standard mercury relays in heater
control circuits. But, depending on the danger posed by a full-on
heater, redundancy is good practice.
....

Good point--the applications I'm familiar with are all just control
(Opto-22, mostly) and no loading to speak of so the thermal problem is
nonexistant. If one has to design in the redundancy that certainly
pretty much defeats the whole point to begin with as well as definitely
going to up the price point...

BTW, a good value for contactors and overloads are the Fujis from
Automation Direct.

OK, thanks; I'll keep them in mind. At present there's nothing new
going on that's "for real"; this is as much entertainment w/ an end
objective of adding the convenience of a remote control station to the
DC in the woodshop. Since I have bunches of old stuff laying around I
can rig up pieces and put them together...I've got this one working w/
the exception of rearranging the couple of connections to add the remote
pushbutton and waiting for the paint on the covers to dry to make it
look a little prettier in its new life. I've also refurbished/cleaned
up several others of various sizes and configurations and figured out
what parts were in the piles of stuff for the most part...who knows when
will dream up a use for some of it? At least half the fun is in simply
being able to reuse the stuff.


--