On Sun, 19 Aug 2007 16:27:02 +1000, Peter Dettmann
wrote:

On Sun, 19 Aug 2007 14:39:50 +1000, Peter Dettmann
wrote:

In the telephony industry these are known as "slugged" relays. Such
relays have a solid copper slug of a specific length - eg. 1/2" or
1"
depending on delay period required - the same diameter as the coil
itself. The slug could either be at the armature end or the heel
end
of the coil depending upon whether a predominantly slow operate or
slow release was required. For ac operation it hardly matters which
end is slugged and if pushed for a part you could use a relay with
a
spare winding on it and simply short circuit this winding to
produce a
"slugging" effect.

No there is a difference here Ross, in the AC relay, the "slug" does
not cover the whole of the magnetic iron path,

I didn't say it did.

it is typically only
applied to about a quarter of the iron circuit. The process is to
delay the decay of flux in that slugged path so that there is a
useful magnetic pull during the time that the un-slugged path has
zero flux,(and therefore zero magnetic pull). Using the DC relay
slug is not really useful for the AC case as it covers the whole
magnetic path.



I beg to differ. As others have described, A relay, whether DC or AC
is simply a coil of wire on a core of suitable magnetic material
(usually soft iron) with a closed magnetic loop which passes through
the pivoted armature. It is only the inclusion of a delaying mechanism
- in the AC case, a shorted copper turn or slug - which results in a
delay to ensure the armature stays held while the AC curent passes
through each half cycle.

For completeness I should have added that we did extensively use
relays on AC fed from a full wave bridge rectifier, and without
capacitor for smoothing. This gave a tendency to chattering as there
is still a pulsating current to the relay, however this chattering
was overcome by the use of an armature end slug (as you describe)
which was only about 1/16" long.

For DC operation an armature end slug produces a predominantly "slow
operate" function because the slug produces an opposing magnetic field
to that produced by the winding when energised. Only after the field
which is set up by the winding has stabilised, and the corresponding
field produced by the slug collapses to zero, does the armature pull
in. While an armature end slug also delays the release of the
armature, it predominantly affects the operate time. If you required
the relay to operate as fast as possible the slug must be on the heel
end and not the armature end. In most AC operation situations, unless
either fast operate or fast release is required, it hardly matters
whether the actual delay occurs on closing or opening the magnetic
circuit.

A longer slug could be used, but fast operating speed was critical.

A longer slug simply increases the length of the delay period. As I
said earlier, for the fastest operation the slug must be on the heel
end (furthest from the armature) of the winding. In cases where fast
operation and slow release (or vice versa) is required, a relay will
often employ a secondary winding which can be shorted or opened as the
case requires, to produce the slugging effect, rather than using a
fixed copper slug.