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Why use a contactor?
Gunner Asch kom med følgende:
On Mon, 13 Jan 2014 14:01:44 +0100, Leif Neland wrote: DaveC sendte dette med sin computer: 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. Why on earth would frind want the saw to turn on after a power failure? In most cases...if the saw is running WHEN the power goes off....it comes back on when the power returns. With a contactor..it stays OFF if properly wired until you turn off the switch..then turn it back on. I agree, but my question is "Why on earth would frind want the saw to turn on after a power failure" which OP claims "the friend would appreciate". When the power is turned off, the material being sawed, tools, fingers etc. could block the saw. I very much prefer the saw does *not* start by itself. In the circuits I've seen, the ON is a "no", normal open pushbutton, which energizes the contactor. An on-switch on the contactor in parallel with this provides current to the contactor. The OFF is a "NC", normal closed pushbutton, removing the voltage to the contactor. All kinds of safety switches could be wired in series, all required to be closed for the saw to run. Leif -- Husk kørelys bagpå, hvis din bilfabrikant har taget den idiotiske beslutning at undlade det. |
Why use a contactor?
On 2014-01-13, Lloyd E. Sponenburgh lloydspinsidemindspring.com wrote:
"DoN. Nichols" fired this volley in : It is at least a *possible* one. Don, I'm not picking on your answer... I had to respond to someone's, and yours got the prize... O.K. I won't take it personally. :-) 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. Though some which are made to mount behind a trim plate in the wall (typical home light switch) may be more open to a buildup of swarf inside it. I've seen various failures in home electrical hardware which I would not have expected. Among those, there was an outlet which failed during a nearby lightning strike. The form of the failure was a breakdown in the hole into which a drive screw went to hold the ground strap which mounts it to the outlet box. It was *supposed* to be a blind hole, but they had used a slightly too long drive screw, or a chip was under it, and there was a little of the inside surface which broke away. Still -- normally not a problem. However, the lightning strike introduced a high enough voltage so the gap to the nearby hot lead was bridged, and the subsequent arc, both partially melted the "hot" inside the socket, and welded it to what was plugged into it (one of those duplex to six outlet plates) -- *and* filled the vicinity with carbon dust. When I got home from work and discovered the power loss in the living room, I went downstairs and switched on the breaker. Five second delay and BZZZZTTT-CLICK. A repeat got the same results. I then walked around the house (old wiring, given breakers showed up in a lot of outlets around the house. I then walked around the house, until I smelled the burnt Bakelite. Of course the outlet was behind a bookcase which I had to empty and move to get to it. Then I discovered that the multi-outlet adaptor would not unplug until I applied a lot of force, and that left one pin of the adaptor in the socket. I then removed the socket, and replaced it with a new one -- and just had to take it apart to see what had happened. If the Bakelite had not flaked around the drive screw, it would not have broken down -- and the failure would have been somewhere else at an even higher voltage -- or maybe not. So -- I don't *depend* on any commercial electrical hardware for the home to do what it should do. And -- once I needed a toggle switch which did not introduce vibration when it was operated to put in the mounting plate for a turntable, so I could cue a record, and then switch on the motor at the appropriate time. At that time, you could get "silent" light switches which contained a puck of mercury in glass and metal end caps. It was designed to work in a vertical orientation, but it was possible to take the switch mounting plate off, and file different notches into the toggle handle so it would hold the puck for horizontal operation. Now, that switch was *not* designed to keep swarf out, so it would have to depend on the decorative plate which goes over it. Granted, no normal person would mount it as I did -- and I never had swarf near the turntable, so it was no problem. But I do remember that switch, and how easy it would be for swarf or sawdust to work its way in. Sawdust would just make it take a bit more force to operate. Metal swarf would bridge the ends of the puck and leave it on full time. Granted -- most of the horizontal/vertical bandsaws use a normal bat-handle toggle switch, but there are variations in quality there. The best have a seal around the ball on which the bat rotate, and have sealed wire entrance on the back. Cheap ones either have terminals on the back (which you screw, solder wires to, or tabs for (hopefully) insulated push-on crimp terminals -- but still metal swarf could bridge that to the frame. 