Finally scored the Big Dust Collector (Oneida 3 hp), and am in the process
of getting it hooked up. I was planning (OK, Hoping) to put microswitches at
the blast gates so I can turn the unit on/off by opening and closing the
gates. Everything went fine in the planning phase, then when I actually
started getting ready to put stuff together, I realized I had no idea how to
build the rely....
Here's the stats:
# HP motor, 220V, 20 Amp (at least that's the circuit breaker it wants).
I was hoping to use some simple low voltage DC for the switching - pretty
much whatever low cost transformer I can find at Rat Shack, and switches to
go with it.
I'm assuming that I just wire the whole thing up so that I've got the
220 feed going into the relay, the plug from the DC goes into the relay, and
the low voltage on/off controls the relay (DC current = on, no current off). SO here's the big question: how do I figure out what size/type of
relay I need? I've dug through all of my old (really old) electronic
textbooks, and found lots of theory, but nothing of real practical value. I
dug through all of my industrial catalogues, and discovered that there are
way more types of relays than I ever imagined. I searched the web, and got
buried (no surprise there...).... So, having pretty much exhausted what my
little brain could come up with, I figured I'd query the collective brain
Thanks to everyone.....
__James J.B.N. DuPrie
On a relay there's two ratings. The first is how much current/volts it can
pass (ie 120 volts AC @ 10 amps) and the second is how much DC it takes to
switch the relay. I think the biggest relay that radio shack has is a 10 or
15 amp one. You'll have to find a 20 amp relay on the net.
You may want to look into a small contactor rather than a simple relay. The
start up current and the
resulting sparks can burn out simple relays. The contactors lugs are amp rated.
: Hi folks...
: Finally scored the Big Dust Collector (Oneida 3 hp), and am in the process
: of getting it hooked up. I was planning (OK, Hoping) to put microswitches at
: the blast gates so I can turn the unit on/off by opening and closing the
: gates. Everything went fine in the planning phase, then when I actually
: started getting ready to put stuff together, I realized I had no idea how to
: build the rely....
: Here's the stats:
: # HP motor, 220V, 20 Amp (at least that's the circuit breaker it wants).
: I was hoping to use some simple low voltage DC for the switching - pretty
: much whatever low cost transformer I can find at Rat Shack, and switches to
: go with it.
: I'm assuming that I just wire the whole thing up so that I've got the
: 220 feed going into the relay, the plug from the DC goes into the relay, and
: the low voltage on/off controls the relay (DC current = on, no current : off). SO here's the big question: how do I figure out what size/type of
: relay I need? I've dug through all of my old (really old) electronic
: textbooks, and found lots of theory, but nothing of real practical value. I
: dug through all of my industrial catalogues, and discovered that there are
: way more types of relays than I ever imagined. I searched the web, and got
: buried (no surprise there...).... So, having pretty much exhausted what my
: little brain could come up with, I figured I'd query the collective brain
: out there......
: Thanks to everyone.....
: __James J.B.N. DuPrie
Contactor is just jargon for a bigger relay.
Not all coils are DC. In fact there is no reason to use a DC relay for this
Does the machine have a "magnetic starter", which is more jargon for a
latching relay setup? If so, we can rig the gate switches to work with it
and all you'll need is wire. At 3 HP, I think it must have the mag.
Rather than microswitches, you'd use cheap pushbuttons...two at each gate,
to work the same as the ones in the mag.
The green button on the mag is normally open. When you push it, you power
the coil, which closes the relay and makes a pair of contacts that then
power the coil and hold the relay in. The red button is normally closed.
It's in series with the coil, so when you push it you break the coil circuit
and drop the relay.
SO, all your greens are normally open and connected in parallel with the big
one in the mag. All your reds are in series with the big one in the mag.
When you push any green, the DC starts. Red stops.
You can mount the buttons in surface mounted plug boxes using solid plates
drilled for the buttons. The wire can be 20 ga, since there is very little
current in the coil. HD should have some control wire rated for 120V.
