Why use a contactor?

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A friend (in USA) with a 220v, 1-phase horizontal bandsaw in his home machine shop wants to replace the 1/4 hp motor with 1/2 hp and rewire with an auto-off switch (when the blade cuts through the metal).
I always default to using a 3-wire control with contactor, but he asks the logical question: "Why?"
Other than the "If the power fails while cutting" answer (which he says seems of limited additional value on a saw with auto-stop feature) I wasn't very convincing.
What are the arguments for using a contactor as opposed to a simple on-off switch of equivalent rating?
Thanks.
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snipped-for-privacy@invalid.net says...

My cheap miller I use has a magnetic latch integrated into the start/stop operator switch. it requires that you feed both the L1 and L2 to it but only switches the L1 to the motor.
I've seen these also for three phase switching of motors so that if you do lose power it will switch off the motor.
I guess it maybe a cost savings since it's much easier to put a simple mechanical latch that is held with small solenoid than putting in that extra crap.
Jamie
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I don't remember any clear reasons for choosing one or the other from my long-ago days designing industrial controls. IIRC at that power level unless we needed 24V for other relays we'd go with a switch. They can be had with two pushbuttons so you can slap the large red one to shut it off. http://community.woodmagazine.com/t5/image/serverpage/image-id/23197i38710354CC71A806/image-size/medium?v=mpbl-1&px=-1
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http://community.woodmagazine.com/t5/image/serverpage/image- id/23197i38710354C

Nice show-and-tell. Where can this switch be bought?
Thanks.
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Here's another one I was looking at on Amazon: (Amazon.com product link shortened)
According to the comments it latches ON magnetically, with a solenoid, and acts like 3-wire control. jsw
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On Sat, 11 Jan 2014 20:49:22 -0500, "Jim Wilkins"

But if you want an auto shutoff, low voltage control to a contactor makes it easy.
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The simplicity and safety of using low voltage controls instead of requiring heavy duty limit switches. One adequately sized contactor and as many microswitches as you want as controls to turn it on or off, running at low voltage and low current.
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snipped-for-privacy@snyder.on.ca fired this volley in

Indeed, and you can add to that (if properly installed) that a contactor will prevent accidental re-powering of equipment if it stops due to a power failure.
With a mechanical switch, if one were to forget to turn off equipment after a power failure, it could come on unexpectedly when power returns.
Lloyd
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"DaveC" wrote in message
A friend (in USA) with a 220v, 1-phase horizontal bandsaw in his home machine shop wants to replace the 1/4 hp motor with 1/2 hp and rewire with an auto-off switch (when the blade cuts through the metal).
I always default to using a 3-wire control with contactor, but he asks the logical question: "Why?"
Other than the "If the power fails while cutting" answer (which he says seems of limited additional value on a saw with auto-stop feature) I wasn't very convincing.
What are the arguments for using a contactor as opposed to a simple on-off switch of equivalent rating?
Thanks.
The reason to use a contactor instead of just an on / off switch is that the insides of the switch can get full of saw dust and turn on the saw with the switch in the off position. I've seen this happen! Regular AC switches are not sealed well enough, but you can buy sealed switches with lower current ratings which is enough to engage the contactor.
Shaun.
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Best answer yet.
Yes, we all know about "during a power failure" but many times this won't be an issue (actually the friend would appreciate the saw continuing the cut after power comes back!), but the "auto turn-on" feature )c: is one I hadn't thought of.
Thanks!
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The problem is that the contactor needs a bulky and expensive sealed enclosure. I've wired my machinery with waterproof outdoor boxes and flex conduit from big-box stores, but they don't carry large JIC boxes with watertight doors. http://www.austinenclosures.com/products/view/JIC_Continuous_Hinge_Boxes/
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On 1/12/2014 8:01 AM, Jim Wilkins wrote:

A small IEC motor starter may be used with a cheap plastic enclosure. 4 screw JIC enclosures will suffice. 6x8x4 inch should be adequate. If the motor has internal overload protection, smaller contactors/ enclosures may be used.
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On 1/12/2014 8:38 AM, SteveF wrote:

Note: The above is in reference to smaller motors, in general. Not something requiring a size 4 motor starter or something on that order.
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That's up to you.
I designed industrial controls to GM's specs which required NEMA-rated enclosures and conduit that gave reasonable protection from a fork lift, and interconnecting wiring no smaller than 16 AWG for mechanical strength.
Hobby machinery may not be subject to the same level of abuse, but it wasn't initially designed with protected spaces for add-on controls and wiring. I've milled connector openings in a lot of 'cheap plastic enclosures', usually ABS Hammond boxes from Digikey, and seen how easily they fracture. Personally I like metal weatherproof outlet boxes on machines that throw wood or metal chips, or plastic for low voltage and safer locations.
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On 1/11/2014 10:33 PM, DaveC wrote:

