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.
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.
On 2014-01-13, Lloyd E. Sponenburgh <lloydspinsidemindspring.com> wrote:
O.K. I won't take it personally. :-)
Though some which are made to mount behind a trim plate in the
wall (typical home light switch) may be more open to a buildup of swarf
inside it. I've seen various failures in home electrical hardware which
I would not have expected.
Among those, there was an outlet which failed during a
nearby lightning strike. The form of the failure was a breakdown in the
hole into which a drive screw went to hold the ground strap which mounts
it to the outlet box. It was *supposed* to be a blind hole, but they
had used a slightly too long drive screw, or a chip was under it, and
there was a little of the inside surface which broke away.
Still -- normally not a problem. However, the lightning strike
introduced a high enough voltage so the gap to the nearby hot lead was
bridged, and the subsequent arc, both partially melted the "hot" inside
the socket, and welded it to what was plugged into it (one of those
duplex to six outlet plates) -- *and* filled the vicinity with carbon
When I got home from work and discovered the power loss in the
living room, I went downstairs and switched on the breaker. Five second
delay and BZZZZTTT-CLICK. A repeat got the same results. I then walked
around the house (old wiring, given breakers showed up in a lot of
outlets around the house.
I then walked around the house, until I smelled the burnt
Bakelite. Of course the outlet was behind a bookcase which I had to
empty and move to get to it. Then I discovered that the multi-outlet
adaptor would not unplug until I applied a lot of force, and that left
one pin of the adaptor in the socket.
I then removed the socket, and replaced it with a new one -- and
just had to take it apart to see what had happened.
If the Bakelite had not flaked around the drive screw, it would
not have broken down -- and the failure would have been somewhere else
at an even higher voltage -- or maybe not.
So -- I don't *depend* on any commercial electrical hardware for
the home to do what it should do.
And -- once I needed a toggle switch which did not introduce
vibration when it was operated to put in the mounting plate for a
turntable, so I could cue a record, and then switch on the motor at the
appropriate time. At that time, you could get "silent" light switches
which contained a puck of mercury in glass and metal end caps. It was
designed to work in a vertical orientation, but it was possible to take
the switch mounting plate off, and file different notches into the
toggle handle so it would hold the puck for horizontal operation. Now,
that switch was *not* designed to keep swarf out, so it would have to
depend on the decorative plate which goes over it. Granted, no normal
person would mount it as I did -- and I never had swarf near the
turntable, so it was no problem. But I do remember that switch, and how
easy it would be for swarf or sawdust to work its way in. Sawdust would
just make it take a bit more force to operate. Metal swarf would bridge
the ends of the puck and leave it on full time.
Granted -- most of the horizontal/vertical bandsaws use a normal
bat-handle toggle switch, but there are variations in quality there.
The best have a seal around the ball on which the bat rotate, and have
sealed wire entrance on the back. Cheap ones either have terminals on
the back (which you screw, solder wires to, or tabs for (hopefully)
insulated push-on crimp terminals -- but still metal swarf could bridge
that to the frame.
Given that he is already "jiggering up" the bandsaw -- replacing
the single phase 120 VAC motor with a higher horsepower single phase 240
VAC motor, you introduce another problem. Ideally (given USA wiring,
where 240 VAC is really two 120 VAC wires 180 degrees out of phase, so
they produce 240 VAC between them), you want a switch which interrupts
both sides of the power -- so a winding failure in the motor does not
leave some part of the system perhaps floating at 120 VAC instead of
near ground as it should be. Now, -- in the UK you don't normally have
240 VAC with a grounded center tap. Instead, one side of the 240 VAC is
*And* -- one which interrupts both sides of the line, since he
is putting in a 240 VAC motor.
Good enough. Remember that the motor is being changed here, so
at least some change in the switch is called for.
For special switches, somewhere I still have a switch designed
for reversing a single phase motor. It runs in either direction
(reversing two of the three circuits), but it enforces a pause when
switching from one direction to the other -- since running single phase
motors can't be instantly reversed just by swapping two wires. They
have to be allowed to slow down enough to come to a near halt (enough so
the centrifugal switch closes to enable the start winding for the
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. :-)
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.
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.
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, 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.
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.
I agree, but my question is
"Why on earth would frind want the saw to turn on after a power
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
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.
Husk kørelys bagpå, hvis din bilfabrikant har taget den idiotiske
beslutning at undlade det.
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.
This may be a duplicate post
What you are describing is known as under voltage protection.
Safety pure and simple is the best reason for using it.
What happens in the event you lose power and as a result you lose
as well as power to the saw?
Without under voltage protection, when power is restored, the saw
will restart automatically which can be disastrous, especially if the
Your insurance company will also like you.
I suspect that because most 220 volt machines are used more in an
industrial setting, in the US, than in the typical home work shop. In
an industrial setting there are numerous machines that are operating all
the time. With a power outage all machines stop but do not come back on
after workers leave. Imagine all of them powering back up with no one
standing around to turn them off when the power is restored.
With the lower voltage machines typically found mostly in home shops it
is unlikely that more than one would come back on by itself nor a group
of people that might be around when the power is restored, hence you
don't often see a magnetic switch being used.
I suspect the magnetic switch is more if an industrial safety measure
that many of us enjoy the benefits from in our home shops.
Having said that and IIRC my relatively new Delta 15 inch stationary
planer does not have a magnetic switch although it is wired for and runs
on 220. I suspect that particular model with its switch configuration
is not available as an industrial unit.
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. :-)
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