If this isn't clear, drop me a note.
I agree that if there is a magnetic switch it would be easier to use
that as a base but see no reason to put 2 buttons everywhere. the
microswitchs can be used with the mag switch directly, there is no need
to use the 'holding contacts" to keep it running. Check inside the
switch box or in the user manual for a schematic.
If you want scan, the schematic in and email to me, I can then let you
know how to reconfigure. Basic instruction would be to remove on wire
from the holding contacts (tape it up) run all the microswitches in
parallel with the start button.
I have a similar set up, 2HP Woodtek DC (manual switch) on a contactor
with a bell transformer and wooden microswitches(I built my own).
firstname.lastname@example.org (remove the XXX)
Wilson Lamb wrote:
How do you stop it if it latches on.
Interesting that you'd build switches. "Microswitch" is a very specific
type of switch with very small throw distance required. Expensive, unless
you have scrounged some.
<BRuce> wrote in message
there is no reason to "latch on" if it happens due to mechanical
failure then the stop button will not help. the part that holds it on
(when the start button is pushed)is the "holding or maintaining" contact
and that was taken out of the circuit. if a microswitch fails on then
it would be the same as the start button failing on. basic control
theory taken in high school.
A microswitch is just that, a very small switch, nothing magical happens
in a microswitch that doesn't happen in a regular switch. just 2 (or
more) contacts activated by a lever or other mechanism.
My switch are wooden forks with screw heads for the contacts, a wedge on
the gate forces the contacts open when the gate is closed. the failure
mode is to not com on when I open the gate but now that they are
"adjusted" that doesn't happen. simple, cheap and take about 10 minutes
to make a new on when I add a new gate. Bell wire all the switches in
parallel and any one closes and the DC comes on.
Wilson Lamb wrote:
OK.. I';ve been back to grainger and MSC catalogues, and the make a lot more
sense now.... Still a question though:
is there such a thing as a 20 amp, 220 volt, dual pole relay/contactor that
uses low voltage DC for the coil? If I've got to run a 120 line to each
switch, it kind of defeats the purpose... I was hoping to run low voltage
wire to each balst gate, use a micro/mini switch (ciruit closes when blast
gate is opened) to control the DC coil current.....
Lots ogf relays/contactors from about $15 to about $500, but they all seem
to want 120VAC (or more) for the coil...
Try <http://www.surplussales.com select 'relays' from the main menu page,
then 'contactors' from that menu.
About 1/4 way down, there's a 24vac coil
240VAC double-pole single throw 'air conditioning' contactor, for $17.
half way down (in the 'medium contacteor' section, is a 25A 240v double-pole,
24VDC coil unit for $13
see also the "HVAC" list, from the main relays page.
I'd consider the 'Honeywell relay center' on the HVAC page as the power-
supply. rip out the relays that are in it, and substutite the $17 one above.
Voila! it's even in a code-compliant enclosure. :)
Or, you can probably get a "open frame' 24V transformer from Radio Shack
for under $10. Would still have to do up an enclosure, etc. -- might save
$10 when you're done. <grin>
Why not use a solid state relay? They will pick from 3vdc to 30vdc with no
modifications, commonly come up to 40a and have no contacts to burn, spark or
get stuck. Hosfelt sells them for less than the price of a decent mechanical
relay. You will still need a "disconnecting means" but that can be the plug if
this is cord and plug connected. I have been using 40a SSRs on my spa with a
2.5hp motor and 5.5kw heat elements for years.
email@example.comGreg (Gfretwell) wrote in message
Glad to hear that you are getting along with SSR's. I would point
out, however, that in some applications SSR's are disallowed as a
typical failure mode is "on". Not to say that a mechanical contactor
won't weld itself closed, but unlike failing SSR's the contacts won't
spontaneously close. Probably doesn't matter for the OP if the DC
fires up in the middle of the night due to a faulty SSR.
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