On 8/16/2012 4:50 PM, Existential Angst wrote:
....
You know, they make relatively high current solid state relays now, with
"coils" that operate at ANY voltage from proly below 120 to over 220-240.
No more contacts!
Some fuses, breakers, heaters, whatever in series, and wala, motor starter!
with mebbe a li'l latching/cutout relay in series with the coil -- which
could actually be just another solid state relay, with the coil powered by
the contacts, instead of the line.
Indeed, I've done some looking into that -- there's some fairly
important engineering to be done that I've not opted to take on,
particularly as long as I've got old pieces-parts to scrounge up for
play-toy applications. If it comes to real gear for actual farm
revenue-generation, then all rules change, of course.
The section I've read on general principles in selecting SSRs for motor
applications says in part--
MOTORS: Motors frequently have severe inrush
currents during starting and can impose unusual
voltages during turnoff. The inrush currents
connected to mechanical loads having high starting
torque or inertia should be carefully determined to verify
that they are within the surge capabilities of the SSR.
A current shunt and oscilloscope should be used to
examine the duration of the inrush current. Motor
starting may frequently reoccur at short intervals and
the affect of repetitive inrush currents on the thermal
operating point of an SSR must be considered. Check
the motor operating current and locked rotor current
versus the SSR motor rating. The possibility of
abnormally stalled rotor conditions which draw much
higher than normal currents should be considered. An
extended stalled rotor condition may require an
oversized SSR or fuse protection. The generated EMF
of certain motors can require an SSR to have a blocking
voltage greater than might be expected from steady
state line voltage. The voltage applied to an SSR by
a motor circuit during turnoff should be examined with
an oscilloscope to verify hat the applied voltages are
safely below the specified SSR blocking voltages.
Otherwise lock-on or erratic turnoff of the motor may
occur. Some motor circuits may require higher than
normal blocking voltage, transient limiting devices, or
other techniques to control the voltage which must be
blocked by an SSR during deceleration or direction
reversal.
All in all, it says you don't just look at a (say) 10 hp motor drawing a
nominal 50A and just buy a 50A SSR and go on assuming everything is just
hunky-dory...
HVAC houses carry these, and I'm sure many other places do, at much lower
prices than these ripoff hvac suppliers.
Any specific parts you know of firsthand for a given/known motor for
comparison purposes? For 240V need either two or a 3P integrated even
though only need two of the three for load. Perhaps there are some but
I've not seen any w/ the auxiliary control contact(s) needed for a
starter circuit so that's another or (I presume) w/ the 3P you could use
the third load contact assuming it'll switch the small coil load
satisfactorily.
The Newark catalog ones of minimal size (10A that I'm sure don't meet
any of the above-discussed conditions for even a 1 hp motor what more 2
or 3 or 5) are about $25 for a single pole. The hp-rated by
manufacturer 2-pole 40A mechanicals were $8+/ea so that seems a
no-brainer on cost. They require another $8 or so control relay but
still that's pretty inexpensive comparably.
But, I'm still open for going forward at some point...
BTW, I got the old Moeller starter going this afternoon!
No smoke
was harmed in the connecting...I did have to clean up a bunch of
terminals to get a good connection thru the start circuit but I'm about
ready to put it on the DC. If I can figure out a way to squeeze the
motor start capacitor that's in the present switchbox, I could replace
it entirely and eliminate the piggyback power feed...there's _almost_
enough room; I'm going to see if I can manage to make just enough more
by a rearrangement or removal of the third pole power feed that don't
need for the purpose. That would "neaten up" things significantly...
--