another name for "magnetic switch"?

[Joe User]

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 transformer.

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....

-j

[Bill Waller]

____________________ Bill Waller New Eagle, PA

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[Rumpty]

Woodwork magazine recently had an article on making your own. The Grizz switches work well, I use them in my shop.

[Andy Dingley]

Lots. There are also two sorts of "magnetic switch" - one good, one cheap.

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 machine vibration.

- 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.

[Tom Quackenbush]

Try "motor starter" or "NEMA starter".

R, Tom Q.

Remove bogusinfo to reply

[Larry Jaques]

On Wed, 15 Jun 2005 09:43:58 -0500, the opaque Joe User spake:

"Magnetic Starter Switch" was the old full name I learned.

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[Roy Smith]

I learned it was called a "locked out relay".

[Mortimer Schnerd, RN]

Do you have a URL to a source of supply?

[Andy Dingley]

No - I buy mine by walking into a local shop. I'm not in the USA either, so various voltages and practices would be inappropriate.

[Duane Bozarth]

Grainger is one...

[CW]

Could that have been "latching relay"? That's what they consist of.

[mschips]

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.

-- Teri

[John Sellers]

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, John Sellers

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