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......
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.
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 job. 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.
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).
BRuce snipped-for-privacy@charter.net (remove the XXX)
Wils> Contactor is just jargon for a bigger relay.
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
There are three 'significant' components in the description of a relay (and several more 'minor' ones -- that are important only if you're trying to match an _existing_ use).
1) the power requirements for the 'coil' -- what it takes to make the relay pull in. This determines/is-determined-by the power on the 'control' circuit. Some are designed to run on AC, some on DC, and voltages are 'anything you might want' -- from a few volts all the way through 440V (or higher, if you get into the 'serious' industrial catalogues ). Then there's the matter of how much _current_ it takes to energize the coil sufficiently for the relay to 'pull in'. typical 'low voltage' (i.e. 48V or less), moderate-power-handling relays need from a few tens of milliamps, to a few hundred milliamps. Devices designed for switching high power levels will draw significantly more power (have to move heavier contacts, and over larger distances, _faster_, all of which requires more power. the 'larger distances, faster' is to minimize the amount of 'arcing' that goes on.) For low-voltage relays, sometimes the current is not expressly given, instead the resistance (in 'ohms') of the coil is specified. OHM's law, describing the relation of current, voltage, and resistance, in a circuit lets one use these interchangably.
2) the power rating for the contacts. There'll be both a voltage, and a current rating. As long as they're both higher than what you're actually switching, everything is fine.
3) the 'configuration' of the contacts. Here you get to deal with 'poles' and 'throws'.
'Poles' is simply how many separate wires you can switch at one time. For switching 120V loads, you only need a single pole (there's only one 'hot' wire), for 240V single-phase, you need a 'double pole' unit (two 'hot' wires), and for 3-phase, you need a 'triple pole' unit. to accomplish a 'latching' action -- i.e., one push-button for 'on', and a separate one for 'off', you'll need one _more_ pole than listed above.
'Throws' is the number of positions in which there is "a path" through the device. Without getting into 'exotic' devices, there are just the 'single throw', and 'double throw' types to consider. Single-throw makes contact in _one_ position only. Either 'normally closed', where energizing the relay *opens* the circuit, or 'normally open', where energizing the relay _closes_ the circuit. "double throw" devices have _both_ kinds of contacts. When the relay is not energized, there is continuity from the 'common' contact(s) to the 'normally closed' one(s). when the relay _is_ energized, there is continuity between the 'common' contact(s), and the 'normally open' one(s).
Just to complictate things, the 'double throw' types come in two varieties -- 'make before break', and 'break before make'. This describes what happens _as_ the relay changes state. In the first variety, there is a 'momentary' period when _both_ sets of contacts (the normally open _and_ normally closed ones) are connected to the common contact. In the second type, there is a momentary period when _neither_ set of contacts is connected to the common.
Assuming you're going to use a switch that is 'on' whenever a blast-gate is open, you'll need a "double-pole, single-throw (normally open), 240V 20A contacts" relay, with a coil voltage to match whatever cheapie transformer you can scrounge. The commonest relays will have 6 or 12 volt coils. You can _usually_ use an AC-rated coil with DC power, although it is a GOOD IDEA to match the specifications. Trying to use a DC-rated coil with AC power will 'almost always' *not* work right -- the typical situation is that the relay pulls in/out 120 times every second. Quite a buzz, lots of arcing, and *lousy* power on the 'out' side of the relay. And the 'lifetime' of the device is virtually non-existant.
Just a slight change in emphasis: for switching 120V loads, you must use only a single pole (don't every switch the neutral). Same for 240: double pole only.
Clarification: A relay with 'more poles than required' is *entirely* suitable for a task, all else being equal. No harm done by letting a set of contacts go 'unused'. Pete is correct about the need for care in wiring -- DO NOT EVER switch the neutral.
The best advice, concerning electrical work: "If you don't *know* what you're doing, _DON'T_DO_IT_! Get a pro, instead."
The work is -not- 'difficult', but 'one little mistake' can have _catastrophic_ results -- somebody gets killed, the house burns down, etc. It isn't worth the risk.
If you _are_ considering "doing something" anyway, be sure to check local _legal_ requirements, *and* potential effect on any insurance coverage.
I've known places where using an extension cord -- *any* extension cord, including 'outlet strips' -- was illegal, by city ordinance. And homeowners insurers who had a fixed policy, in the event of fires attributable to a 'defective' piece of equipment (household appliance, or whatever), of going after the manufacturer to recover their payout.
Wow.. Great info, but no magnetic starter (that I know of). I'm pretty sure that buying it was option, and I didn't get it because I figured I'd be doing the blast gate switch thing.....
You better unplug your PC and never use it again. If it has a hardware switch on the PS it will usually switch both wires. That is because these are usually
120/240 capable. The code only says that if you do have a switch or breaker on the neutral that it has to switch all conductors on the same handle and that no pole can operate independently 240.22 & 404.2(B). It is a violation to put a fuse in the neutral (except as motor overload protection on 3p corner grounded delta 340.36). These provisions are in the code to allow 120/240v equipment like PCs or voltage tappable motor tools while still maintaining the same switches and O/C-O/L devices.
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...
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.
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.
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