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.

In article ,
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.


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

"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.

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...v=mpbl-1&px=-1

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

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

Thanks.

"DaveC" wrote in message
...

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

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

Thanks.

Here's another one I was looking at on Amazon:
http://www.amazon.com/Woodstock-D416...569239-2528519

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

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

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


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

--

-Mike-

On Sat, 11 Jan 2014 16:48:51 -0800, DaveC 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.
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.

On Sat, 11 Jan 2014 20:49:22 -0500, "Jim Wilkins"
wrote:


"DaveC" wrote in message
al-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...v=mpbl-1&px=-1


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

"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.

fired this volley in
:

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.


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

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!

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...

--

-Mike-

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


"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.


Not likely.

On 2014-01-12, DaveC 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?

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: | (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 ---

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 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.

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.

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.

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...


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."

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

Can you get just thermal overloads without a contactor?

"DaveC" wrote in message
...
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!

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/prod...s_Hinge_Boxes/

On 1/12/2014 8:01 AM, Jim Wilkins wrote:


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/prod...s_Hinge_Boxes/


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.

On 1/12/2014 1:56 AM, DaveC wrote:
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

Can you get just thermal overloads without a contactor?


Yes.

On 1/12/2014 8:38 AM, SteveF wrote:
On 1/12/2014 8:01 AM, Jim Wilkins wrote:


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/prod...s_Hinge_Boxes/


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.

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

On 1/12/2014 12:45 AM, John Robertson wrote:
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...


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 :-#(#


Regardless, there is still electricity in that switch, just a lower
voltage. Burnt bits of saw dust can burn at a lower voltage sparks as
well as a higher voltage spark.

I suspect that the previous poster witnessed a defective or broken switch.


FWIW this links to the purpose and or how a magnetic switch/contactor works.

http://www.ask.com/question/how-does...contactor-work

"Burnt bits of saw dust ..."

Bandsaws are also used to cut metal. No burning necessary.

"SteveF" wrote in message
...
On 1/12/2014 8:01 AM, Jim Wilkins wrote:


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/prod...s_Hinge_Boxes/


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.

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.

On Sat, 11 Jan 2014 22:45:56 -0800, John Robertson
wrote:

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...


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 :-#(#
Or dust physically jamming the motion of the switch contacts - - -

On 2014-01-12, 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.

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.

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.

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 would probably be
hard to convince your friend their is a real risk here. For good reason...

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.

--
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 ---

On 2014-01-12, Ignoramus13867 wrote:

[ ... ]

On 2014-01-12, DaveC 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?"

[ ... ]

What are the arguments for using a contactor as opposed to a simple on-off
switch of equivalent rating?

[ ... ]

The best argument for using a contactor in a small bandsaw, is that
you can get proper thermal overloads to protect the motor.

i

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.)

A proper motor starter (of which the contactor is a part)
includes thermal sensors for the current through each winding, and if
any one of the three goes well above the normal full-load running
current (as will happen if one phase dies), it will interrupt the power
to the motor and stop it before the motor is damaged. And it also
protects the windings from burning out when trying to start with one
phase of three missing.

And actually, some starters only measure the current through two
of the three legs, because if one is out, one of the two monitored will
be over-current enough to trip it.

So -- with a single phase 240 VAC motor, the over-current from
loss of a phase would not apply.

But the other reasons for using a motor starter/contactor still
stand.

Enjoy,
DoN.

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On 2014-01-12, DaveC wrote:
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

Can you get just thermal overloads without a contactor?

Not really. The thermal overloads sense the current in each
winding, and generate heat to soften either a wax or a low temperature
solder to allow a wheel to rotate and a low-current contact to open.
All of the phases have their contacts in series, and in series with the
coil of the contactor. Any one of them opening drops the contactor out
to protect the load.

With single phase, a slow-blow fuse in series with each hot lead
should sufficient, but with three phases, you need to sense any
condition of over-current in any single phase, and to drop everything
once it is sensed. (Of course, if you are running the three-phase motor
from a VFD, that incorporates the functions of the starter (current
sensing and automatic shutdown) in the electronics of the VFD.

Enjoy,
DoN.n

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DoN. Nichols wrote:

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.


You are absolutely correct Don - and thank you for that correction. I got
sidetracked by the one poster who mentioned that he had once seen sawdust
create a short. While I disagree that such is a common or even an
anticipated occurance, I do acknowkedge your clarification - you point is
well taken.


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.

Yes... and the earth could have been flat before centrifigul force changed
all that. My point Don is that yes - things are possible - probable, or
even common in the real world is something different.




It is at least a *possible* one.

I will concede that anything is possible - even my wife deciding... well...
we won't go there... Just because something is possible does not put it in
the realm of probable, practical, or even worth thinking about. I am not
saying that your comments are not worth consideration with that statement,
I'm simply trying to make your statement that it is at least possible as
meaningless in its context as I can.

--

-Mike-

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.

i

On 2014-01-12, Gunner Asch wrote:
On Sat, 11 Jan 2014 23:56:48 -0800, DaveC wrote:

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

Can you get just thermal overloads without a contactor?

Yes..but they are NOT..NOT fuses or circuit breakers. They are
designed to carry SMALL current loads..often times much smaller than
the motors they are protecting.

They may read the current..but the internal switches are too small to
handle repeated ons/offs


http://www.sensata.com/klixon/thermo...ision-7bt2.htm

they describe it as a "thermostat" and rate the contacts at 10A 240V.

probably enough for a 1/2 HP motor.

but if you need the cut-out to switch both terminals that's not going
to suit.

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"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

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?

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On 13/01/14 12:11, Lloyd E. Sponenburgh 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...


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.

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.


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

David Billington fired this volley in
:

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.

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

"David Billington" wrote in message
...

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.

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.