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. Given that he is already "jiggering up" the bandsaw -- replacing the single phase 120 VAC motor with a higher horsepower single phase 240 VAC motor, you introduce another problem. Ideally (given USA wiring, where 240 VAC is really two 120 VAC wires 180 degrees out of phase, so they produce 240 VAC between them), you want a switch which interrupts both sides of the power -- so a winding failure in the motor does not leave some part of the system perhaps floating at 120 VAC instead of near ground as it should be. Now, -- in the UK you don't normally have 240 VAC with a grounded center tap. Instead, one side of the 240 VAC is grounded. 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. *And* -- one which interrupts both sides of the line, since he is putting in a 240 VAC motor. 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. Good enough. Remember that the motor is being changed here, so at least some change in the switch is called for. For special switches, somewhere I still have a switch designed for reversing a single phase motor. It runs in either direction (reversing two of the three circuits), but it enforces a pause when switching from one direction to the other -- since running single phase motors can't be instantly reversed just by swapping two wires. They have to be allowed to slow down enough to come to a near halt (enough so the centrifugal switch closes to enable the start winding for the reversal.) So -- yes with the right switches -- no problem. However, if you want the weight of the arm of a horizontal/vertical bandsaw to switch it off (at least he 4x6" ones) -- you want a switch with not much operating force -- which is less likely to be capable of switching both sides of the line for the 240 VAC motor. (I'm still not sure why he feels the need for a larger motor -- perhaps it was one of those Chinese import motors which are almost all empty housing, and very little frame, made to *look* like a bigger motor, but bound to burn out with any serious use. My 4x6 HV bandsaw came from MSC, and apparently they spec'd a better quality motor -- and that one is still running, and does not get hand-burning hot with a long cut. :-) So perhaps all he needs is a motor which really is the nameplate horsepower on what he had -- and for that, a 120 VAC motor should be sufficient and the original switch might even do well. (I'll have to look under the base of mine to see what the switch looks like there. :-) Enjoy, DoN. -- Remove oil spill source from e-mail Email: | (KV4PH) Voice (all times): (703) 938-4564 (too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html --- Black Holes are where God is dividing by zero --- |
Why use a contactor?
On 2014-01-13, David Billington wrote:
On 13/01/14 12:11, Lloyd E. Sponenburgh wrote: [ ... ] 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. 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. Routing the wiring around the machine tool to provide both the "stop" switch at the end of cut condition, and multiple "stop" buttons within convenient reach if something goes wrong, plus more than one start button as well. With no high voltage on those (if design makes the back easy to contact). You can use smaller gauge wire to route it where you need it without adding stiffness to the machine's movement. (I would like stop and start buttons on the moving arm of the H/V bandsaw, so I don't have to bend over as far to reach the power switch.) But this is just me. :-) Even the Chinese 12x40" lathes have relays and a transformer to provide the control voltages at 24V instead of the 120 or 240 VAC (the latter more likely for that large a lathe). And -- the wiring might be for 480 VAC as well, depending on where the machine will be installed. Just move some jumpers to change operation from 240 VAC to 480 VAC, and the control buttons don't get any more voltage than 24 VAC. I discovered how this was set up when I helped a friend convert his to a three-phase motor with a VFD. Even added a "jog" feature to enable rotating the chuck until the proper wrench socket is facing out. :-) And the VFD actually uses even lower voltages -- 5 VDC or at worst 10 VDC for all the signal leads. Enjoy, DoN. -- Remove oil spill source from e-mail Email: | (KV4PH) Voice (all times): (703) 938-4564 (too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html --- Black Holes are where God is dividing by zero --- |
Why use a contactor?
On 2014-01-13, Gunner Asch wrote:
On Mon, 13 Jan 2014 06:11:15 -0600, "Lloyd E. Sponenburgh" lloydspinsidemindspring.com wrote: [ ... ] 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 Last year.. I replaced 5 switches in Delta /Milwaukee table saws alone. Wood sawdust packed in the switches? What kind of switches? Perhaps the standard wall switch? Did you take them apart for the fun of finding out what the failure mode was? (Detail photos would go a long way towards settling the debate ranging here. But at least is is about *metalworking* (and woodworking), not politics. Enjoy, DoN. -- Remove oil spill source from e-mail Email: | (KV4PH) Voice (all times): (703) 938-4564 (too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html --- Black Holes are where God is dividing by zero --- |
Why use a contactor?
