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.
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!
On Mon, 13 Jan 2014 06:11:15 -0600, "Lloyd E. Sponenburgh"
<lloydspinsidemindspring.com> wrote:
>"DoN. Nichols" <[email protected]> fired this volley in
>news:[email protected]:
>
>> 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
Last year.. I replaced 5 switches in Delta /Milwaukee table saws
alone.
__
"Anyone who thinks Obama is doing a good job
is either stupid or a perpetual societal leech"
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On 1/11/2014 9:29 PM, Shaun wrote:
>
>
> "DaveC" wrote in message
> news:[email protected]...
>
> 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 1/13/2014 7:01 AM, 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?
>
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.
On Mon, 13 Jan 2014 14:01:44 +0100, Leif Neland <[email protected]>
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.
__
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is either stupid or a perpetual societal leech"
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On 13/01/14 12:11, Lloyd E. Sponenburgh wrote:
> "DoN. Nichols" <[email protected]> fired this volley in
> news:[email protected]:
>
>> 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
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)
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(604)872-5757 or Fax 872-2010 (Pinballs, Jukes, Video Games)
www.flippers.com
"Old pinballers never die, they just flip out."
In article <[email protected]>,
[email protected] 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
[email protected] fired this volley in
news:[email protected]:
> 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
"DoN. Nichols" <[email protected]> fired this volley in
news:[email protected]:
> 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
David Billington <[email protected]> fired this volley in
news:[email protected]:
> 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
"Shaun" <[email protected]> fired this volley in
news:[email protected]:
> 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
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.
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:
> 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
>
On Sat, 11 Jan 2014 16:48:51 -0800, DaveC <[email protected]> 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.
"Lloyd E. Sponenburgh" wrote in message
news:[email protected]...
"DoN. Nichols" <[email protected]> fired this volley in
news:[email protected]:
> 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
On Sat, 11 Jan 2014 22:45:56 -0800, John Robertson <[email protected]>
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 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-a-magnetic-contactor-work
On 2014-01-13, DoN. Nichols <[email protected]> wrote:
> On 2014-01-12, Ignoramus13867 <[email protected]> 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 14 Jan 2014 03:42:58 GMT, "DoN. Nichols" <[email protected]>
wrote:
>On 2014-01-13, Gunner Asch <[email protected]> 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?
Nope..the standare Delta toggle switches and in 2 of them..push button
switches. Red for stop, green for run, with a mechanical mechanism
between them.
>
> 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.
Nope..I hit em with a BFH and dumped the switch in the trashcan..after
fanning away a cloud of fine sawdust from the guts of the switch.
The pushbotton switches..I cleaned the copious amount of sawdust out
of them..and replaced the switches inside
Gunner
__
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"DaveC" <[email protected]> wrote in message
news:[email protected]...
>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
"DaveC" <[email protected]> wrote in message
news:[email protected]...
>>
> 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.
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
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?
>
Look on grizzly's web site - they sell them. I got one for my table saw and
love it.
--
-Mike-
[email protected]
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-
[email protected]
"DaveC" <[email protected]> wrote in message
news:[email protected]...
>> 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/products/view/JIC_Continuous_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/products/view/JIC_Continuous_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 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/products/view/JIC_Continuous_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 1/11/2014 10:33 PM, 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.
Disagree completely.
The likelihood of the failure the other respondent noted being sawdust
"turning on" the saw is remote at best. Almost certainly it was a
failure in the switch, not the sawdust imo.
Sawdust is far more likely to cause a switch to fail to make contact.
The prime advantage of motor contactors for the small motors of the ilk
in home shops is the one noted of fail-safe operation on restart in the
(admittedly unlikely) case of power failure.
Second is the low-voltage contacts are more reliable over the longer
haul, but again for most recreational/hobby users the demand for direct
switching is simply not enough to make it be a real issue.
I use them universally simply because I like the tactile feel of the
starter button vis a vis the mechanical switch.
The one other advantage but is only really useful for the DC is that
it's much simpler to wire a remote umbilical cord to it for access from
afar than it is to wire inline switches.
