On Wed, 15 Jun 2005 09:43:58 -0500, Joe User wrote:

I guess what I'm getting around to is a question: Is there another name
for this type of switch?

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.


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