Re: [CAD_CAM_EDM_DRO] Spindle Relay Question

Jan,
I guess I should have been more specific, in that I would suggest a good
old fashioned electromechanical DPDT relay to do the switching of the
power leads to the motor, and driving the coil of the DPDT power relay
with a simple solid state relay as an amplifier for the TTL output of
the computer.

Nearly all solid state relays use triacs or back to back discrete SCRs
which remain conducting once triggered until the next zero crossing of
the current passing through them. They can be triggered on by a very
brief spike of noise that would never even be noticed by an
electromechanical relay. If you get a spike at the wrong instant, it
can trigger an SSR on at the wrong time and create the short circuit you
described, and let the magic smoke out :o) That's probably what
happened to your original system.

In my experience, the successful mixing of solid state devices with
inductive power loads borders on black magic. My 4 HP servo motors are
living proof of the fact that is possible, but I sure wouldn't want to
have to design those control circuits.

Dave Speck

JanRwl@... wrote:

>In a message dated 5/25/2005 8:07:08 P.M. Central Daylight Time,
>volitan@... writes:
>
>Does a DPDT DC relay with that rating exist, or do I need a different
>circuit altogether?<<
>
>I never heard of a DPDT SSR. I once put six AC SSR's in "one box" for
>on/off and reverse of a 3-phase Bridgeport, and it worked for a day or so, then
>one set went open. I replaced the "bad one", and it worked again, but for no
>more than a day. I had wired this sensibly so that it was impossible for
>"both sides" to go on at once (would be a dead-short across the incoming lines),
>but somehow "noise" or whatever would trigger the "off ones" when the "on
>ones" were selected. I finally gave up and now use three SSR's only to switch
>the three phases, and use a mechanical DPDT lever-switch to select direction
>BEFORE turn-on.
>
>With that experience, I suspect it might be very, very difficult to wire two
>pairs of DC SSR's so that only one pair at a time could be switched on for
>one of two directions. One would THINK so "on paper", but there seems to be
>them thar lektrikul gremlins in thar! Jan Rowland
>
>