Why use a contactor?

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.

Reply to
DaveC
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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

Reply to
Maynard A. Philbrook Jr.

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.

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Reply to
Jim Wilkins

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Nice show-and-tell. Where can this switch be bought?

Thanks.

Reply to
DaveC

Here's another one I was looking at on Amazon:

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According to the comments it latches ON magnetically, with a solenoid, and acts like 3-wire control. jsw

Reply to
Jim Wilkins

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.

Reply to
clare

But if you want an auto shutoff, low voltage control to a contactor makes it easy.

Reply to
clare

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.

Reply to
Shaun

snipped-for-privacy@snyder.on.ca fired this volley in news: snipped-for-privacy@4ax.com:

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

Reply to
Lloyd E. Sponenburgh

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!

Reply to
DaveC

Not likely.

Reply to
Leon

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.

Reply to
DoN. Nichols

??

Reply to
DaveC

Reply to
Ignoramus13867

It does not happen, or happens so rarely that it is deemed unnecessary 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.

Reply to
Leon

Can you get just thermal overloads without a contactor?

Reply to
DaveC

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.

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Reply to
Jim Wilkins

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.

Reply to
SteveF

Yes.

Reply to
SteveF

Note: The above is in reference to smaller motors, in general. Not something requiring a size 4 motor starter or something on that order.

Reply to
SteveF

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