Why use a contactor?

D

DaveC

Guest
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.
 
DaveC wrote:
http://community.woodmagazine.com/t5/image/serverpage/image-
id/23197i38710354C
C71A806/image-size/medium?v=mpbl-1&px=-1

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

Look on grizzly's web site - they sell them. I got one for my table saw and
love it.

--

-Mike-
mmarlowREMOVE@windstream.net
 
clare@snyder.on.ca fired this volley in
news:a924d9t33tnjrtqblj749uprvfo3j88tpk@4ax.com:

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.
Click to expand...

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
 
On 2014-01-12, DaveC <invalid@invalid.net> wrote:
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?
Click to expand...

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.

--
Remove oil spill source from e-mail
Email: <BPdnicholsBP@d-and-d.com> | (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 ---
 
On 1/11/2014 9:29 PM, Shaun wrote:
"DaveC" wrote in message
news:0001HW.CEF726730137FDD4B01029BF@news.eternal-september.org...

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.
Click to expand...

Not likely.
 
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
On 2014-01-12, DaveC <invalid@invalid.net> wrote:
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.
Click to expand...
 
On 1/11/2014 10:57 PM, DaveC wrote:
Not likely.

??
Click to expand...
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.
 
On 12.01.14 6:36, Leon wrote:
On 1/11/2014 10:57 PM, DaveC wrote:
Not likely.

??

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.
Click to expand...
Thats just silly. 99 percent of switches in
european houses work just fine with 240 volts.
Only the door bell and the thermostat
runs at low volts.....
 
In the US the 240 volts is balanced and you are alot better off switching both hots for a few reasons related to safety. Also about not restarting after a power outage. This shold be enough to get you to pony up the twenty bucks for a relay. the kind they use in an AC condensing unit should do fine.
 
On 01/11/2014 8:41 PM, Mike Marlow wrote:
DaveC wrote:
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.

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!

It may not get you much mileage with your friend. Despite the previous
poster's claim to have "seen this", you really have to consider the
likelihood of such a thing. Wood is an... insulator. Though he may have
"seen this", do you think your friend is going to believe it to be a real
enough possibility for him to consider? There are millions of simple
switches in shops all over the world - and they work. It would probably be
hard to convince your friend their is a real risk here. For good reason...
Click to expand...

Wood may be an insulater but carbon certainly isn't. I suspect what the
provious poster was talking about was burnt bits of sawdust being
conductive. They would be burnt by the sparks present when the switch
was opened...leading to a potentially hazardous situation!

John :-#(#

--
(Please post followups or tech inquiries to the newsgroup)
John's Jukes Ltd. 2343 Main St., Vancouver, BC, Canada V5T 3C9
(604)872-5757 or Fax 872-2010 (Pinballs, Jukes, Video Games)
www.flippers.com
"Old pinballers never die, they just flip out."
 
"DaveC" wrote in message
news:0001HW.CEF726730137FDD4B01029BF@news.eternal-september.org...

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.
 
On Sat, 11 Jan 2014 16:48:51 -0800, DaveC <invalid@invalid.net> wrote:

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.
Click to expand...
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.
 
"DaveC" <invalid@invalid.net> wrote in message
news:0001HW.CEF7435F013EC536B01029BF@news.eternal-september.org...
http://community.woodmagazine.com/t5/image/serverpage/image-
id/23197i38710354C
C71A806/image-size/medium?v=mpbl-1&px=-1

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

Thanks.
Click to expand...

Here's another one I was looking at on Amazon:
http://www.amazon.com/Woodstock-D4160-110-Volt-Paddle-Switch/dp/B005W17HYY/ref=pd_sim_hi_1/191-4569239-2528519

According to the comments it latches ON magnetically, with a solenoid,
and acts like 3-wire control.
jsw
 
>
http://community.woodmagazine.com/t5/image/serverpage/image-
id/23197i38710354C
> C71A806/image-size/medium?v=mpbl-1&px=-1

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

Thanks.
 
"DaveC" <invalid@invalid.net> wrote in message
news:0001HW.CEF726730137FDD4B01029BF@news.eternal-september.org...
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.
Click to expand...

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
 
In article <0001HW.CEF726730137FDD4B01029BF@news.eternal-september.org>,
invalid@invalid.net says...
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.
Click to expand...

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
 
On Sat, 11 Jan 2014 20:49:22 -0500, "Jim Wilkins"
<muratlanne@gmail.com> wrote:

"DaveC" <invalid@invalid.net> wrote in message
news:0001HW.CEF726730137FDD4B01029BF@news.eternal-september.org...
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.

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
Click to expand...
But if you want an auto shutoff, low voltage control to a contactor
makes it easy.
 
DaveC wrote:
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.

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!
Click to expand...

It may not get you much mileage with your friend. Despite the previous
poster's claim to have "seen this", you really have to consider the
likelihood of such a thing. Wood is an... insulator. Though he may have
"seen this", do you think your friend is going to believe it to be a real
enough possibility for him to consider? There are millions of simple
switches in shops all over the world - and they work. It would probably be
hard to convince your friend their is a real risk here. For good reason...

--

-Mike-
mmarlowREMOVE@windstream.net
 
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.
Click to expand...

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!
 
You must log in or register to reply here.

Welcome to EDABoard.com

Sponsor

Back
Top