Disagree completely.
The likelihood of the failure the other respondent noted being sawdust "turning on" the saw is remote at best. Almost certainly it was a failure in the switch, not the sawdust imo.
Sawdust is far more likely to cause a switch to fail to make contact.
The prime advantage of motor contactors for the small motors of the ilk in home shops is the one noted of fail-safe operation on restart in the (admittedly unlikely) case of power failure.
Second is the low-voltage contacts are more reliable over the longer haul, but again for most recreational/hobby users the demand for direct switching is simply not enough to make it be a real issue.
I use them universally simply because I like the tactile feel of the starter button vis a vis the mechanical switch.
The one other advantage but is only really useful for the DC is that it's much simpler to wire a remote umbilical cord to it for access from afar than it is to wire inline switches.
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The only machines I own that can feed themselves into the cut are a wood planer and a 4" x 6" horizontal bandsaw. The spring-balanced bandsaw doesn't mind being shut off and restarted in the cut. Would a hydraulically damped bandsaw start safely after power had been out?
That might be a good test just before you replace the blade. If the motor won't start you'll be there to switch it off.
I don't trust my bandsaw to run unattended anyway. jsw
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A)    Note the newsgroups to which this is cross-posted:
        sci.electronics.misc         sci.electronics.repair         rec.crafts.metalworking         rec.woodworking
    And while wood may be an insulator, horizontal bandsaws (which I     remember from the earlier articles in this thread, and which I     use) are more commonly used to cut metal. And metal swarf is     certainly conductive.
B)    Toggle switches typically have a spring-loaded actuator which     bears on a metal plate which rocks back and forth to bridge     contacts or not bridge them.
    Even a buildup of wood sawdust or plastic dust *could* trap the     metal rocker in a position to keep the bridging happening, even     when the toggle is in the "off" position.

    And most of those switches are not located near the point of generation of metal swarf as they are on the typical inexpensive horizontal bandsaw. The airborne sawdust in a wood-working shop is not likely to be a problem. However, metal chips dribbled from the blade (after going almost a full turn around the path of the blade) are a possibility, depending on the construction of the switch. (And, they could also conduct from the switch terminals to the chassis of the saw, if they got under the frame.) Certainly a sealed limit switch (more commonly found on serious machine tools) would be free of that particular problem. They are a small MicroSwitch module inside a metal housing with O-rings to keep out oil as well as chips. They are operated by a cam, also sealed, or an external plunger -- operating through a sealed path. (Honeywell is one maker -- and they *are* quite expensive, but the best thing where swarf will build up.) The wiring comes in through a fitting which normally has a tapered gasket which compresses around oil-proof wiring, so there is no path for the chips in through there either. And yes, the are best used at low voltages and low currents. I recently re-designed the limit switches in a conversion of a CNC milling machine, and needed to get switches of the same physical mounting but which had two separate circuits depending on which direction they were operated in.)

    It is at least a *possible* one. Now, if the power is fed to this through a wall-mounted disconnect switch, so you could stop the bandsaw without having to unplug it (which might be difficult to reach with the saw still running), that would be less of a problem. It would be nice to have a warning LED near where you would reach to change the clamping of the workpiece, so you know the saw is still running, especially of other noisy tools are being run at the same time in the shop. A horizontal bandsaw is typically fairly quiet, unless it is cutting relatively thin metal.
    Enjoy,         DoN.
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Don, I'm not picking on your answer... I had to respond to someone's, and yours got the prize...
This is the dumbest discussion I've ever heard (short of all the political spew on here).
Even half-quality garbage plastic switches from China are NOT prone to accidentally 'switching on' from accumulations of dust or swarf.
That it's a remote possibility, I won't deny. But such switches typically serve for years to decades without a malfunction in the dusty, dirty, swarf-filled environments in which we use them, and it's dumb to think of the average home craftsman's going to the trouble to re-wire, retrofit, and otherwise jigger-up his equipment with low voltage contactors and safety circuits. That some would or even could is beside the question.
This discussion should be turned to "What's the best-quality switch I can buy affordably that will suit the safety needs of the application." For that, I recommend a good industrial-quality safety-style switch that requires a simple swipe of the hand to turn off, and a positive 'de- locking' action to turn on.
My old (1970s) Shopsmith came with one. When I finally wore it out in the 1990s, I replaced it with the same-quality switch from a US maker (IIRC it was a Square-D safety switch, specifically for table saws). You could knock it off easily, but had to pull the bat out manually to turn it back on -- heavy-duty thing. It lasted more than 20 years of nearly daily use, and the replacement is still on the machine, still working.
LLoyd
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On 13/01/14 12:11, Lloyd E. Sponenburgh wrote:

What's the benefit of a low voltage contactor and safety circuit? I have various equipment with safety circuits and they use either the 230V or 415V supply for the contactor and safety circuit.

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It's simple, cheap, and easy to rig remote switching and sensors for things like safety guards. It requires only bell wire and some thoughtful routing, rather than running live power everywhere a switch is required.
LLoyd
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