I wast just browsing a DC thread where "magnetic switches" are
discussed. Not the first time I've seen the term, and I know what they
were talking about, but I DAGS and the first 40 or 50 hits mostly
related to burglar alarm type switches, and none referred to the "punch
red for off, punch green for on, don't power the tool up after a power
failure" type switch that makes sense for woodworking and other
machinery. Grizzly's web site lists them as magnetic switches, but a
search at Grainger's web site for "magnetic switch" returned alarm type
switches and relays, and at McMaster-Carr returned alarm switches. I
didn't keep searching very long, so I didn't find what I was looking for
at either site.
I guess what I'm getting around to is a question: Is there another name
for this type of switch?
They're very simple devices. The actual switch for power to the tool is
a relay. The relay's coil gets power either through a set of contacts
on the relay itself, or applied via a normally open momentary switch
(the "on" button). A normally closed momentary switch in series with
the coil power is the "off" switch.
A relay with a 110VAC coil and appropriately rated contacts, a NC
pushbutton (red for off) and a NO pushbutton (green for on) and an
appropriate enclosure are all you need to make one of these.
If one wanted a separate start/stop switch for the dust collector at
every tool, it would be easy for the electrically inclined woodworker to
build a switch that would support a remote start/stop pair for the DC at
every station. All of the start buttons would be in parallel, while all
of the stop buttons would be in series (this could be obnoxious if you
have a bad connection on the "stop" side). I'd probably use a 24VAC for
the control side, which would mean a 3 pole relay or an extra 24VAC
There are also current sensing controls. Attach one of these devices to
a power wire on a tool (there is no electrical connection, it senses the
magnetic field caused by the current flowing through the wire) and use
it to apply power to the DCs relay. I couldn't find these right off,
either. anyone know what they're called?
just random thoughts and questions....
Lots. There are also two sorts of "magnetic switch" - one good, one
Most of the names are generic though and apply equally to either sort of
switch. In the UK they're frequently called NVR switches (no-volt
release, because they automatically turn off if the power fails, so the
machine can't start unexpectedly if the power comes back).
You may also hear them called "starters". These are inaccurate shorthand
for "motor starters", aka "star-delta switches", a complex time-switch
used for 3-phase motors. They start the motor in high-torque mode, then
re-arrange the supply to the three windings for more efficient running.
The good ones are "contactors" (actually the contactor is just one
component inside) These are electrically activated switches, moved
entirely by an electromagnet's field. They're controlled by two small
low current pushbuttons, which may be mounted remotely.
The bad ones are usually called "magnetic switches". The contact is made
by pressing the on button, and held in place by a mechanical latch
(just like a non-magnetic switch). However this latch relies on a
magnetic solenoid too, hence the name and their no-volt release
behaviour. They have several drawbacks:
- They're often cheaply made. They can tend to switch off owing to
- You can only have one "on" button, and it's mounted directly on the
switch. You can't remote control them.
- Their NVR behaviour isn't reliable. They're not permitted for many
industrial machines acccording to UK HSE rules.
- Although it's theoretically possible, you usually can't access the
connection need to add further "off" buttons. With a real contactor you
can easily add knee or pedal E-stop switches to a machine.
Either type can also have over-load windings added to protect the motor.
Personally I only use the contactor sort. I've replaced the magnetic
catch sort, except I think for just one left - and that's going when I
get round to it.
Cats have nine lives, which is why they rarely post to Usenet.
Use a regular relay or contactor. Energize coil with 2 momentary
contact switches, one a normally open (Start) and the other a normally
closed (Stop), in series. One end of switches in series to AC hot
(Black wire) and other end of series switches to switched output of
relay or contactor. Add appropriate resistor in line to avoid burning
out coil in case it's not the voltage you're switching.
On Wed, 15 Jun 2005 09:43:58 -0500, the opaque Joe User
"Magnetic Starter Switch" was the old full name I learned.
Crapsman tools are their own punishment
http://diversify.com Comprehensive Website Design
Make a search for "magnetic contactor" - that's the correct term for
switches used for most woodworking machines. Andy gave a pretty
comprehensive description. Motor starters are contactors for use when
the motors don't have integral overload devices. Starters combine a
contactor with an overload device and an incoming line disconnect
device (either a fused switch or circuit breaker).
Hope this helps,
HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.