--
"SteveF" <[email protected]> wrote in message
news:[email protected]...
> 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/products/view/JIC_Continuous_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.
"dpb" <[email protected]> wrote in message
news:[email protected]...
>
> The prime advantage of motor contactors for the small motors of the
> ilk in home shops is the one noted of fail-safe operation on restart
> in the (admittedly unlikely) case of power failure.
>
The only machines I own that can feed themselves into the cut are a
wood planer and a 4" x 6" horizontal bandsaw. The spring-balanced
bandsaw doesn't mind being shut off and restarted in the cut. Would a
hydraulically damped bandsaw start safely after power had been out?
That might be a good test just before you replace the blade. If the
motor won't start you'll be there to switch it off.
I don't trust my bandsaw to run unattended anyway.
jsw
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-
[email protected]
On 2014-01-12, Gunner Asch <[email protected]> wrote:
> On Sat, 11 Jan 2014 23:56:48 -0800, DaveC <[email protected]> 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/thermostat-precision-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.
--
For a good time: install ntp
--- news://freenews.netfront.net/ - complaints: [email protected] ---
"David Billington" <[email protected]> wrote in message
news:[email protected]...
>
> 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.
"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
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
"Larry W" <[email protected]> wrote in message
news:[email protected]...
> 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
>
> --
> Make it as simple as possible, but not simpler. (Albert Einstein)
>
> Larry W. - Baltimore Maryland - lwasserm(a)sdf. lonestar. org
---
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On Sat, 11 Jan 2014 23:56:48 -0800, DaveC <[email protected]> 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
__
"Anyone who things Obama is doing a good job
is either stupid or a perpetual societal leech"
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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?
--
Husk kørelys bagpå, hvis din bilfabrikant har taget den idiotiske
beslutning at undlade det.
Gunner Asch kom med følgende:
> On Mon, 13 Jan 2014 14:01:44 +0100, Leif Neland <[email protected]>
> 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
--
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beslutning at undlade det.
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
On Sat, 11 Jan 2014 20:49:22 -0500, "Jim Wilkins"
<[email protected]> wrote:
>
>"DaveC" <[email protected]> wrote in message
>news:[email protected]...
>>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
>
>
But if you want an auto shutoff, low voltage control to a contactor
makes it easy.
"DaveC" wrote in message
news:[email protected]...
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 Sun, 12 Jan 2014 13:56:33 -0500, [email protected] wrote:
>On Sat, 11 Jan 2014 22:45:56 -0800, John Robertson <[email protected]>
>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 - - -
Indeed.
I do service work on the side for a So Cal tool buyer/seller and the
number of switches Ive pulled out of wood working machinery jammed
solid with sawdust is huge. Even so called "sealed" switches.
Gunner
__
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is either stupid or a perpetual societal leech"
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On Mon, 13 Jan 2014 21:04:37 +0100, Leif Neland <[email protected]>
wrote:
>Gunner Asch kom med følgende:
>> On Mon, 13 Jan 2014 14:01:44 +0100, Leif Neland <[email protected]>
>> 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"
I would hope that it wouldnt.
>
>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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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 <[email protected]> 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 2014-01-12, DaveC <[email protected]> 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.
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--- Black Holes are where God is dividing by zero ---
On 2014-01-12, Mike Marlow <[email protected]> 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.
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On 2014-01-12, Ignoramus13867 <[email protected]> wrote:
[ ... ]
> On 2014-01-12, DaveC <[email protected]> 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 <[email protected]> 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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On 2014-01-13, Lloyd E. Sponenburgh <lloydspinsidemindspring.com> wrote:
> "DoN. Nichols" <[email protected]> fired this volley in
> news:[email protected]:
>
>> 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.
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On 2014-01-13, David Billington <[email protected]> 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.
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On 2014-01-13, Gunner Asch <[email protected]> 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.
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On 2014-01-13, Ignoramus28861 <[email protected]> wrote:
> On 2014-01-13, DoN. Nichols <[email protected]> wrote:
>> On 2014-01-12, Ignoramus13867 <[email protected]> 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.
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