On 2014-01-13, Ignoramus28861 wrote:
On 2014-01-13, DoN. Nichols wrote: On 2014-01-12, Ignoramus13867 wrote: [ ... ] The best argument for using a contactor in a small bandsaw, is that you can get proper thermal overloads to protect the motor. This is more of a factor with three phase motors, where a motor once started will continue to run if one phase of power fails, but the current drawn in each remaining winding will be higher, and will be likely to burn out the motor. (And he was going to add a 240 VAC single phase motor to the bandsaw in question.) No, on bandsaw, the main problem is that it runs unattended and can easily get stuck. O.K. I've not experienced that on mine -- but given some of the motors on the import H/V bandsaws, that is a possibility. And *that* is a good reason for replacing the motor with one which delivers the nameplate horsepower, instead of being mostly empty air in a too-large housing trying to look like an adequate motor. :-) I've read the reports of motors getting so hot that they burn the hands of the user, but mine never gets that hot. MSC got a proper motor put into it, apparently. Enjoy, DoN. -- Remove oil spill source from e-mail Email: | (KV4PH) Voice (all times): (703) 938-4564 (too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html --- Black Holes are where God is dividing by zero --- |
Why use a contactor?
"Lloyd E. Sponenburgh" wrote in message . 3.70... "DoN. Nichols" fired this volley in : It is at least a *possible* one. 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 The AC switch that was used on the table saw that became permanently turned "ON" was a standard house hold light switch that wasn't sealed. The saw dust accumulated behind the contacts inside the switch casing. When the saw was wheeled in and out of the garage, the saw dust pushed the contacts closed. When I cracked open the AC switch it was literally stuffed full of saw dust. I replaced the switch and covered up all openings in the metal box that the switch was mounted in. Shaun |
Why use a contactor?
"Shaun" fired this volley in
: The AC switch that was used on the table saw that became permanently turned "ON" was a standard house hold light switch that wasn't sealed. Shaun, I think you just proved my point. The switch was not intended for that exposure, nor was it intended to switch inductive loads. Your situation is common, but not to be compared even diagonally with installations that have designed-to-purpose switches. LLoyd |
Why use a contactor?
Am 12.01.2014 01:48, schrieb DaveC:
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. 1.) Safety issues. It is a lot safer to have the machine shut off, when the power is cut. Otherwise the saw might start up again, while you have your fingers on the blade or so. Or the blade is stuck and starting the saw can ruin the blade/motor/mechanics. 2.) When using a simple on-off-switch, there is really no good way to have it stop, other than turning the switch. When you use a contactor, you can start and stop the saw with multiple switches, for example a two-button-start, to ensure both hands are away from the blade, or one stop-button and a sensor/switch, that stops the saw, when it hits a certain point or so. In our company we have special motor-switches, that have an under voltage-switch attached. The switch has two buttons (on and off; who would have thought :-) and the under voltage-switch is attached on the side and turns the switch off, when the voltage drops below a certain value, via a mechanical rod assembly. And that on machines, that, theoretically will just give you a good whack if it starts up and you have a body part in there. Might also be a good reason to install something like that in a band saw. Greets Tom |
Why use a contactor?
and the under voltage-switch is attached on the
side and turns the switch off, when the voltage drops below a certain value, via a mechanical rod assembly. How does a low-voltage switch work via a mechanical rod assembly? Can't picture this... |
Why use a contactor?
"DaveC" wrote in message and the under voltage-switch is attached on the side and turns the switch off, when the voltage drops below a certain value, via a mechanical rod assembly. How does a low-voltage switch work via a mechanical rod assembly? Can't picture this... Picture this: a "linear rheostat". ;)} The shoe could be moved by the rod ass'y. pdk |
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