Re: a/c switches

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Nick Andrews by way of Terry Fritz <twftesla-at-qwest-dot-net>" <nicothefabulous-at-hotmail-dot-com>

Another possible source for contactors is spa/hot tub repair places.  When I 
worked on them, I replaced the contactors on old units fairly often.  Most 
were 240V, 20A-100A units with 120V coils, used to switch the heaters and 
motors/pumps.  They use a simple air switch to actuate the 120V to the coil, 
further isolating the operator, a great idea when you're sitting in 500gal 
of water...  I kept the old parts, of course.  If you ask the service guys 
real nice, they may save them for you.

Nick Andrews
Littleton, CO


>From: "Tesla list" <tesla-at-pupman-dot-com>
>To: tesla-at-pupman-dot-com
>Subject: Re: a/c switches
>Date: Sat, 03 Aug 2002 16:36:05 -0600
>
>Original poster: "Terry Fritz" <twftesla-at-qwest-dot-net>
>
>Hi Adam,
>
>At 04:40 PM 8/3/2002 -0400, you wrote:
>........................
>
> >
> >what exactly does a power relay do?
> >Adam
>
>In a simple toggle switch, the contacts are very small and the forces and
>distances that open and close them are small.  Fine for less than 5 amp
>stuff that does not require too much current.
>
>However, suppose we want to switch 100 amps at 480 three phase with a small
>panel switch.  No "switch" made will have the size and power to switch
>that.  A "fault" could blow a one foot diameter hole in the panel and
>vaporize any switch.  And, ruin the guy that flipped it's day...  If a big
>power substation contactor "goes bad", it can knock down all the chain link
>fences when it explodes...
>
>So we get a power relay.  Basically it will have very large precious metal
>contacts like 1/2 inch in diameter and 1/4 inch thick.  The contacts may
>have a travel of 1/2 inch and they will slam together very hard.  A big
>electromagnetic coil moves the contacts.  This coil is controlled by our
>little panel switch :-)
>
>So we switch a few amps into the electromagnetic coil with the tiny switch
>which moves these giant contacts into each other in the large contactor
>(preferably "far away").  If the big contactor explodes, we are perfectly
>safe since the tiny switch and coil are very well isolated from the mess.
>
>Power contactors (BIG relays are also referred to as "contactors") come in
>all shapes and sizes from very small to giant ones used in power
>substations.  They switch like 380kV at 2000 amps there, no panel switch
>will do that!
>
>Here is a link that has more than anyone really wants to know about them:
>
>	http://relays.tycoelectronics-dot-com/lexicon/index.stm
>
>But don't worry about all that stuff.  They are really very simple.  Best
>way to learn about them is just get one.  You will figure it out right
>away.  I could not find any really good links about them but here is a guy
>that made a high voltage one that basically tells all:
>
>	http://members.tm-dot-net/lapointe/HVRelay.html
>
>Of course, our little 30 amp 120 VAC does not require very extreme
>relays...  But the little $12 one from DigiKey will switch about 10X the
>power of any "toggle switch" in perfect safety.  Radio Shack also has a
>few, but I am not sure the current.  The electromagnetic coils on them run
>from 5, 12, 24 volts DC and 24 and 120 volts AC.  You probably want the
>120VAC coils in this case so you can just power the relay's coil from the
>AC line voltage.  The like 5 volt DC ones are for transistor control
>circuits and such.
>
>BTW - Ken had a great idea about checking out "home" circuit breakers and
>such at the hardware store!
>
>Currents above 15 amps start to get really dangerous just from an explosive
>standpoint.  Thus, all the rules change and it starts requiring some
>serious hardware to control it.  It's not too bad really once you get into
>it.  With power relays, you can control just as much power as you want with
>just a "little" switch.
>
>Cheers,
>
>	Terry