Why use a contactor?

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Low voltage control confines the high voltage to the control box and the motor, so it's less exposed to physical damage. There isn't much if any benefit if you have only a single short run to the power switch and the motor draws less than the switch's rating, for the cost of a 24V transformer and a larger control enclosure.
I can't remember ever seeing a contactor and low voltage controls on consumer power tools although it's pretty much standard for industry.
The difference may be the risk of forklift accidents, since humans can't crush conduit or puncture sheetmetal.
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    [ ... ]

    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.
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On 2014-01-13, Lloyd E. Sponenburgh <lloydspinsidemindspring.com> wrote:

    O.K. I won't take it personally. :-)

    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.

    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.

    *And* -- one which interrupts both sides of the line, since he is putting in a 240 VAC motor.

    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.
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    [ ... ]

    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.
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"Lloyd E. Sponenburgh" wrote in message

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
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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
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Readers of this thread may be interested in this and similar devices:
http://www.ebmag.com/Products/Safety-Codes/jds-products-unveils-sensing-saf-start.html
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Make it as simple as possible, but not simpler. (Albert Einstein)

Larry W. - Baltimore Maryland - lwasserm(a)sdf. lonestar. org
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Check out the price on those units. How about around 113 dollars for the privilege of buying their gadget?
I like the 12 buck switch that does the same thing and comes with the off paddle.-- Jim in NC

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The relay just drops out and stays out until the trip is released.
Very important in some cases. Especially when an order of power up is required.
Martin
On 1/17/2014 5:41 PM, Larry W wrote:

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DaveC sendte dette med sin computer:

Why on earth would frind want the saw to turn on after a power failure?
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On 1/13/2014 7:01 AM, Leif Neland wrote:

IIRC the machine saw was a horizontal band saw, It may be one that saws logs unattended so to speak. If it completes its run it may automatically shut off at that point. In this situation the saw finishing its task would be desirable.
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Gunner Asch kom med følgende:

"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
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On 1/11/2014 9:29 PM, Shaun wrote:

Not likely.
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??
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On 1/11/2014 10:57 PM, DaveC wrote:

on 110 volt machines. It's an industrial safety feature to prevent automatic start up after a power failure. Think multiple numbers of machines in an industrial setting starting back up all on their own.
Additionally like your AC thermostat in your home the 220 volts going to the condensing unit out side does not come near the person adjusting the thermostat. You really don't want a 220 volt switch shorting or electrocuting some one. Better that the switch be lower voltage.
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This may be a duplicate post
Lew --------------------------------------------------------------- "DaveC" wrote:

What you are describing is known as under voltage protection.
Safety pure and simple is the best reason for using it. ---------------------------------------------------

What happens in the event you lose power and as a result you lose lighting as well as power to the saw?
Without under voltage protection, when power is restored, the saw will restart automatically which can be disastrous, especially if the lights haven't restarted.
Your insurance company will also like you. ------------------------------------------

See above.
Lew
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On 1/11/2014 10:44 PM, Lew Hodgett wrote:

industrial setting, in the US, than in the typical home work shop. In an industrial setting there are numerous machines that are operating all the time. With a power outage all machines stop but do not come back on after workers leave. Imagine all of them powering back up with no one standing around to turn them off when the power is restored.
With the lower voltage machines typically found mostly in home shops it is unlikely that more than one would come back on by itself nor a group of people that might be around when the power is restored, hence you don't often see a magnetic switch being used.
I suspect the magnetic switch is more if an industrial safety measure that many of us enjoy the benefits from in our home shops.
Having said that and IIRC my relatively new Delta 15 inch stationary planer does not have a magnetic switch although it is wired for and runs on 220. I suspect that particular model with its switch configuration is not available as an industrial unit.
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    Mostly -- the contactor would (or should, if wired correctly) break power to both sides of the motor. And -- he can use a lighter duty switch for the "cut complete" switch, so the weight of the arm would be more likely to trip it. A heavy duty switch capable of handling the current of a 1/2 HP motor, and switching both sides of the 240 VAC feed might take too much operating force.
    Also -- the "cut complete" switch can be a momentary contact one, so you lift the arm, reposition the stock for the next cut, let the blade down in contact with the workpiece, and push a button more conveniently located to re-start the motor.
    *And* -- a mushroom switch can be conveniently located to power the motor down if it starts spitting smoke. :-)
    Enjoy,         DoN.
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The best argument for using a contactor in a small bandsaw, is that you can get proper thermal overloads to protect the motor.
i cc

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Can you get just thermal overloads without a contactor?
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